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Studies in the thermal decomposition of the isomeric hexanes Chrysochoos, John 1964

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STUDIES IN THE THERMAL DECOMPOSITION OF THE ISOMERIC EEXANES "by JOHN CHRYSOCEOOS DIPLOMA of CHEMISTRY, The University of Athena, 19 M.So.,The University of B r i t i s h Columbia, 196E A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of Chemistry We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1964 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that per-m i s s i o n f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i -c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission.. A c t i n g Chairman, Department of Chemistry Department of The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8, Canada Date ABSTRACT An investigation has "been made of the pyrolysis of n-hexane, S-methyl-pentane, 3-methyl-pentane, 2, 8-dimethyl-"butane and 2,3-dimethyl-butane i n a s t a t i c system at temperatures "between 490° and 530°C. The normal pyrolysis and also the pyrolysis i n h i b i t e d "by n i t r i c oxide have "been studied. The effect of v a r i a t i o n of i n i t i a l hydrocarbon pressure on the d i s t r i b u t i o n of products and on ener-gies of a c t i v a t i o n and frequency factors was determined for the uninhibited reactions. The effect of v a r i a t i o n i n surface-to-volume r a t i o on rates and on product d i s t r i b u t i o n has also "been investigated. A study of factors governing o v e r a l l reaction rates has provided information on the kineti c s of chain termination processes. A l l pyrolyses at low pressures are s i g n i f i c a n t l y i n h i b i t e d "by reactions occurring on the surface of the quartz vessel. Mech-anisms have been proposed to account q u a l i t a t i v e l y for observed decomposition products and the kineti c s of pyrolysis of each of the isomers. In the reactions in h i b i t e d by n i t r i c oxide, the effect of the pressure of n i t r i c oxide on the rates and product d i s t r i b u t i o n s has been examined. The effect of an increase i n surface-to-volume r a t i o on reaction rates and on product d i s t r i b u t i o n s has been i n -vestigated. The pressure of NO required for maximum i n h i b i t i o n appears to be independent of the p a r t i a l pressure of the hydrocar-bon and also of the nature of the l a t t e r . i i i The kinetic and a n a l y t i c a l evidence indicates that n i t r i c oxide acts "both homogeneously and heterogeneously as an i n h i b i t o r . Significant consumption of n i t r i c oxide was found for a l l branched isomers. The reduction i n i n h i b i t i o n observed as the branching of the isomers increases i s attributed i n part to increased heter-ogeneous i n i t i a t i o n reactions, from an increase i n the extent of adsorption of the more branched isomers r e l a t i v e to the less-branched isomers. A mechanism i s proposed for the i n h i b i t e d pyrolysis of each of the isomers, which accounts q u a l i t a t i v e l y for the experimental r e s u l t s . A major conclusion drawn from the present study i s that n i t r i c oxide acts as an i n h i b i t o r through reactions occurring on the surface of the reaction vessel as w e l l as through reactions occurring i n the gas phase. W. A. Bryce xx iv ACKNOWLEDGMENT The present investigation was car r i e d out under the supervision of Dr. W. A. Bryoe, to whom the author i s greatly indebted. The author i s also indebted to the University of B r i t i s h Columbia (Chemistry Department) for the teaching assistantship offered to him during the completion of the present work (1962-1964). i v CONTENTS Page. CHAPTER I. INTRODUCTION The uninhibited decomposition of paraffins • 2 Heterogeneous reactions i n the hydrocarbon pyrolyses? The i n h i b i t e d decomposition 12 Theories of i n h i b i t i o n 19 Objectives of the present study 25 CHAPTER I I . EZPERIMENT AL Materials and methods 26 Gas chromatographic apparatus 27 Infra-red analysis 28 The vacuum system 28 Single furnace 30 Double furnace 30 Description of a t y p i c a l experiment 31 CHAPTER I I I . RESULTS Uninhibited pyrolysis 35 Rates of reaction 35 " ,T " for n-hexane 36 " " " for 2-methyl-pentane 40 " " " for 3-methyl-pentane 40 V Page. Rates of reaction for neohexane 40 « » " for diisopropyl 46 The effect of.hydrocarbon structure on rate 46 Order of reaction 46 n tt tt f o r n-hexane 56 » » for 2-methyl-pentane 56 " " " for 3-methyl-pentane 62 " " " for neohexane 62 " •» » for diisopropyl 62 Energies of a c t i v a t i o n and frequency factors for the o v e r a l l reaction 67 - " - for n-hexane 70 - " - for 2-methyl-pentane 70 - " - for 3-methyl-pentane 71 - " - for neohexane 72 - " - for diisopropyl 72 A n a l y t i c a l r e s u l t s 73 " " for n-hexane 73 n " for 2-methyl-pentane 74 " " for 3-methyl-pentane 75 " " for neohexane 77 " " for diisopropyl 79 The v a r i a t i o n of product d i s t r i b u t i o n with time 81 - " - for n-hexane 81 - " - for 2-methyl-pentane 81 v i Page. The v a r i a t i o n of product d i s t r i b u t i o n with time - » - for 3-methyl-pentane 81 - " - for neohexane 82 - " - for diisopropyl 82 Variation of product d i s t r i b u t i o n with time 87 - " - for n-hexane 87 - " - for 2-methyl-pentane 87 - ,T - for 3-methyl-pentane 87 - " - for neohexane 91 - " - for diisopropyl 91 Variation of product ratios with pressure 91 - " - for n-hexame 91 - " - for 2-methyl-pentane 94 - " - for 3-methyl-pentane 95 - " -r for neohexane 97 - " - for diisopropyl 98 Effect of packing on product d i s t r i b u t i o n 99 - M - for n-hexane 99 - " - for 2-methyl-pentane 99 - " - for diisopropyl 101 Inhibited pyrolyses 102 The effect of n i t r i c oxide on reaction rates 102 - n - for n-hexane 102 - " - for 2-methyl-pentane 103 - " - for 3-methyl-pentane 111 - " - for neohexane 111 - " - for diisopropyl 116 v i i Page. The effect of structure on rates 125 A n a l y t i c a l r e s u l t s 127 " » for n-hexane 128 « " for 2-methyl-pentane 128 » " for 3-methyl-pentane 130 " " for neohexane 131 ,T " for diisopropyl 131 Var i a t i o n of product d i s t r i b u t i o n with time for the f u l l y i n h i b i t e d decomposition 131 - " - for n-hexane 132 - " - for 2-methyl-pentane 132 - " - for 3-methyl-pentane 135 - " - for neohexane 135 - " - for diisopropyl 135 Variation of product d i s t r i b u t i o n with n i t r i c oxide pressure and surfaee-to-volume r a t i o 135 - " - for n-hexane 139 - " - for 2-methyl-pentane 139 - " - for 3-methyl-pentane 146 - " - for neohexane 158 - n - for diisopropyl 158 Variat i o n of product r a t i o with n i t r i c oxide pressure and surface-to-volume r a t i o 168 - " - for n-hexane 168 - " - for 2-methyl-pentane 172 - " - for 3-methyl-pentane 172 v i i i Page. Variation of product r a t i o with n i t r i c oxide pressure and surface-to-volume r a t i o - » - for neohexane 182 - " - for diisopropyl 182 Comparative adsorption of isomeric hexanes with respect to n i t r i c oxide on s i l i c a wool 191 - " - : n-hexane 192 - " - : 2-methyl-pentane 193 - " - : 3-methyl-pentane 194 - " - : neohexane 195 - " - : diisopropyl 196 Consumption of n i t r i c oxide during pyrolysis 197 Summary of re s u l t s 198 CHAPTER IV. DISCUSSION The uninhibited pyrolysis 203 The mechanism of pyrolysis of n-hexane 211 The o v e r a l l mechanisms for n-hexane pyrolysis 223 The mechanism of pyrolysis of 2-methyl-pentane 225 The o v e r a l l mechanism for 2-methyl-pentane 234 The mechanism of pyrolysis of 3-methyl-pentane 238 The o v e r a l l mechanism for 3-methyl-pentane 243 The mechanism of pyrolysis of neohexane 245 The o v e r a l l mechanism for neohexane 250 The mechanism of pyrolysis of diisopropyl 253 The o v e r a l l mechanism for diisopropyl pyrolysis 257 i x Page. The i n h i b i t e d decomposition 259 The mechanism of the i n h i b i t e d pyrolysis of n-hexane _ 263 The mechanism of the i n h i b i t e d pyrolysis of 2- methyl-pentane 270 The mechanism of the i n h i b i t e d pyrolysis of 3- methyl-pentane 273 The mechanism of the i n h i b i t e d pyrolysis of neohexane 275 The mechanism of the in h i b i t e d pyrolysis of diisopropyl 277 Summary 278 REFERENCES 282 APPENDIX I 288 APPENDIX I I 294 X LIST OF TABLES Table. Page. 1 Varia t i o n of a c t i v a t i o n energy and frequency faotor with i n i t i a l pressure of n-hexana 70 2 Va r i a t i o n of a c t i v a t i o n energy and frequency faotor with i n i t i a l pressure of 2-methyl-pentane 71 3 Variation of a c t i v a t i o n energy and frequency factor with i n i t i a l pressure of 3-methyl-pentane 71 4 Va r i a t i o n of a c t i v a t i o n energy and frequency factor r.-:.t with i n i t i a l pressure of neohexane 72 5 Variation of a c t i v a t i o n energy and frequency factor with i n i t i a l pressure of diisopropyl 73 6 A n a l y t i c a l r e s u l t s (mole fo) for the decomposition of n-hexane 73 7 A n a l y t i c a l r e s u l t s (mole fo) for the decomposition of 2- methyl-pentane 74 8 A n a l y t i c a l r e s u l t s (mole fo) for the decomposition of 3- methyl-pentane 76 9 A n a l y t i c a l r e s u l t s (mole fo) for the decomposition of 3-methyl-pentane 76 10 A n a l y t i c a l r e s u l t s (mole fo) for the decomposition of 3-methyl-pentane 77 11 A n a l y t i c a l r e s u l t s (mole fo) for the decomposition of neohexane 78 12 A n a l y t i c a l r e s u l t s (mole fo) for the decomposition of neohexane 79 x i Table. Page. 13 A n a l y t i c a l r e s u l t s (mole fo) for the decomposition of neohexane 79 14 A n a l y t i c a l r e s u l t s (mole fo) for the decomposition of diisopropyl 79 15„..iiAnalytieal re s u l t s (mole fo) for the decomposition of diisopropyl 80 16 Variation of product d i s t r i b u t i o n (mole fo) with time for the decomposition of 3-methyl-pentane 81 17 Va r i a t i o n of product r a t i o s with pressure for n-hexane 94 18 Va r i a t i o n of product r a t i o s with pressure for 3-methyl-pentane 94 19 Varia t i o n of product r a t i o s with pressure for 2-methyl-pentane 95 20 Va r i a t i o n of product r a t i o s with pressure for 3-methyl-pentane 96 21 Variation of product r a t i o s with pressure for 3-methyl-pentane 96 22 Variation of product r a t i o s with pressure for 3-methyl-pentane 97 23 Va r i a t i o n of product r a t i o s with pressure for neo-hexane 98 24 Variat i o n of product r a t i o s with pressure for neo-hexane 98 25 Varia t i o n of product r a t i o s with pressure for diisopropyl 99 • x i i Table. Sage, 26 Variation of product r a t i o s with pressure for n-hexane i n packed and unpacked vessels 100 27 Variation of product r a t i o s with pressure for 2-methyl-pentane i n packed and unpacked vessels 100 28 Variation of product r a t i o s with pressure for diisopropyl i n packed and unpacked vessels 101 29 Va r i a t i o n of percentage reduction i n rate with i n i t i a l hydrocarbon pressure and S/V r a t i o s for various isomeric hexanes 127 30 A n a l y t i c a l r e s u l t s (mole fo) for various NO pressures i n the decomposition of n-hexane 129 31 A n a l y t i c a l r e s u l t s (mole fo) for various NO pressures i n the decomposition of 2-methyl-pentane 129 32 A n a l y t i c a l r e s u l t s (mole fo) for various NO pressures i n the decomposition of 3-methyl-pentane 130 33 A n a l y t i c a l r e s u l t s (mole fo) for various NO pressures i n the decomposition of neohexane 131 34 A n a l y t i c a l r e s u l t s (mole $>) for various NO pressures i n the decomposition of diisopropyl 132 35 Variat i o n of n-hexane/NO r a t i o with i n i t i a l pressure of the mixture af t e r adsorption on quartz wool. 192 36 Variation of n-hexane/NO r a t i o with i n i t i a l pressure of the mixture after adsorption on quartz wool 193 37 Variation of 2-methyl-pentane/NO r a t i o with i n i t i a l pressure of the mixture af t e r adsorption on quartz wool 193 x i i i Table. Page, 38 Variation of 2-methyl-pentane/NO r a t i o with i n i t i a l pressure of the mixture after adsorption on quartz wool 194 39 Variation of 3-methyl-pentane/NO r a t i o with i n i t i a l pressure of the mixture after adsorption on quartz • wool 194 40 Variation of neohexane/NO r a t i o with i n i t i a l pressure of the mixture a f t e r adsorption on quartz wool 195 41 Variation of neohexane/NO r a t i o with i n i t i a l pressure of the mixture after adsorption on quartz wool 195 42 Variation of diisopropyl/NO r a t i o with i n i t i a l pressure of the mixture after adsorption on quartz wool 196 43 Variation of diisopropyl/NO r a t i o with i n i t i a l pres-sure of the mixture after adsorption on quartz wool 196 44 Variation of butene-l/NO r a t i o with i n i t i a l pressure of the mixture af t e r adsorption on quartz wool 197 45 Energies of ac t i v a t i o n and frequency factors for the isomeric hexanes 204 46 Calculated electronic energies of isomeric hexanes 207 47 P o t e n t i a l barrier for rotations of CH3 group i n var-ious hydrocarbons 209 48 Overall order of reaction for various types of i n i t -i a t i o n and termination processes. 220 49 Decomposition rates i n m i n - 1 for 100 mm of n-hexane calculated from Norrish and Pratt rate expression 269 xiv Table. Page. 50 Rate constants for various pressures and temperatures for n-hexane 288 51 Overall energies of a c t i v a t i o n i n kcal/mole for var-ious i n i t i a l pressures of n-hexane 288 52 log A for various i n i t i a l pressures of n-hexane 289 53 Rate constants for various pressures and temperatures for 2-methyl-pentane 289 54 Overall energy of a c t i v a t i o n i n kcal/mole for various i n i t i a l pressures of 2-methyl-pentane 290 55 Log A for various i n i t i a l pressures of 2-me-pentane 290 56 Rate constants for various pressures and temperatures for 3-methyl-pentane 290 57 Overall energies of a c t i v a t i o n i n kcal/mole for var-ious pressures of 3-methyl-pentane 291 58 Log A for various i n i t i a l pressures of 3-me-pentane 291 59 Rate constants for various pressures and temperatures for neohexane 291 60 Overall a c t i y a t i o n energies i n kcal/mole for various i n i t i a l pressures of neohexane 292 61 Log A for various i n i t i a l pressures of neohexane 292 62 Rate constants for various pressures and temperatures of diisopropyl 292 63 Overall a c t i v a t i o n energies i n kcal/mole for various i n i t i a l pressures of diisopropyl 293 64 Log A for various i n i t i a l pressures of diisopropyl 293 XV LIST OF FIGURES Figure Page 1 Schematic outline of thermal decomposition apparatus 29 2 Diagram of the pressure transducer 33 3 Pressure i n mm vs. response i n mm for the. transducer 34 4 Relationship "between i n i t i a l manometric rate and i n i t -i a l a n a l y t i c a l rate for the isomeric hexanes 38 5 Dependence of i n i t i a l rate on i n i t i a l pressure for n-hexane 39 6 Pressure-time curves for packed and unpacked vessels, recorded "by the pressure transducer, for n-hexane 41 7 Percentage reduction i n rate due to the packing for various i n i t i a l pressures of n-hexane 42 8 Dependence of i n i t i a l rate on i n i t i a l pressure for 2-methyl-pentane 43 9 Pressure-time curves for packed and unpacked vessels for 2-methyl-pentane 44 10 Percentage reduction i n rate due to the packing for various pressures of 2-methyl-pentane 45 11 Dependence of i n i t i a l rate on i n i t i a l pressure for 3-methyl-pentane 47 12 Pressure-time curves for packed and unpacked vessels, for 3-methyl-pentane 48 13 Percentage reduction i n rate due to the packing for various i n i t i a l pressures of 3-methyl-pentane 49 14 Dependence of i n i t i a l rate on i n i t i a l pressure for neohexane 50 xv i Figure Page 15 Presaure-time curves for packed and unpacked vessels for neohexane 51 16 Percentage reduction i n rate due to the packing for various i n i t i a l pressures of neohexane 52 17 Dependence of rate on i n i t i a l pressure for diisopropyl 53 18 Pressure-time curves for packed and unpacked vessels for diisopropyl 54 19 Percentage reduction i n rate due to the packing at various i n i t i a l pressures of diisopropyl 55 20 Dependence of i n i t i a l rate on i n i t i a l pressure for a l l isomeric hexanes 57 21 Order of reaction plot for n-hexane 58 22 Order of reaction plot for the packed and unpacked vessel for n-hexane 59 23 Order of reaction plot for 2-methyl-pentane 60 24 Order of reaction plot for the packed and the unpacked vessel for 2-methyl-pentane 61 25 Order of reaction plot for 3-methyl-pentane 63 26 Order of reaction plot for the packed and the unpacked vessel for 3-methyl-pentane 64 27 Order of reaction plot for neohexane 65 28 Order of reaction plot for the packed and the unpacked vessel for neohexane 66 29 Order of reaction plot for diisopropyl 68 30 Order of reaction plot for the pecked and the unpacked vessel for diisopropyl 69 31 Variation of the product d i s t r i b u t i o n with time for 100 mm of n-hexane 83 x v i i Figure 3?age 32 Variation of the product d i s t r i b u t i o n with time for n . c 100 mm of 2-methyl-pentane 84 33 Vari a t i o n of the product d i s t r i b u t i o n with time for 100 mm of neohexane 85 34 Variation of the product d i s t r i b u t i o n with time for 100 mm of diisopropyl 86 35 Va r i a t i o n of the product d i s t r i b u t i o n with i n i t i a l pressure for n-hexane 88 36 Va r i a t i o n of the product d i s t r i b u t i o n with i n i t i a l pressure for 2-methyl-pentane 89 37 Vari a t i o n of the product d i s t r i b u t i o n with i n i t i a l pressure for 3-methyl-pentane 90 38 Variation of product d i s t r i b u t i o n with i n i t i a l pres-sure for neohexane 92 39 Vari a t i o n of the product d i s t r i b u t i o n with i n i t i a l pressure for diisopropyl 93 40 Dependence of i n i t i a l rate on NO pressure i n the unpacked vessel f o r n-hexane 104 41 Dependence of i n i t i a l rate on NO pressure i n the packed vessel for n-hexane 105 42 Dependence of i n i t i a l rate on NO pressure ir/jhe packed vessel for n-hexane 106 43 Dependence of i n i t i a l rate on NO pressure at various S/V r a t i o s for n-hexane 107 44 Dependence of i n i t i a l rate on NO pressure i n the unpacked vessel for 2-methyl-pentane 108 x v i i i Figure Page 45 Dependence of i n i t i a l rate on NO pressure i n the packed vessel for 2-methyl-pentane 109 46 Dependence of i n i t i a l rate on NO pressure at various S/V r a t i o s for 2-methyl-pentane 110 47 Dependence of i n i t i a l rate on NO pressure i n the un-packed vessel for 3-methyl-pentane 112 48 Dependence of i n i t i a l rate on NO pressure i n then-packed vessel for 3-methyl-pentane 113 49 Dependence of i n i t i a l rate on NO pressure i n the packed vessel for 3-methyl-pentane 114 50 Dependence of i n i t i a l rate on NO pressure at various S/v" r a t i o s for 3-methyl-pentane 115 51 Dependence of i n i t i a l rate on NO pressure i n the un-packed vessel for neohexane 117 52 Dependence of i n i t i a l rate on NO pressure i n the packed vessel for neohexane 118 53 Dependence of i n i t i a l rate on NO pressure i n the packed vessel for neohexane 119 54 Dependence of i n i t i a l rate on NO pressure at various S/V rat i o s for neohexane 120 55 Dependence of i n i t i a l rate on NO pressure i n the un-packed vessel for diisopropyl 121 56 Dependence of i n i t i a l rate on NO pressure i n the packed vessel for diisopropyl 122 57 Dependence of i n i t i a l rate on NO pressure i n the packed vessel for diisopropyl 123 x i x Figure Page 58 Dependence of i n i t i a l rate on NO pressure at various S/V r a t i o s for diisopropyl 124 59 Dependence of i n i t i a l rate on NO pressure for various isomeric hexanes 125 60 Variation of the product d i s t r i b u t i o n with time for the f u l l y i n h i b i t e d decomposition of n-hexane 133 61 Variation of the product d i s t r i b u t i o n with time for the f u l l y i n h i b i t e d decomposition of 2-methyl-pentane 134 62 Variation of the product d i s t r i b u t i o n with time for the f u l l y i n h i b i t e d decomposition of 3-methyl-pentane 136 63 Variation of the product d i s t r i b u t i o n with time for the f u l l y i n h i b i t e d decomposition of neohexane 137 64 Variation of the product d i s t r i b u t i o n with time for the f u l l y i n h i b i t e d decomposition of diisopropyl 138 65 Varia t i o n of the product d i s t r i b u t i o n with NO pressure for n-hexane 140 66 Variation of hydrogen with NO pressure for n-hexane 141 67 Variation of CgHg y i e l d with NO pressure and S/V r a t i o for n-hexane 142 68 Variation of I - C 4 H 3 y i e l d with NO pressure and S/V r a t i o for n-hexane 143 69 Variation of C H 4 y i e l d with NO pressure and S/V r a t i o for n-hexane 144 70 Variation of the product d i s t r i b u t i o n with NO pressure for 2-methyl-pentane. 145 71 Variation of hydrogen y i e l d with NO pressure for 2-methyl-pentane 147 X X Figure Page 72 Variation of C H 4 y i e l d with NO pressure and S/V r a t i o for S-methyl-pentane 148 73 Variation of CgHg y i e l d with NO pressure and S/V r a t i o for 2-methyl-pentane 149 74 Variation of i-C^Hg y i e l d with NO pressure and S/V r a t i o for 2-methyl-pentane 150 75 Variation of the product d i s t r i b u t i o n with NO pressure for 3-methyl-pentane 151 76 Variation of hydrogen y i e l d with NO pressure for 3-methyl-pentane 152 77 Variation of CgHg y i e l d with NO pressure and S/V r a t i o for 3-methyl-pentane 153 78 Variation i n y i e l d of i-C 4H Q plus 1-C4HQ with NO pres-sure and S/V r a t i o i n 3-methyl-pentane 154 79 Variation of CH4 y i e l d with NO pressure and S/V r a t i o for 3-methyl-pentane 155 80 Variation of trans-C^Hg y i e l d with NO pressure and S/V r a t i o i n 3-methyl-pentane 156 81 Variation of eis-C^Hg y i e l d with NO pressure and S/V r a t i o for 3-methyl-pentane 157 82 Variation of the product d i s t r i b u t i o n with NO pressure for neohexane 159 83 Variation of hydrogen y i e l d with NO pressure for neohexane 160 84 Variation of CLEL y i e l d with NO pressure and S/V r a t i o o O for neohexane 161 XXI Figure Page 85 Variation of i-C^Hg y i e l d with NO pressure and S/V r a t i o for neohexane 162 86 Variation of CH4 y i e l d with NO pressure and S/V r a t i o for neohexane 163 87 Variation of the product d i s t r i b u t i o n with NO pressure for diisopropyl 164 88 Variation of hydrogen y i e l d with NO pressure for diisopropyl 165 89 Variation of C3Hg y i e l d with N8 pressure and S/V r a t i o for diisopropyl 166 90 Variation of CgBTg y i e l d with NO pressure and S/V r a t i o for diisopropyl 167 91 Variation of CH4 y i e l d with NO pressure and S/V r a t i o for diisopropyl I 6 9 92 Variation of CgH 6/l-C 4H 8 r a t i o with NO pressure for n-hexane 170 93 Variation of CgH^/CgHg r a t i o with NO pressure for n-hexane 171 94 Variation of "QBL^ /C^ Eg r a t i o with NO pressure for n-hexane 173 95 Variation of CgB^/CgHg r a t i o with NO pressure and S/V r a t i o for n-hexane 174 96 Variat i o n of GH^/CgHg r a t i o with NO pressure for 2-methyl-pentane 175 97 Variation of Cg H4/ C2 H6 r a t i o w i t h W 0 Preasure for 2-methyl-pentane 176 x x i i Figure Page 98 Variation of CgHg/CgHg r a t i o with NO pressure for 2-methyl-pentane 177 99 Variation of C^/CgHg r a t i o with NO pressure and S/V r a t i o for 2-methyl-pentane 178 100 Variation of CgH^CgHg r a t i o with NO pressure for 3-methyl-pentane 179 101 Varia t i o n of CH^/CgH^ r a t i o with NO pressure for 3-methy1-pentane 180 102 Va r i a t i o n of CgH 4/C 2H 6 r a t i o with NO pressure and S/V r a t i o for 3-methyl-pentane 181 103 Varia t i o n of CgH4/CgH6 r a t i o with NO pressure ftfr neohexane 183 104 Variation of CH4/CgHg r a t i o with NO pressure for neohexane 184 105 Variation of CH4/CgH4 r a t i o with NO pressure for neohexane 185 106 Variation of C3Hg/i-C4Hg r a t i o with NO pressure for neohexane 18& 107 Variation of GgijGgL^ r a t i o with NO pressure and S/V r a t i o for neohexane 187 108 Variation of C3Hg/C3B:g r a t i o with NO pressure for diisopropyl I 8 8 109 Variation of C3Hg/2-methyl-'butene-2 r a t i o with NO pres-sure for diisopropyl l 8 ^ 110 Variation of C 3H 5/i-C 4Hg r a t i o with NO pressure for diisopropyl 190 x x i i i Figure Page 111 Relationship "between the pressure of NO i n the system and the pressure of NO recovered by gas chromatographic analysis 199 118 Dependence of n i t r i c oxide consumption on i n i t i a l pres-sure of n i t r i c oxide for the isomeric hexanes 200 I. INTRODUCTION The thermal decomposition of hydrocarbons has been tinder investigation for several decades. The occurrence of free r a d i c a l s i n many of these decompositions has been established by the Paneth metallic mirror technique (1 - 4) as w e l l as by mass spectrometric methods ( 5 - 8 ) . It i s also known that the decomposition of certain hydrocarbons i s accelerated by methyl (9, 10) and a l l y l (11, 12 ) ra d i c a l s as we l l as by some traces of impurities as oxygen (13 - 15). Rice was the f i r s t to treat the decomposition of a paraf-f i n , ethane, as a free r a d i c a l mechanism (16). The so-called Rioe-Hertzferd mechanisms, along with alt e r n a t i v e s , proposed by Kiihle and Theile (17) have formed the basis for the inves-t i g a t i o n of the thermal decomposition of hydrocarbons since the early 1930 Ts. Most of these decompositions show common ch a r a c t e r i s t i c s . A marked feature i s the apparent chain nature of the mechanisms as wel l as the occurrence of the phenomenon of i n h i b i t i o n . The raites of decomposition, for almost a l l hydrocarbons, are reduced markedly by the addition of inhib-i t o r s of which the most important are n i t r i c oxide, propylene and butylene. I n h i b i t i o n by NO was discovered and exhaustively investigated by Hinshelwood and his co-workers for many organic compounds ( 1 9 - 2 9 ) . They compared the r e l a t i v e i n h i b i t i n g effects of NO and z propylene, and found that to a t t a i n the same extent of i n h i b i t i o n , twelve times as much propylene must he used as n i t r i c oxide, which means/that NO i s twelve times more effective than propylene (18). E a r l i e r the phenomenon of s e l f - i n h i b i t i o n by unsaturated decom-position products had been observed (30). Stepukhovlteh has com-pared qu a n t i t a t i v e l y the action of isobutylene and propylene as i n h i b i t o r s for several hydrocarbons (31-33) and found that the same l i m i t i n g rate i s obtained for these two i n h i b i t o r s when the reactions are f u l l y i n h i b i t e d . 1. The uninhibited decomposition of the paraffins In the normal decomposition the main problem a r i s i n g i s the nature of the mechanism. It has been known for a long time that the majority of gas phase reactions involve considerable a c t i v a t i o n energy, i n the range from 30 to 50 kcal/mole. Semenov (34) correlates the a c t i v -ation of a given reaction,E A, with the energy b a r r i e r , E 0 , f o r the reverse reaction, and the heat of reaction, Every molecular decomposition proceeds through a t r a n s i t i o n state and for an endothermic reaction we have, as indicated i n the diagram pot e n t i a l energy S reactants reaction coordinate 3 The energy ba r r i e r , i s a small quantity for free r a d i c a l reactions, i n general not higher than 10 Kcal/mole and usually 3 to 6 K c a l / mole (34-36) . It can be expressed, for endothermic reactions, by the empirical formula (34) E 0 - A- of\qv where A has a value of approximately llskc'al/mol and <*' i s approximately 0.25 for the majority of free r a d i o a l reactions. Both A and o( depend to some extent on the t#pe of the reaction. Consider the reaction A B — * C The rate i s given by R m o l e o ^ k ^ B ] where k i s the bimoleoular rate constant. Thus where 1 0 " c m /sec i s the average preexponential factor for a simple bimoleoular reaction. Suppose that the above reaction proceeds through a free r a d i o a l mechanism R-L B > R2~'" M l ^ Rg +- A > R].-*- M 2 (2) where R^, Rg are ra d i c a l s produced from A,and M^ , Mg are mole-cules. Since A and B disappear at the same rate we obtain Vain ^ W I O " 1 0 e " B l / R T LRJIBI where E-^  i s the ac t i v a t i o n energy of the step with the lowest 4 rata. Then by forming the r a t i o Rmolec/ Rchain - ^ we obtain a c r i t e r i o n by which we can d i s t i n g u i s h between molec-ular and chain mechanisms. The concentration of R^ • can be estimated from the rate of i t s formation and disappearance. I f the r a d i c a l reaction i s photoinitiated a value of can be calculated. I f I i s the i n t e n s i t y of the l i g h t we obtain A i—=»R1^-Rg * M (3) d[Rj dt - <f>I - k 2 [ R l ] a - 0 Thus ^ I ' f c g t R f j 2 ( 4 ) where I i s the in t e n s i t y of l i g h t i n quanta/sec and 0 the quantum e f f i c i e n c y i n molecules/quantum, which tends to unity for a pure primary photochemical process. Thus i f -=• I' molec/sec then the new quantity I' has unit^l of rate while i t s value i s close to that of I. From (4) we obtain [ R j r ( I ' A a ) * (5) Then the r a t i o \ takes the form ^ _ l O ^ V W R I ^ l / l O - l O e - E l / R T ^ J f B ] ^ U ] e 5 Substituting (5)) into (6) we obtain _ [A] B l - B A B I - B A 6 - e RT - \A ( f c a / l 1 ) * e RT (7) (I ' A j j ) * 3 3 For a molecular reaction E A has a large value, about 40 kcal/mol, while for a chain reaction E^ i s about 5 to 10 kcal/mole. Therefore (7) becomes i -30,000 With strong i l l u m i n a t i o n I w i l l have a value of approximately I O 1 5 quanta/sec while for weak il l u m i n a t i o n I — 1 0 1 2 quanta/sec. Considering the weakest i l l u m i n a t i o n , we can write without loss of the accuracy that I T — l O ^ 2 quanta/sec since $ varies more commonly from 10" 2 to 1 molecules/quanta. Bearing i n mind that kg i s a very small quantity, usually of the order of 10" 1 0 cm3/sec, while [A] — l O ^ 9 moleoules/cm^ i n the gas phase we obtain ^ - 10" 5 at T - 600°k and ^ - 10" 1 1 at 400°k. So one can see e a s i l y that the molecular reaction w i l l p articipate very l i t t l e under the conditions of the experiment. Values of y , vary from IO 4 to 10" 6 (34) i n hydrocarbon decompositions. The above treatment can be extended to any kind of decomposition, either thermal or photoinitiated. The photo-i n i t i a t e d was used here because i t i s simpler to calculate ^ i n a photochemical reaction than i n a thermal reaction. Calcu-lations of t h i s kind indicate that decompositions i n the gas 6 phase w i l l proc.eed mainly through chain mechanisms, rather than molecular mechanisms. For the reactions B r E g =>2HBr ^-0.1 (34) I 2 Hg =>2EI ^ - 200 (34) Thus for the reaction of hydrogen with iodine the main process i s a moleculdr mechanism. In a chain mechanism three stages can he distinguished. I n i t i a t i o n , propagation and termination. In thermal decompositions of hydrocarbons i n i t i a t i o n w i l l take place through a carbon-car Don s p l i t provided that D(C-C) i s s i g n i f i c a n t l y less than D(C-H). The decomposition can take place either unimolecularly or bimolecularly. The observed frequency factors can be consid-ered as a c r i t e r i o n of the unimoleoularity of the decomposition. Suppose that the decomposition of a molecule A to product B can be considered to be either unimolecular or bimoleoular. The rates i n the two cases w i l l be = 1 0 1 3 e' E l / R TW where 1 0 ^ sec~^ and 1 0 " ^ om^ seo"-'- are taken to be the preex-ponential/faotors of the unimolecular and bimoleoular rate con-stants respectively. For the unimolecular reaction 1 0 1 3 a e c - l corresponds approximately to the frequency of the bond breaking while for the bimoleoular reaction, 10" 1 0cm 3sec" 1 corresponds 7 approximately to the number of c o l l i s i o n s per cm16 per sec at S.T.P. By convention the preexponential factor A can be equated to PZ where P i s the s t e r i c faotor found by several workers to have value from 10~ 3 to IO""4 for hydrocarbons (3). Knowledge of s t e r i c factors would be of great value i n the consideration of reaction mechanisms i n the hydrocarbons but unfortunately they have not been extensively treated t h e o r e t i c a l l y . Comparing a unimolecular decomposition with a bimoleoular one of the same a c t i v a t i o n energy we obtain V i m . / ' W l . = 1 ° 1 S e - B l / M [ n T / l 0 S 1 0 6 - E l / M W » = W*»(l/n> where n i s the number of molecules per cm3 approximately 1 0 1 9 at S.T.P. Therefore ^anim./ Rbimol. - 10 4 . Thus the unimole-cular decomposition w i l l take place much faster then the bimo-leoular one. Unimolecular i n i t i a t i o n processes w i l l therefore be of major importance i n the present study. Unimolecular reactions are favored at lower pressures where energy loss by c o l l i s i o n s i s low. In these cases molecules can use a l l of t h e i r kinetic energy to pass the energy b a r r i e r leading to decomposition . Heterogeneous phenomena may also be important since the energy needed for a heterogeneous reaction i s much lower than for the corresponding homogeneous reaction. Heterogeneous reactions i n the hydrocarbons pyrolyses Heterogeneous i n i t i a t i o n has been examined for the simple reaction (37). C l 2 - f H 2 — » 2HC1 8 •using vessels with different diameters. At low pressures and small diameters i t was found that the r a t i o of the rates, R, i n the two diff e r e n t vessels relates to the diameters, d, as follows R 2 d2 Rn ° < -| q At higher pressures dependence of the rates on diameter was s t i l l observed hut atf larger diameterrno r e g u l a r i t y existed. The rates are presumably higher for larger surface area since less energy i s needed for reaction occurring on the surface, beoause of the weakening of the bonds that occurs when molecules are adsorbed on surface s i t e s . By using the technique of d i f -f e r e n t i a l calorimetry, by which one can measure the heat of reaction i n both the center of the vessel and on the w a l l by a d i f f e r e n t i a l thermocouple (38), i t was found that i n i t i a t i o n and termination of the chains take place on the wall for the thermal reaction (34) H 2 -+- 01 2 >2HC1 A theory of heterogeneous-homogeneous ca t a l y s i s proposed by Polyakov (39) has been applied to oxidation reactions. The c a t a l -y s i s depends on both the nature of the w a l l and the nature of the compound undergoing oxidation. The p r o b a b i l i t y for the capture of a r a d i c a l or an atom by a surface s i t e was found by various workers (40-43) to be between IO" 5 and 1. This p r o b a b i l i t y was found to increase with temperature (41-44). The a c t i v a t i o n energy for adsorption of radicals varies between 4 to 12 kcal/mole. 9 It appears therefore that the adsorption i s a chemical process i n which bonds are formed between the r a d i c a l and the wal l s i t e . What happens i n many cases i s not simply sorption, but reaction with the w a l l . For example Mo03 becomes blue under the action of H and 0 atoms. Mirrors of antimony are removed by a l k y l r a d i c a l s , etc. In the case of wall reactions, Semenov (34) discusses the p o s s i b i l i t y of bond formation between the free r a d i c a l or atom and the unbalanced charge of the surface. He correlates the magnitude of heterogeneous reactions with the required energy for electron transfer i n the surface. A surface with unbalanced charges i s thus formed. I f U i s the p o t e n t i a l energy of such a surface then heterogeneous i n i t i a t i o n i s favored i f tr^D(C-C) and heterogeneous termination i f TJ<D(C-C) . Heterogeneous i n i t i a t i o n , e v e n i f not marked, at least appears to promote the i n i t i a t i n g action of impurities. PQltorak and Voevodskii (45) found that acceleration by Og i n the cracking of Propane at low pressures, strongly depends on the pretreatment of the vessel. A number of workers have t r i e d to determine whether or not wall effects exist i n hydrocarbon pyrolyses. The r e s u l t s are contradictory. L a i d l e r and Wojciechowski• (46,47) looked for w a l l effects i n ethane decomposition with negative r e s u l t s . How-ever, they found rates to decrease s l i g h t l y for the packed vessel. Similar r e s u l t s were obtained by them for the propane decomposi-t i o n , i n which a sl i g h t decrease i n rate occurs with packing as we l l as some change i n the order of the reaction (48,49). 10 P u r n e l l and Q,uinn (50) found no effect for different w a l l coatings i n the pyrolysis of n-butane. In contrast, Wall and Moore (51) have found evidence of oatalysis "by the surfaoe i n the packed vessels i n the mixed pyrolysis of CgHg and Cgfllg. The r e s u l t s were obtained by mass spectrometric analysis. Surfaoe effects were also found i n the pyrolysis of the isomeric pen-tanes (53). Wall oat a l y s i s occurs for isopentane and s l i g h t reduction i n rate was obtained i n n-pentane. Pratt and Purnell (53) found w a l l c a t a l y s i s i n the pyrolysis of acetaldoxime. A com-parison of the various r e s u l t s obtained to the present shows that the evidence regarding w a l l effects i s contradictory. The w a l l may affeot both i n i t i a t i o n and termination processes. Propagation i s believed to take place homogeneously through such processes as substitution: R.^ RgRg * R i R 2 R 3 ^ decomposition of r a d i c a l s : R^ » Rg -+- o l e f i n (b) isomerization reactions: CHgCHCHg »'CHgCH£CHg (c) For reaction (a) the a c t i v a t i o n energy can be obtained within an accuracy of 1 to 2 koal from thermochemical data, through the relationship: E A C ^ A H ~ &H f(RiRg) AH f(R 3) - AHffR!)- £H f(R 2Rg) The reverse reaction i s exothermic but because Rg i s unstable i f i t i s a large r a d i c a l , the reverse reaction w i l l occur with low p r o b a b i l i t y . Enthalpies of formation as w e l l as heat capacities are w e l l known for molecules but not for radioals. T a c i t l y , we 11 use the values of AH f found for lower temperatures, assuming they remain unchanged. One can also calculate them using various empirical formulae, such as that proposed by Voevodskii D(C-H) - D(C-H) - B - D(O-H) + EB Pnw. Sec- *eni-The constant B can be calculated for a p a r t i c u l a r series. This method w i l l be followed i n connection with thermoohemical data and d i s s o c i a t i o n energies found experimentally to calculate a c t i v a t i o n energies for i n d i v i d u a l steps i n the mechanisms pro-posed i n the present work. For reaction (b) more than one mode can be obtained. The choice between them i s a function of th e i r a c t i v a t i o n energies frequency factors etc. Their r e l a t i v e p r o b a b i l i t i e s are commonly determined from a n a l y t i c a l data. Isomerizations, as i n (c) are very important. In those reactions the angle of attaok plays a very s i g n i f i c a n t r o l e . Rough quantum mechanical calculations (34) led to the qua l i t a t i v e r e s u l t that the attaok of a S"-bond i s more favorable i f the approaching r a d i o a l and the bond attacked are oolinear. A per-pendicular attaok requires approximately twice as much energy. In the case of a ^r-bond a perpendicular attaok i s more favorable. In isomerization reactions, one electron of a 6"-bond s h i f t s under the influence of the free r a d i c a l . The <S -bond i n question i s either a C-H or C-C bond. In th i s case, however, the electron s h i f t i s accompanied by an atomio displacement, as a resu l t of which the € -bond breaks and another one i s formed. The molecule requires a special geometry. For instance, the reaction IS CH3CH2CB:2» *' CH2CH2CH3 proceeds very e a s i l y while CHgCHCHg =>CH2CH2CH2» requires more energy. Schematically i t i s explained as follows The problem of isomerization of free radioals has been discussed as a common property (54,55). Isomerizations processes w i l l be considered as important i n the present study. Such alternate reactions as CH3,CHCH3 >(CH3CH2CH2)^ ^CgB^ + CHg (l) CH3CHCH3 > CH3CH: CHg-*- H (2) have been found (56) to have r a t i o s k-L-./lcg £ 0.07 at 500°C S. The i n h i b i t e d decomposition The decomposition of hydrocarbons can best be understood on the basis of free r a d i c a l mechanisms. The nature of the products i s a function of the chain length, which i s governed by the re l a t i v e importance of the propagation and termination steps. The capture of the propagating r a d i c a l s by an i n h i b i t o r i n e v i t a b l y stops the chain. Inhibitors are molecules which are either rad-i c a l s themselves, such as n i t r i c oxide with i t s unpaired electron, 13 or compounds to which radicals can add, such as o l e f i n s . The i n h i b i t i o n "by o l e f i n s i s often termed s e l f - i n h i b i t i o n since these compounds appear among the products, and th e i r effect i s obvious from the shape of the pressure-time curves or from the v a r i a t i o n of the product d i s t r i b u t i o n with time. The general features of the i n h i b i t i o n of organic pyrolysis by n i t r i c oxide are now w e l l known. N i t r i c oxide i n r e l a t i v e l y small proportions suppresses markedly the rate of reaction so that a l i m i t i n g rate i s attained. Higher concentrations of NO cause the rate to inorease again. The nature of the residual reaction i n f u l l y - i n h i b i t e d py-yo l y s i s has been a point of controversy for many years. One point, established so f a r , i s that for each hydrocarbon there appears to exist under certain conditions a de f i n i t e l i m i t i n g i n h i b i t e d rate of decomposition, depending only on the addition of s u f f i c i e n t amount of i n h i b i t o r , but independent of the nature of the i n h i b i t o r . A thorough investigation of the i n h i b i t i o n for a variety of hydro-carbons under the same experimental conditions should give i n f o r -mation on whether i n h i b i t i o n i s related to the nature of hydro-carbon or to the condition under which the experiments were done. Hinshelwood and his coworkers (57) assumed that the residual reaction i s simply a molecular reaction. In the i n h i b i t i o n of n-butane by NO for example they proposed the following scheme: C 4H 1 0 k x — > ER H"'"C4H10 fc2-J> R + Products 2R ^ > M (c) (a) (b) 14 R-I.NO — = 4 — > s (a) S ^—> R+NO (e) I f (a) and (e) oome into equilibrium at onoe them the stationary concentration of R w i l l not be affected by NO and no i n h i b i t o r y effect w i l l be observed because R i s regenerated. The same idea was proposed by Echols and Pease (58) for the l a t e r stages of the reaction when i n h i b i t i o n i s no longer e f f e c t i v e . Why t h i s e q u i l i b -rium should occur i n the la t e r stages and not i n the beginning of the reaction i s not apparent. Hinshelwood introduced the concept of the apparent chain length, defined as the r a t i o : rate i n the absence of i n h i b i t o r / r a t e of the f u l l y i n h i b i t e d decomposition, assuming that the re s i d u a l reaction i s a molecular mechanism. However, chain lengths have been found by other methods to be much longer than those given by the above r a t i o . For the i n h i -bited reactions the chain length i s a function of pressure (59). Hinshelwood and his co-workers have always assumed that the n i t r i c oxide had no effect oh' the products obtained i n the p a r a f f i n py-rolyses. They have accounted for t h i s i d e n t i t y of decomposition products i n one discussion (60)« by suggesting that two separate free r a d i c a l mechanisms occur,one suppressed by NO and the other unaffected by i t . However, the hypothesis seems improbable. Various modifications of t h i s approach have been put forward by Hinshelwood and his co-workers (61,6S). 15 An important question i s the nature of the reaction of NO with a l k y l radioals and with hydrocarbons. Thomson and Meissner (63) photographed the absorption spectrum of NO with organic compounds during decomposition and found evidence of C-N bonds. Other workers (64,65) proved the existence of CHgNO and CHg- NOH i n the photochemical decomposition of t - b u t y l - n i t r i t e i n the presence of NO. B i l l i n g e and Gowenloek (66) found nitroso-oompounds/in the reaction i - C 3 H 7 + NO * l -CgH ? N0 which was trapped as a monomer and gives the dimer on warming at room temperatures. Similar reactions have been assumed to take place between CHg and NO (67,68) CHg-*- NO * CHgNO CHgNO i s supposed to react further CHgNOCgHs * CH 4 + CgH? + NO The reaction of a l k y l r a dicals with NO and expecially of CH has been reviewed i n the past (69). Bryce and Ingold (70), 3 by using a mass spectrometer found CHgNO or CHg:N0H i n the reaction of NO with methyl radioals from (CHg)gHg i n the tem-perature range 4 8 0 - 9 0 0 ° C . Products l i k e HCN, HgO, NHg, CO, Kg COg and CHgCN were also found. Recently the formation of CHgNO was again observed i n the photochemical decomposition of t - b u t y l -n i t r i t e following the reaction by infra-red absorption (71). The addition of NO favors the formation of CHgNO which indioates that the compound i s formed primarily through the reaction CHg-*- NO * CHgNO 16 Comparatively l i t t l e i a known so f a r , regarding the s t a b i l i t y of these nitrogen-containing compounds. Clement and Ramsay (7 2) found that the upper l i m i t i n the di s s o c i a t i o n energy of HNO i s 48.6 kcal/mole while Thrush (73) has shown that such reactions as x+H+NO »HN0 +x has an a c t i v a t i o n energy of 0.7± 0.3 kcal/mole at 600°K. Thus a value of 48 kcal/mole appears to he reasonable for D(H-NO) (74, 75). Gray (76) has obtained a value of 57 kcal/mole for D(CH3-N0g). Gowenlock at a l (77) suggested the same value for D(CHg-NO). How-ever, Gowenlock seems to favor a lower value of about 30 kcal/mol on the ground that D(CHg-NO) should be about 15 koal less than D(H-NO) (78). Batt and Gowenlock (79) have shown that CHgNO i s o -merizes into formaldoxime, CHg:N0H, with an a c t i v a t i o n energy of about 40 kca./mole. Pratt and Purnell (80) suggested that similar reactions occur between CgHg and NO C 2H 5 - NO » CgH4-HN0 CgHg + NO » C 2 H 5 N 0 CH„CHoN0 ^ NO » CH^ CHN* ^  HN0o 3 2 j3 i 2 CHg +• HCN OH* +N0 Let us return to the nature of the residual reaction at f u l l i n h i b i t i o n . F i r s t of a l l i t has been assumed by previous workers that the products were i d e n t i c a l i n both the uninhibited and i n -hib i t e d decompositions. Differences reported i n the l i t e r a t u r e were considered to be due to a n a l y t i c a l error. Pc^ltorak (14) 17 found i d e n t i t y i n the products of decomposition of propane i n both the uninhibited and the f u l l y i n h i b i t e d decompositions. However, i n the re s u l t s he obtained the sum of C2H4 and CgHg had values of 16.5$ and 22$ for the uninhibited and f u l l y i n h i b i t e d reactions respectively for 5fo of conversion at T = 590°C and a propane pressure of 25 mm. This means an approximate 30$ increase i n CgHg-»- CgH4 for an increase i n NO pressure from 0 to 5 mm. Rice and P.olley (81) as well as Goldanskii (82) have not supported the idea of a moleoular mechanism for the residual reaction and have used the hypothesis that molecules capable of ending chains are at the same time capable of generating them. It i s d i f f i c u l t to apply t h i s hypothesis to the problem of i n h i -b i t i o n because of the existanee of l i m i t i n g (residual) rates which appear to be independent of the nature of the i n h i b i t o r used. A direct attempt to check the idea of a molecular residual reaction was made by Wall and Moore (51). They decomposed ther-mally a mixture of CgHg and CgJDg i n the absence of NO and with 2.5$ of NO pressure. They found i d e n t i c a l products including Hg, Dg , and HD. This i s l o g i c a l for a chain reaction but not for a molecular mechanism i n which HD would not be expected. Unfor-tunately they did not extend t h i s investigation over a broad range of NO pressures and Hinshelwood questioned the re s u l t s since i n s u f f i c i e n t NO was used to reach f u l l i n h i b i t i o n (83). But according to the work of Hinshelwood himself, i t i s obvious that maximum i n h i b i t i o n i s approached for small NO pressures, so that even i f the NO pressure was not s u f f i c i e n t i n Wall and 18 Moore's experiment for f u l l i n h i b i t i o n , the rate was very similar to the rate of the f u l l y i n h i b i t e d reaction.Pocltorak and Voev-odskii t r i e d to c l a r i f y t h i s work (45). They studied the pyrolysis of CgHgin the presence of D2« The exchange of Dg could give i n f o r -mation about the chain mechanism. Lack of exchange would validate the molecular reaction. Exchange was found i n both i n h i b i t e d and uninhibited decompositions. Rice and ¥@rnerin (84) completed the previous studies by decomposing CgDg i n the presence of CH^. The r a t i o CHgD/CH4 was found to be i d e n t i c a l i n both the i n h i b i t e d and uninhibited decompositions. On the basis of the above informations the molec-ular reaction seems to be excluded. The problem of the alleged i d e n t i t y of the products from the two types of pyrolysis remains. Another problem, not settled yet i s the question of the extent of consumption of NO. The information i s contradictory. Wall and Moore (51) found consumption of NO i n ethane pyrolysis. Hinshelwood and his colleagues also found large consumption of NO i n ethane pyrolysis (85). Extensive consumptions of NO were also observed i n the pyrolyses of diethyl-ether when the amount of NO was larger than the amount of ether (86-88). Consumption of NO was also found i n isopentane decomposition (92). However, ne g l i g i b l e consumption or no consumption at a l l was found by others (89,90). This problem i s therefore not resolved up to the present time. Under these circumstanses the formation of any general theory of i n h i b i t i o n seems to be premature because the 19 following points are uncertain; i ) The effect of the surfaoe on the decomposition i i ) The relationship between the amount of i n h i b i t o r required for maximum i n h i b i t i o n and the nature of the hydrocarbons. i i i ) The effect of the conditions of the decomposition on" the amount of i n h i b i t o r required for f u l l i n h i b i t i o n . iv) The effect of the i n h i b i t o r on the product d i s t r i b u t i o n . v) The nature of the reactions between a l k y l radicalsand NO. vi$ The extent of consumption of n i t r i c oxide. Theories of i n h i b i t i o n Although r e l i a b l e answers to these questions have not been available a number of attempts have been made to solve the prob-lem of the i n h i b i t i o n i n pyrolysis of organic compounds. A b r i e f summary of the more recent ones w i l l be given here chron-o l o g i c a l l y . Voetfodskii and Poltorak (45,91) attempted to correlate the phenomenon of i n h i b i t i o n with reactions occuring on the surface of the reaction vessel. They postulated the existence of i r r e -versible decomposition processes on the w a l l . In t h e i r experiments they used vessels pretreated by HF, i n which they pyrolysed C 3 H Q with very small amounts of Og. At low pressures acceleration of the rate was observed. Voevodskii introduced the suggestion that decomposition might be i n i t i a t e d and terminated on the walls. This introduces a new concept of i n h i b i t i o n since the NO pressures required might be related to the a c t i v i t y of vessel walls and not 20 to the amount of hydrocarbon. At the same time Voevodskii accept the existence of two heterogeneous chain i n i t i a t i o n s , one revers i b l e and one i r r e v e r s i b l e . This idea appears to be ar b i t r a r y . L a i d l e r and Wojoieohowskii (93) suggested that the decompo-s i t i o n of organic compounds i n h i b i t e d by NO, proceeds/through a special type of free r a d i c a l mechanism i n which the i n h i b i t o r i s involved i n both i n i t i a t i o n and termination. I n i t i a t i o n i s as-sumed to take place through hydrogen-abstractions by NO while termination takes place between i p O or NO and the ohain c a r r i e r e x i s t i n g i n the highest concentration. This mechanism has been applied to various paraffins. They observed that the o v e r a l l order for decomposition of ethane i s unity for higher pressures but' increases to 3/2 at lower pressures and temperatures. This change i n order i s accounted for by the following mechanism pro-posed for the i n h i b i t e d decomposition. 0 z H 6 + N O ^ 0 2 H 6 +HN0 kg CgHg » C gH 4 i.H H + C g H 6 CgH^Hg k 4 H + NO *=±=* HNO k»4 CgHg+BNO CgHg + NO A steady-state treatment leads to the rate expression rate = ( k ^ g k ^ / k . ^ ) * [c2H6] with f i r s t order k i n e t i c s . At lower pressures they assumed that termination takes plac E l also through the reaction H •+• HNO k 6 > Hg + N 0 and the o v e r a l l rate becomes rata = ^ ( f c ^ / k ^ ) * [G^ 3 / 2 This mechanism has been also applied to the pyrolysis of methane (93), propane (48),butane and other organic compounds. The rates i n the above expression are independent of NO pressure. These mechanisms can be applied s t r i c t l y to the unin-h i b i t e d reaction and to that region of the f u l l y i n h i b i t e d de-composition i n which the rate remains constant. The rate expres-sion given above does not permit c a l c u l a t i o n of the rate as a function of NO pressure during the early stages of the i n h i b i t i o n or i n the region i n which the rate i s increased by the reaction of NO. These mechanisms become very d i f f i c u l t to manipulate for higher paraffins i n which the p a r t i c i p a t i o n of a great many radi c a l s must occur. Blackmore and Hinshelwood ( 9 4 ) continued to support the idea that the molecular mechanism i s predominant i n the f u l l y i n h i b i t e d reaction. However, they contend It i s masked at high NO pressures by a chain reaction induced by NO. I f the NO pressure i s less than the required for the l i m i t i n g rate, the molecular reaction i s again affected by the i n i t i a l chain re-action which i s not completely suppressed by NO. The conclusion to be draiwn from t h i s contention i s that the molecular reaction w i l l be predominant for only an extremely narrow range of NO 22 pressure, which cannot "be accurately defined. A major c r i t i c i s m of the above work i s that the extremely high pressure of NO used, has changed the character of the reaction, so that one i s no longer dealing with the i n h i b i t e d decomposition. Syring et a l (67) have examined the problem of i n h i b i t i o n and have proposed a rather complex mechanism based on a number of assumptions. These include the following: i ) The uninhibited decomposition proceeds through the general scheme M (1) ^ #+P 2 (3) chain ends — * P 4 (4) where (5 and are r a d i c a l s which can only react bimoleoularly and unimoleoularly respectively, M i s the organic compound and P^ i s the reaction product. i i ) The i n h i b i t i o n takes plaoe through the capture of either the $ or the V*- r a d i o a l by an i n h i b i t o r I. i i i ) The i n h i b i t o r i s not oonsumed at a l l or i s consumed to a very s l i g h t extent. iv) The ooncentration of # ,V> and the ( ^ I ) complex obey the p r i n c i p l e of the steady state. v) Organic compounds decompose by both a chain and a molecular mechanism so that the o v e r a l l rate, r, i s the sum of the molecular and the chain rate, R m and R r = R- R m r = R * R m 23 r o * V Rm where r ^ i s the f u l l y inhibited, rate and r 0 i s the uninhibited rate. Using the above assumptions,inhibition due to the removal of |3 radioals(£ - i n h i b i t i o n ) i s aooounted for by the scheme: 0 + I ,( $X) ( i ) (( ? l ) » P ~ I (-i) (> i) i = r c — > P 5 ( J ) (/?I) + M =>L.+Pg-*- I (2») where C i s the isomeric compound of ( P i ) . The step (J) accounts for the consumption of the i n h i b i t o r i f any occurs. In the case of the i n h i b i t i o n due to the removal of p> radioals ( h - i n h i b i t i o n ) the mechanisms proceed as follows: Y- -Hi » ( r-l)- ( i ) ( YD * Y - I (-i) ( W I ) ^ = ? C — * P 6 (J) (t~l) ^ ^ P 3 - I (3 f) Using the above schemes the r a t i o r - r ^ / r Q - r i s expressed as a function of concentrations of both the organic compound and the i n h i b i t o r . Plots of the r a t i o r - r /r - r obtained from t h i s mechanism against NO pressure agrees cl o s e l y with experimental r e s u l t s obtained by. Hinshelwood and his co-workers. Norrish and Pratf (95) consider that the important point i n the i n h i b i t e d decomposition i s the nature of the reaction of 24 NO with a l k y l radioals. The sigmoidal shape i n preasure-time curves i n the i n h i b i t e d decompositions r i a s explained through such schemes as NO +- CgHg 'CgB^O + CHgCHNOH NO CHgCHNOH » (CHgOHN) «+HN02 (CHg CHN) • »CHg -»- HCN HNOg >OH» + NO The degenerate chain branching proposed i n t h i s mechanism accounts for the self-a c c e l e r a t i n g nature of the paraffins pyrolysis i n h i b i t e d by NO. Their mechanism for the i n h i b i t e d pyrolysis takes the form: P » R L - R S (!) R S ^ P ?M^R L (2) R L *01-R S (3) R^ *end products (4) RL-^ -NO => RNOTO^ (5) 0x-H NO — : * R s - product H')) Ox * product r (7) where P i s the organic compound undergoing p y r o l y s i s , M i s an inactive product, n 0 1 w i s an o l e f i n i c compound, "0 X" i s the oxime formed from the isomerization of the a l k y l - n i t r o z o compound RND. R^ and Rg are large and small radioals respectively. Solving for the steady state concentrations of R L, R s and 0 X, they obtained the expression for the o v e r a l l rate i n terms of P and NO pressure as follows: 25 1 a[P] 2 k l ( ^(kgky/kskg) + (kgA5)[N0i + [NO] 2 | [P] dt I (k4k 7/k 5k 6) ^ [ ( k 7 / k 6 ) + (fc4/fc5)] [NC]j Excellent agreement i a obtained between the rates of pyrolysis predicted for the pentanes- by t h i s expression and the rates ob-served by Blackmore and Hinshelwood (94). A, number of the rate constants involved had to be assumed. Objectives of the present study The present investigations of the pyrolysis of the isomeric hexanes was undertaken to provide information on the following points: i ) The general mechanism of the uninhibited pyrolysis including both homogeneous and heterogeneous reactions. i i ) The nature of the i n h i b i t e d reaction including both homo-geneous and heterogeneous processes. i i i ) The effect of NO on the mechanism as revealed by detailed analysis of products. iv) The effect of surfaoe on the product d i s t r i b u t i o n i n the i n h i b i t e d p y r o l y s i s . v) The effect of structure of the isomeric hexanes on the i n h i -b i t i o n . v i ) The extent of consumption of NO i n the i n h i b i t e d decom-positions. 26 I I . EXPERIMENTAL 1. Material and methods  Hydrocarbons The hydrocarbons studied were l i q u i d s (n-hexane, 2-methyl-pentane, 3-methyl-pentane, 2-2-dimethyl-butane, 2-3-dimethyl-butane, 2-methyl-butane-1 and 2-methy1-butane-2). Certain C^ to C 4 hydrocarbons were also used as c a l i b r a t i n g gases i n the gas chromato^raph and infra-red spectroscopic analysis. A l l hydrocarbons were Research grade obtained from P h i l l i p s Petroleum Company, B a r t l e s v i l l e , Oklahoma. Their purity as supplied varied from 99.5 to 99.9$ . For further p u r i f i c a t i o n successive trap-to-trap d i s t i l l a t i o n was done using two temp-eratures, -196°C. ( l i q u i d Ng) so that traces of v o l a t i l e com-pounds l i k e CH^ , H 2 and a i r were removed, and -78°C. ( s o l i d CO and methanol) so that less v o l a t i l e compounds were kept i n a s o l i d condition. The p u r i f i e d hydrocarbons were stored i n globes on the vacuum system. N i t r i c oxide N i t r i c oxide was supplied i n a cylinder by the Matheson Company, East Rutherford, N.J. By successive trap-to-trap d i s -t i l l a t i o n i t was p u r i f i e d from the more v o l a t i l e impurities by condensing i t at -196°C. Nitrogen dioxide was removed by evaporating the n i t r i c oxide at -78°C. Special care was paid to the p u r i f i c a t i o n of the 27 NO because impurities l i k e NOg affect markedly the decomposition of hydrocarbons since NO generates 0 atoms at high temperatures. It was impossible to remove a l l traces of nitrous oxide, NgO, because i t has the same freezing point as NO. I t s concen-t r a t i o n however was very -slow, barely detectable by infra-red analysis. Traces of NgO were considered to have no effect on the hydrocarbon decomposition, because of the high s t a b i l i t y of t h i s oxide. 2. Gas chromatographic apparatus Two diffe r e n t columns were used for the analysis of the products. A s i l i c a gel column, 60/80 mesh, 2 meters long, was used for the analysis of and Cg hydrocarbons and n i t r i c oxide, with helium as the c a r r i e r gas. The same column was used for the analysis of Hg, using nitrogen as the c a r r i e r gas. For Cg to Gg hydrocarbons 30$ HMPA (hexamethylphosphoramide) on 60/80 mesh Columpack was used. The packing was obtained from Fisher S c i e n t i f i c Company. The container was copper tubing 3 meters long and helium was used as the c a r r i e r gas. The thermal conductivity detector used was designed by Ryce et a l (96). The c e l l current was always kept at 90 ma. The c e l l temperature and s e n s i t i v i t y of the recorder were kept con-stant for the entire investigation, at 44°C and 1 mV respectively, except i n the analysis of neohexane and diisopropyl i n which a s e n s i t i v i t y 4 mV was used. The s e n s i t i v i t y of the ohromatograph was cheeked d a i l y using a standard sample of CgHg, and was found £8 to be constant within the l i m i t s of experimental error. The temperatures of the columns were kept constant at 0° and 34°C for HMPA and s i l i c a gel respectively. g. Infra-red analysis Confirmation of the i d e n t i t y of the products was obtained by analysis of the infra-red spectra of a l l products using a Perkin Elmer Model E l instrument. The spectra of a l l compounds were taken i n the region of 650 to 4,000 cnf^ . The gaseous sample was kept i n a glass c e l l with NaCl windows, polished from time to time to maintain high transmittance. 4. The vacuum system The apparatus used was a conventional s t a t i c system shown schematically i n F i g . 1. The hydrocarbons were pyrolysed i n quartz reaction vessels held at specified temperatures i n an e l e c t r i c a l l y heated furnace. In the i n i t i a l phase of the work a single furnace was used and the pressure readings were made with a mercury manometer, while i n the f i n a l phase a double furnace was used with two i d e n t i c a l reaction vessels, one unpacked and the other packed. In t h i s phase of the investigation a pressure transducer was used for the measurement of the pressure increase. The hydrocarbons, being studied were stored i n globes, G, and could be admitted to the mixer, K,, with the use of two toepler pumps, B. The mixer was a s t i r r i n g vessel, K, with a paddle driven lay a magnetic s t i r r i n g motor, for premixing of the SCHEMATIC OUTLINE OF THERMAL DECOMPOSITION A P P A R A T U S -20 gasea where neeeaaary. From the mixer the hydrocarbons were ad-mitted to the reaction veaael, V, through a stopoock, S-^ .The. prea-aure i n the reaction veaael could be read on a manometer,M, or by meana of the preaaure tranaducer (not shown). Samples for gaa chromatographic analysia were withdrawn into a detachable sample pipette, P, from a stopcock Sg. 5. Single furnace The reaction vessel consisted of a c y l i n d r i c a l quartz vessel with a volume of 300 ml. It was placed into a brass cy-linder wound with heating wire and covered with insu l a t i n g material. Several inches of vermioulite provided the thermal i n s u l a t i o n . Jl quartz c a p i l l a r y tube connected the reaction vessel to the vacuum system. A variable voltage was supplied to the heating element of the furnaoe by means of a c o n t r o l l e r power supply. A chromel-alumel thermocouple was used to measure the temperature at one end of the brass cylinder and a second thermocouple measured the temperature of the brass cylinder i n the centre of i t . A Honey-well Electronic M i l l i v o l t Controller Model No. Y156C18-V(5)H-61 was used to oontrol the temperature to within 1° at 500°C. 6., Double, Furnace Two c y l i n d r i c a l quartz vessels, each of 160 ml, capacity, one packed with f i r e - p o l i s h e d quartz tubes and the other unpacked, were placed i n the furnace described above. Both vessels were connected through a three way stop-cock to a pressure transducer 31 and a manometer. The pressure transducer consists of unbonded strain-gauge windings i n a four-arm bridge, so that the r e s i s -tance i s variable. Pressure exerted against the diaphragm of the transducer produoes a displacement of the sensing element changing the resistance of the two active arms and causing an e l e c t r i c a l output proportional to the applied pressure. The diagram of the transducer i s given i n F i g . £. The response of the transducer was linear with pressure change i n the pressure range from 0 to 60 mm (Fig. 3). For pressures higher than 60 mm the s e n s i t i v i t y was not constant, and the transducer was therefore not usedfor pres-sure measurements above 60 mm. The highest s e n s i t i v i t y obtained was 9 mm response i n displacement of the recording pen per mm of mercury on the manometer. Description of a t y p i c a l experiment After pumping the reaction vessel and mixing system to a "black vacuum" the compounds were admitted to the toepler B (Fig. l ) , where the p a r t i a l pressures were measured. For the hydro-carbonrrnitric oxide mixture the compound with the smaller p a r t i a l pressure, that i s the NO, was admitted to the mixer f i r s t , and then the hydrocarbon was admitted. The gases were mixed thoroughly for S or 3 minutes. The mixture was then transferred from the mixer to the reaction vessel. A manometer, d i r e c t l y connected with the reaction vessel, was used to measure the i n i t i a l pres-sure ;of the reacting system and also the v a r i a t i o n of pressure 32 with time. The transducer waa not used i n t h i s phase of the investigation. After the desired extent of decomposition was at-tained, which was determined by the attainment of a given value of the r a t i o AP/P^ , the products were withdrawn from the reaction vessel into an evacuated globe connected to the apparatus and analyses were then done either by gas chromatography or by infra-red absorption. STRAIN GAl'GS -b 6-R3C.0RD*2R F i f . "2: Diagrar of the pressure transducer 34 1* o o o o o o o o CO O >p LO v< CO w ,-1 J}{ un sans sejcf 35 I I I . RESULTS 1. Uninhibited pyrolyses The pyrolyses of the isomeric hexanes, n-hexane, 2-methyl-pentane, 3-methyl-pentane, 2 ?2-dimethyl-butane and 2,3-dimethyl-butane were done at temperatures around 500°C. The following investigations were made for the uninhibited or normal decom-position of each of the five hexanes. i ) The effect of the v a r i a t i o n of i n i t i a l pressure of hydrocarbon on the i n i t i a l reaction rate and on the product,distribution for a constant extent of reaction. i i ) The eff e c t of v a r i a t i o n of surface area to volume r a t i o (S/V) of the vessel on the rate of reaction and on the product d i s -t r i b u t i o n at various temperatures. i i i ) The effect of v a r i a t i o n i n extent of decomposition on the product d i s t r i b u t i o n for constant pressure. iv) The effect of v a r i a t i o n of i n i t i a l hydrocarbon pressure and of the structure of the hydrocarbon on the energy of activ-ation and the frequency factor for the o v e r a l l reaction. A.) Rates of reaction The pressure-time curves appear regular (Fig. 6, 9, 12, 15, 18) but they show a tendency to l e v e l o f f before completion of the p y r o l y s i s , which oan be attributed to s e l f - i n h i b i t i o n by the products. At low pressures, and e s p e c i a l l y at lower temper-atures, the curves have a s l i g h t sigmoidal shape. This may be a 36 consequence of the d i f f i c u l t y i n measuring the small pressure changes which occur i n the e a r l y stages of the reaction. I n i t i a l rates were measured by drawing tangents to the pressure-time ourves. To test the v a l i d i t y of t h i s method, i n i t i a l rates were also obtained from rate ourves obtained by analyzing for the hexane at various time i n t e r v a l s i n the pyrolysis. A comparison of the i n i t i a l rates obtained from a n a l y t i c a l r e s u l t s , with the i n i t i a l rates obtained manometrioally i s given i n Fig.4. for each of the isomeric hexanes. It can be seen that Rii x <x R^anal) i n which the constant i s unity for n-hexane while for d i i s o -propyl i t i s 0.9 . For the other three hexanes o( varies between 0.9 and 1. Therefore the use of R^( &P) as a measure of i n i t i a l rate of pyrolysis i s considered to be j u s t i f i e d . Thus a l l rates obtained i n the present study are based on pressure measurements, and w i l l be simply denoted as Rj_ . A l) n-hexane For n-hexane, l i n e a r i t y i s observed i n the plot of i n i t i a l rate against i n i t i a l pressure only for low pressures from 0 to 40 mm (Fig. 5). Some curvature was observed for higher pressures. At lower pressures the relationship applies Ri = (3 * i where {3 i s a constant depending on the temperature. The effect of packing: To evaluate the effect of packing two similar vessels were 37 used, one unpacked and the other packed with f i r e - p o l i s h e d quartz tubes so that the area-to-volume r a t i o of 1 for the unpacked, vessel became 11 for the packed vessel. Both vessels were kept i n the same furnace at the same temperature. Hydro-carbons were admitted to both of them simultaneously and after the pressures were balanced the i n i t i a l pressure was measured using the pressure transducer, the output of which was displayed on a m i l l i v o l t c o n t r o l l e r . Using a three-way stop-cock the vessels were oonnected to the transducer alternately so that the pressure change i n each vessel oould be recorded. Two press-ure-time curves were thus obtained simultaneously (Fig. 6), i n whioh the dotted l i n e s show the extention of the curves between recorded sections. I n i t i a l rates obtained from these recorded pressure-time curves were converted to rates i n millimetres of mercury per minute by using a conversion factor obtained by determining the s e n s i t i v i t y of the pressure transducer as di s -cussed i n Chapter I I . Packing the vessels reduced the rates of pyrolysis at low pressures. At higher pressures the rates i n the packed and un-packed vessel became almost i d e n t i c a l . The percentage reduction i n rate as a function of the i n i t i a l pressure i s given i n Fig . 7 for n-hexane i n terms of the r a t i o Ri (unpacked!}) - R^ (packed) ^ 3C -LvJv/ Ri(unpacked) R i ( p n a l )ir.rr/rrin F i g . 4: R e l a t i o n s h i p between i n i t i a l rrenorne t r i c r 9 t a and i n i t i a l a n a l y t i c a l r s t e f o r t h e 3.2 520°C P i tr.m F i g . 5: Dependence of i n i t i a l r a t e on i n i t i e l pressure for n-hexane 40 A2) 2-methyl-pentane A plot of i n i t i a l rate against i n i t i a l pressure (Fig. 8) i s s i m i l a r to that for n-hexane. The ourves are lin e a r at lower pressures hut "become s l i g h t l y sigmoidal at higher pressures, esp e c i a l l y at lower temperatures. The effect of packing: The method outlined for n-hexane was used to examine the effect of increase i n S/V r a t i o for a l l of the isomers. A ty-p i c a l AP-time curve for 2-methyl-pentane i s given i n F i g . 9. The reduction i n rate "between unpaoked and packed vessel (Fig. 10) appears to he less than i n the case of n-hexane. A3) 3-methyl pentane The plot of i n i t i a l rate against i n i t i a l pressure (Fig.11) i s s i m i l a r to the corresponding r e s u l t s for n-hexane and 2-methyl-pentane except that i t has a more markedly sigmoidal shape. For four different temperatures curves similar i n shape were obtained. The effect of packing: An increase i n S/V r a t i o appears to have an effect on rates sim i l a r to that for 2-methyl-pentane (Fig. 12). The reduction i n rate decreases with increasing pressure and p r a c t i c a l l y va-nishes at pressures above 100 mm (Fig. 13). The points are more scattered than i n n-hexane and 2-methyl-pentane. A4) 2,2-dimethyl-butane (neohexane) A plot of i n i t i a l rates against i n i t i a l pressures (Fig. 14) is similar to those obtained for the other isomers. The effect of packing: 80 70 60 50 40 10 O 532°C @ 540°C ® 510°C 0 520°C 10 ~20~ 30 40 50 ? i r.m F i g . 7: P e r c e n t a g e r e d u c t i o n i n r a t e due t o t h e p a c k i n g f o r v e r i o u s i n i t i a l p r e s s u r e n-hexane 52C°G 0 20 40 60 80 100 120 140 160 180 200 210 ? i rem F i g . 8: D e pendence o f i n i t i a l r a t s on i n i t i a l p r e s s u r e f o r 2- r e t h y l - p e n t s n e o o o o o o t o l O o o 001 x 46 Packing has a very small effect on i n i t i a l rates (Fig. 15). The reduction due to packing appears to he much less marked than i n the cases of the less branched isomeric hexanes (Fig. 16). A5) 3,3-dimethyl-butane (diisopropyl) Diisopropyl decomposes at a much higher rate than do the previous hydrocarbons but thecurves r e l a t i n g i n i t i a l rates to the i n i t i a l pressure appear to be' similar to those for the other hydrocarbons (Fig. 17). The effect of packing Packing affects the rate i n a manner similar to that for neohexane (Fig. 18). The reduction i n rate caused by packing the vessel i s less for diisopropyl than for any of the other hexanes (Fig. 19). A6) The effect of hydrocarbon structure on rate The effect of the extent of sk e l e t a l branching on the rates of o v e r a l l pyrolysis of the isomeric hexanes i s indicated by the results presented i n Fig . 30, i n which i n i t i a l rates are plotted against i n i t i a l pressures at 510@C. The f i v e isomers f a l l appro-ximately into three groups i n this comparison. N-hexane has the slowest rate of pyrolysis and diisopropyl the fastest. The other three isomers have rates which are very similar at pressures up to approximately 80 mm at which pressure 3-methyl-pentane decom-poses at a s i g n i f i c a n t l y greater rate than do neohexane and 3-methyl-pentane. B) Order of reaction Reaction orders were obtained by p l o t t i n g logR^ against a i m / u r n ^ H 80 ® 530°C O 520°C (Z) 510°C w.m TPig.lZ: P e r c e n t a g e r e d u c t i o n i n r a t e due o t h e p a c k i n g f o r v a r i o l a i n i t i a l p r e s s u r e s of ? - r r e t h y l - p e n t a n e 4 . 8 k F i g . 14: Dependence o f i n i t i a l r a t e on i n i t i a l p r e s s u r e f o r n e o h e x a n e 18 16 14 T = 530 C P i - 34 mm 12 E e < 10 unpac feed plTc ked 13 14 •5M c- . 1 : 7 8 t i r e rr i n D r a p p u r A t i r e c u r v e s f o r p a c k e d and u n p a c k e d v e s s e l s f o r d i i s o p r o p y l 80 h 70 l o o 60 — AS <D CO O. 50 N-t OS l •—-» p M * O CD tt: C Pi •tH a ! 20 h 10 L F i g . P^ ir.rr 19: P ercentage r e d u c t i o n i n r a t e due t o the p a c k i n g at v a r i o u s i n i t i a l pre d i i s o p r o p y l 100 110 s s u r e s o f 56 l o g f i , that i s using the common rate equation Rate - kP n where n i s the o v e r a l l reaction order. Bl) n-hexane The order of reaction calculated for four different tempe-ratures was found to he approximately unity (Fig. E l ) . For the lowest temperature (T -490°C) the order appears to change to a value of 0.7 at approximately 100 mm pressure although has not been established with certainty. The effect of packing: The reaction orde> i n the packed vessel appears to be s l i g h t -l y higher than i n unpacked vessels at least at the lower pressures (Fig. EE). At lower pressures n — 1 for the unpacked vessel, while n — I.S for the paoked vessel. At higher pressures the order appears to f a l l s l i g h t l y for both vessels. BE) S-methyl-pentane Some i r r e g u l a r i t i e s i n reaction order were observed for S-methyl-pentane (Fig. 33) . For three d i f f e r e n t temperatures to order appears to have two values h - 1.3 at pressures up to 60 mm and n - 0.9 at higher pressures. The ef f e c t of packing: The ef f e c t of packing oa reaction order i s simi l a r to that observed for n-hexane. (Fig. 34). The order i s approximately unity for the unpaoked vessel while for the packed vessel the value i s about 1.3. P i mm F i g . 20: D e p e n d e n c e o f i n i t i a l r a t e on i n i t i a l p r e s s u r e f o r a l l i s o m e r i c h e x a n e s O unpac ked O packed T = 532°C slope c 1 slope -1.2 slope = 0.85 Q' 0.4 0. 0.8 1.0 1.2 1.9 2.0 2.2 1.4 1.5 l o g Pi F i g , 22: Order of r e a c t i o n p l o t for the packed and the "unpacked v e s s e l for n-hexane 2.4. 2.6 0.6 o. 4 - o . ± unpac ked pac ked T - 520°C 0.4 0.6 0.9 1.0 1.2 1.4 1.6 l o g P i 1.9 2.0 2.2 2.4 2.6 Fig. 24; O r d e r o f r e a c t i o n p l o t f o r t h e p a c k e d and the u n p a c k e d v e s s e l f o r 2 - r e t h y l - p e n t 8 n a 62 B 3) 3-methyl-pentane No i r r e g u l a r i t i e s i n reaction order were observed i n g-methyl-pentane. The order calculated for four d i f f e r e n t temperatures appears to be uniformly 1.2 over a pressure range from 10 to 140 mm (Fig. 25). The effect of packing: The effect of packing on the o v e r a l l order i s similar to that observed for the other two isomeric hexanes. For lower pressures the order for the unpacked vessel i s about 1.2 while for the packed vessel i t i s about 1.3. For pressures higher than 100 mm the order becomes approximately 0.7 for both vessels (Fig. 26). B 4) 2,2-dimethyl-butane (neohexane) The i r r e g u l a r i t i e s which were encountered i n 2-methyl-pentane are more pronounced i n neohexane. The order of decomposition c a l -culated for three d i f f e r e n t temperatures has the value 1.5 for lower pressures but for higher pressures the value changes from 1.5 to 1.10 (Fig. 27). The effect of packing: The packing appears to affect the order of the reaction i n neohexane also. A s h i f t from n = 1 to n = 112 i s observed be-tween the unpacked and the packed vessel (Fig. 28). B 5) 2,3-dimethyl-butane (diisopropyl) I r r e g u l a r i t i e s similar to those observed for the previous hydrocarbons were observed for diisopropyl. The order of reaction calculated for four d i f f e r e n t temperatures was found to be con-siderably greater than unity (approximately 1.5) for the lower 520°C 0.5 1.0 1.5 2.0 2.5 l o g P i F i g . 25: Order of r e a c t i o n p l o t f o r ?-irethyl-pentane • 1 1 I I I I I 1 I I I I 1 — C. .'. 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 l o g P i F i g . 26: Order of r e a c t i o n p l o t f o r the packed and the unpacked v e s s e l f o r 3-rrethyl-pentane 65 0.8 6 7 pressures, while for the higher pressures i t changes to 1.0 (Fig. 29). A comparison of a l l f i v e isomers suggests that the more branch-ed, the isomeric hexane the larger the difference i n order between the lower and the higher pressures. This observation i s of interest i n considering the mechanisms of decomposition of these compounds. The effect of packing: The same effect i s observed as i n the case of neohexane. The order i n the unpacked vessel i s approximately unity while for the packed vessel i t i s about 1.2 (Fig. 30) C ) Energies of a c t i v a t i o n and frequenoy factors for the o v e r a l l  reaction Energies of actiivatipn were 0a3.0li13.ated frp# t h e Arrhenius equation E A(Kcal/mole) _ 2.303TiT*>log(kP/ki) - Tg - T i using rate constants calculated from rate ourves obtained at either 3 or 4 temperatures. The preexponential factors were calculated from the expression :: k ^ Ae-SA/R* Details of the calculations of rate constants, energies of a c t i -vation and preexponential factors are given i n Appendix I. The energies of ac t i v a t i o n and frequenoy factors show v a r i -ations with pressure reaching more or less l i m i t i n g values at higher pressures (Tables 1 - 5 ) Such variations have been found 1.3 5S0°C Q t i i i » L 0 .8 1.1 1.4 1.7 2.0 2 .3 2.6 l o g P i F i g . 29: O r d e r o f r e a c t i o n p l o t f o r d i i s o p r o p y l 70 before i n the iaomerio pentanea (92) and for 2- and 3-methyl-pentanes (97). In a l l of the iaomerio hexanes exoept diiaopropyl the frequency faotora have values around l O ^ s e c for pressures up to approximately 60mm but have lower values at higher pressures. C l ) n-hexane Activa t i o n energies are calculated from rate constants meas-ured at temperatures 490°C, 510°C and 520°C. The re s u l t s obtained are averages of three calculated values and are given i n Table 1. Table 1 - Variat i o n of ac t i v a t i o n energy and frequency factors with i n i t i a l pressure of n-hexane. (mm) E^ (kcal/mole) A (sec""-1-) 0 - 4 0 64 ± 1 1.7 x IO} 4 4 0 - 6 0 60 t 2 1.1 x 1 0 1 3 60 - 80 59.5 ±2.5 8.2 x 1 0 1 2 90 59 t 4 5.3 x 1 0 1 2 100 59 i 4 3.5 x IO* 2 120 58.5± 3.5 3.3 x I O 1 2 Average values over the whole pressure range appear to be mean-ingless. The rate constants are related to the A factors and ac-t i v a t i o n energies by a general expression of the form k . A ( P 1 ) 9 - E A ( P l , / f f i f which indicates the dependence of A and E^ on pressure. Similar r e s u l t s have been obtained by other workers, whose work w i l l be refe'rred to i n Chapter IV. C 2) 2-methyl-pentane In the study of 2-methyl-pentane three diff e r e n t temperatures 71 were used, 500, 510 and 520°C. Energies of a c t i v a t i o n given i n the following table are average values. Variations of E^ and A with hydrocarbon pressure similar to that found for n-hexane were ob-served. The results,are given i n Table 2. Table 2 - V a r i a t i o n of a c t i v a t i o n energy and frequency faotor with i n i t i a l pressure of 2-methyl-pentane (min) E A (Kcal/mole) A (sec* 1) 0 - 3 0 30 - 60 60 - 90 90 -100 120 It can be seen that the aotivation energy f a l l s to lower values as the isomeric hexane becomes more branched* C3) 3-methyl-pentane Energies of aotivation and frequency factors were calculated from rate constants obtained at four different temperatures. Again a reduction i n both E A and A with increasing pressure was observed. The energy of aotivation appears to be lower than the energy for n-hexane and 2-methyl-pentane, except for very low pressures. The r e s u l t s are given i n Table 3. Table 3 - Variation of a c t i v a t i o n energy and frequency faotor with i n i t i a l pressure of 3-methyl-pentane P i (mm) E A (kcal/mole) A (sec" 1) 62.0 60.0 58.0 58.5 56.0 1 2 3 4 2 4.7 x 5.5 x 1.2 x 5.5 x 8.4 x 10 10 10 10 10 13 13 12 12 11 0 - 3 0 40 65.0 t 2 59.0 i 2 9.5 x 1 0 1 3 4.0 x I O 1 2 72 50 59.0 ^ 7 4.4 x lOf^ 60 -80 55.5 ± 3.5 6.6 x lOtt 90 54.5 i 5 8.0 x l6„ 100 54.0 ± 4 6.6 x 10 ft 120 54.0 * 4 2.5 x 1 0 1 X C4) 2,2-dimethyl-butana (neohexane) Values of E A and A for neohexane given i n Table 4 are averages of s i x values calculated from rate data obtained at four diff e r e n t temperatures. The energies of act i v a t i o n are lower than i n the oase of the less branched isomers and the frequenoy factors are unusually low, much less than the normal values for homogeneous unimolecular reaction. Table 4 - Variation of act i v a t i o n energy and frequency factor with i n i t i a l pressure of neohexane P i (mm) E A (kcal/mole) A (sec"'1') 0 - 30 56.0 - 2 5.0 X 1 0 1 0 i o 9 1 0 30 - 50 51.0 t 2 5.0 X 50 - 80 47.0 t 3 1.6 X 80 -100 - 47.0 4 6.6 X io* 100 -120 45.5 1 3.5 3.3 X 10 9 120 -150 45.5 * 2.5 1.3 X 10 9 C5) 2,3-dimethyl-butane (diisopropyl) In the case of diisopropyl rates obtained at four temperatures were used to calculate the kinetic parameters. The ac t i v a t i o n energy and the frequency factors are lower than i n neohexane. Diisopropyl displays many p e c u l i a r i t i e s as far as the rate of decomposition i s concerned i n comparison with the other isomeric 73 hexanes. The results for B A and A given i n Table 5 are averages of s i x different values at each pressure. Table 5 - Var i a t i o n of act i v a t i o n energy and frequency factor with i n i t i a l pressure for diisopropyl P i (mm) E A (kcal/mole) A (sec" 1) 0 -30 51.5 •+• E.5 1.7 X 10 1 1 30 -50 51.0 •+ 5 4.E X 50 -86 48.0 5 1.4 X 80-100 48.0 4 1.1 X By 100-130 47.5 5 1.1 X 130-150 47.5 5 6.4 X i o 9 D ) A n a l y t i c a l r e s u l t s  Dl) n-hexane The main products i n n-hexane decomposition are CH4, 0£H4» CgHg. OgHg, and I - C 4 H 8 (Table 6). Hydrogen was formed i n small amounts, less than zfa of the other decomposition products, and CgHg only i n traces. An almost complete mass balance was obtained i n d i c a t i n g that polymerization of unsaturated products i s n e g l i g i b l e at the temperatures which were used. A r a t i o of the pressure increase to the i n i t i a l pressure, Ap/p i t of 0.S0 was adopted for a l l unin-h i b i t e d pyrolyses i n an attempt to achieve a constant extent of reaction. Table 6 - A n a l y t i c a l r e s u l t s (mole $) for the decomposition m c A A o 0 of n-hexane . / „ _ T = 500 C A P/Pi - 0.SO Products P i - 1 1 S mm P i - 100 mm P i - 70 mm P i - 5 0 mm CH 4 4.SO 4.00 3.80 3.70 74 C2 H6 2.50 2.60 1.90 2.00 5.40 5.20 4.80 4.25 °3H6 9.60 8.50 7.80 7.00 1-C4H8 5.00 4.20 3.80 3.70 H 2 6.50 0.50 0.50 0.50 C3 H8 traces traces traces traces n-hexanea 72.50 74.00 77.00 78.00 Total 99.7 99.0 99.6 99.1 D2) 2-methyl-pentane The main products i n the decomposition of 2-methyl-pentane are: CH£, C 2H 6,QC 2Hp CgHg?. i-C 4Hg and H 2 (Table 7). Propane i s formed i n small hut measurable quantities.The mass balance i n a l l cases i s above 99$, which excludes any p o s s i b i l i t y of poly-merization. Table 7 - A n a l y t i c a l r e s u l t s (mole fo) for the decomposition of 2-methyl-pentane T - 510°C kP/Pi = 0.20 Products P t n ll2V*ux P A Pi 170 mm 150mm 120 mm 108mm 81mm 48mm CH 4 6.00 5.50 5.30 5.60 4.30 3.00 C2 H6 5.70 5.00 5.50 5.60. 4.50 3.40 C 2H 4 3.70 3.20 3.60 3.80 4.20 3.00 G 3H 8 1.80 1.60 1.30 1.20 1.00 0.70 °3H6 7.50 7.80 7.40 8.40 7.20 6.50 75 i-C 4H Q 6.20 6.60 7. 00 7.10 5.00 4.80 H 2 5.50 5.50 5. 00 5.00 4.50 4.00 2-methyl-pentane 63.50 64.00 64.00 63.00 6 69.00 74.00 Total 99.50 99.20 99.10 99.70 99.70 99.40 T - 520 °C &P/Pi - 0.20 produc t s - 181 mm P 'i = 154 mm P A r 102 mm P ^ 80 mm CH 4 6.00 5.60 4.70 4.00 C 2H 4 5.80 5.60 4.20 3.20 C2 H4 3.20 3.20 2.80 2.20 C3 H8 2.10 1.90 1.60 0.90 °3H6 9.00 7.60 6.80 7.20 i-C 4H 8 7.20 6.00 5.60 5.60 H 2 5.20 5.00 4.50 4.00 2-methyl-pentane 61.00 67.00 69.00 7 2.00 Total 99.8 99.5 99.2 99.1 D3) 3-methyl-pentane The a n a l y t i c a l results for 3-methyl-pentane d i f f e r from those for the other hexanes i n that, there are very few major products hut a number of products appear i n almost equal amounts. The re s u l t s are given i n Tables 8 - 1 0 . 76 Table 8 - A n a l y t i c a l r e s u l t s (mole$>) fpr the decomposition of 3-methyl-pentane T r 490°C Ap/Pi - 0.20 Products P i = 90 mm TZ^ ~ 80 mm P^- 70 mm CH 4 6.90 7.00 6.40 CgHg 4.70 4.70 4.20 0 2H 4 4.00 3.10 2.10 C 2H 6 2.80 2.70 2.70 1- C 4H 8+ i-C 4H 8 1.10 1.00 1.30 trans-butene-2 2.60 2.70 " 2.70 cis-butene-2 1.70 2.10 2.10 2- methyl-butene-l 3.70 4.20 4.30 H 2 1.00 1.00 1.00 CgHg traces traces traces 3- methyl-pentane 71.00 71.00 72.00 Total 99.5 99.6 99.3 Table 9 - A n a l y t i c a l r e s u l t s (molejS) for the decomposition of 3-methyl-pentane T - 500°C AP/Pi = 0.20 Products Pj^ - 110 mm P i = 100 mm P i - 91 mm CH4 7.00 6.20 5.80 CgH6 3.80 3.50 3.50 CgH4 2.60 2.40 2.20 C 3H 6 3.20 3.20 3.10 1-C4H8+ i- g 4 H 8 1.60 1.40 1.80 77 trans-butene-2 3.10 3.10 2.60 ois-butene-2 2.80 3.00 ;; 2.10 23methyl-butene- 1 4.30 3.70 3.70 H 2 1.50 1.50 1.50 3-methyl-pentane 70.00 71.00 73.00 Total 99.7 99.0 99.3 Table 10 - A n a l y t i c a l results (moles$) for the decomposition of 3-me thy 1- pentane T = 510°C &P/Pi =i 0.20 Products P i =118 mm ' Pi_- 98 mm P^ - 51 mm P i ~ 44 mm CH 4 6.80 7.20 4.90 4.70 C 2 H6 3.80 4.20 3.20 2.00 C 2H 4 3.40 3.80 3.00 2.00 G3 H6 4.70 3.00 2.60 2.60 1-C4H8 * i-C 4H 8 1.60 1.50 1.50 1.20 trans-butene-2 3/30 2.50 2.80 2.00 cis-butene-2 2.80 2.20 2.00 1.50 2- me t hy 1- bu t e ne -1 5.20 3.80 2.50 3.60 H 2 2.00 1.90 1.50 1.50 3-methyl-pentane 65.00 69.50 75.50 78.00 Total 99.1 99.6 99.5 99.1 D4) 2,2-dimethyl-butane (neohexane) The a n a l y t i c a l r e s u l t s for the pyrolysis of neohexane are given l n Tables 11, 12 and 13. 78 Table 11 - A n a l y t i c a l r e s u l t s (mole#) fpr the decomposition of neohexane Products T - 500eG ^ P / ^ i = 0.20 P P. P P- P 180 mm 146 mm Ilk mm 9§ mm 6 6 mm CH 4 7.40 6.10 5.40 4.00 4.90 °2H6 3.00 3.30 2.50 2.00 1.80 C 2 H4 3.80 3.60 3.60 2.50 2.60 C3 H6 0.90 0.60 0.60 0.70 0.60 i-C 4H 8 8.50 7.80 8.00 8.20 8.00 2-methyl-butene-2 3.20 3.40 2.50 3.30 3.20 5.00 5.00 4.50 4.50 3.50 Neohexane 67.00 69i00 72.00 74.00 75.00 Total 98.8 99.3 99.1 99.7 99.6 Table 12 - A n a l y t i c a l r e s u l t s (mole$>) for the decomposition of neohexane T - 510°C * p/Pi = 0.20 Products P.-^  180 mm p£ = 143 mm Pj_ - 120 mm p£ = 60mm CH 4 7.20 6.20 6.50 5.50 CgHg 3.30 2.30 2.50 2.20 C 2H 4 3.50 3.00 4.10 4.10 C 3H 6 1.10 0.80 0.20 0.60 i-C 4H 8 7.90 8.00 8.00 8.00 2-methyl-butene-2 3.70 3.20 3.20 2.60 H„ 5.00 5.00 4.50 4.00 79 neohexane 67.00 71.00 70.00 73.50 Total 99.S 99.5 99.7 99.5 TAble 13 - A n a l y t i c a l r e s u l t s (mole$) fpr the decomposition of neohexane T _ 5S0°C O.SO Products 17&mm 156mm is6mm 100mm 84mm 60mm CH 4 9.E0 9.00 8.40 7.40 7.60 7.70 CsH6 4.00 3.10 S.80 S.50 E.10 S.00 5. SO 3.60 4.60 4.90 3.60 4. SO 1.30 1.00 0.70 0.70 0.40 0.30 i-C 4H 8 8.50 8.70 8.70 8.50 8.40 7.90 2- me thy 1- hut ene - S 3.E0 3.80 4.00 3. SO 3.40 3.70 5.00 5.00 4.50 4.50 4.00 3.70 neohexane 63.00 65.00 66.00 70.00 70.00 70.50 Total 99.4 99.7 99.7 99.6 99.5 99.8 D5) 3,3-dimethyl-butane (diisopropyl) The a n a l y t i c a l r e s u l t s for the pyrolysis of diisopropyl are given i n Tahles 14 and 15. Table 14 - A n a l y t i c a l results (mole $) for the decomposition of diisopropyl T - 510OC ^ P / ^ = 0.80 Products Pj = 160mm Pj = 180mm P j - 100mm Pj •=. 60mm CH4 9.SO ? 8.60 9.50 6.50 C 2H 8 1.S0 1.30 1.S0 0.90 80 C 3H 6 6.20 6.40 6.80 6.40 i-C 4Hg 0.70 0.60 1.30 0.50 2-methyl-butane-2 6.70 7.30 9.30 9.50 H 2 5.00 5.00 4.00 3.50 C 2H 4 trace trace trace trace CgHg trace t t r a c e trace trace diisopropyl 66.00 70.00 68.00 71.00 Total 95.0 99.2 99.8 98.8 Table 15 - A n a l y t i c a l r e s u l t s (mole fo) for the decomposition of diisopropyl T - 520°C - 0.20 Products P i - 100mm P i - 80mm P i - 60mm P i -. 50mm CH4 11.00 8.90 9.50 7.50 °3H8 1.00 0.90 0.40 0.40 C3 H6 6.40 7.10 6.70 6.70 i-C 4H 8 2.00 ? 0.90 1.00 0.80 2-methyl-butene-2 8.00 11.00 0.60 8.30 H 2 5.00 4.50 4.00 4.00 0 2 % trace trace trace trace C2 H4 trace trace trace trace diisopropyl 66.00 66.00 67.50 71.00 Total 99.4 99.2 99.7 98.7 81 E ) The v a r i a t i o n of product d i s t r i b u t i o n with time The v a r i a t i o n of the composition of the reaction mixture with time, for a fixed i n i t i a l hydrocarbon pressure was measured at regular i n t e r v a l s for a time period of up to half an hour. E 1) n-hexane The v a r i a t i o n i n product d i s t r i b u t i o n with time i s given i n Fig. 31. The percentages of a l l products show a tendency to l e v e l off at higher extents of reaction, presumably because of the oc-currence of s e l f - i n h i b i t i o n . E 2) 2-methyl-pentane Analogous r e s u l t s were obtained i n the pyrolysis of 2-methyl-pentane (Fig. 32) E 3) 3-methyl-pentane In the pyrolysis of 3-methyl-pentane, many products are formed i n approximately the same proportions. The a n a l y t i c a l r e s u l t s are therefore presented i n the following table, rather than graphically. Table 16 - Var i a t i o n of product d i s t r i b u t i o n (mole fo) with time for the decomposition of 3-methyl-pentane T - 520°C - 100mm time 1-C 4 %r c i s - 2-methyl-min CH 4 C 2 H 6 C 2H 4 C3Hg *i-C 4H 8 trans-C 4H 8 C 4H 8 butene-1 2 2.50 1.30 1.50 0.65 - 0.86 - 1.20 4 3.30 2.60 - 1.60 0.82 1.90 1.70 2.30 6 - - - 1.75 0.50 1.80 1.20 2.00 7 6.00 3.20 2.70 2.20 0.80 2.60 1.95 3.00 82 8 6.20 3.20 2.00 2.10 0.72 2.20 1.70 2.70 10 7.40 3.20 3.20 - - - - -12 8.70 4.40 3.00 2.90 1.20 2.50 2.30 2.70 15 8.40 4.50 4.10 3.30 1.35 2.80 2.40 3.70 20 12.00 5.60 4.60 3.20 1.50 3.70 3.40 3.40 Methane, CgHg and CgHg show a marked increase with time while for the other products the increase i s not so pronounced. 2-methy1-butene-l appears to a t t a i n a steady state concentration early i n the reaction. B 4) 2,2-dime thy1-hutane (neohexane) The v a r i a t i o n of the product concentrations with time i s given i n Pig. 33. The amount of isohutylene increases very mark-edly with time while 2-methyl-butene-2 increases rapidly i n the early stages hut levels o f f quickly. The concentrations of CH4, CpH^, CgHg and Hg display an almost l i n e a r dependence on time. E 5) 2,3-dimethyl-hutane (diisopropyl) The r e s u l t s showing the v a r i a t i o n i n the product d i s t r i b u t i o n with time are given i n F i g . 34. 2-methyl-butene-2 appears to reach a maximum concentration at an early stage i n the reaction which indicates that decomposition of i t occurs i n the system. The concentrations of CH4 and CgHg show marked increases with time while the variations of CgH8, i - C 4 H 8 and Hg with time are less pronounced. 0 2 4 6 8 IC 12 14 16 18 20 22 time rrin F i g . 31: V a r i a t i o n of the pro.uct d i s t r i b u t i o n with titr.e f o r 100 mm of n-hexane u n i n h i b i t e d decomposition T " 520°C 100 mm 12 -time min F i g 32: V a r i a t i o n o f the product d i s t r i b u t i o n with time f o r 100 mm of 2-methyl-pentane 10 QJ i H O E uninhibited reaction 520°C Pj_ - 100 mm 10 12 time min Fig. 34: Variation of the product d i s t r i b u t i o n with time for 100 mm of diisopropyl §7 F ) Varia t i o n of product d i s t r i b u t i o n with pressure The complete v a r i a t i o n with pressure of the product d i s t r i -bution was investigated for a i l isomeric hexanes. I t was expected that t h i s investigation would provide additional information on the mechanisms of the reactions. F 1) n-hexane The v a r i a t i o n i n product d i s t r i b u t i o n with pressure i s given i n Fig. 35 for a constant extent of reaction given by AP PA - 0.20 . The percentages of both l-C 4Hg and CgHg increase r a p i d l y at low pressures but tend to l e v e l o f f at pressures of about 80mm. Methane and CgHg show a pressure' dependence but l e v e l o f f at higher pressures. The v a r i a t i o n of C2H4 with pressure i s almost l i n e a r . I t was not possible to investigate the v a r i a t i o n of Hg with pressure because of the small amounts produced. F 2) E-methyl-pentane The v a r i a t i o n of the product d i s t r i b u t i o n with pressure i s given i n F i g . 36. Propylene appears to be almost pressure inde-pendent, in d i c a t i n g that i f might be formed e n t i r e l y from spontaneous decomposition of a larger r a d i o a l . Isobutylene shows some dependence on pressure but i t soon levels o f f . A l l other produots are s l i g h t l y pressure dependent tending to l e v e l off at higher pressures. F 3) 3-methyl-pentane The v a r i a t i o n of product d i s t r i b u t i o n with pressure appears more in t e r e s t i n g i n the case of 3-methyl-pentane (Fig. 37. There f i g . 37: V a r i a t i o n of the product d i s t r i b u t i o n with i n i t i a l pressure f o r 3-rethyl-pentnne 91 are only two products, C H 4 and Cg^e* w h i o 1 1 a i l 0 W pressure dependence. A l l other products are pressure independent i n d i c a t i n g that they are formed from spontaneous decomposition of larger r a d i c a l s . F 4) 2,2-dimethyl-butane (neohexane) The v a r i a t i o n of product d i s t r i b u t i o n with pressure i s shown i n F i g . 38. Ethane and C„H.. are s l i g h t l y pressure dependent while a l l other products display either a very s l i g h t pressure-depen-dence, l e v e l l i n g o f f at higher pressures, or are pressure inde-pendent. Methane appears to have a very s l i g h t pressure dependence for a wide range of pressures. F 5) 2,3-dimethyl-hutane (diisopropyl) The v a r i a t i o n of product d i s t r i b u t i o n with pressure i s shown i n F i g . 39. Methane displays marked pressure dependence while the other products are either pressure independent or s l i g h t l y dependent tending to l e v e l o f f i n the middle of the pressure range. Propane displays a s l i g h t l i n e a r dependence on pressure, which can be attributed to the propagation of decomposition by CgH^ r a d i c a l s . G ) Variat i o n of product r a t i o s with pressure The v a r i a t i o n of the r a t i o s of cer t a i n products with pressure was investigated i n a l l isomers over a wide range of pressures and at differ e n t temperatures. These r a t i o s provide information about competing reactions i n the o v e r a l l mechanisms. G- l ) n-hexane The v a r i a t i o n of the r a t i o s : 16 T - 520°C run F i g . 38: V a r i a t i o n o f p r o d u c t d i s t r i b u t i o n w i t h i n i t i a l p r e s s u r e f o r n e o h e x a n e 94 GH4/CgH4 , C2 H4/ G2 H6 and C 3H 6/l-C 4H 8 have heen investigated i n the pyrolysis of n-hexane. The results are given i n Table 17. Table 17 -• Var i a t i o n of product : ratios with pressure for n-hexane T - 500°C Ap/p, - 0.20 pressure mm CH4/CgHg CH4/C2H 4 CgH 4/C 2H 6 C3H 6/i-C 4H 8 31 _ 0.74 44 2.20 0.75 2.40 1.90 52 1.87 0.85 2.16 1.65 72 2.15 0.80 2.60 ? 1.85 82 1.90 0.86 2.10 102 1.60 0.78 2.00 1.75 110 1.60 0.77 1.85 1.75 G 2) 2-methyl-pentane The variations of product r a t i o s with pressure i n the pyroly-sis of 2-methyl-pentane are given i n Tables 18 and 19. Table 18 - Variat i o n of product r a t i o s with pressure for 2-methyl-pentane T - 510°C *P/Pt = 0. 20 pressure CH 4/ C H 4 / C2H4/ 0 3 % / G 3 H 6 / H 2 / mm CgHg CgH4 C 2H 6 C 3H 8 i - C 4 H 8 CH 4 33 - - - 11.8 1.85 -48 0.88 1.0 0.90 9.3 1.35 1.33 62 - - - 9.6 1.37 mm 81 0.95 1.05 0.94 7.0 1.44 0.92 108 1.00 1.46 0.85 8.4 1.21 0.89 120 0.98 1.47 0.66 5.8 1.08 0.94 150 1.10 1.72 0.64 4.9 1.18 1.00 170 1.05 1.87 0.56 4.2 1.20 0.93 9jS Table 19 - Variat i o n of product r a t i o s with pressure for 2-methyl-pentane T = 520°G /AP/P. - 0.20 pressure mm CH4/. C2 H6 CH 4/ °2H4 C 2H 4/ C2 H6 °3H8 C 3 H6/ i-C 4H 8 * CH 4 (52 1.20 1.80 0.69 7 ? 1.45 mm 62 1.10 Mi 9.70 1.60 -80 1.24 1.80 0.69 8.00 1.30 1.00 102 1.12 2.10 0.64 4.25 1.22 0.96 121 0.98 1.98 0.49 4.80 1.16 0.98 154 1.00 1.75 0.57 3.14 1.20 0.89 181 lv§4 1.88 0.55 4.20 1.24 0.92 Prom the above tables we can see that both CH3» and CgHg* radioals seem to participate equally i n the propagation beeause there i s no clear increase or decrease of CH4/C2He with pressure. G 3) 3-methyl-pentane The pyrolysis of 3-methyl-pentane produces many more products than do the pyrolyses of the other isomers. Therefore more analy-t i c a l data are required to gain an insight into the mechanism of the decomposition. The product d i s t r i b u t i o n i s largely pressure independent (Fig. 37), and therefore i t appears that the products are formed largely by the decomposition of higher r a d i c a l s . The variations of product r a t i o s with pressure for 3-methyl-pentane are given i n Tables 20, 21, and 22. 96 Table 20 - Variati o n of product r a t i o s with pressure for 3-methyl-pentane pressure mm T -CH 4/ C2 H6 510°C C2H4/ AP/P. trans-C4H8 0.20 C 3 H 6 / cis - C 4 H 8 trans-C 4H 8/ CgHg/ cis - C 4 H 8 2-me-butane -3 30 1.80 0.98 —> 44 2.30* 1.00 1.30 1.73 1.34 0.72 51 1.63 0 .94 0.93 1.30 1.39 1.04 91 1.70 0.96 1.05 1.20 1.30 98 1.86 0.92 1.20 1.36 1.16 0.80 118 1.78 0.92 1.40 1.65 1.18 0.90 * Doubtful value Table 21 - Variation of produot r a t i o s with pressure for 3-methyl-pentane T - 500°C kP/P; - 0.20 pressure CH4 CgH4 CgHg CgHg trans-C4H8 CgHg mm - •• , • _ _ _ _ _ CgHg CgHg trans-C4H 8 c i s - C 4 H 8 ois-C^Hg 2-me-butene 41 - 1.05 1.37 1.30 0.75 53 1.75 0.94 -72 1.53 0.68 - -81 1.73 0.62 -91 1.65 0.63 1.20 1.48 1.24 0.84 100 1.77 0.69 1.04 1.08 1.04 0.86 110 1.84 0.69 1.04 1.08 1.04 0.75 97 fable 22 - Variat i o n of product r a t i o s with pressure for 3-methyl-pentane T - 490°C kp/p. = o.20 pressure mm CH 4 "^ 2^ 6 C2H4 °2H6 °3H6 C3H6 trans-C 4Hg °3H6 trans-C 4Hg c i s - 0 4 H 8 c i s - C 4 H 8 2-me-butene-l 33 1.04? mm mm • mm 40 - - 1.08 1.35 1.26 0.94 52 •-• - 1.08 1.20 1.10 0.84 70 1.50 0.50 1.00 1.28 1.28 0.65 80 1.49 0.67 1.00 1.22 1.28 0.64t 90 1.47 0.85 1.07 1.64 1.52 0.76 100 - 0.85 - - - -116 1.55 0.81 1.16 1.33 1.15 0.78 The concentration of methane and ethane appear to be equally af-fected by temperature and pressure. The values of r a t i o s CgHg/ cis-C 4Hg , CgHg/trans-C4Hg and trans-C 4Hg/cis-C 4Hg are approx-imately oonstant although a good deal of scattering e x i s t s , which can be attributed to the fact that complete separation of the gas chromatographic peaks for c i s - and trans-C 4Hg was not achieved. G- 4) 2,2-dimethyl-butane (neohexane) The variations of product r a t i o s with pressure for neohexane are given i n Tables 23 and 24. 98 Table 23 - Variation of product r a t i o s with pressure for neohexane T - 500°C kp/Pi- 0.20 pressure CH4 CH 4 C2H4 i-C 4Hg H 2 mm °2H6 C2 H4 °2H6 2-me-butene-2 CH 4 60 2.70 1.88 1.45 2.50 0.71 99 2.00 1.60 1.25 2.50 1.00 116 2.14 1.50 1.42 3.20 ? 0.83 140 2.85 1.67 1.10 2.50 0.82 180 - 0.94 1.25 2.60 0.68 Table 24 - Va r i a t i o n of product r a t i o s with pressure for neohexane T - 520 °C kp/P.= 0.20 pressure CH4 CH 4 C 2H 4 i-C 4H 8 f - L mm u 2 % °2H4 °2H6 2-me-butene-2 CH4 61 3.80 1.83 2.10 2.10 0.48 84 3.60 2.10 1.70 2.40 0.52 100 2.96 1.50 1.95 2.60 0.60 120 2.98 1.82 1.64 2.20 0.53 150 2.90 - - 2.30 0.56 170 2.30 1.76 1.30 2.60 0.55 G 5) 2,3-dimethyl-butane (diisopropyl) In the pyrolysis of dii s o p r o p y l , ethane appears i n negli g i b l e quantities so the p a r t i c i p a t i o n of the CgHs* r a d i c a l i n propagat-ion can be excluded. Propane appears however i n s i g n i f i c a n t quantities which suggests the occurrence of chain propagation by G 3H 7. r a d i c a l . The v a r i a t i o n of product r a t i o s for diisopropyl i s given i n Table 25. 99 Table 25 - Variation of product r a t i o s with pressure for diisopropyl T = 520°C - 0.20 pressure mm C 3 H 5 CgHg C3 H6 H 2 CgHg i - C ^ s 2-me-butene-2 "fiT 52 16.7 8.35 0.81 0.54 60 15.0 6.75 0.64 0.42 80 7.5 8.10 0.65 0.51 103 6.4 - 0.79 0.46 T = 510°C AP/Pi - 0.20 60 7.15 12.7 ? 0.67 0.53 104 5.65 6.8 0.72 0.42 120 4.90 9.8 0.87 0.58 156 5.15 8.9 0.93 0.53 H ) Effect of packing on product d i s t r i b u t i o n The effect of packing of the reaction vessel on the d i s t r i -bution of products was investigated for n-hexane, 2-methyl-pentane and diisopropyl. The double furnace was used i n t h i s study with an S/V r a t i o of 11 i n the packed vessel. H l ) n-hexane The r e s u l t s for n-hexane are given i n Table 26. From Table 26 i t i s apparent that the r a t i o s CH4/C2H6 and C2H4/C2Hg are s i g n i f i c a n t l y affected by packing, being lower for reactions i n the packed vessel. H 2) 2-methyl-pentane The r e s u l t s for 2-methyl-pentane are given i n Table 27. Table 26 - Variation of product r a t i o s with pressure for n-hexane i n packed and unpacked vessels T _ 520°C Ap/^i - 0.20 pressure mm CH4 CH4 C 2H 4 C 2H 6 + C 2H 4 |. C 3H 6 D2H4 ^ 2 % G 3 H 6 j I - G 4 H 8 unp. pack. unp. pack. unp. pack. I unp. pack. | unp. pack. 40 50 60 70 80 90 100 2.20 2.00 1.75 1.40 1.37 1.40 1.40 1.60 1.50 1.30 1.27 0.91 1.00 0.62 0.65 0.70 0.66 0.58 0.61 0.76 0.60 0.60 0.70 0.61 0.48 0.72 3.50 3.20 2.50 2.02 2.36 2.30 1.85 2.80 2.70 1.90 2.10 2.00 1.56 1.62 1.50 1.85 1. 80 1.50 1.56 1.79 -1.45 1.40 1.76 1. 85 1.50 1.52 -1.75 1.62 1.76 1. 74 1.76 1.76 1.85 1. 78 Table 27 - Variation of product ra t i o s with pressure for 2-methyl-pentane i n packed and unpacked vessels pressure mm CH4  C2 H6 T = 520°C C£H4 °2H4 °2H6 kB/T. = 0.20 1 C 2H 4 f C 2H 6 uniT. pack, j unp. pack. -unp. pack, unp. pack. °3H6 °3H6 ^3^8 unp. pack. C3H6 i-C 4H 8 unp. pack. 50 0.86 0.74 1.50 1.58 0.60 0.51 0.68 0.71 9.1 8.6 1.25 1. 28 60 0.85 0.64 1.48 1.49 0.58 0.50 0.88 0.80 8.5 - 1. 28 -80 0.77 0.70 1.55 1.53 0.50 0.46 0.80 0.77 8.6 6.1 1.40 1. 26 90 0.72 0.67 1.45 1.38 0.50 0.44 0.65 0.68 - - 5.8 — 1. 20 100 0.73 0.59 1.43 1.55 0.51 0.37 0.69 - 5.5 mm 1.33 1. 10 120 0.85 0.60 1.50 1.35 0.56 0.50 0.95 0.98 .5.0 4.4 1.20 1. 22 101 The r a t i o s CH^/CgHg and C2H4/C2H6 exhibit a sli g h t dependence on packing. The r a t i o C 3H 6/C 3H g varies s l i g h t l y with packing. H g) 2,8-dimethyl-hutane (diisopropyl) The results for diisopropyl are given i n Table 28. Table 28 - Variation of product r a t i o with pressure for diisopropyl i n packed and unpacked vessels T - 520°C ^ S / S t - 0.20 pressure O^/CgHg C 3H 6/i-C 4H 8 H 2 / G H 4 unp. pack. unp. pack. unp. pack. 50 0.47 0.48 CO 60 5.4 4.9 9.6 9.4 0.48 0.50 70 5.8 5.2 9.2 9.0 0.48 0.50 80 5.4 4.9 8.2 9.1 0.48 0.46 90 4.0 2.7 - - 0.48 0.47 100 5.0 4.7 9.2 9.0 0.46 0.45 120 4.4 4.1 8.6 8.0 — The r a t i o H2/CH4 appears more or less independent of both pressure and packing. The value o f CgHg/CgHs depends only s l i g h t l y on packing and the value of the r a t i o C 3Hg/i-C 4H 8 i s unaffected by a change i n the S/V r a t i o . 102 2. Inhibited Pyrolyses In the present work the following investigations were done of the i n h i b i t e d decomposition of the isomeric hexanes. i ) The effect of n i t r i c oxide pressure on reaction rate at various hydrocarbon pressures and on the product d i s t r i b u t i o n at oonstant extent of reaction. i i ) The ef f e c t of v a r i a t i o n of surface-to-tfolume r a t i o on the rates of the i n h i b i t e d pyrolyses, on the amounts of NO required to a t t a i n the l i m i t i n g rate, and on the product d i s t r i b u t i o n . i i i ) The effect of v a r i a t i o n i n extent of the reaction on the product d i s t r i b u t i o n <£6v constant amount of NO i n the f u l l y i n h i b i t e d decomposition. iv) The comparative adsorption of NO and the isomeric hexanes on quartz wool. v) The effect of hydrocarbon structure on the rates of the i n h i -b i t e d reactions. v i ) The extent of consumption of NO i n the i n h i b i t e d pyrolyses. A ) The effect of n i t r i c oxide on reaction rates A 1) n-hexane The effect of NO on rates was investigated at d i f f e r e n t pressures of hydrocarbon to determine whether the l i m i t i n g pressure of NO i s related to the hydrocarbon pressure. In F i g . 40 the i n i t i a l rate i s plotted against n i t r i c oxide pressure. The l i m i t i n g rate i s attained with from 12 to 14 mm of NO at a l l three hydro-carbon pressures. It appears that the l i m i t i n g pressure of NO does not depend on the p a r t i a l pressure of the hydrocarbon, 103 otherwise the pressure of NO required for f u l l i n h i b i t i o n should have been leastaat the lowest pressure of the hydrocarbon. To determine whether or not the l i m i t i n g pressure of NO i s a function of the surface-to-volume r a t i o for the vessel, the l a t t e r was packed, so that the S$T r a t i o was increased from 1 for the unpacked vessel to values of 5 and 10 respectively. Again three d i f f e r e n t pressures of hydrocarbon were used. A common l i m i t i n g value of NO pressure was again obtained at about 16mm for S/V - 5 (Fig. 41) and about 18mm for S/V - 10 (Fig. 42). In Fig. 43 the rates are plotted against n i t r i c oxide pressure for the same n-hexane pressure at 520°C for d i f f e r e n t S/V r a t i o s . Although a l l the l i m i t i n g rates are very low, a small difference i n the l i m i t i n g n i t r i c oxide pressure can be detected. The pres-sure of n i t r i c oxide required to cause maximum i n h i b i t i o n appears to increase s l i g h t l y with increasing S/V r a t i o . A 2) 2-methyl-pentane In the pyrolysis of 2-methyl-pentane the same investigation was carried out as was done for n-hexane. The curves showing rate plotted against NO pressure are simi l a r to those for n-hexane but the reduction i n rate i s less steep. The l i m i t i n g NO pressure appears again to be independent of hydrocarbon pressure (Fig. 44) occurring at a pressure of 12 to 14 mm i n each case. Increasing the S/V r a t i o from 1 to 5 appears to increase the l i m i t i n g NO pressure to 18,mm (Fig. 45) for both i n i t i a l hydro-carbon pressures. In Fig. 46 the rate i s plotted against NO pressure for different S/V r a t i o s at the same pressure and 1.6 Q i 104 1.4 1.2 1.0 C.8 0.6 0.4 O1^0 ffifi 0 75 mm O 50 mm 8 - 520 C 12 16 20 K© pressure mm 24 P i s . 40: Dependence of i n i t i a l rate on K0 pressure i n the unpacked 105 T I T 0 j / U l i I • H 106 2.2 2.0 1.8 1«6i 1.4 1 . 8 loO 0.8 0 . 6 0.4 0.2 0 0N O 100 nun 0 80 mm 0 50 mm S/V - 10 oni" 1 T - 520°S 12 16 20 24 KO pressure mm 42j Dependence of I n i t i a l rata on NO preeaur© i n the vessel for n-Jsesan© packed 4 k s/v _ 1 1 0 S/7 - 5 err" 1 O s/v r 10 crr- 1 P i - 100 rrrr T - 520°C 16 40 20 34 28 ? 2 36 KO p r e s s u r e rnr F i g . 43: Dependence o f i n i t i a l r a t e on KO p r e s s u r e a t v a r i o u s s/V r a t i o s f o r n - ^ e x a n e 44 3.2 V 2.8 0 ICO rrrr 0 7 5 n:n"! 0 50 air ^ ^ ^ ^ ^ ^ T - 520°G o CD _L 12 32 7.6 4.0 16 20 24 28 KO p r e s s u r e trrr F i g . 44: Dependence o f i n i t i a l r a t e on NO p r e s s u r e i n t h e u t p a e k e d ' v e s s e l f o r 2 - r e t h y l -p e n t e n e 3 . 2 h p e n t s n e i i i temperature. Again there i s an i n d i c a t i o n that the l i m i t i n g NO pressure increases with increasing S/V r a t i o although the effect i s small. A 3) 3-methyl-pentane The v a r i a t i o n of the rate with NO pressure becomes less and less marked as the isomer becomes more and more branched. The l i m i t i n g NO pressure for three d i f f e r e n t hydrocarbon pressures i s again between 12 to 14 mm (Fig. 47). The effect of surface-to-volume r a t i o appears to be the same as for the previous isomers. That i s an increase of S/V from 1 to 5 causes a s h i f t i n the l i m i t i n g NO pressure to approximately 16 mm which i s again inde-pendent of the p a r t i a l pressure of hydrocarbon (Fig. 48). For an S/V r a t i o of 10 the l i m i t i n g NO pressure s h i f t s to about 19mm for both pressures of hydrocarbon, (Fig. 49). In F i g . 50, the rates are plotted against NO pressure for various S/V r a t i o s at the same hydrocarbon pressure at 520°C. In a l l oases, the higher the S/V r a t i o the lower the rate at a given hydrocarbon pressure which i s i n agreement with the effect of S/Y r a t i o on the rates of the uninhibited reactions. In the absence of NO the rates are almost the same at 100 mm pressure of hydrocarbon for a l l three S/V r a t i o s . At t h i s pressure of hydro-carbon, packing has no s i g n i f i c a n t effect on the uninhibited reac-ti o n . A 4) 2,2-dimethyl-butane The rate of pyrolysis of neohexane, a highly branched isomeric hexane, i s less affected by NO than are the rates of the previous 3.8 fl fci — O — 100 rm 0—0 80 nan 0, 50 rr.m S/V = 1 cm" 1 T - 5200C _L AT ± 0 4 b 12 16 20 24" 28 p r e s s u r e trm F i g . 47: Dependence o f i n i t i a l r a t e on NO p r e s s u r e i n t h e u n p a c k e d v e s s e l f o r 3 - n e t h y l -penter.e 26 40 S/V r 5 err - 1 NO p r e s s u r e r m F i g . 4 8 : Dependence o f i n i t i a l r a t e on KO p r e s s u r e i n t h e p a c k e d v e s s e l f o r 3 - r r e t h y l - p e n t a n e 114 NO pfesaur© mm F i g , 49; Xtepondene© of i n i t i a l rat© en ®Q prescma?© i n th© packed vessel f@3? 3-mQthy3»°p@ntanQ 3. 2 2.8 2.4 T - 5 2 0 U C P i - 100 mm S/V = 1 cm" 1 S/V-5 c r j - 1 JL 0 8 16 32 40 44 20 24 28 KO p r e s s u r e mrr> P i g . 50: Dependence o f i n i t i a l r ^ t e on KO p r e s s u r e a t v a r i o u s S/V r a t i o s f o r 3 - r e t h y l - p e n t a n e 116 i s o m e r s . A s i m i l a r e f f e c t h a d been o b s e r v e d i n the h i g h l y b r a n c h e d h y d r o c a r b o n n e o p e n t a n e ( 9 2 ) . The s h i f t o f l i m i t i n g NO p r e s s u r e s w i t h i n c r e a s i n g S/v" r a t i o f o r n e o p e n t a n e and i s o p e n t a n e have a l s o b e e n o b s e r v e d ( 5 0 ) . The l i m i t i n g NO p r e s s u r e i n neohexane was f o u n d t o be the same f o r f o u r d i f f e r e n t p r e s s u r e s o f t h e h y d r o c a r b o n , about 12 t o 13 mm ( F i g . 5 1 ) . The e f f e c t o f s u r f a o e -t o - v o l u m e r a t i o i a a n a l o g o u s t o t h a t o b s e r v e d i n a l l p r e v i o u s c a s e s ( F i g ' s 52 and 5 3 ) . Common l i m i t i n g NO p r e s s u r e s o f a p p r o x -i m a t e l y 17 and 19 mm were o b s e r v e d i n the p y r o l y s i s o f neohexane f o r S/V* r a t i o s o f 5 and 10 r e s p e c t i v e l y . I n F i g . 5 4 , r a t d s a r e p l o t t e d a g a i n s t NO p r e s s u r e a t d i f f e r e n t S/V r a t i o s f o r t h e p y r o l y s i s o f 100 mm o f neohexane a t 520°C. A g a i n t h e r e a r e i n d i c a t i o n s t h a t t h e l i m i t i n g NO p r e s s u r e i n c r e a s e s w i t h i n c r e a s i n g S/T r a t i o . A 5) 2 , 3 - d i m e t h y l - b u t a n e ( d i i s o p r o p y l ) D i i s o p r o p y l a p p e a r s t o be more i n h i b i t e d b y NO t h a n i s n e o -h e x a n e , b u t t h e r e d u c t i o n o f the r a t e w i t h NO p r e s s u r e i s more g r a d u a l t h a n f o r t h e l e s s b r a n c h e d i s o m e r s . To i n v e s t i g a t e t h e dependence o f t h e l i m i t i n g NO p r e s s u r e on t h e p a r t i a l p r e s s u r e o f the h y d r o c a r b o n , f o u r d i f f e r e n t p r e s s u r e s were u s e d . A l l o f them d i s p l a y the same l i m i t i n g NO p r e s s u r e o f a b o u t 12 mm ( F i g . 5 5 ) . I n c r e a s i n g the S/V r a t i o t o v a l u e s o f 5 and 10 o a u s e s a s h i f t o f t h e l i m i t i n g NO p r e s s u r e t o a b o u t 17 mm ( F i g . 56) and 20 mm ( F i g . 57) r e s p e c t i v e l y . The l i m i t i n g NO p r e s s u r e s i n e a c h c a s e a p p e a r s t o be i n d e p e n d e n t o f t h e p a r t i a l p r e s s u r e o f h y d r o -c a r b o n . I n F i g . 5 8 , the r a t e s a r e p l o t t e d a g a i n s t NO p r e s s u r e a t O 120 mm Q 100 mm Q 75 mm 0 50 mm T S/V = 520°C - 1 c i r - 1 J-L. 28 ZZ 36 F i g . 51: Dependence neohexane 20 24 KO pressure mm of I n i t i a l r s t e on KO pressure i n the unpacked v e s s e l f o r .8 .4 O 100 mm .O SO pm 5 0 -mm 28 s/V - 5 c i r " 1 T - 520°C 32 3 6 40 16 20 KO pressure mr-? l g . 52: Dependence of i n i t i a l r a t e on KO pressure i n the packed v e s s e l f o r neohexane 119 4.4 4.0 S . 6 3.2 2.8 2.4 2.01 O 100 mm 0 70 mm (2) 50 mm S/V - 10 em T =. 520°C -1 O 0 —o ^<Z>^0___0 LA. -O 0. O-CD— _<2L_ 8 12 16 RO pressure mm 20 24 Fig . 53: Dependence of i n i t i a l rat© on NO preaaure i n the peeked vessel for neohexane. G S/V - 1 cr r - l 0 S/V - 5 cm- 1 O S / V - 10cm" 1 T - 520°C = 100 mm •o-• <2>. •o-- O -0 a 8 12 16 28 32 36 40 20 24 NO P r e s s u r e mm D -^ndenoe of i n i t i a l r a t e on KO p r e s s u r e at v a r i o u s S/V r a t i o s f o r neohexane O 120 mm 0 100 mm © 75 mm 0 50 mm T - 520°C S/V - 1 orr,*1 6.4 C.Ol I I I L—i! ! I 1 S ! S U 0 4 8 12 16 20 24 28 32 36 40 44 MO pressure mm Fig. 56: Dependence of i n i t i a l rate on NO pressure in the packed vessel for diisopropyl 103 4,4 4.0 3.6 to O 100 mm 0 75 mm 0 50 mm T - 830°0 S/V - 10 cm"1 8 34 13 U 20 NO preaaura mm Fig. 57: Depandence of i n i t i a l rate on NO prtsanrQ ln the packed vessel for diisopropyl 134 4.4L Q . 4 L ~ l 1 I i 1 i 1 • ' i 4 8 12 16 20 24 NO pressure rogj Fig. 58: Dependtnoo of i n i t i a l rate on NO pressure at varioua § A r a t i o s for diioopropyl 1E5 different S/V r a t i o s for a hydrocarbon pressure of 100 mm at 520°C. Increasing of the S/V r a t i o causes an increase i n the l i m i t i n g NO pressure as was observed for a l l other isomers. A 6) The effect of structure on rates The structure of the isomer has a marked effect on the rates of the i n h i b i t e d pyroSbyses. This effeot of structure i s demon-strated i n Fig. 59 i n which i n i t i a l rates are plotted against NO pressure for 100 mm of the hydrocarbon at 5S0°C. The l i m i t i n g NO pressure i s approximately the same for each isomer, but pro-nounced differences exist i n the shapes of the rate ourvesq. Another difference among the isomers i s observed i f the percentage reduction i n rate i n the f u l l y i n h i b i t e d pyrolysis i s calculated for each hydrocarbon. The percentages can be given i n terms of the r a t i o Ri(uninhib.) - R i ( l i m i t i n g ) - A A ~ Ri (uninhibited) x 1 0 0 The r e s u l t s for the various isomers are given i n Table 29. The percentage reduction i n rate at a l l S/V r a t i o s decreases as we go from n-hexane to diisopro p y l . I t should be also pointed out that i n neohexane and diisopropyl the percentage reduction i n -creases with higher S/V" r a t i o . This v a r i a t i o n i s not observed for n-hexane or 2-methyl-pentane and 3-methyl-pentane because for these isomers the l i m i t i n g rate i s very small, and a l l l i m i t i n g rates are approximately of the same magnitude. The e f f e c t , i f any, of increasing S/V r a t i o on reduction i n rate cannot be ...observed for these two isomers. fcs « l 1 O" — 0 — O -P j -Z 100mm T = 520°C S/V - 1 o i r - 1 -O— di i sopropyl •O neohexane 2 - m e t h y l - p e n t e n e ^ -0 © 0 — Q 2 - m e t h y l - p e n t a n e 12 16 20 24 2 8 I- •> o-0 32 36  NO p r e s s u r e mm F i g . 59: Dependence o f i n i t i a l r a t e on NO p r e s s u r e f o r v a r i o u s i s o m e r i c h e x a n e 9 40 44 187 Table 89 - Variation of percentage reduction i n rate with i n i t i a l hydrocarbon pressure and S/V r a t i o s for various isomeric hexanes T - 580°C isomeric hexane pressure mm S/V^-lcnr 1 S/V = 50 m"1 S/V; lOonf 1 50 98$ 98$ T^ ;v 97$ n-hexane 80 96 97 97 100 97 95 94 3-methyl-pentane 50 97 94 75 95 - -100 94 94 mm 50 93 83 3 - me t hy 1- p ent ane 75 - 83 -80 91 mm 98 100 87 81.5 95 50 67 75 87 75 64 - 86 8,8-dimethyl- 80 mm 70 -butane 100 54 68 78 130 54 — — 50 58 58 79 8,3-dimethyl- 75 65 74 76 butane 100 71 73 78 180 68 — — B) A n a l y t i c a l r e s u l t s The mass-balances i n the analysis of reaction mextures were almost complete i n a l l cases where small amounts of NO were used (Tables 30 to 35). As the amount of NO used increased the mass-balances became increasingly incomplete. This observation can be attributed to the fact that at high NO pressure the n i t r i c oxide 128 may disappear by conversion to nitrogen-containing products i n s i g n i f i c a n t amounts. Notutrogen-containing compounds were detect-ed i n the present study which may be a consequence of the methods of analysis used. Such obvious products.as nitroso-methane and nitroso-ethane are so unstable that they would undergo pyrolysis at temperatures used i n the present work. The s t a b i l i t y of a l k y l -nitroso-compounds follows the sequence (98-100) CHgNO <C2H5N0 <i-propyl-N0 <tert-butyl-NO Reactions of NO with a l k y l r a d i c a l s have been investigated i n the past(70). Thus the p o s s i b i l i t y of formation of some nitroso-compounds does exist and may become more important as one goes from n-hexane to diisopropyl. Pyrolysis of these higher nitroso-compounds would probably occur, however, at the temper-atures used i n the present study. B l ) n-hexane The a n a l y t i c a l mass balance obtained at various NO pressures for n-hexane i s given i n Table 30. B 2) 2-methyl-pentane The a n a l y t i c a l r e s u l t s for the i n h i b i t e d decomposition of 2-methyl-pentane are given i n Table 31. Again the mass balance becomes less complete as the NO pressure increases. 129 Table 30 - A n a l y t i c a l r e s u l t s (mole fo) for various NO i n the decomposition of n-hexane pressures T = 530 °C AP/P. ^  0.30 r 100mm NO pressure (mm) 0 4 • 6 10 18 CH4 10.50 11.00 11.60 11.00 11.40 °2H6 6.00 5.70 5.50 4.60 4.90 °2H4 IE. 00 12.50 13.00 13.50 13.50 C3 H6 6.50 6.80 7.20 6.40 7.25 1-C4H8 3. 50 3.60 3.60 3.90 3.50 H 2 1.30 .1.30 1.80 1.50 1.60 n-hexane. 60.00 58.90 57.00 58.00 57.00 Total 99.8 99.8 99.7 98.9 99.1 Table 31 - A n a l y t i c a l r e s u l t s (mole fo) for various NO pressures in the decomposition of 2-methyl-pentane T - 520°C fcP/PjL - 0.25 P A = 100 mm NO pressure (mm)  0 3 5 10 12 15 CH4 5.50 5.80 6.50 8.70 8.60 8.80 °2H6 6.80 6.70 5.30 4.60 4.50 4.50 °2H4 3.90 4.00 4.70 5.90 5.60 6.80 C3 H8 1.40 1.30 1.20 1.10 1.00 1.00 C3 H6 7.50 7.10 7.50 7.30 7.50 8.00 i-C 4Hg 5.40 5.50 5.30 5.90 6.20 6.40 H 2 4.70 4.70 5.20 4.70 4.70 4.60 2-me'©pentane 64.60 64.50 64.00 . 61.00 61.00 59.00 Total 99.8 99.6 99.7 99.2 99.1 99.1 130 From Table 31 i t can be seen that the amounts of CH 4 and C2H4 increase markedly with increasing .NO pressure, while CgHg and CgHg decrease. Propylene and i -C4Hg display a s l i g h t dependence on NO pressures. B 3) 3-methyl-pentane As i n the case of the uninhibited decomposition 3-methyl-pentane yields a variety of products (Table 32). Methane, CgHg, and CgH^ show dependence on NO pressure. The mass balance becomes more incomplete at higher NO pressures. Table 32 - A n a l y t i c a l r e s u l t s (mole fo) for various NO pressures i n the decomposition of 3-methyl-pentane T = 520°C k P ^ - 0.25 P i = 100 mm NO pressure (mm)  0 3 5 8 10 12 18  13.60 14.00 6.50 7.10 5.20 4.80 3.50 3.30 1.80 1.60 2.60 2.70 2.10 2.50 2.80 2.90 1.80 1.80 59.00 57.00 Total 9978 9974 9878 9974 9973 9871 9777 0H 4 9.80 CS H4 5.60 C2 H6 5.20 °3H6 3.20 l-C 4Hg + i-C4Hg trans-C 4H 8 cis - C 4 H g , 1. 50 3.00 .2J60 2-me-but-ene-1 H 2 2.00 1.90 3-me- 65.00 pentane 10.80 12.10 6.50 5.30 6.20 5.80 3.40 3.20 1.20 1.40 2.90 3.00 2.60 2.80 2.90 2.40 1.90 1.80 61.00 61.00 13.00 13.10 6.00 7.20 5.40 5.60 3.30 3.60 1.30 1.60 2.40 2.90 2.40 2.00 2.90 3.10 1.70 1.70 61.00 57.50 131 B 4) 2,2-dimethyl-"butane (neohexane) The a n a l y t i c a l r e s u l t s for the i n h i b i t e d decomposition of neohexane are given i n Table 33. Table 33 - A n a l y t i c a l r e s u l t s (mole fo) for various NO pressures i n the decomposition of neohexane T - 510°C AP/Pi = 0.25 100 mm NO pressure (mm) 0 3 8 10 12 CH4 10.60 10.60 11.00 11.80 12.50 C2 H6 3.20 3.00 2.70 2.60 2.40 C2 H4 6.50 6.50 7.40 6.80 7.50 C3 H6 0.60 0.60 0.60 0.80 0.80 i" c4 H8 8.00 8.10 8.30 8.70 9.00 2-me- 3.50 3.50 3.50 3.20 3.00 butene-2 H2 4.40 4.40 4.00 4.00 3.90 neohexane 63.20 63.00 62.00 . 60.00 59.00 Total 99.8 99.7 99.5 97.9 98.1 B 5) 2,3-dimethyl-butane ( d i i s opropyl) The a n a l y t i c a l r e s u l t s for diisopropyl are given i n Table 34. C ) Variat i o n of product d i s t r i b u t i o n with time for the f u l l y  i n h i b i t e d decomposition The v a r i a t i o n of product d i s t r i b u t i o n with time was inves-tigated for the f u l l y i n h i b i t e d decomposition. The p a r t i a l pres-sure of NO used was 20mm to ensure that complete i n h i b i t i o n had been attained. The analyses were done at various extents of re-action for AP/P* r a t i o s up to 0.40 for a l l isomeric hexanes. 132 Table 34 - A n a l y t i c a l results (mole fo) for various NO pressures i n the decomposition of diisopropyl T - 520°C AP/^ = 0.25 Pj_ _ 100 mm NO pressure (mm) 0 3 5 10 12 15 CH4 13.00 13.40 14.00 14.00 15.00 15.00 C 3H 8 1.30 1.10 1.20 1.30 1.30 1.10 C gHg 6.60 6.90 7.50 8.20 9.10 7.80 i-C 4H 8 0.30 0.30 0.40 0.80 0/70 . 0.80 2-me- 7.00 butene-2 H 2 4.50 7.20 4.50 6.20 4.30 6.40 4.70 6.70 4.30 7.70 4.60 dii s o p r o p y l 67.00 66.00 66.00 63.50 62.00 61.00 Total 99.7 99.4 99.6 98.8 98.1 98.0 C 1) n-hexane The v a r i a t i o n of product d i s t r i b u t i o n with time Is given i n Pig. 60. It i s important to note that the amounts of a l l products vary l i n e a r l y with time. S e l f - i n h i b i t i o n i s not observed although this phenomenon was very pronounced i n the uninhibited reaction. C 2) 2-methyl-pentane The v a r i a t i o n of product d i s t r i b u t i o n with time i s given i n Fig. 61. The realtionship between product d i s t r i b u t i o n and time i s almost l i n e a r , although some deviations are obvious particu-l a r l y at larger extents of reaction. However, there i s some scatter i n the a n a l y t i c a l r e s u l t s , as , for instance, for CgH4 and CgHg. A d i s t i n c t contrast exists between the i n h i b i t e d and uninhibited reactions (see Fi g . 32.). 32 h t i me m i n F i g . 60: V a r i a t i o n o f t h e p r o d u c t d i s t r i b u t i o n w i t h t i m e f o r t h e f u l l y i n h i b i t e d d e c o m p o s i t i o n o f n-hexane 25 30 t i me min F i g . 6 1 : V a r i a t i o n of the product d i s t r i b u t i o n with time f o r the f u l l y i n h i b i t e d decomposition of 2-methyl-pentane 135 C 3) 3-methyl-pentane The v a r i a t i o n of product-distribution with time displays the same cha r a c t e r i s t i c s as i n n-hexane with the exception of the product 2-methyl-butene-l, the concentration of which goes through a maximum (Fig. 62). It appears that 2-methyl-butene-l undergoes pyrolysis i n the l a t e r stages of the reaction. C 4) 2,2-dimethyl-butane (neohexane) The v a r i a t i o n of product d i s t r i b u t i o n with time i n the f u l l y i n h i b i t e d decomposition of neohexane d i f f e r s somewhat from the variations observed with the less branched isomers (Fig. 63). The v a r i a t i o n of CH4 with time i s linear but i-C^Hg, CgH4 , and Hg do not vary l i n e a r l y with time. The phenomenon of s e l f -i n h i b i t i o n may occur with this system. C 5) 2,3-dimeibhyl-butane (diisopropyl) The results obtained for diisopropyl are s i m i l a r to those for neohexane, but deviations from l i n e a r i t y are more marked for the major decomposition products (Fig. 64 ). D ) Vari a t i o n of product d i s t r i b u t i o n with n i t r i c oxide pressure  and surface-to-volume r a t i o Previous workers have suggested that the product d i s t r i -butions i n the f u l l y i n h i b i t e d and uninhibited decompositions of the paraffins are i d e n t i c a l . A care f u l analysis for a l l products i n the present investigation shows that this i s not the case (Fig. 65, 80, e t c . ) . This observation was made i n the study of the decomposition of isomeric pentanes (52,92). In general, a l l of the products formed i n decompositions of isomeric hexanes can 32 V-tiree ffnn F i g . 62: V a r i a t i o n of the product d i s t r i b u t i o n with t i r e f o r the f u l l y i n h i b i t e d decomposition o f 3-irethyl-pentane time min ? i g . 5?: V a r i a t i o n of the product d i s t r i b u t i o n with tirre f o r the f u l l y i n h i b i t e d decomposition of neohexane 24 L time min F i g . 64: V a r i a t i o n of the product d i s t r i b u t i o n with tirre f o r the f u l l y i n h i b i t e d decomposition of d i i s o p r o p y l 139 be divided into three groups aocording tothe way i n which t h e i r formation i s affected by n i t r i c oxide. i ) Products whose formation i s favoured by NO such as methane and ethylene. i i ) Products whose formation i s reduced by increasing NO pressure such as ethane and propane-;:-; . i i i ) Products whose production i s independent of NO pressure. D 1) n-hexane The v a r i a t i o n of product d i s t r i b u t i o n with. NO pressure i s given i n Fig. 65 for a fixed extent of decomposition corresponding to a AP/Pi r a t i o ' o f 0.25. Ethylene shows marked increase with NO pressure while CgHg and l-C^Hg decrease i n i t i a l l y and then l e v e l o f f . Methane and CgHg are more or less independent of NO pressure. Hydrogen appears to be independent of NO pressure but shows strong dependence upon temperature (Fig. 66). The v a r i a t i o n of SfV r a t i o affects the product d i s t r i b u t i o n i n d i f f e r e n t ways. No s i g n i f i c a n t effect i s observed on the amounts of CgHg and I - C 4 H Q at differ e n t NO pressures and S/T r a t i o s of 1, 5, 10, (Fig. 67 and 68). Methane shows a s l i g h t increase with NO pressure and i s favoured by the increased surface (Fig. 69). D 2) 2-methyl-pentane In the i n h i b i t e d pyrolysis of 2-methyl-pentane some products appear to be markedly affected by the NO pressure while others are independent of i t (Fig. 70). Methane and C ?H 4 increase 8 1 0 12 14 16 18 20 22 KO p r e s s u r e rrrr, F i g . 66: V a r i a t i o n o f h y d r o g e n w i t h KO p r e s s u r e f o r n-hexane r>0 i, e i o o i J . M . _1_ rH rH r H 1 1 1 fcj £ o o c. o r-H IO rH /I II -c; > CO CO CO o o e be t: o fc m o 0.! o O • CVJ O O m rH 1' rr »r P-il •* PH to to 0.1 03 C 0 0 CD —( CO X <D x: i fl o tD O r-H -r-i 4^ CO tV , > CO E b fl u P 0 5 co cn m ro >H r H UJ O CO OJ T 3 r H ff! * >» CD O I r H « M o fl o OB •rH cr. > CO til, _ l _ OJ 146 markedly with NO while the formation of CgHg and CgHg i s reduced. Propylene and i-C 4Hg are independent of NO pressure. The amount of hydrogen i s reduced slowly with NO pressure (Fig. 71). The effect of varying S/V r a t i o on the products i s shown i n Fig. 7£. The y i e l d of methane i s s l i g h t l y reduced at the higher S/V r a t i o while CgHg and i-C 4Hg are almost independent of NO pressure and are affected only s l i g h t l y by the increase i n S/7 r a t i o (Fig. 73 and 74). The higher value of s/V r a t i o favors the production of CgHg but reduces the y i e l d of i-C4.Hg. D 3) 3-methyl-pentane Methane and CgH4 are favored by increased NO pressure while CgHg i s reduced as was observed for the previous isomers. S-methyl-butene-1 goes through a maximum (Fig. 75). Hydrogen displays a sli g h t reduction i n y i e l d with increasing NO pressure (Fig. 76) and i t i s again mora affected by the temperature than by NO pressure. The effects of surface-to-volume r a t i o were investigated for the three diff e r e n t values 1, 5, and 10. Propylene and the com-bined l-C^Hg and i-C^Hg are independent of NO pressure and S/lf r a t i o (Fig. 77 and 78). The y i e l d of methane increases on changing the S/V r a t i o for 1 to 5. A further increase has no effect (Fig. 79). Trans- and c i s - butene, although affected s l i g h t -l y by NO pressure, appear to be unaffected by an increase i n s/V r a t i o (Fig. 80 and 81). V -Pj - 100 rir. - 0.25 O 520°U 0 16 2 4 6 8 10 12 14 NO p r e s s u r e nrr. 1: V a r i a t i o n o f h y S r o p e n y i e l J w i t h KO p r e s s u r e f o r 2 - i r e t h y l - p e n t a n e 18 20 0 O s/v 1 O S / V - 5 G I T . " 1 - 520°C =. 100 rrr: 8 10 16 18 • 12 14 KO pressure nrn v i g . 72: V a r i a t i o n of CH 4 y i e l d with KO pressure and s/V r a t i o f o r 2-re thy 3 - pentane 10 12 1.0 p r e s s u r e rrn; F i g . 73: V a r i a t i o n o f C ^ g y i e l d w i t h !".0 p r e s s u r e and S/V r a t i o f o r 2 - n e t h y l - p e n t a n 16 n 1 1 1 ' ' • 1 1 » i ,_ L 2 4 6 ? IC 12 14 16 18 20 22 KO p r e s s u r e nrr, F i g . 7 5 : V a r i a t i o n o f the p r o d u c t d i s t r i b u t i o n w i t h KC p r e s s u r e f o r 2-trethyl-pentan° a. o o o o o o c o o CM rH Si Si o o e J J CV) b Si O 0.7 a • 1—1 < H II P* PT <J H oo if O l •o a: rr O TT O l ° II 8 1 0 ^ 07 r H r-i r-l 1 1 1 c j b fc: o o o o i — i LO r-t II II ll i > co cn CO eso o © e ecfo t i r- O • O O 03 O l o • >Q r-H C II I ' II P H I —' a, 6E> o 6 © 6 O] O O l fl i x: b l CQ r« o + 3 55 5H m co 0 1 C / J !4 i — I a: C O o. a> u UI ll) a< f-. p . o o n < e x o j o x: 4 J ai •rH r>3 r ^ CQ o O C D CO • r H 4.0 i.O 00 2. 5 o i r-H vf O I 0-'' o T - 520°C i>. -l - 100 ir. 0. 25 * i O S/V = 1 err." 1 CD S/V r 10 cm' 1 10 12 14 NO p r e s s u r e err ? i g . 78: V a r i a t i o n i n y i e l d o f i - C 4 ? T g p l u s l-Ca/Tgwith K 0 p r e s s u r e and S/V r a t i o i n 3 - r e t h y l -p e n t a n e 1 vJvJ 6 r CO %< 5 o i OJ fl w U A v> 4 o s/v 0 s/v 0 s/v i CMl" 1 5 e i r " 1 = 10 orr -1 T -p i -- 520^0 100 nr. A P xi _ - C.25 i 10 12 14 I\0 p r e s s u r e t rr. 16 13 F i g . 80: V a r i a t i o n o f t r a n s - C 4 H g y i e l d w i t h K 0 p r e s s u r e and S/'i r a t i o i n H - r e t h y l - p e n t a n e rH r-l rH 1 1 1 t_ E E o o o c» rH lO r-H II II II ^ > > \ C0~ cn cn o o e C 3 o E o LO o C: C ( Ol o • ' O rH o l l II II P H I cn CD fl co fl I r H >> XI 4-5 ai fc. i o 4J CO r H CO fl CO CO p W 0 7 G • r H CO • r H >.> 00 tr rr o i w • r H O <;_. o rt o •,-H co 158 D 4) S,2-dimethyl-butane (neohexane) The v a r i a t i o n of product d i s t r i b u t i o n with NO pressure i s simil a r to that observed for the other isomers although less marked. Most of the products appear to be either independent of NO pressure or s l i g h t l y dependent on i t (Fig. 82). Hydrogen i s reduced a l i t t l e by NO pressure and depends on the temperature (Fig. 83). The effect of packing has been investigated for three d i f -ferent values of S/v" r a t i o . Propylene appears unaffected by the surface (Fig. 84) while i-C 4Hg seems to be a l i t t l e reduced by increasing the packing, although there i s enough scatter to cast doubt i n the v a l i d i t y of th i s conclusion (Fig. 85). Methane seems again to be favored a l i t t l e at higher surface area (Fig.86). D 5) 2,3-dimethyl-butane (diisopropyl) A picture si m i l a r to that obtained for neohexane i s obtained for diisopropyl so far as the v a r i a t i o n of product d i s t r i b u t i o n with NO pressure i s concerned. Methane, CgHg and i-C 4Hg appear to be favored a l i t t l e by increased NO pressure while CgHg may decrease very s l i g h t l y . 2-methyl-butene-2 i s more or less inde-pendent of NO pressure but there i s much scattering i n the analy-t i c a l r e s u l t s (Fig. 87). Hydrogen decreases very s l i g h t l y with NO pressure (Fig. 88). The effect of packing on the product d i s t r i b u t i o n has been investigated using the three diff e r e n t values of S/V r a t i o . Pro-pane appears to be favoured by higher s/V r a t i o (Fig. 89) while C3Hg, i s affected to a lesser degree by s/v th£n CgHg i s (Fig.90). 161 r-i I ti o t> o o rH •o rH It l< II "—fc > CO cn co o o e o o CJ o 01 CM o rH o 11 U ll l~H 1 -CM 01 o o n 01 ^ I ° J r - t I I E E o o rH lO I I I I CO CO o o e E o E c L O o O C M w c • m <—t o II II II p-tl •* PH 16 14 12 o T -- 520 JC ~ ~ ~ 0.25 cm O s/7 r 1 0 5 / 7 =• 5 Q)S/vr t 10 cm" 1 cm 10 12 14 16 18 pressure rir _i 2C • 9 9 Fir. 86: V a r i a t i o n o f C H 4 y i e l d w i t h I»0 p r e s s u r e and S/V r a t i o f o r n e o h e x a n e -o-? i 100 mm - 0.25 $—e—e o 0 Q 530°C 0 520°C 0 2 4 6 8 , 10 F i p . 88: V a r i a t i o n o f h y d r o g e n y i e l d w i t h KO p r e s s u r e f o r d i i s o p r o p y l 12 14 KO p r e s s u r e mm 16 - O -18 20 16 16 F i g . 82: V a r i a t i o n o f CgHg y i e l d 8 10 12 14 KO p r e a s u r e rm w i t h KO o r e s s u r e and s/V r a t i o f o r d i i s o p r o p y l 0 S/V r 5 c i r " 1 1.4- T r520°C (5 S/V z 10 cm" P i - 100 mm 6 k 4 k P. k > 4 • 1 I l I 1 I 1 L_ L_ 0 3 4 6 8 10 12 14 16 18 20 22 Y.O p r e s s u r e mm Tiff. 90: V a r i a t i o n o f C3JI5 y i e l d w i t h NO p r e s s u r e and s/V r a t i o f o r d i i s o p r o p y l 168 A sim i l a r v a r i a t i o n i s observed i n the formation of CH4 (Fig. 91). Generalizing, one can say that increasing NO pressures increase product y i e l d s i n the following sequence: C 2H 4^CH 4> C 3H 5^ i-C 4H 8-1-C 4H 8> c i s - C 4 H 8 ~ trans-C 4H 8 and reduces the produet y i e l d i n the sequence C2H6> °3H8> H 2 ^ i - C 4 H 8 Increasing the surface-to-volume r a t i o increases the y i e l d of products according to the sequence C 2H 6> CgHg > CH 4>CgH 6~ i - C 4 H 8 - o i s - C 4 H 8 , trans-C 4H 8 The effect on the l a t t e r three products i s n e g l i g i b l e . B ) Var i a t i o n of product r a t i o s with n i t r i c oxide pressure and surfaoe-to-volume r a t i o The v a r i a t i o n of ce r t a i n product r a t i o s with NO pressure and with S/V r a t i o was,investigated to provide additional i n f o r -mation on the decomposition mechanisms. Ratios such as CgH4/CgHg, CgHg/CgH8 and CgHg/C4Hg were plotted against NO pressure at constant extent of reaction to give some indication of the r e l a -tive importance of the competitive reactions by which the products i n the respective r a t i o s are produced. E 1) n-hexane The r a t i o CgHg/l-C 4H 8 appears to be independent of NO pressure (Fig. 92), which indicates no effect of NO on the main modes of decomposition which produce CgHg and 1-C4H8. The r a t i o Cg^/CgHg increases with NO pressure (Fig. 93) due both to an inorease i n CgH^ and a decrease i n CgHg with NO pressure. A s i m i l a r change i s observed i n the v a r i a t i o n of the r a t i o CH4/CgHg with NO pressure 2 0 L 18 16 «—I o E 10 T ~ P i "• 520°C 100 rjin 0.25 Q s / v - l © S / Y _ 5 err cm -1 J L 0 8 16 18 10 12 14 KO pressure rrm Fig. 91: Variation of y i e l d with KO pressure and S/V.ratio for diisopropyl 6 8 10 12 2:0 pressure mm V a r i a t i o n o f C j ^ / l - C ^ p r s t i o with ¥.0 pressure f o r n-hexane 4. 2K 172 (Mg. 94). The va r i a t i o n i n the r a t i o C2H4/C2Hg has been inves-tigated for various s/V r a t i o s . Higher S/V ra t i o s reduce the product r a t i o , suggesting that increased surface area favors s l i g h t l y the formation of C 2H 6 (Fig. 95). A similar result was observed i n isopentane (50). S 2) 2-methyl-pentane In the i n h i b i t e d pyrolysis of 2-methyl-pentane the ra_tio CH 4/C 2H 6 (Fig. 96) as we l l as the r a t i o C 2H 4/C 2H 6 (Fig. 97) display marked increase with NO pressure which can be attributed to the fact that the formation of both CH4 and CgH4 are favored by i n -creased NO pressure while CgHg i s reduced. A similar dependence on NO pressure i s observed i n the va r i a t i o n of the r a t i o CgHg/ CgHg (Fig. 98) due to a decrease of CgHg with NO pressure. The surface area seems to have no effect on the r a t i o C2H4/CgHg for thi s isomer although the results are s u f f i c i e n t l y scattered to be inconclusive (Fig. 99). S 3) 3-methyl-pentane The r a t i o OH /CgHg appears to be dependent upon NO pressures as with previous isomers though i t does not seem to depend upon temperature (Fig. 100). The r a t i o CH4/C2H4 appears to be inde-pendent of both NO pressure and temperature, which can be a t t r i -buted to an equal effect of NO on the production of both products (Fig. 101). The effect of packing on the C2H4/C2Hg r a t i o i s sim-i l a r to that observed for n-hexane. A higher r a t i o favors the formation of C„Hfi (Fig. 102). 1 ° 0.8 0.4 P i - 100 mm. — 5 - = 0.25 8 10 16 12 14 NO p r e s s u r e mm F i g . S4: V a r i a t i o n o f C H 4 / C 2 H 6 r a t i o w i t h NO p r e s s u r e f o r n -hexane 13 20 22 16 18 12 14 KO p r e s s u r e wrr F i g . 9 5: V a r i a t i o n o f C 2 H 4 / c 2 H 6 r a t i o w i t h KO p r e s s u r e and s/V r a t i o f o r n -hexane 20 22 1.6 20 r 10 L P L ~ 100 rrrr 8 - &p -0.25 6 -4 J i • i i 1 1 1 i 1 1 0 2 4 o 8 10 12 14 16 18 20 NO p r e s s u r e rrrr. F i g . 98: Variation of C 2K 6 / c 2H 8 r a t i o with K0 pressure for 2-ir.ethyl-pentane 0 . 2 r | . I I 1 1 1 1 ' ' -0 3 4 6 S 10 12 14 16 18 20 22 IvO pressure irrr F i g . S9: V a r i a t i o n of C 2 H 4 / C 2 H 5 r a t i o with NO pressure an-o" S./V r a t i o f o r 2-methyl-pentane 1.6 1.4 1.2 l . | o 0.8 rp CM 0. 0.4 0.2 0. 0 520°C © ? i - 100 mm -p~ - 0. 25 0 8 10 14 16 18 KO p r e s s u r e mm * i g . 100: V a r i a t i o n o f C 2 V c 2K 6 r 9 t i o * 0 p r e s s u r e f o r 2 - m e t h y l - p e n t a n e 20 22 2. 2 2.0 1.3 1.6 o ^.1.4 tr O O 510°C © 520°C 0 530°C P j - 100 mm A P ' i - 0.25 P<6 8 10 12 14 16 18 20 22 r.O pressure rrir F i g . 101: V a r i a t i o n o f CH4/C2H4 r a t i o w i t h R0 p r e s s u r e f o r 3 - i T . e t h y l - b u t e n e 4.0 3.5 3.0 8.5 T -A p P* 520°C 100 rcrc _ 0.25 O S/V = 1 err" 1 0 S/V - 5 cir- 1 Q S/V - 10 c r " 1 <0 CY) O r}< O l O 2.0 1. 5 t -» 0.51; 0 2 4 6 8 10 12 14 16 18 20 82 K0 pressure vm F i g . 102: V a r i a t i o n of C g ^ / C r / I ^ r a t i o with NO pressure and S/V r a t i o f o r 3-rrethyl-pentane 182 E 4) 2,2-dimethyl-butane (neohexane) The r a t i o CgH4/CgHg ^ a s s a m e dependence upon NO pressure as i n a l l other isomeric hexanes (Fig. 103). The CH4/CgHg r a t i o has a smaller dependence on NO pressure (Fig. 104). The r a t i o s CH 4/C 2H 4 and 0 3H 6/i-C 4H 8 appears to be independent of both NO pressure and temperature (Fig. 105) and (Fig. 106). This suggests that both CH 4 and CgH4 are equally affected by NO while CgHg and i-C 4Hg are formed i n processes whose rates are independent of NO pressure. Packing the vessel has no marked effect on the G2H4/C2H6 r a t i o (Fig. 107). E 5) 2,3-dimethyl-butane (diisopropyl) No CgH4 or CgHg are found i n the pyrolysis of diisopropyl. The dependence of the CgHg/CgHg r a t i o on NO pressure (Fig. 108) i s similar to that observed for 2-methyl-pentane and res u l t s from the decrease i n CgHg. Higher temperatures favor the formation of CgHg. The value of CgHg/2-methyl-butene-2 shows some depen-dence upon NO pressure (Fig. 109) while increased temperature favors the formation of 2-methyl-butene-2. The r a t i o CgHg/i-C4Hg increases•with NO pressure (Fig. 110). In summary the following common features occur i n the i n -h i b i t e d pyrolyses of the isomeric hexanes. i ) The r a t i o s CH4/CgHg, CgH^CgHg and CgHg/CgHg increase with NO pressure i n a l l cases. i i ) The r a t i o CgHg/C4H8 i s either independent of NO pressure or i t i s s l i g h t l y dependent on i t . i i i ) The r a t i o s CgH4/CgHg and C„Hfi/CgHg either decrease with r f o 1. 0.* 0.4h © 520°C O510°C 100 rrm 0. 25 10 •16 12 14 KO p r e s s u r e rojr. F i g . 1 0 ? : V a r i a t i o n o f C g ^ / C p H g r a t i o w i t h K0 p r e s s u r e o f n e o h e r ^ n ? 18 20 7 \-9 O 2.0 ^ J *o:> ^ 1 . 5 r r »--' o 1.2 0.8 C.4 - 100 rrr. A ? - 0.85 O 510°C 0 520°C 0 530°C 10 16 12 14 KO p r e s s u r e rrr! F i g . 105; V a r i a t i o n o f C H 4 / C 2 H 4 r a t i o w i t h K 0 p r e s s u r e f o r n e o h e x e n e 0 510°C 0 520OC 0 bZO°G 10 12 14 KO pressure r r r F i g . ICS: V a r i a t i o n of C ? H 6 / i - C 4 H 8 r a t i o with KO pressure for neohexane 0 2 4 6 8 10 12 14 16 18 20 22 KO p r e s s u r e rrm F i p . 107: V a r i a t i o n o f C ^ / C ^ r a t i o w i t h KO p r e s s u r e and S/V r a t i o f o r n e o h e x a n e 191 increased surface-to-volume r a t i o at fixed NO pressure or are independent of S/V r a t i o . iv) The r a t i o s C H 4 / C 2 H 4 and C 3 H 5 / C 4 H 3 show a variable dependence upon S/V although most commonly they are independent of S/V r a t i o . F ) Comparative adsorption of isomeric hexanes with respect to  n i t r i c oxide on s i l i c a wool In order to obtain additional information about the surface effects i n the decomposition of hexanes, the adsorption of a l l isomeric hexanes on quartz wool was investigated r e l a t i v e to the adsorption of NO. The experiment was very simple i n p r i n c i p l e and was intended to provide a q u a l i t a t i v e picture of the r e l a t i v e extents of adsorption of the isomeric hexanes i n the system under investigation. A glass tube was packed with quartz wool, increasing the S/V r a t i o by a factor of about 60. The tube was heated to temperatures i n the range 300 to 350°C. Higher temper-atures were not used because of the danger of decomposition of the hydrocarbons. The tube was thermally insulated with a one-inch coating of plaster and the temperature was checked with a thermocouple extending to the centre of the adsorption tube. Known binary mixtures of NO and each of the isomeric hexanes were prepared. After complete evacuation of the tube, known pressures of the mixtures were admitted to the adsorption tube. The gaseous mixture was i n contact with the quartz wool for exactly two minutes i n a l l experiments. A sample of the mixture i n equilibrium with the surface was obtained by rapid expansion into an evacuated f l a s k and was analysed by gas chromatography. The expansion was 192 not instantaneous so that some desorption occurred which would contribute to the quantitative u n r e l i a b i l i t y of the r e s u l t s . The sampling technique was constant for a l l mixtures, however. Differences i n the extent of adsorption of the various isomers i n the presence of NO would, i n p r i n c i p l e , cause variations i n the composition of the binary mixtures i n the f i n a l analysis. No special e f f o r t was made to use a f i x e d composition for a l l binary systems, because the experiments were not intended to give a quantitative picture of the r e l a t i v e extents of adsorption of the isomeric hexanes on quartz wool. The result s of these rather primitive adsorption studies are now presented. F l ) n-hexane A mixture of n-hexane and NO was used i n whioh the i n i t i a l n-hexane/NO r a t i o was 2.50. The r e s u l t s of the analysis of samples obtained after passing the mixture through the tube at various pressures are given i n Tables 35 and 36. Table 35 - Variation of n-hexane/NO r a t i o with i n i t i a l pressure of the mixture after adsorption on quartz wool. (n-hexane/NO) i n l t # =2.50 T - 350°C I n i t i a l pressure of the mixture (n-hexane/NO) f i n a 3 _ mm  18 2.85 27 2.66 36 2.70 51 2.70 68 2.70 93 2.50 193 Table 36 - Va r i a t i o n of n-hexane/NO r a t i o with i n i t i a l pressure of the mixture after adsorption on quartz wool (n-hexane/NG) i n i t l a l = 3.00 T = 310°C I n i t i a l pressure of the mixture (n-hexane/NO) f j _ n a i mm 11 20 30 41 57 75 104 4.0 4.3 3.9 3.4 3.2 3.7 3.6 The r e s u l t s show that the r a t i o of n-hexane/NO after passing through the adsorption tube i s higher than the i n i t i a l value. The results indicate that r e l a t i v e l y more NO i s adsorbed than n-hexane, since i t s p a r t i a l pressure i s lower i n the gas phase. The en-hanced adsorption of NO was more marked at the lower pressures. At higher pressures the composition of the equilibrium mixture approaches the i n i t i a l pre-adsorption value. F 2) 2-methyl-pentane The r e s u l t s of the adsorption of a mixture of 2-methyl-pentane and NO on quartz wool are given i n Tables 37 and 38. Table 37 - Var i a t i o n of 2-methyl-pentane/NO r a t i o with i n i t i a l pressure of the mixture after adsorption on quartz wool (2-methyl-pentane/N0) i n l t =1.50 T = 350°C I n i t i a l pressure of the mixture (2-methyl-pentane/ilO)fi n ai mm . • 20 1.75 30 1.70 54 1.29 ? 79 1.62 109 1.52 194 Table 38 - Var i a t i o n of 2-methy1-pentane/NO r a t i o with i n i t i a l pressure of the mixture after adsorption on quartz wool (2-me thyl-pent ane /SO) i n l t '•= 1.82 T - 310°C I n i t i a l pressure of the mixture (2-methyl-pentane/N0) f i n a l mm  11 2.00 31 1.94 44 1.88 61 1.86 66 1.86 96 1.80 It i s obvious that the f i n a l r a t i o i s higher at a l l pressures than the i n i t i a l value, but the change i n r a t i o caused by adsorp-t i o n i s less marked than i n n-hexane. F 3) 3-methyl-pentane The result s of the adsorption of a mixture of 3-methyl-pen-tane and NO are given i n Table 39. Table 39 - Var i a t i o n of 3-methyl-pentane/NO r a t i o with i n i t i a l pressure of the mixture after adsorption on quartz wool (3-methyl-pentane/N0) l n i t - 2.12 T r; 310°C I n i t i a l pressure of the mixture (3-methyl-pentane/WO)fi n ai • mm ; '  22 2.10 30 2.14 47 2.22 64 2.04 97 2.16 After the series was completed the composition of the i n i t i a l mixture was checked again. It was found to be 3-methyl-pentane/NO ~ 2.14. From the above table one can see that adsorption has no 195 effect on the composition of the mixture. F 4) 2,2-dimethyl-hutane (neohexane) The results of the adsorption of a mixture of 3-methyl-pentane and NO are given i n Tables 40 and 41. Table 40 - V a r i a t i o n of neohexane/NO r a t i o with i n i t i a l pressure of the mixture after adsorption on quartz wool (neohexane/NO) i nit - 3 » 2 4 T = 310°C I n i t i a l pressure of the mixture (neohexane/NO)final mm :  17 2.77 39 3.15 55 3.00 70 3.00 100 3.30  Table 41 - V a r i a t i o n of neohexane/NO r a t i o with I n i t i a l pressure of the mixture after adsorption on quartz wool. (neohexane/NO) i n i t - 4.66 T - 350°C I n i t i a l pressure ;he mm of t mixture (neohexane/NO) f i n a l 38 3.70 56 3.90 79 4.00 99 3.80 From the above tables i t can be seen that the r a t i o (neohexane/ f i n a l i s always less than the i n i t i a l value and t h i s difference i s much more marked at lower pressures. It can be attributed to the p o s s i b i l i t y that neohexane-is more strongly adsorbed than NO and much more strongly adsorbed than the other isomers. 196 F 5) 2,3-dimethyl-butane (diisopropyl) The r e s u l t s of the adsorption of a mixture of diisopropyl and NO are given i n Tables 42 and 43. Table 42 - Variat i o n of diisopropyl/NO r a t i o with i n i t i a l pressure of the mixture after adsorption on quartz wool (diisopropyl/NO),-. + - 6.0 T - 350°C I n i t i a l pressure of the mixture ( d i i s o p r o p y l / N O ) f _ n a l mm 15 5.3 20 4.8 31 4.6 45 5.4 60 4.9 86 5.4 Table 43 - Variat i o n of diisopropyl/NO r a t i o with i n i t i a l pres-sure of the mixture after adsorption on quartz wool (diisopropyl/NO). =6.5 T - 310°C mit I n i t i a l pressure of the mixture (diisopropyl/NO)final mm 14 5.5 26 5.8 54 5.9 77 6.1 The adsorption of t h i s isomer resembles the adsorption of neohexane. Generalizing,the r e l a t i v e extents of adsorption of the isomeric hexanes appear to be given by the sequence; neohexanediisopropyl> 3-methyl-pentaneNO> 2-methyl-pentane > n-hexane 197 In all-opyrolyses many -unsaturated compounds are formed; i t was therefore of interest to measure the adsorption of a t y p i c a l o l e f i n r e l a t i v e to n i t r i c oxide. For t h i s purpose a binary mix-ture of butene-1 and NO was investigated. The r e s u l t s appear i n Table 44. Table 44 - V a r i a t i o n of butene-l/NO r a t i o with i n i t i a l pressure of the mixture after adsorption on quartz wool (butene-l/NO) i n i t 2.80 T - 310°C I n i t i a l pressures of the mixture (butene-l/NO) f _ _ n a _ . mm 10 1.94 20 1.30 26 1.42 33 1.72 38 2.07 43 2.10 53 2.07 It can be seen that the unsaturated hydrocarbon i s adsorbed more than NO by the quartz wool. 0 ) Consumption of n i t r i c oxide during pyrolysis In a l l previous studies NO was reported to be either s l i g h t l y consumed or not consumed at a l l during the pyrolysis of various organic compounds. In the study of the isomeric pentanes done i n t h i s laboratory (52,92) consumption of NO was observed. In the present investigation a very c a r e f u l gas chromatographic e s t i -mation was attempted for a l l pyrolysis using a s i l i c a g e l column. To cheek the v a l i d i t y of NO analysis a binary mixture of NO and neohexane was; used i n a series of c a l i b r a t i n g experiments. The composition of the mixture was 1 to 1. Various i n i t i a l pressures 198 were admitted to the reaction vessel and after a constant time, the mixture was analysed i n the HM3?A column. Although only four di f f e r e n t i n i t i a l pressures were used, i t can he seen (Fig. I l l ) that a l l the amount of NO i n the mixture was recovered i n the analysis. No consumption of NO was found for NO pressures up to 5 mm. For higher NO pressures, consumption of NO was observed which varied with the structure of the hydrocarbon (Fig. 112). The con-sumption of NO i s much more marked for the more branched isomers. It varies from 4 to 5 fo for n-hexane to about 25$ for diisopropyli at a constant NO pressure of say 20 mm. It i s important to mention that under the same conditions the mass-balance was less complete for the highly branched isomers as was shown before. The con-sumption of NO may therefore involve the formation of some stable nitroso-compounds which could not be detected gas chromatograph-i c a l l y . Summary of results The resul t s obtained i n the present study can be summarized as follows: i ) The decompositions of the isomeric hexanes follow f i r s t order mechanisms at-pressures above 100 mm while at lower pressures the order of the reaction i s greater than unity. This change i n the order becomes more marked with the more branched hexanes. i i ) The packing increases the order s l i g h t l y at lower pressures but has no effe c t at higher pressures. i i i ) The packing increases the rates at lower pressures but the reduction diminishes as the pressure increases. The effect of the I s o m e r i c h e x s n s s 201 packing disappears at pressures around 100 mm. The reduction i n rate due to the packing decreases with increased branching i n the structure of the isomers. iv) The energies of aotivation and the frequency factors are pressure dependent tending to constant values at higher pressures. Both k i n e t i c parameters decrease with increased pressure and with increased hranohing of the isomers. v) The present r e s u l t s indicate that the n i t r i c oxide pressure required for maximum i n h i b i t i o n i s independent of the pressure of the hydrocarbon for a l l isomeric hexanes. This pressure of KO Increases s l i g h t l y with increase i n S/? r a t i o i n the reaction vessel. The l i m i t i n g NO pressure i s approximately the same, from 10 to 12 mm,for a l l isomers i n the unpacked vessel. v i ) The effect of NO on the reaction rate decreases from n-hexane to neohexane. v i i ) The produot d i s t r i b u t i o n s are not i d e n t i c a l i n the uninhibited and i n h i b i t e d decompositions but vary with NO pressure. The y i e l d s of c e r t a i n products such as CH4, C2H4 and CgHg are depen-dent on the pressure of NO i n the system, where as the y i e l d s of the higher molecular weight products such as CgHg and C 4 H 8 tend to be independent of NO. v i i i ) Increasing the S/V r a t i o affects the product d i s t r i b u t i o n to some extent i n both the uninhibited and the i n h i b i t e d decom-positions. However, t h i s effect i s more marfeed for decompositions which are i n h i b i t e d . 202 ix) N i t r i c oxide i a not consumed i n the i n h i b i t e d reaction when i t s i n i t i a l pressure i n the system does not exceed 5 mm but some consumption occurs at higher pressures. The consumption of NO for a given extent of decomposition increases as the s k e l e t a l structure of the hexane becomes more branched. 203 IT DISCUSSION The uninhibited pyrolyses The general features of the pyrolysis of hydrocarbons have been considered i n Chapter 1. The mechanisms of pyrolysis of each of the isomeric hexanes w i l l be considered i n the present chapter. Throughout the present study i n i t i a l rates were used instead of maximum rates. I n i t i a l rates were chosen so that the rates of pyrolysis would be compared for the same extent of reaction for a l l Isomers. Maximum rates, when differ e n t from I n i t i a l rates, may not always occur at the same extent of reaction for a l l isomers. Significant differences i n the rates of i n i t i a t i o n reactions may be masked i f maximum rates are used. Furthermore, most rates referred to i n the l i t e r a t u r e are i n i t i a l rates. R e l a t i v e l y l i t t l e work has been done i n the past on the py-r o l y s i s of the isomeric hexanes. Frey and Hepp (101) investigated the decomposition of n-hexane at 525°C and 575°C and the decom-position of 2,3-dimethyl-butane at 575°C. They concluded that the rate constant for n-hexane pyrolysis i s given by the equation « „ ,/OP -55.000 _ i k -a 2.7 x l O 1 ^ e — sec J-Dintses and h i s coworkers (102-104) investigated the decomposition of n-hexane, but t h e i r k i n e t i c parameters d i f f e r markedly from those given above. Their expression for the rate constant was 204 Pearl et a l (97) found the energies of aotivation and frequenoy factors for the pyrolysis of the iaomerio hexanes at high pres-sures given i n Table 45. Table 45 - Energies of aotivation and frequenoy faotorsfor the iaomerio hexanes Compound E. (kcal/mole) logA n-hexane 60 13.8 2-methyl-pentane 55 12.0 3-methyl-pentane 48 10.2 diisopropyl 55 12.2 The existence of radicals i n these decompositions can be inferred from the fact that n i t r i c oxide and propylene i n h i b i t t h e i r decomposition. The present r e s u l t s show that the rates of normal decomposition increase from n-hexane' to di i s o p r o p y l . The o v e r a l l energy of a c t i v a t i o n decreases from n-hexane to diisopropyl. Assuming that the energy of ao t i v a t i o n required for the primary di s s o c i a t i o n of the molecule, namely D(C-C) w i l l be a giajor factor i n governing the o v e r a l l a c t i v a t i o n energy, an investigation of the d i s s o c i a t i o n energies of various primary C-C rupture processes i n the isomeric hexanes w i l l be necessary. The above assumption w i l l be v a l i d provided that the mechanisms of the de-composition of a l l isomeric hexanes involve reactions with simi-l a r energy requirements, except for the primary C-C d i s s o c i a t i o n . 205 The values calculated or obtained from the l i t e r a t u r e for C-C bond d i s s o c i a t i o n energies are given below. Details of the c a l -culations/used are given i n Appendix I I . i ) n-hexane i l ' °6 H14 * ^ a " " n~°5 Hll* D(C-C) = 78 kcal/mole (la) » C 2H 5+n-C 4H g. D(C-C) - 78 kcal/mole (lb) » 2 i^-CgH7.) D(C-C) = 76 kcal/mole (lc) i i ) 2"roethyl-pentane CH3CHCH2CH2CH3 * CHg •+• CHgCHCH2CH2» D(C-C)- 72-75 kcal/mole (2a) CH, CH, — CEg^CHgCHCH2CH2CH3 D(C-C)=79 kcal/mole (2b) -» C 2H 5-CH 3CH-CH 2. D(C C)-76 kcal/mole (2c) CH_ ->n-C 2H 7-i-C 3H 7 l D(C-C)-76 kcal/mole (2d) i i i ) 3-methyl-pentane CHgCH2CHCH2CH3 »CHg + CHgCH2CHCH2- D(C-C)-72-75 kcal/mole (3a) CH. CH. -* C2Hfj •+• CHgCHCH2CHg D(C-C)=79 kcal/mole (3b) -»CHg^ CHgCH2CHCH2CHg D(C-C)^79 kcal/mole (3c) iv) neohexane CH, CH3C-CH2CH3 CH. CHg HCHgC8H2»*CH3 CHg CHg D(C-C)'80 kcal/mole (4a) CHgC* -*-C2H5 CH. D(C-c)-73 kcal/mole (4b) 206 CH2 D(C-C)=75 kcal/mole (4c) v) diisopropyl CH„CHCHCH 2/ \ 3 CHg CHg * CHgCHCHCHg-'-CHg CHg 2 i—CgHy 3)(C-C) =70 kcal/mole (5a) D(C-C) -66.5 kcal/mole (5b) The energy required for the i n i t i a t i o n of pyrolysis of diisopro-pyl i s s i g n i f i c a n t l y lower than that required for a l l other isomers. The r e l a t i v e l y rapid decomposition observed for t h i s isomer i s doubtless related to t h i s fact. However one should expect that since there i s a s i g n i f i c a n t difference i n the o v e r a l l a c t i v a t i o n energy, the rate w i l l be much faster i n diisopropyl. Because of the redu-c t i o n i n frequency factor that i s observed, t h i s difference i n rate of decomposition between diisoprjopyl and n-hexane i s reduced to a factor of about 2 to 3. From the above D(C-C) values one can see that no decrease i s observed from n-hexane to neohexane which might account for the reduction i n E A ( o & e r a l l ) . Consequently the difference i n the i r reactivity,nust be attributed to other factors. One of these factors may be the t o t a l electronic energy of the various isomers. These energies have been calculated by Hoffman using an extended Huckel theory (105) vtfhich considers the contributions made by electrons i n the 2s and 2p o r b i t a l s of carbon and i n the Is o r b i t a l of hydrogen, including a l l possible interactions. The wave function of the molecule was expressed by 807 where o_j!s are constants whose values must he determined and v|/j i s the wave function for the molecule i n the j t h state. Mini-mizing the energy of the system we obtain the following set of secular equations ^_ (H i tj - ES_y )c_j - 0 j - 1, 2,. . ..n where ~y(_ i s the Hamiltonian operator of the system and y7 the adjoint of v^/ These equations gave the r e s u l t s tabulated below when applied to the isomeric hexanes (105). Table 46 - Calculated electronic energies of isomeric hexanes Isomeric compound E^, ev/mole n-hexane - 660.504 3-methyl-pentane - 660.326 2-methyl-pentane - 660.070 neohexane - 660.200 diisopropyl - 660.160 It can be seen r e a d i l y that no s i g n i f i c a n t differencesyexist between isomers. The energy of a molecule can be partitioned into various forms. The t r a n s l a t i o n a l energy of the isomeric molecules w i l l be the same at a given temperature, and therefore no differences are expected from isomer to isomer. Decomposition oectms by the accumulation of v i b r a t i o n a l energy i n a s p e c i f i c bond. I f the time required for the r e d i s t r i b u t i o n within the molecule of the -energy acquired by c o l l i s i o n s i s greater than the time required E08 for the molecule to decompose, d i s s o c i a t i o n w i l l occur i f the energy i s s u f f i c i e n t to break the bond. The r e d i s t r i b u t i o n of energy can take place through a v i b r a t i o n a l mod.e, or by conversion of v i b r a t i o n a l to r o t a t i o n a l energy. Hence i t i s obvious that the higher the resistance to i n t e r n a l r o t a t i o n , the less probable w i l l the energy r e d i s t r i b u t i o n pro'cess become, and therefore the higher w i l l be the p r o b a b i l i t y of decomposition after c o l l i s i o n a l a c t i v a t i o n . Rotation i s i n general governed by a sp e c i f i c energy ba r r i e r which varies from molecule to molecule. The po t e n t i a l function, Y, hindering i n t e r n a l r o t a t i o n i s given by the f o l -lowing expression (106,107) V - &v" 0(l - cosn^6) where V^is the ba r r i e r height and n i s the number of equivalent minima, that i s the number of peaks and valleys i n the pot e n t i a l energy function per revolution. <j£>is the angle of i n t e r n a l r o t a t -ion. This equation has been applied to the solution of the Sc.hr.o-dinge.Tequation (108-110) and the r e s u l t s appear to be inte r e s t i n g . The method has been used to calculate the po t e n t i a l b a r r i e r to in t e r n a l r o t a t i o n of the methyl-group i n various hydrocarbons (106,111-113). Some of the results obtained are given i n Table 47. These values cover a l l p o s s i b i l i t i e s i n the paraffins. N-hexane consists mainly of groups corresponding to (b), 2-methyl-pentane consists of groups corresponding to (b) and (c) while neohexane consists of groups corresponding to (b) and (d). N-hexane requires the least amount of energy for i n t e r n a l r o t a t i o n , 209 and t h i s amount increases with branching of the chain, being almost twice as much i n heohexane. Table 47 - P o t e n t i a l b a r r i e r for rotations of GH3 group i n various hydrocarbons. Hydrocarbon T Q , kcal/mole 0H 3—CH 3 (a) 2.875 10.125 (106) CHgCHg—CHg (b) 3.400 (average) (111) CB2CH—CH* (c) 3.260 (average) (112) CHg CH3 CH3C—^-CH, (d) 4.800 (average) (113) CH3 3 Therefore, energy can be used up i n rota t i o n more e a s i l y i n n-hexane than i n neohexane. In other words, neohexane appears more r i g i d than n-hexane. Therefore, c o l l i s i o n s would be more effective i n a c t i v a t i n g the molecule. Consequently the vibra-t i o n a l energy can be dissipated more e a s i l y i n n-hexane than i n neohexane. From t h i s point of view, i t i s expected that the more branched isomers w i l l decompose more rapi d l y than straight-chained isomers because of the greater tendency for the branched isomers to r e t a i n t h e i r l o c a l i z e d energy. Other groups w i l l con-tribute to the energy d i s s i p a t i o n but th e i r contribution w i l l be of minor importance. In a l l isomeric hexanes the aotivation energies drop as the pressure increases tending to constant values at higher pres-210 sures. This observation cannot be attributed a f a l l - o f f i n uni-molecular rate with decreasing pressure. In such a complex system i t i s absolutely impossible to determine the extent to which uni-molecular and bimoleoular mechanisms take place. An indi c a t i o n can be obtained as to whether or not the unimolecular mechanism w i l l predominate. In the simplest form the t o t a l energy of the molecules can be considered as the sum of energies associated with i n d i v i d u a l bonds. The pr o b a b i l i t y that energy E_ i s i n the i ^ * 1 bond i s given by the expression (57) xwns. \ -Ei/BT -Eo/RT . -_Ej/RT B(E_) - e i / s e ...etc e j where J i s a running integer. The chance that the energy l o c a l -ized i n a spec i f i c bond w i l l surpass the c r i t i c a l energy required for decomposition, i s very small. I f this energy remains l o c a l i z e d long enough, decomposition w i l l take plaoe. For low pressures the r e s t r i c t e d number of c o l l i s i o n s helps to l o c a l i z e the energy and thus the molecule accumulates enough energy to decompose uni-molecularly. The required energy i s higher than that required for a bimoleoular decomposition under the same conditions. The increase i n pressure reduces the chance for the accumulation of energy i n the molecule and consequently reduces the chance for unimolecular reaction. The pr o b a b i l i t y of bimoleoular decomposition increases with pressure and therefore the energy required i s reduced. Active surfaces w i l l have a r e l a t i v e l y greater effect on decomposition at lower pressures. For a l l isomers reduction i n E l l rate was observed on packing the vessel. This indicates that the surfaoe may play an important role i n both termination and i n i t -i a t i o n of chains. Adsorption of r a d i c a l s on the w a l l w i l l reduce the rate of homogeneous propagation and w i l l promote termination reactions for which a t h i r d body i s required. The mechanisms of pyrolysis of each of the isomeric hexanes w i l l now be considered i n d e t a i l . 1. The mechanism of pyrolysis of n-hexane We can accept with certainty that the decomposition of n-hexane involves a free r a d i c a l mechanism. The nature of t h i s reaotion i s unknown nor i s i t known whether the reaction takes place i n the gas phase completely or i s i n part heterogeneous. The possible contributions of heterogeneous i n i t i a t i o n and hetero-geneous termination must be considered. There i s no doubt that propagation proceeds l a r g e l y by homogeneous reactions, because, the p r o b a b i l i t y of c o l l i s i o n of r a d i c a l s with hydrocarbon mole-cules i s high. Heterogeneous i n i t i a t i o n undoubtedly would dimin-i s h the energy of aotivation. Active s i t e s of the w a l l w i l l there-fore aooelerate the i n i t i a t i o n . The net ef f e c t on the o v e r a l l rate w i l l depend upon the r e l a t i v e effects of active centers on i n i t i a t i o n and termination. However, i f no effeet of surface on rate i s observed, i t does not mean that surfaoe reaotions can be completely excluded. Molecules can be adsorbed on the surfaoe either physically or chemically. In the l a t t e r case the bonds between atoms of the 212 molecule w i l l be weakened and di s s o c i a t i o n w i l l take place more re a d i l y . The ef f e c t of surface adsorption on the bonding i n o l e f i n s has been studied by L i t t l e et a l (114). They measured the infra-red spectra of the four isomeric butanes adsorbed on porous Vyoor glass at room temperature. Rapid cis-trans isomerization was found for butene-2. There was an increase i n the number of C-H bonds. On outgaasing at room temperatures small amounts of cracking products were collected. Recently other workers have also demonstrated the effect of surface on the rate of hydrooarbon p y r o l y s i s . Ethane and ethylene were decomposed when adsorbed on i r i d i u m films at 27° and 100°C (115). Analogous r e s u l t s were obtained by applying the flash-desorption technique to trans-butene-2 adsorbed on alumina (116). I t was found that two diff e r e n t s i t e s of ohemi-sorption exist whose r e l a t i v e a c t i v a t i o n energies for desorption were 12.1 and 16.2 kcal/mole respectively. The effect of surface adsorption i n accelerating pyrolysis would be expected to be r e l a t i v e l y greater at low temperatures. The i n i t i a t i o n step i n n-hexane pyrolysis has been considered before. These i n i t i a t i o n steps can take place either homogeneously or heterogeneously. The l a t t e r w i l l be r e l a t i v e l y more important at low pressures. The p r o b a b i l i t y that a molecule w i l l dissociate heterogeneously under a given set of conditions w i l l be a fundtion of the a c t i v a t i o n energy of the decomposition which may be af-fected by the structure of the molecule. Since the bonding i n w a l l 213 of the iaomerio hexanes i s of the same type, the size of the mole-cule may play a decisive role i n the extent of adsorption. We have seen (Chapter III) that n-hexane i s adsorbed less than a l l other hexanes on quartz wool. This may be attributed to the fact that the smaller the size of the molecule the higher the probab-i l i t y that more active centers w i l l be used up. Molecules w i l l occupy a portion of minimum pot e n t i a l on the surface and i t i s u n l i k e l y that multilayers w i l l be formed at the temperatures used. In n-hexane, since i t i s l i k e l y that adsorbed molecules w i l l be d i s t r i b u t e d i n a random fashion, some active centers w i l l be prevented from adsorbing molecules, and therefore a r e l a t i v e l y larger proportion of the centers w i l l not be used than for the more branched isomers. In addition the straight chain may cause multiple adsorption of single molecules. However, because of the reduced adsorption of n-hexane r e l a t i v e to other isomers,heter-ogeneous reactions w i l l be r e l a t i v e l y less important i n the py-r o l y s i s of t h i s molecule. The i n i t i a t i o n can occur by means of several reactions CHgCHgCHgCHgCHgCHg *CH3-n-C5Hi:_« (la) ^CgHs-n-C4H9« (lb) » 2n-GgH7» (lc) We have already seen that the dis s o c i a t i o n energies for ( l a ) , ( l b ) , and (lo) are 78, 78 and 76 kcal/mole respectively. Methyl and e-thyl radioals are considered to be the chain propagating r a d i c a l s . S14 The absence of propane i n the products indicates that n-C 3H 7 r a d i c a l disappears mainly by a decomposition reaction. Methyl r a d i c a l s w i l l be formed mainly from reaction (la) and from the pyrolysis of n-CgH7 r a d i c a l , while the C 2H 5 r a d i c a l w i l l be formed from reaction ( l b ) . The y i e l d s of some of the products can be expressed i n terms of the p r o b a b i l i t i e s of the various carbon-carbon rupture pro-cesses that can occur i n the decomposition of n-hexane and of large r a d i c a l s l i k e n-CgH__g . Thus the sum of CgHg and CgH4 can be used as a measure of CgHg« formation which can be related to the p r o b a b i l i t y of reaction ( l b ) , that i s for P(C 2$Cg) rupture where P represents the p r o b a b i l i t y . The y i e l d of CgH4 w i l l be also related to reaction (lo) because of the reaction ni-CgH 7 ^CHg+CgH4 S i m i l a r l y the y i e l d of methane w i l l be related to the p r o b a b i l i t i e s for C_,"cg a n < i ^3~c4 ^ p t u r e . Therefore we obtain for the product r a t i o : CH4 _ gg(0i-C g) - P ( C S - C A ) C 2H 6- CgH4 2P(C 2-C S) -P(C 3-C 4) where P(C-C) represents the pr o b a b i l i t y of a p a r t i c u l a r rupture. The experimentally observed value for t h i s r a t i o was 0.55 (Table-6 ). Hence SP(C 1-C 2) +0.45 P(C 3-C 4)- 1.10 P(C 2-C 3) = 0 Since EPfCj-Cg) - EP(Cg-Cg) + P(C 3-C 4) - 1 we can express the r e l a t i v e p r o b a b i l i t i e s of the various C-C ruptures processes i n terms of one process. Thus we obtain E15 P(C 2-C 3) - 0.32 - 0.175 P(C 3_C 4) ^(C^Cg) - 0.175 - 0.32 P(C 3-C 4) From the above two relations we conclude that reaction (lb) i s predominant. Assigning to P(C 2-C 3) :the value of 0 we obtain E(C3-C 4) -0.54 , while by assigning a value 0 to P(C 3-C 4)we obtain P(C 2-C 3) ± 0.32 . The conclusion i s that P(C3-C 4)never exceeds a value of 0.54 . This means, i n spite of the lower d i s s o c i a t i o n energy of reaction ( l e ) , i t never accounts for more than 50$ of the primary d i s s o c i a t i o n while reaction (la) never accounts for more than 30$. The propagation by CH3 w i l l proceed as follows: n- C 6H 1 4 CH3 ^ CH4-r.n-C6H13» (2a) , CH4 CHgCHgCHgCHgCHCHg (2b) > CH4 -r CH CHCHCHCH CH_ (2c) 3 2 2 2 3 The acti v a t i o n energy for H-abstraction by CH3 i n paraffins i s approximately 6-8 kcal/mole (117 ). Trotman,- Dickenson and Steacie calculated that the abstraction of a secondary hydrogen by CH3 requires 2 kcal/mole less than primary H-abstraction (117). The energy of a c t i v a t i o n for reaction (2a) can be calculated from the equations n-C6H14-»- CH3 > CH 4-n-C 6H 1 3 SA_z ? n-C 6H 1 4 •• * H+n-C 6H 1 3 D(C-H) - 91 kcal/mole A value for D(C-H) of 91 kcal/mole i s assumed on the basis that D(CH3-H) z l O l kcal/mole, D(C2H5-H) - 98 kcal/mole, D(n-C3H7:-:H) = 216 95 kcal/mole and E(n-C4Hg-H) - 93 kcal/mole We therefore obtain the following E A - AH f (CH4)->-91-^Hf (H) - 8.5 kcal/mole. An average value for E^ for reactions (2a-2c) of 811 kcal/mole has been found (118). Therefore the E__ for reactions (2b), (2c) including:secondary hydrogen abstractions by CHg w i l l be approx-imately 6.5 kcal/mole. The propagation can also occur through H-abstraction by C2H5 r a d i c a l s C 2H 5 + n-C 6H 1 4 ^OgHg-n-OgH^ (3a) * C 2H 6 - CHgCHgCHgCHgCHCHg (3b) » CgHg - CHgCHgCHCHgCHg (3c) Assuming that D(CHg-H) =101 kcal/mole and D(C2H5-H) - 98 kcal/mole the a c t i v a t i o n energies for H-abstraction of the types CHg RH > CH4 - R CgH 5*RH—^CgHg-R w i l l d i f f e r by about 3 kcal/mole. Hence the energies of a c t i v a t i o n for processes (3a), (3b), (3e) w i l l be 3 kcal/mole higher than the corresponding energies for (2a), (2b) &(2c) namely 11.5 , 9.5 and 9.5 kcal/mole respectively. Propagation by n-CgHy would require s t i l l higher energies of act i v a t i o n of about 13-14 kcal/mole. The ne g l i g i b l e amounts of CgHg found indicates that propagation by n-CgH7 i s i n s i g n i f i c a n t at least at low pressures. The large r a d i c a l s formed by hydrogen abstraction w i l l 217 decompose, propagating the reaction further. n ~ C 6 H i 3 * i-C 3H 7 -rCgHg (4a) — » C 2 H 5 -GH2:CHCH2CH3 (4h) — * C 2 H 5 ^CH3CH:CHCH3 (4c) — * CH +CH,_CH:CHCH0CH„ (4d) —» CHg - CH2: CHCH2CH2CHg (4e) Reactions (4c), (4d), and(4e) are ruled out because the products predicted by these reactions are not observed. Therefore we can assume that the n-hexyl r a d i c a l decomposes by reactions (4a) and (4b). No r e l a t i v e p r o b a b i l i t i e s can be worked out for these two modes because CgHg and l-C 4H g are formed from the decomposition of a l l hexyl-radicals. The decomposition of the other hexyl r a d i c a l s w i l l proceed by reactions: CHgCH2CH2CH2CHCHg — * C2Hg-t-CH2: CHCH2CH3 (5a) > n-CgH7 * CgHg (5b) and .# CHgCH2CH2GHCHgCHg > C 2H 5 -^ -CH2: CHCHgCHg (6a) > n- CgH7 - CgHg (6b ) Reactions (4a,4b), (5a,5b) and (6a,6b) w i l l occur at different rates, because although the products are the same, the heats of formation of the decomposing ra d i c a l s are d i f f e r e n t . However, thi s difference becomes very i n s i g n i f i c a n t as the size of the molecule increases. From the equations n-C 6H 1 4 =»n-CgH^ 3•+• H D(O-H) - 9 1 kcal/mole n-C 6H 1 4 —>C2H5^-n-C4H(3 D(C-C) -78 kcal/mole 218 we obtain the quantity £*H f(n-hexyl) - -1 kcal/mole therefore for reaction (4a) we obtain D ( C 3 - C 4 ) ^ (E A) - 24 kcal/mole Since the 5H_.(CgH 5 ) i s greater than the & H f(n - C g H 7 ) and the &H_. (CgHg) i s greater than __ H_» {1-C 4 Hg) we can infer that reaction steps (4a) and (4b) w i l l be almost isoenergetic. The same argu-ment holds for (5a) and (5b) and (6a) and ( 6 b ) . Propagation i a also promoted by the decomposition of such radioals as n - C g H ^ , n-C^Hcj and n -CgH 7 produced i n the various steps. Thus n - C * H 7 ^CH* E__:_r26^4 kcal/mole 3 7 3 2 4 A (/_8) ( ? a ) >C<5H6A-H E A - 3 8 kcal/mole (118) * (7b) Reaction (7b) can be investigated as follows (34). The heat of reaction w i l l be the difference between D(C-H) and the energy required for formation of a TT-bond i n CgHg. Thus Q, _ D(C-H)-Q,_ r 95-57 ~ 38 kcal/mole. The reaction i s therefore endothermic. Bearing i n mind that E-E 0 - . IQ,1 (34) where E Q i s the energy barrier for the reverse reaction and as-suming with Semenov that EQ^Ekcal/mole i n the above reaction, we obtain ( E _ k ) 7 ^ _ _ 4 0 kcal/mole The higher aotivation energy of reaction (7b) makes i t less impor-tant. The rate of pyrolysis of propyl r a d i c a l s w i l l be high at the temperature used i n the present study. Previous workers have 219 found Hg i n the products of pyrolysis of propyl-raaicals produced hy Hg-photosensitized decomposition of propane. (119). In general i t appears that for a l l decompositions of a l k y l - r a d i c a l s the pre-exponential factor i s approximately l O ^ s e o " 1 and E 0 i s a small quantity or may he zero. Thus reaction (7b) w i l l contribute s l i g h t l y , which could account for the small amount of H 2 found. F i n a l l y n-.C4H$ -- * C H 3 ^ C 3 H 6 EA -24 koal/mole (118) (8a) ° 2 V 0 A EA - 26 keal/mole (118) (8b) -* 1-C 4H 8^H EA -40 kcal/mole (118) (8c) and n*"C5H____ - r CHg ~r-1— C^Hg E A - 30 kcal/mole (9a) 1 C2 H5"'" C3 H6 E A - 28 koal/mole (9b) * CgH7 +- CgH4 E A = 27 koal/mole (9o) f C 5 H 1 0 ^ H E A - ? (9d) Reaction (9d) i s ruled out beoause CgH^Q i s not found i n the decomposition products. The above values of E^ , have been c a l -culated by methods similar to those discussed e a r l i e r , or have been obtained from the l i t e r a t u r e . Reactions of the type H +RH > Hg — R w i l l participate s l i g h t l y and they are s l i g h t l y endothermio, 4-5 kcal/mole. It i s assumed they are not very important i n the present study. Reactions of the type H-^ RH * CH4+ R' are u n l i k e l y beoause they w i l l require a perpendicular attaok on a 6"-bond which requires twice as much energy as a oolinear attaok. 220 The reaction H H+ H-'C-CH2CH2 -'—: > CH4 4 CH2-CH2 ---H requires a Walden inversion with a r e l a t i v e l y high energy. Such reactions have been found to have acti v a t i o n energies of about 14-16 kcal/mole (120). For example for the reaction RT R ? Rn R„ \ / 2 \/ I + C - I * I - C • I R 3 R 3 E__ i s 14 kcal/mole. The fact that the order of decomposition of n-hexane i s approximately unity with some increase with packing can be con-sidered i n terms of various types of termination reactions/according to the terminology of Goldfinger et s i ( 1 2 1 ) . According to th i s approach, the order of any reaction mechanism i s determined by the orders of i n i t i a t i o n and termination steps. The various pos-s i b i l i t i e s are given i n Table 48. Table 48 - Overall orders of reaction for various types of i n i t i a t i o n and termination processes. F i r s t order i n i t i a t i o n Second order i n i t i a t i o n  third-body simple third-body o v e r a l l simple termination termination termination termination order 2 W W W& 3/2 £ W ' P P M \4~ ' 1 — ^ 1/2 221 where p and ^ r a d i c a l s are those which react only bimolecularly and unimoleeularly respectively. N\ i s a t h i r d body (molecule or w a l l ) . The r e l a t i v e l y low concentration of p or K r a d i c a l s sug-gests a very low p r o b a b i l i t y for (? P PA or |3|*N\ or when M i s a molecule.relative to simple bimoleoular reactions. The pro-b a b i l i t y of these reactions increases i f (A i s an aetive s i t e , because, an adsorbed r a d i c a l remains,for a c e r t a i n time period, "frozen" on the w a l l . Under these circumstances the p r o b a b i l i t y becomes a function of the gas-phase concentration of t h i s r a d i c a l . Where M i s a molecule, the p r o b a b i l i t y i s a function of the con-centrations of both radicals andtA . It can be argued that the ad-sorption of r a d i c a l s on surfaoe s i t e s favors the p r o b a b i l i t y that M i n t h i r d body termination reactions w i l l be the w a l l of the reaction vessel. For f i r s t order i n i t i a t i o n whioh probably w i l l predominate at lower pressures two p o s s i b i l i t i e s e x i s t for termination: Bimoleo-u l a r , $Y termination and trimoleoular ?ptA termination. Second order i n i t i a t i o n whioh i s more probable at higher pressures re-quires bimoleoular YY termination and t h i r d body ppfA termination. The r a d i c a l s H, CHg and C2H5 are considered as (? r a d i c a l s and n-CgH7 as a u r a d i o a l . The increase i n order for the paoked vessel i s attributed to a t r a n s i t i o n to a termination for f i r s t order i n i t i a t i o n s at lower pressures. Hence the termination at lower pressures i n the unpacked vessel can take place as follows: 222 CHg+CHg - i l ^ * c 2 H 6 (10a) » CgHg (10b) CH g^C 2H 5 — ^ C 3 H 8 (11a) - ^ U CH4 - C 2H 4 ( l i b ) C2 H5^°2 H5 " — *C4H10 (12a) - i ^ - > C 2 H 4 - C 2 H 6 (12b) H-CH g •» CH 4 (13) H-C 2H 5 ^C 2H 6 (14) CH3-r-n-CgH7 *04Hio (15a) J ^ C H 4 ^ C g H 6 (15b) S L C2H5-^n-CgH7 — >C2H4 + C 3H 8 (16a) i c H + C H (16b) 2 6 3 6 H r H ~ ^ H 2 ( 1 7 ) Reaction (10a) appears to p r e v a i l at higher surface areas because increased S/V r a t i o s increase the formation of C2Hg (Table 26). The energy of a c t i v a t i o n i s assumed to be zero for reactions (10a), (10b) and (11a) whereas some act i v a t i o n energy, perhaps 4 to 5 kcal/mole w i l l be required for reaction ( l i b ) . I t has been suggested that reaction (12b) has ac t i v a t i o n energy of 4.8 k c a l / mole (122). The value of (BA) 1 2b-^ EA^12a h a s 1 3 9 6 1 1 f o u n a t(> be (123') 223 0.8^ 0.2 koal/mole. A value of 4.8 koal/mole has been reported for th i s difference by other workers (122). However the absence of n-C4H__o from the products rules out (12a). Reactions(lS) and (14) w i l l be of minor importance. Reactions (15b), (16a) and (16b) w i l l require energies of act i v a t i o n comparable to that for reaction (12b), while (15a) i s ruled out because <3f the absence of n-C4H__Q among the products. Reaction (17) may account for some of the trace amount of H 2 found. The o v e r a l l mechanisms for n-hexane pyrolysis The o v e r a l l mechanisms must be extremely complex as i t appears to include the following reactions I n i t i a t i o n reactions n-0 6Hi4 > CH,,^  n-C5H1:i« D(0-0) =78 kcal/mole (la) ° heter. ° x x heter ' °2H5 ^ n - g 4 H 9 * D(0-0.)= 78 kcal/mole (lb) h o m»—>2n-C~H„ D(C-C) - 76 kcal/mole (lc) heter. 2 7 Propagation reactions CH 3-n-C 6H 1 4 > CH 4-n-C 6H 1 2 E A — 8.5 kcal/mole (2a) • » CH4 -^-CH2CH2CH2CH2CHCH3 B__-= 6.5 kcal/mole (2b) -> CH4+ CH3CH2CH2CHCH2CH3 E a=6.5 kcal/mole (2c) C 2H 5^n-C 6H 1 4 -^ C2 H6^ n " G 6 H i 3 E A ~ 11.5 kcal/mole (3a) ^ C 2H 6-CH 3CH 2CH 2CH 2eHCH 3 E A~9.5 kcal/mole (3b) > C 2H 6-CH 3CH 2CH 2CHCH 2CH 3 E An9.5 kcal/mole (3c) 224 Radical decompositions n-C 6H 1 3. _^C 2H 5-1-C 4H 8 E A = 24 kcal/mole (4a) n-C 3H 7 +C 3H 6 -24 kcal/mole (4b) CH CHgCHgCHgOHOH g — * C2H5 -H-1-C4HQ EA — ? (5a) ^n-CgH? ^ CgH 6 EA — 9 (5b) CH3CH2CH2CHCH2CH3 _^C 2H 5 ^1-C 4H 8 S A — - <? (6a) —*n-C 3H 7- CgH6 E A — 9 (6b) n-C 5H i ; L J CHg-^ - 1-C4H8 E A z.30 kcal/mole (7a) —>C 2H 5 * C 3H 6 E A = 28 kcal/mole (7b) -^n-CgH7^-CgH4 S A = 27 kcal/mole (7c) n-C4He — C H 3 - C3 H6 EA = 24 kcal/mole (8a) 5 C2 H5^°2 H4 E A ~26 kcal/mole (8b) n-C 3H 7 — ? CHg -i- C 2H 4 EA = 24 kcal/mole (9a) — H - CgH6 = 40 kcal/mole (9b) 'mination reactions CHg- CHg > C 2H 6 EA - 0 kcal/mole (10a) 1 G2 H6 3 A — 0 kcal/mole (10b) CHg+-CEH5 CgHg E A — 0 koal/mole ( l l a ) E A - 4 kcal/mole ( l i b ) C 2H 5-.C 2H 5 _ l ^ U c 2 H 4 - C 2 H 6 E A - 4.8kcal/mole(12) H * CHg CH4 E A = 0 kcal/mole (13) H + C 2H 5 - ^ C 2 H 6 • - 0 kcal/mole (14) CHg^n- CgH7 J l ^ CH 4^CgH 6 \ - 4.8kcal/mole(15) 225 CgEg-n-CgH.j, — ^ — > CgH^ CgHg 4.8kcal/mole (16a) — — — ~ > C 2 H 6 ^ ° 3 H 6 E A= 4.8kcal/mole (16b) H -*-H — ^— > H 2 E A - ° kcal/mole (17) The occurrence of small amounts of Eg can be accounted for by sl i g h t pyrolysis of various radioals such as n-CgH7, n-C4H9 and n-C5H____ by the general mechanism R * o l e f i n * H The fact that Hg i s found i n small quantities indicates that reactions of t h i s type are not of major importance i n the mechanism. This mechanism accounts for a l l of the observed pyrolysis products but i t i s much too complex to be treated qu a n t i t a t i v e l y i n a way which would y i e l d a t h e o r e t i c a l expression for the rate of disappearance of n-hexane. Although the values of energies of aotivation which have been c i t e d are useful i n i n d i c a t i n g the r e l a t i v e p r o b a b i l i t i e s of the various reaction steps, r e l i a b l e information on the pre-exponential factors for these steps are i n general not available and therefore prediction of the rate constants of these reactions cannot be made with any certainty. 2. The mechanism of pyrolysis of 2-methyl-pentane We s h a l l assume that pyrolysis of 2-methyl-ipentane .. involves a free r a d i c a l mechanism only. Since 2-methyl-pentane appears to be adsorbed more strongly than i s n-hexane, i t i s expected that heterogeneous i n i t i a t i o n w i l l participate to a higher extent i n t h i s pyrolysis than i n 226 n-hexane. We s h a l l asume that the net effect of surfaces on pyrol-y s i s w i l l depend on the r e l a t i v e contributions made by the surface to the termination and i n i t i a t i o n rates, and that t h i s net effe c t i s given by the expression net e f f e c t on o v e r a l l rate r effect on rate of termination minus effect on rate of i n i t i a t i o n The result of increased heterogeneous i n i t i a t i o n for 2-methyl-pentane i s that the reduction i n rate due to the packing i s less marked than i n n-hexane. Heterogeneous termination should be of the same importance i n both compounds since similar r a d i -cals are involved. Thus, increased i n i t i a t i o n w i l l r e sult i n a lower reduction i n rate. The i n i t i a t i o n process has been considered e a r l i e r . I t i s assumed to involve the following reactions CH3CHCH2CH2CH3 » CHgCHgCHCH^Hg* D(C-C) - 72-75 (la) I i kcal/mole CHg CHg =>CHg -CHgCHCH2CH2CHg D(C-C) - 79kcal/ mole (lb) »C2H5-CHgCHCH2* D(C-C) =76kcal/ \ mole (lc) CHg » i-CgH 7 -n-CgH 7 D(C-C) ~76 k c a l / mo le (Id) Propane w i l l be produced as a consequence of reaction (Id) while CH4 i s formed through reaction ( l a ) , (lb) and (Id), where i t i s assumed that the propyl r a d i c a l decomposes into CHg and C 2H 4. S i m i l a r l y C2H4 and C2Hs w i l l be produced through reaction (lc) and (Id). These l i g h t products w i l l be also produced from 237 the decomposition of the hexyl r a d i c a l s i n the same fashion. We can therefore relate the r e l a t i v e p r o b a b i l i t i e s of various C-C ruptures to the various product r a t i o s as i n the case of n-hexane, as follows: CH4 P(C 4-C 5)+ 2 P ( C 1 - C 2 ) * P(C 2-C 3) C SH 8 - E(C 2-C 3) This r a t i o was found to vary with temperature; an< .average value of 3 . 6 i s assumed. Also CH4 ^ P(C 4-C 5) i - 2P(C_-C g) + P(C 2-C 3) ° 2 p 6 * ° 2 H 4 ~ *(Cg-C 4) ~ P(C 2-C 3) which r a t i o was found to have a value approximately 0 . 6 . From the above rel a t i o n s we obtain P(C 4-C 5) * 2 P(C 1-C 2 ) - 2 . 6 P(C 2-C 3) - 0 (1) P(C 4-C 5) * 2P(C 1-C 2) -r- 0 . 4 P(C 2-C 3 ) - 0 . 6 P(C 3-C 4) - 0 (2) Since for the t o t a l p r o b a b i l i t y we obtain 2 P ( C r C 2 ) - P(C 2-C 3) - P(C 3-C 4) + P(C 4-C 5) = 1 (3) we obtain a set of three equations with four unknowns. Therefore we can express the three of the p r o b a b i l i t i e s i n terms of the fourth. We can e a s i l y obtain the expression P(C 2-C 3) 0 . 2 P(C 3-C 4) (4) and also 7 . 5 P(C 4-C 5) * 1 5 P ( C r C 2 ) z 3 . 9 P(C 3-C 4) (5) Using (4) and (5) i n (3) we obtain the following values P(C 3-C 4) 0 . 5 8 P(C 2-C 3) 0 . 1 2 P(C__-C2) •__ 0 . 1 5 - 0 . 5 P(C 4-C 5) 828 The l a s t value can he expressed, i n the form P ( C r G 2 ) < o.i5-c.;::\ which i s obtained by assigning an extreme value of zero to P(C 4-Cg) The high value of P(C 3-C 4) i s i n agreement with the lower dis-sociation energy for the reaction ( l c ) . The CHg rad i c a l s produced can propagate the decomposition as follows: CH3 i - OHgCHOHgOHgOHg > CH 4 - CHgCHCHgCHgCHg* (2a) CH3 CH3 »CH4 - CH3CHCH25HCH3 (2b) CH3 > CH4 -r- CHg CHCHCH gCHg (2c ) CH3 >CH4 + CH3CCH2CH2CH3 (2d) CH3 T CH4 - CH3CHCH2CH2CH3 (2e) CHg-The energies of a c t i v a t i o n for these processes w i l l be about 7-8 kcal/mole. By using the equations CHgCHCHgCHgCHg - CHg * CH4 - CHgCHCHgGHgCHg- E A - ? CH2 CH3 CHgCHCHgCHgCHg > H -CHgCHCHgCHgCHg. D(C-H)-91 kcal/mole CH3 CH3 we obtain, by the method previously used E A — 8.5 kcal/mole 229 For the abstraction of a secondary H, the energy required i s about 2 kcal/mole less than for the primary H (117), while for a t e r t i a r y hydrogen i t i s about 4 kcal/mole less than for the primary. However, E__ for the reaction i s not expected to be much diff e r e n t than the corresponding value for n-hexane, namely 8 kcal/mole. The ac t i v a t i o n energies for reactions (2b) and (2c) w i l l be about 6 kcal/mole. For reaction (2d) i t i s about 4-5 kc a l / mole while for reaction (2e) i t i s almost the same as for reaction (2a), namely 8 kcal/mole. Hydrogen-abstraction by CgHg and n-CgH7 w i l l require an act i v a t i o n energy higher than that required for H-abstraction by CHg, by amounts of 3 and 6 kcal/mole respectively. In fact t h i s difference appears i n the bond di s s o c i a t i o n energies. D(CH3-H) r 101 , D(C 2H 5-H)r98 , D(n-C3H7-H) - 95 kcal/mole This difference w i l l be encountered i n reactions of the type RH CH3 > R ^CH 4 RH - CgH5 —> R -C 2H 6 RH + n-C3H7 —» R -C 2H g Propagation w i l l therefore also be carried out by CgHg and n-C 3H 7 radicals as follows CgH5+-CH2CHCH2CHgCH2 > CgHg-r- CH3CHCHgCHgCHg» Ej_ —11.5kcal/ I I mole (3a) CH3 -^^ 3 >CgH6+- CH3CHCHgCHCH3 B__-9 kcal/mole (3b) I CH3 >C2H6 -r-CH2CHCHCHgCH2 E A =9 kcal/mole (3c) CH2 230 . ^HQ^CHgCHGHCHgCHg E A = 8 kcal/mole (3d) CHg » 0 2H 6 CH3CHCHgCHgCHg E A - 11.5kcal/mol3e) CHg-n-C 3H ? ^ CH3CHCH2CH2CH3 »CgHg^ CHgCHCHgCHgCHg' (4a) CH^ CH3 E A = 13 kcal/mole >C 3 H 8 + CH3 CHCH2CHCH3 (4b) CHg E A _ 11 kcal/mole CgHg-+CHgCHCHCH2CH3 (4c) CH3 E A — 1 1 kcal/mole > CgHg + CH_ftCH.CH.CHg (4d) CHg E A — 10 kcal/moaie > C 3H Q - CH2CHCH2CH2CH3 (4e) CH2» E A - 13 kcal/mile The heats of formation of n-C 3H 7 and i-CgH? d i f f e r by some 6 kcal/mole (34) and i t w i l l therefore be important to consider the extent to which propagation i s promoted by n-CgH7 or i-C 3H 7. The corresponding H-abstractions by i-CgH 7 w i l l require an average a c t i v a t i o n energy of 17-19 kcal/mole in.comparison with value of 13 kcal/mole estimated for n-CgHy. Hydrogen i s a s i g n i f i c a n t product of the pyrolysis and i s l i k e l y to be formed by reactions of H-atoms, produced i n spontan-eous decomposition of higher radicals, with hydrocarbon molecules. Since D(H-H) - 103 kcal/mole and D(CHg-H)z 101 kcal/mole, i t i s expected that H-abstraction by H atoms w i l l require about 3 k c a l / 231 mole leas energy than the corresponding a c t i v a t i o n energy for H-abstraction by CHg. Thus we can estimate a c t i v a t i o n energies for the reactions H -^ -CHgCHCHgCHgCHg H^g -- CHgCHCHgCHgCHg* S A _ 5 kcal/mole (5a) CHg CHg -rHg -•-CHgCHCHgCHCHg S A_r3.5 kcal/mole (5b) CH„ —> H g --CHgCHCHCHgCHg E A_:3. 5 kcal/mole (5c) CHg r Hg -vOHgOCHgCHgCHg E A_2.5 koal/mole (5d) CHg 1 Hg -r-CHgCHCHgCHgCHg E A 5 kcal/mole (5e) CHg-The large r a d i c a l s produced w i l l propagate the decomposition further through th e i r pyrolysis CHgCHCHgCHgCHg* > GzRd'i' i~°4H8 ^6A^ CHr* 7 i - C g H 7 - C g H 5 (6b) — » C H g -butene-1 (6e) > CHg -r-butene-2 (6d) Processes (6c) and (6d) are eliminated beoause of the absence of butene-1 and butene-2 i n the products. The energies of a c t i -vation for reaction (6a) and (6b) w i l l be comparable to the cor-responding energies i n n-hexane. Similar reactions can occur for the other hexyl-radicals. 832 CHgCHCHgCHCHg >» C 2H 5 i-C 4 H g (7a) C H 3 => 1-C3H7 ^  C 3H 6 (7b) CH3CHCHCH20H3 >» C 2 H 5 ^ i - C 4 l 8 * 8 a^ CH3 * i - C 3 H 7 C 3H 6 (8b) CH3CCH2CH2CH3 > C 2 H 5 ^ i - C 4 H 8 (9a) C H 3 > i-C 3H 7 - r-C 3H 6 (9b) and CH3CHCH2CH2CH3 > C ^ i - C ^ (10a) CH2» > n-C 3H 7 -J-OgHg (10b) Although, the product r a t i o CjjHg/i - ^ H s w i l l be determined by reactions (6a) and (6b), (7a) and (7b) etc., i t i s not possible to calculate the r e l a t i v e p r o b a b i l i t i e s of the various C-C rup-ture reactions, beoause the contribution to the t o t a l mechanism of reactions l i k e (6), (7), (8), (9) and (10) has not been esta-blished. A l l of reactions referred to above produce the same products. Smaller r a d i c a l s w i l l continue the propagation by meohanisms such as CH3CHCH2CH2' —•> C 2H 5 — CgH6 ( l l ) CH 3 CH3CHCH2CH3 • C 2H 5 C 3H 6 (12) CH3CHCH2* => CH3 -J-CgHg E A • r E3 kcal/mole(118) l l 3 a ) CH3 » H i-C 4H 8 E A _. 40 kcal/mole (13b) n - C 3H 7 ¥ CH3 +CgH 5 E A - 20 kcal/mole (118) (14a) v.H -+-GgH6 E a - 38 kcal/mole (14b) 333 i-CgH 7 » C H3-*- C2 H4 S A-20lcoal/mole (34) (15a) => H -T-G 2H 6 E =.38koal/mole (15b) Reaction (15b) may account for the formation of Hg because H-abstraction by i-CgH 7 w i l l require a r e l a t i v e l y high energy of aotivation. Therefore a large amount of i-CgH 7 may undergo pyro-l y s i s , while CgHg i s formed from H-abstraction by n-CgH7. The r e s u l t s of the present study show that the order of decomposition i s a l i t t l e higher than unity at low pressures and i t drops almost to unity at higher pressures. Packing the vessel increases the order to a small extent. The following possible terminations are suggested by the general scheme out-li n e d i n Table 47. At low pressures , py_. andlV*™ terminations can ooeur. At higher pressures YY and Ywware probable. Recombina-tions of the $Y and ?Y* types may be of increased importance for the packed vessel. The methyl r a d i c a l and H atom are considered as I5 r a d i c a l s . The r e l a t i v e l y large amount of Hg formed indicates that n- and i-CgH 7 r a d i c a l s as w e l l as C2H5 may be considered asY - r a d i c a l s . Therefore the termination w i l l follow the following scheme at low pressures i n the umpacked vessel. CHg -v- CHg - 1 L C 2H 6 J °2H6 CHg - C 2 H 5 ! L , CgH8 f>Y •> CH| -^C 2H 4 CH3 -CgH 7 CH 4 +C 3H 6 234 H - T - 0H g - 1 * CH4 CH 4 H - 0 2H 5 > CgH6 H 2 ^ 2 = 4 H - i - C3H7 C3H8 H -1- H * H Hg -r-CgHg 2 - H 3 The faot that packing favours the formation of CgHg suggests that the termination i s favoured against any possible PK reac-t i o n i n whioh the same P r a d i c a l i s involved. The o v e r a l l mechanism for 2-methyl-pentane pyrolysis The o v e r a l l mechanism must he more complex than i t i s for n-hexane because there are great many chain c a r r i e r r a d i c a l s , and more hexyl r a d i c a l s are produced than for n-hexane. I n i t i a t i o n reaotlons hom. CHgCHCHgCHgCHg » CHg — CHgCHCHgCHgCHg.I)(C-C )—72-75ko/m (la) i heter. I CHg CHg hom. . CHg -*- CHgCHCHgCHgCHg D(C-C)-79 kc/mol (lb) heter. hom. CgH5 -T- CHgCHCHg* D(C-C)-76 kc/mol ( l c ) heter. I CHg hom. > i-C 3H 7 - n-CgH7 D(C-C) = 75 kc/mol (Id) heter. 235 Propagation reactions CHg •+- CHgCHCHgCHgCHg <3H4 -^CHgCHCHgCHgCHg' CHg ^CH4 - 0H3CHCH2CHCH3 CH, EA = CgHg CHgCHCHgCHgCHg CH, -%CH4 -T- CH3CHCHCH2CH3 CH„ E * -C H 4 v CH3CCH2CH2CH2 CH, •^CH4^ CH3CHCH2CH2CH3 CHo» Ej -> C 2H 6 -+- CH3 CHCH2CH2CH2« I CH3 E A --> CgHg -+ CH3CHCH2CHCH3 CH, E. -~* CgHg-1- CHgCHCHCHgCH3 C H g EA " C 2H 6 -*-CH3CCH2CH2CH3 CHg E A — -* C gHg —CHg CHCHg CHg CHg CH2- E A ~ n-C3H7-CHgCHCHgCHgCHg > CgHg-^CHgCHCHgCHgCHg' CH, CHg E A •^ CgHg-1- CHgCHCHgCHCHg (Ea) 8.5 kcal/mole (2b) 6.5 kcal/mole (2c) 6.5 kcal/mole (2d) 4-5kcal/mole (2e) 8 kcal/mole (3a) 11 koal/mole (0b) 9 koal/mole (3c) 9 koal/mole (3d) 8 kcal/mole (8a) 11 koal/mole (4a) 13 kcal/mole (4b) I > :<CHg E A - 11 koal/mole 236 * CgHg - CHgCHCHCH2CHg (4c) CHg B A — 1 1 K O A L / M O L E > CgHg-r-CHgCCHgCHgCHg (40.) C H 3 EA — 1 0 koal/mole •> CgHg •* CHg CHCHgCHgCHg (4e) CH2? E A ~ 13 kcal/mole H•+ CHgCHCHgCHgCHg » Hg.'+CHgCHCHgCHgCHg* (5a) CHg CHg E A 3 5 koal/mole * Hg -r-CHgCjHCHgCHCHg (5b) CHg E A — 3.5koal/mole * Hg CHg CjHCHCHgCHg (5o) CHg E A z 3.5koal/mole r Hg-r CHgfiCHgCHgCHg (54) CHg E A — 2.5koal/mole » H -CHgCHCHgCHgCHg (5e) I CHg» E A - 5 koal/mole Radioal deoompoaitiona CHgCHCHgCHgCHg* » CgHg +- i-C 4Hg E A-26 koal/mole (6a) CHg » i-C 3H 7 ---CgHg E A — 20 koal/mole (6b) CHgCHCHgCHCHg ^ CgHg+-i-C4H8 E A = 26 koal/mole (7a) CHg M--CgH7 -CgH 6 E A = 20 koal/mole (7b) CHgCHCHCHgCHg > C 2 H5- ^Vs E A ~ 2 6 fcoal/mo19 { 8 a * CHg -> i-CgH 7 4- CgHg . E A =20 koal/mole (8b) 337 CHg C CH g OH g CHg CHg CHgCHCH2CH2CHg CHg-CHgCHCHgCHg* CHg CHgCHCH2CH2CHg CH-CHCH • o , 2 CH, n-C 3H 7 i-CgH 7 °2H5 -^ CgHg + 1-C.H, 4 8 -> n-C H - C_H 3 7 3 6 ~* °2 H5" i"°4 H8 ^n-CgH 7-CgH 6 -* C 2 H 5 " C3H6 -^ CHg + 1-C4H8 " C 2 H 5 + C 3 H 6 -CHg ^ 1-C 4H Q -> C H 3 * C3 H6 H ^ i - C 4 H 8 C H3"°2 H4 •> H - CgH6 * CHgr- C2H4 H t CgH6 - H-C 2H 4 Termination reactions CHg - CH 3 CH 3- C 2H 5 PI* C2H6 ^ C 2H 6 P k ' C3 H8 * CH 4.C 2H 4 .26 kcal/mole (9a) E A = 20 kcal/mole (9b) EA = 26 kcal /mole (10a) EA = 20 kcal/mole (10B) 23 koal/mole ( l l a ) ( l i b ) EA = • 23 koal/mole (12a) E A ~ ? (12b) EA - 23 kcal/mole (13a) EA ~ 40 kcal/mole (13b) 20 kcal/mole (14a) E A ~ 40 kcal/mole (14b) E A ~ 20 kcal /mole (15a) EA- 38 kcal/mole (15b) E A " 40 koal/mo le (16) EA = : 0 koal/mole (17a) EA = : 0 kcal/mole (17b) EA *" : 0 koal/mole (18a) EA = = 4 kcal/mole (18b) 238 H -^ CH„ H *CH 3 7 H H >0H4 * Hg CgH4 (Ik C3 H8 Ho EA EA E 'A E A -EA ~ EA = (19) 0 kcal/mole (20a) 0 koal/mole (20b) 0 kcal/mole (21a) (21b) 0 kcal/mole (22a) (22b) 0 kcal/mole (23a) 0 kcal/mole (23b) The: mechanism of pyrolysis of 3-methyl-pentane The o v e r a l l a c t i v a t i o n energy for the pyrolysis of 3-methyl-pentane i s approximately equal to the value for 2-methyl-pentane. The order of reaction remains almost unaltered with changing pressure. It has a value s l i g h t l y higher than unity. This isomer i s adsorbed somewhat more than i s 2-methyl-pentane and therefore heterogeneous i n i t i a t i o n may be s t i l l more important. Consequently the net reduction i n rate due to packing w i l l be s t i l l less. I n i t i a t i o n reactions for the hydrocarbon decompositions have been considered e a r l i e r . The estimated energies of acti v a t i o n are as follows: CH3CHgCHCH2CH3 »CH3-^CHgCHgCHCHg' D(C-C)r72 kcal/mole? (la) CH, CHr. CgH5^CH3CHCHgCH3 D(C-C) -79 kcal/mole (lb) CH3+ CH3CH2CHCH2CH3 D(C-C) -79kcal/mole (lc) Through the o v e r a l l mechanism CH3 r a d i c a l s are formed from C-j-Cg 239 and Cg-Cg ruptures (Cg i s the carbon of the methyl side group). Eth y l radicals are formed i n C2-Cg rupture reactions. Therefore the r a t i o of the products can be related to the r e l a t i v e p r o b a b i l i t i e s for various C-C ruptures as follows: CH4 ^ 2P(C-,-0P,)-+ 3?(C3-0fi) _ K 02=6 *° ^ 4 2P(C2-Cg) where K i s found to have an average value of approximately 1/ Therefore 2P(C 1-C 2)+ P(C 3-C 6)- 2P(C 2-Cg)r 0 Since 2 P ( C r C 2 ) + P(C 3-C 6) + 2P(C2-Cg) = 1 we obtain from the above equations P(C 2-Cg) - 0.25 and 2P(C 1-C 2)+ £ ( Cg-C 6) - 0.5 Assigning to P(Cg-Cg) the extreme value of 0 we obtain P(C 1-C 2)< 0.25 Therefore reaction (lb) appears to participate more than reaction (la) i n spite of the higher a c t i v a t i o n energy required. The methyl r a d i c a l w i l l promote the propagation by H-abstract-ion reactions CHg -rCH3CH2CHCH2CHg >CH4 -+ CHgCHgCHCHgCHg- (2a) 1 I C H 3 CHg — * CH4 +. CHgCHgCHCHCHg ( 2b) CHg 240 CH 4^CH 3CH 2CCH 2CH 3 (2c) > CH4-r-CH3CH2CHCH2CH3 (2d) CH2« Processes (2a) and (2d) are comparable with those considered for 2-methyl-pentane. Both w i l l have an ac t i v a t i o n energy of approx-imately 8 kcal/mole. The ac t i v a t i o n energy for (2d) i s about 6 kcal/mole (secondary Er) while E A for reaction (2c) i s 4-5 k c a l / mole(tertiary H)/ Propagation w i l l also be promoted by C2H5 r a d i c a l s . The re-quired energy w i l l be about 3 kcal/mole higher than the value required for H-abstraction by CHg. C2H5-r-CH3CH2CHCH2CH3 —» CgHg - CHgCHgCHCHgCHg' (3a) CH3 CH3 E A - llkcal/mole *C 2Hg - CH3 CH2CHCHCH3 (3b) I CH3 s i . - 9 koal/mole * C2Hg + CH3CH2CCH2CH3 (3c) CH3 2 A - 7 kcal/mole * CgHg-CH3CH2CHCH3 (3d) \ CH2« E A - 1 1 kcal/mole The large r a d i c a l s formed w i l l undergo spontaneous decom-position to form small r a d i c a l s by a large variety of reactions. CH3CH2CHCH2CH2. > CH •+• pentene-2 (4a) CH3 -> CH3-CH2:CCH2CH3 (4b) t CH3 241 •*C 2H 5^CH 3CH:CHCH 2 CgHg- CH 2:CHCH 2CH 3 (4c) (4a) Pentene-2 was not found i n the products and therefore reaction (4a) i s eliminated. Reaction (4d).however,is of minor importance since very small amounts of butene-1 are formed. Other reactions included are: CHgCHgCHCHCHg CH 3 rCKg-v CHg-.CCHgCHg ^ CgHg -»-CHg: CHCHgCH3 -> C^Hc; GE.n CH: CHCH^ & O o g CH 3 CHgCCH 2CH 3 CH„ C H 3 ^ CH g:CCHgCH 3 CH„ C 2 H 5 - v C H 3 C H : C H C H 3 (5a) (5b) (5c) (6a) (6b) A further p o s s i b i l i t y would be CH 2. CH 3CH 2CHCH 2CH 3 3 • CH g:CCHgCH 3 —•> CgHg -v CHgj CHCHgCH3 (7a) (7b) Smaller r a d i c a l s w i l l decompose further by reactions of the types > C H g - v C H g : C C H g ( 8 a ) CH 3CH 2CHCH 2' CH. '3 CH, C 2 H 5 - C g H 6 CHg CHgCH: CHCHg (8b) (8c) (Reaction 8a accounts for the appearance of the small amounts of i - C 4 H 8 ) 342 CH 3 CH 2 CHGH 3 » C H 3 - C 3 H 6 E A = 33 koal/mole (118) (9a) ^ G2 H 5"" G2 H4 E A ~ 2 3 koal/mole (118) (9b) CH 3 CH 2 CHCH 2 CH 3 * CH 3 -»- CH 3 CH: CHCH3 (10a) * 8 2 H 5 - C 3 H 6 (10b) Small amounts of H2 can be accounted for by reactions such as CH 3 CHCH 2 CH 3 H +CH 3 GH:CHCH 3 (9c) CgHg » H •+ @ 2 H 4 ( l l ) These atoms w i l l form Hg by reactions of the type H-r- RH * Hg* R The order of reaction i a s l i g h t l y higher than unity, namely 1.3 . The termination w i l l therefore be of the types , $Y and PPM at lower pressures and P>W , , M and PKM at higher pressures. At higher S/V r a t i o s pPM termination w i l l be favored. Again we can consider C 2 H 5 to be a Y r a d i c a l and CH 3 and H to be |3 r a d i c a l s . Termination may therefore take place as follows at lower pressure and i n the unpacked vessel. e>P> C H 3 - r C H 3 — — > C 2 H 6 (13a) - ^ - > C 2 H 6 U3b) C H 3 ^ - C 2 H 5 l h _ C 3 H 8 (13a) — C H 4 - CgH 4 (13b) C H g - H ^ C H 4 (14a) . -e*i OH 4 d«) 243 C2 V H °2H6 " H2-°2 H4 Hg » H< (15a) (15b) (16a) (16b) The o v e r a l l mechanism for 3-methyl-pentane The o v e r a l l mechanism i s again very complex due to the large number of alternative' reactions which must be included. Energies of a c t i v a t i o n are given for the various reaction steps where es-timates can be made. I n i t i a t i o n reactions CH3CHgCHCH2CH3 CH^ -+- CHgCHpCHCHg* D( C-C) ~ 72-75 kcal/mole 3 i 2 3 heter. 3 3 2 i CH3 CHg (la) .hom. c g CH^CH.CHCH, ])(C-C) - 79 kcal/mole heter. <2 o c - c o (lb) h ° m ' > CH CH CH CH CH CH D(C-C) - 79 kcal/mole heter. ^ o <c do (lc) Propagation reactions CHg-* CH3CH2CHCH2CH3 —*CH 4 t- CHgCHgCHCHgCHg* CH3 CHg CH 4~ CHgCHgCHfiHOHg CHg -> CH4 -+ CHgCHgftCHgGHg I CH3 E^— 8 kcal/mole (2a) E A - 6 kcal/mole (2b) E A~4-5 kcal/mole (2c) 244 OH. -* CH„CH CHCH CH E A - 8 koal/mole CH, (2d) C 2 H 5 ^ C H 3 C H 2 f C H 2 C H 3 * °2H6 * C H 3 G H 2 ° H C H 2 C V CH. C > H3 ~* CgHg •*- CHg OHgCH&HCHg CH, C 2H 6- CHgCHgCCHgCHg CH, »CgH 6^ CH3CHgCHCHgCH3 CHg-Radioal deoompo9itions CHgCHgCHCHgCHg. •CgHg CHgCH: CHCHg CHg + OH,^ CH0:CCHftCH g - V - g . - w - g ^ g CHg C2Hg —CHgCHgCH :CH2 Uilg CH«j CHdlHCHg CH, - CH 3-CH 2:CCH 2CH 3 CH 3 * CgH5-*-CH2:CHCHgCH3 CgH5-v-CHgCH: CHCHg CH3CHgCCHgCH3 -» CHg -CH2:CHCH2CHg CHg CHgCHgCHCHgCHg CH2. -> CgHg —CH3CH:CHCH3 ->CH3- CH2:CCHgCH3 CHg E A =11 kcal/mole (3a) E A = 9 kcal/mole (3D) E A — 7 kcal/mole (3c) EA=. 11 kcal/mole (3d) E A—26 kcal/mole (4a) (4b) (4c) (5a) (5b) (5o) (6a) (6b) (7a) CgHg^-CHg^CHgCHg (7b) 245 CH3CH2CHCH2. > CHg _0!H2:C'CHg (8a) CH3 CH3 »>• CgHg^-CgHg E A - 2 3 koal/mole (8°) CHgCHCHgCHg * CH 3-CgH 6 E A - 2 3 koal/mole (9a) CCH * C.H E A - 2 3 kcal/mole (9b) * H + CHgCH:CHCHg E^r 39 kcal/mole (9c) CHgCH2CHCH2CH3 » CHgCHg CH: CHCHg (10a) *C 2H 5 -CgH6 (10b) Termination reactions CHg-*- CH3 C:2H6 E A ~ ° kcal/mole -(Ha) C 2H 6 S A ^ 0 kcal/mole ( l i b ) CHg C2Hg -lb_> CgHg E A- 0 koal/mole (12a) P k » CH^CgHg E A - 4 kcal/mole (12b) CgH8 E A - 0 kcal/mole (12e) CH 4- C2Hg E A z 4 kcal/mole (124) CgHg-r-CgHg P f i > C2H4-^C2Hg E A r 4 . 8 kcal/mole (13a) • ^ U CgH4-i- CgHg E A r 4 . 8 kcal/mole (13b) H—CHg CH4 E A z 0 kcal/mole (14a) CH4 E A= 0 kcal/mole (14b) H-^CgHg CgHg E A 3 0 kcal/mole (15a) . CgHg S A = ° kcal/mole (15b) Hv H J^ U Hg E A~ 0 kcal/mole (16a) »H 2 E A - 0 kcal/mole (16b) The mechanisms of the pyrolysis of 2,2-aimethyl-butane (neohexane) The pyrolysis of neohexane appears to be analogous to that 246 of 2-methyl-pentane and of 3-methyl-pentane. The o v e r a l l energy of aotivation i a lower than that observed for the other iaomera while the change i n order between higher and lower pressures i s more marked. Neohexane i s adsorbed on quartz much more than any of the other isomeric hexanes and therefore packing would be ex-pected to favor heterogeneous i n i t i a t i o n of pyrolysis more than i n the other isomers. The net reduction i n rate due to packing i s less than for a l l other isomers. The i n i t i a t i o n process has been considered previously. It i s assumed to involve the following reactions. CH3 CH3 i i CH3C-CH2C|f3 ».CH3C*0H2-* CHg D(C-C)-80 kcal/mole CH3 CH3 (la) CH3 i •*CH3C» +C2R~5 D(C-C)-73 kcal/mole CH3 (lb) *CH3C*CH2CH3 - CH3 D(C-C)= 75 kcal/mole CH3 (lo) Propagation by methyl r a d i c a l s w i l l occur by the reactions CH3 CH3 i I CH3 «- CH3C-CH2CH3 — * CH4 - CH3C-CH2CH2« (2a) CH3 CH, CH3 <c-r CH 4 *CH3C-CHCH3 (2b) CH3 847 CH3 i >CH4 -v CH3C*-CH2CHg (So) CH2. For reactions (2a) and (2c) the a c t i v a t i o n energy i s comparable to the energy required for a primary H-abstraction by CH g.in n-hexane. Therefore i t i s about 8 kcal/mole. For reaction (2b) the ac t i v a t i o n energy w i l l be about 6 kcal/mole. The propagation w i l l be also carried out by CgH^radicals. CHg CHg C 2H 5 ^ CHgC-CHgCHg — C 2 H 6 ^-CHgC-CHgCHg. (3a) CHg CH, CHg C2H6-CHgC-CHCHg (3b) CHg CHg — » C2H6-CHgC*-CH2CHg (3o) CH2. For the above processes the energy of aotivation w i l l be as follows ( E A ) S a - 11 kcal/mole ^sA^3b - 9 kcal/mole ( E A ) g Q - 11 kcal/mole As we have seen, CHg r a d i c a l s are formed by C]_-C2 and by Cg-C4 rupture reactions while CgHg comes from a C2~Cg rupture. Therefore CH4 ^ gBtCn-Og)* P(Cg-C 4) °2 H4- , , C?6 P(C 2-Cg) 248 An average value of 0.9 can be used for t h i s r a t i o (Tables 11-.1Z). Thus 3P(Ci-C 2)+ P(Cg-C 4)-0.9P(C 2-Cg) •= 0 Since 3P(C1-Cg)-»P(C2-C3) • P(C 2-C 4) - 1 we obtain P(C 2-C 2) - 0.52 and ZtlCi-Cg) + P(C g-C 4) . 0.48=> PtC^Og) < 0.16 The propagation w i l l be completed with further decomposition of the large radicals formed from the parent molecule. CH2 i CH2C-CHCHg » CHg T- CH3C:CHCHg (4a) CHg CHg > C 2H g-CH 2:CCH g (4b) CHg * CHg - CH2:CCH2CH3 (4o) CHg Reaction (4c) i s ruled out because 2-methyl-butene-l i s not found i n the reaction products. The other hexyl r a d i c a l s w i l l decompose as follows: CHg i CHgC- CH2CH2» >CHg t- CHgC: CHCHg (5a) I i C % CH3 C 2H 5 + CH2:CCH3 (5b) » CHg Reaction (5b) participates to a small extent. 249 CHg*C*CH2CH3 .—, CHg ^  CHg:C-CHgCHg (6a) CHg» CHg -vCgH5*-CH2:C*CH3 (6b) CHg Reaction (6a) i s ruled out by the a n a l y t i c a l r e s u l t s . The smaller radicals w i l l undergo pyrolysis as follows: CHg CHg-C-CHg- » CHg -r CHg J C»CHg (7) CHg CHg CHgCCHgCHg * CHg -r CHg: C~CHg (8a) I CHg CH£ and — * C g H ^ C g H 6 (8b) CHg CHg*C' —> H >CH 2:C»CH 3 S A - 40 koal/mole (9a) CHg CHg — * CHg CgH6 (9b) Reaction (8b) and (9b) appear to be the major source of CgHg production while reaction (8a) should participate s i g n i f i c a n t l y i n spite of i t s high endothermioity. The amount of H g produced i s too high to be formed solely from termination processes. It probably comes larg e l y from reactions l i k e H.»- RH — ^ H 2 * R As we have seen P(Cg-Cg) ~0.52 which means that the r a d i c a l CHot CHgC< C Hg i s formed i n high concentration i n the decomposition. 250 Thus reaction (9a) beoomes s t i l l more important. The observe! change i n order of decomposition from 1.5 to 1.0 as the pressure i s increased indicates that at low pressures termination w i l l preoeed through recombinations of the type Is!3 , Pk , and while at high pressures the following types of termination w i l l occur: kV* andPp^M .(Table 48). Hydrogen atoms and CHg are considered as P r a d i c a l s while CgHg i s considered as a k - r a d i c a l . The pyrolysis of the ethyl r a d i c a l CgHg CgH4-H H E A — 40 kcal/mole (118) should be important because of the high concentration of CgHg i n the system r e s u l t i n g from a Cg-Cg rupture. Thus the termination w i l l be as follows at low pressures CHg •+• CHg H - CHg K - C 2 H 5 H - H ^ C 2 H 6 (10a) - i ^ C g H g (10b) -iiKCgH 8 (11a) ^ C H 4 - C g H 6 ( l i b ) ^ CH 4 (12a) J » C H 4 (12b) ^ CgH6 (13a) ^ H 2 + °2B4 (13b) (14a) ^>Hg (14b) The o v e r a l l mechanism for neohexane pyrolysis I n i t i a t i o n reaction 251 CHg CHg CHgCCHgCHg J^I^UCHgCCHg* +-CHg D(C-O) ^ 80 kcal/mole (la) heter. CHg CHg CHg hom. h e t Q r ^ C H g C . - C 2 H 5 D(C-C) 373 kcal/mole (lb) CHg h o m - ->CH2CCH2CHg ^-CHg D(C-C)z75 kcal/mole (lc) heter. > CHg Propagation reactions CHg CHg CHg^CHg C CHgCHg ^ CH 4-»-CHgC»CH 2CH 2» E A — 8 kcal/mole (2a) CHg CHg CHg \ *3 CH, -,CH4^ CH C-CHCHg E A= 6 kcal/mole (2b) 3 CH \ 3 * CH4*- CH3C~CH2CHg E A _ 8 kcal/mole (2c) CH 2. CHg CHg C 2H 5^ CH 2C»CH 2CHg ^>C2H6->-CHgOCH2CH2. E A ~ 11 koal/mole (3a) CHg CHg CHg ^CgHg ^  CHgC-CH#-CHg S A _ 9 koal/mole (3b) CHg CHg C2H6i-CHgC*CH2CHg E A = 11 kcal/mole (3c) CH 2. 252 Radical decompositions CH 3 CCHCH 3 > C H 3 ^ C H g C H ^ C C H g (4a) C H 3 G H 3 — T C 2 H 5 - * . C H 2 T C C H 3 { 4 B ) C H 3 CH, CH 30CH 2GH 2*—=>CHg -t- CHgCH - CCHg ( 5 a ) GH„ CH 3 3 — * CpH 5 -h C H 2 - CCH 3 (5b) C H 3 CH, CH 3 CCH 2 CH g > c 2 H 5 ~\~ C H 2 ^ CCH 3 (6) °V C H 3 C H 3 C H 3 C C H 2 ' ^ CHg -»- C H g ^ CCH g (7) i i CHg CHg CHgCCHgCHg *CH 3 + 0 H 2 ^ CCHg (8a) C H 3 C H 3 — . C g H g - v C g H g (8b) CH 3 CHgC* —> H -»• C H 2 r CGHg E A ^ 40 kcal/mole (9a) CH, CHg 253 Termination reactions - C H 3 -- J U L — , °2H6 S A = 0 kcal/mole (10a) °2H6 *A = 0 kcal/mole (10b) CH 3 + 0 2 H 5 p k y C 3 H 8 0 kcal/mole ( l l a ) °V °2H6 E A ~ 4 kcal/mole ( l i b ) H - 0H 3 E A = 0 kcal/mole (12a) C H 4 E A = e kcal/mole (12b) H •+• ^ > °2H6 H 2-C 2H 4 EA ~ 0 kcal/mo le (13a) (13b) H H H 2 EA ~ 0 kcal/mole (14a) — => H 2 EA - O kcal/mole (14b) 5. The mechanisms of the pyrolysis of 2,3-dimethyl-butane (diisopropy] Diisopropyl appears to be the most "active" as far as decom-position i s concerned. It decomposes almost twice as fast as n-hexane. As we have seen before, i t s low o v e r a l l a c t i v a t i o n energy i s oonsistant with the r a p i d i t y of decomposition and the low value of D(C-C) for the primary p y r o l y s i s . Heterogeneous i n i t i a t i o n may be as important as i t was i n neohexane. The o v e r a l l order decreases from a value of 3/2 at low pressures to unity at high pressures. The i n i t i a t i o n processes have already been discussed, and the f o l -lowing reactions are proposed. CH3CHCHCH3 > CH3 CH3CHCH~CH3 3)(C-C) z 70 kcal/mole (la) CH3 CH3 CH3 » 2 i-C 3H 7 D(C-C)r 66.5 kcal/mole (lb) 254 Methyl radicals are formed mainly through a C^-Cg rupture and p a r t i a l l y through a Cg-Cg rupture followed by i—CgH 7 — * CHg— GgH^ But since CgH4 appears only i n traces we can ignore the second source of CHg r a d i c a l s . Therefore CH 4 ' _ 4P(Ci-C 2) CgHg" _ ^ Cg-Cg) The r a t i o has an average value of 6 (Table - 15). Thus SP(C 1-C 2) 3P(C 2-C g) 4P(C 1-C 2)- ?(C 2-C 3) - 1 and since we obtain 7P(C2-Cg) - 1 r£} P(C 2-Cg) = 0.143 and p ( c l - c 2 ^ ~ °* 2 1 Hydrogen i s a major product. It i s assumed to be formed through the pyrolysis of CgH7 r a d i c a l s . i-CgH 7 — * CgH6* H Thus we obtain CE 4 4P(C 1-C 2) C 3 H 8 * H 2 " P ^ C 2 - V because C2-Cg rupture gives formation to either CgHg or H 2. The r a t i o has an average value of 1. Thus P(C 2-C 3) -0.5 and P(C^-Cg) 0.125 Since for a C2-Cg rupture an a c t i v a t i o n energy of 66.5 kcal/mole i s required i n comparison with 70 kcal/mole for a C^-Cg rupture, Cg-Cg rupture w i l l predominate and therefore the second set of p r o b a b i l i t i e s would be closer to the r e a l values. 255 The large yi e l d s of methane and hydrogen indicate that CHg and H are the major chain c a r r i e r s . CH3 •+ CHgCHCHCHg * CH4 -vCHgCHCH«CH?/ (2a) CH3CH3 CHgCHg »  CH4 + CHc, CH5 -OH, ( 2h) CHgCHg Reaction (2a) requires an energy of ac t i v a t i o n of about 8 kcal/mole. A value of 6 kcal/mole i s probable for reaction (2b). Provided that 12 primary and 2 secondary hydrogens are a v a i l -able an average value i n a c t i v a t i o n energy, for H-abstraction by CHg, would be about 7.7 kcal/mole. This value i s i n good agreement • \vith a value of 7.8 kcal/mole found experimentally (117). Hydro-gen atoms w i l l compete with CHg r a d i c a l s i n propagating the reaction H •*- CHgCHCHCHg * Hg — CHgCHCHKJHg' (2a) CHgCHg CHgCHg > Ho ^ CHgCHCHCHg (3b) I \ CHgCHg The energy required for these processes would be 2-3 kcal/mole less than the energy required for the corresponding H-abstraction by CHg, since D(H-H).— 103.5 kcal/mole and D(CHg-H) - 101 k c a l / mole. Analogous reactions were examined by Schiff and Steacie (124) using n-pentane and both H and D atoms. On - the. assumption that the s t e r i c factor i s 0.1 , a c t i v a t i o n energies of 8.5 and 7.8 kcal/mole were found respectively. A few experiments were also done on the reactions of n-hexane with H atoms (124). An a c t i v a t i o n 256 energy of 9 kcal/mole was found on the assumption that the s t e r i c factor i s 0.1 . The isopropyl r a d i o a l w i l l also participate i n the propagation i-C 3H 7 - CHgCHCHCHg CgH8 -v CHgCHCH*CH2* (4a) / N / X CHgCHg CHgCHg CgH8 - CH3CH*C-CHg (4h) CH2 CHg Assuming that D(CH3-H) —101 kcal/mole and D(i-C 3H 7-H) — 89kcal/ mole, a difference of about 10 - 11 kcal/mole i s expected to be found between reactions (4a) and (2a) as well as between (2b) and (4b). Therefore the energy of a c t i v a t i o n for H-abstraction by i-C H w i l l be i n the range of 16 - 18 kcal/mole. O t The r a d i c a l s produced w i l l decompose spontaneously, propagat-ing the reaction further CHgCHCHCHg —>CHg CHgCH:C-CHg (5a) CHgCHg CH3 >i-CgH 7^CgH 5 (5b) CH3CH(>CHg » CHg -v- CHgCH:C—CHg (6a) CHgCH3 CH3 >i-CgH7 ^CgH6 (6b) The propagation w i l l proceed further by pyrolysis of smaller r a d i c a l s v CH3CHCHGH3 >CH3 -CH2:C'CH3 (7a) CHg >C 2H 5-CgH 6 (7b) £57 i-C 3H 7 * H ^ C 3H 6 (8) The fact that the order changes from 3/2 to unity as pressure increases, suggests that at lower pressures termination w i l l take place through recombination of the types , $\*, , and ."PPM, while at higher pressures, recombinations of the type and pMvf w i l l occur (Table 48). Again CH3 and H are considered as P ra d i c a l s and i-C 3H7 as a - r a d i c a l . Therefore the termination at low pres-sures w i l l be as follows: GErf CH3 (9 a) 0 2H 6 (9b) CHg ^ i'-CgH7 - i t * CH4 + CgHg (10) - P K , C 3H 8 (11a) Hg^CgEg ( l i b ) H-tCH3 -^-^ C H 4 (X2a) • ^ C H 4 (12b) PI3 , * H>H -I-U H 2 (13a) ^ H g (13b) The o v e r a l l mechanisms for diisopropyl pyrolysis I n i t i a t i o n reactions CH3CHCHGH3 h^ m^,\ CH3-, CH3CHCHCH3 D(C-C) = 70 kcal/mole (la) CH3CH3 0H 3 2i-C,H„ D(C-C) Z-66.5 kcal/mole (lb) heter. c ' Propagation reactions CHg ^  CHgCHCHCHg —=> CH4 CHgCHCHCHg* E A - 8 kcal/mole (2a) CHgCHg CHgCHg 258 H CH,CHCHCH, CHgCHg ->>CH4 CHgCHC-CHg B A — 6 kcal/mole (2b) / \ CHgCHg . H 2 ^-CHgCHCHCHg' E A — 6 kcal/mole (3a) CHgCHg ->H -vCH CHC-CHg 2 A - 4 kcal/mole (3b) CHg CHg i-CgH 7 - CHgCHCHCHg > CgHg ^  CHgCHCHCHg' E A = 18 kcal/mole CHsCHg CHgCHg (4a) >CgH8 4- CHgCHC-CHg E, -16 kcal/mole CHgCHg Radioal decompoaitions CH,CHCHCH9« CHgCHg CH_CHCCH„ V \ * CHg CHg CHgCHCHCHg \ CHg CHg-r CHgCH:C-CHg CH, -,i-CgH 7*C 3H 6 CHg - CHgCH:C-CHg CHg ->i-C 2H 7 ^ -CgH| -> CHg - CHg :C-CHg CH, i-CgH ? Termination reactions "*°2H5 "* °3H6 * H CgH6 C%^CHg CgH 6 C 2 H6 CHg -1- i - C g^ 7 P K * CH 4-CgH 6 E A ^ 40 kcal/mole E A— 0 kcal/mole EA — 0 kcal/mole EA - 4 kcal/mole (4b) (5a) (5b) (6a.) (6b) (7a) (7b) (8) (9a) (9b) (10) 259 H -H.-C3H7 i * ^ > C gH 8 E A = 0 kcal/mole (11a) H 2 + CgH6 ( l i b ) H -* CHg W > CH4 E A - 0 kcal/mole (12a) — ^ CH4 E A - 0 kcal/mole (12b) H-+H H 2 E A - 0 kcal/mole (13a) ^/?/y> Hg E A - 0 kcal/mole (13b) Although t h i s o v e r a l l mechanism involves fewer steps than those proposed for the other isomers, i t i s s t i l l much too complex to allow a quantitative treatment based on a steady state c a l c u l a t i o n of r a d i c a l concentrations, to be done. The i n h i b i t e d decomposition N i t r i c oxide was used as the i n h i b i t o r i n a l l cases. I t was preferred to propylene because i t i s a much more potent i n h i b i t o r and secondly because CgHg i s a common pyrolysis product of the hexanes. Previous workers found i d e n t i c a l products for the normal and f u l l y i n h i b i t e d decompositions. However from t h e i r own r e s u l t s i t i s obvious that the product d i s t r i b u t i o n s are affected by NO. In the present study a s i g n i f i c a n t effect of NO on the product d i s -t r i b u t i o n s has been established. The amount of NO required for maximum i n h i b i t i o n appears to be more a function of the surface area of the reaction vessel than of the p a r t i a l pressure of hydrocarbon. This suggests that NO has an effect on heterogeneous processes as w e l l as on homo-260 geneous processes. The action of WO has been mainly attributed by other workers to the capture of chain-propagating radicals such as CHg and C2H5. Most of the t h e o r e t i c a l approaches to the i n h i -b i t i o n problem are based on t h i s assumption. However, some very int e r e s t i n g problems a r i s e , using t h i s hypothesis. For many com-pounds, p a r t i c u l a r l y the n-paraffins, a very small amount of i n h i b i t o r , approximately 2-5$, i s s u f f i c i e n t to suppress the reaction almost completely. The r e l a t i v e p r o b a b i l i t i e s of capture of a CHg r a d i c a l by NO or by the hydrocarbon when the l a t t e r i s i n large excess are of significance. Even allowing for a s i g n i f -icant difference i n a c t i v a t i o n energy between the two reactions, both processes should occur with reasonable e f f i c i e n c y . The extent of i n h i b i t i o n should therefore be proportional to the percentage of the NO i n the system. On t h i s basis i t i s d i f f i c u l t to under-stand the reduction i n rate i s so marked for small amounts of NO i n the paraffins. The effects of the surface are contradictory. Other workers have considered a reduction of about 20$ i n rate on packing to be i n s i g n i f i c a n t (46-49). On the basis of the large reductions i n rate observed i n packed vessels i n the present study i t seems improbable that i n h i b i t i n g e f f ects can a l l be accounted for on the basis of reactions of a l k y l r a dicals with NO i n the gas phase only. As we have seen (Chapter III) the r e l a t i v e adsorption of various hexanes with respect to NO i s given by the following sequence: 261 n-hexane ^ 2-methyl-pentane ^ 3-methyl-pentane ~ NO < diiaopropyl <: neohexane I f surface i n i t i a t i o n i s important i n a l l pyrolyses one should expeot less effect of surfaces on n-hexane pyrolysis than i n neohexane. Further, i f i n h i b i t i o n i s due even p a r t i a l l y to the adsorption of NO on i n i t i a t i n g s i t e s one would expeot a more abrupt reduction i n the rate of the i n h i b i t e d reaction i n n-hexane than i n neohexane. A competition for active s i t e s w i l l be established for the hydrocarbon-NO mixture. In n-hexane the hydro-carbon w i l l be mainly replaced by NO, and therefore heterogeneous i n i t i a t i o n w i l l be suppressed almost completely. This could r e s u l t i n a sharp reduction i n rate to a small value. The reduction w i l l be less marked i n neohexane. An adsorption equilibrium w i l l be established and therefore heterogeneous i n i t i a t i o n w i l l continue even i n the f u l l y - i n h i b i t e d decomposition, so that the l i m i t i n g rate w i l l have a much higher value for the branched isomer than for n-hexane. The behaviour of the other isomeric hexanes i s ex-pected to l i e between these two extremes. I f i n i t i a t i o n i s purely homogeneous one could argue that the l i m i t i n g p a r t i a l pressure of NO would be a function of the p a r t i a l pressure of hydrocarbon. The l i m i t i n g NO pressure was found to be e s s e n t i a l l y independent of hydrocarbon pressure. After the reaction s t a r t s , either homogeneously or hetero-geneously, NO w i l l c e r t a i n l y react with r a d i c a l s i n the gas phase. After the i n i t i a l retardation, the rate increases again as i n d i -cated by the sigmoidal &£-time ourves. This may be beoause NO 862 does not d r a s t i c a l l y suppress the propagation except i n the i n i t i a l period. Furthermore the p a r t i c i p a t i o n of NO i n gaseous reactions may reduce the surface concentration of NO and thus increase het-erogeneous i n i t i a t i o n . For small amounts of NO (up to 5$) no consumption was found. The oxide may he regenerated from the unstable nitroso compounds formed i n reactions such as CH 3-N0 >• CHgNO For the above reaotion i t was found (185) that E A — 5.7 kcal/mole for P - 1 or E A - 0 kcal/mole for P r l . 5 x 10" 4 This compound i s unstable (98,100) and w i l l decompose to various products auoh as HCN, HgO, NH3, CO, N 2, C0 2, CHgCN etc (70). Analogous reactions can take place between CgHg and NO lead-ing either to C 2H 4 and HNO or to CgHgNO. The presence of CgHgNO has not been observed i n the present study. I t s existence was suspected i n the i n h i b i t e d decomposition of isopentane (98). The existence of HNO has been v e r i f i e d by flash-photolysis (186). From infrared chemiluminesence of HNO a lower l i m i t for D(H-NO) of 56 kcal/mole has been obtained (187). For higher pressures of NO, consumption ranging from 5 to 80$ was observed i n the present study. This can be attributed to reactions of NO with more complex r a d i c a l s leading to more stable nitroso-compounds which w i l l tend also to decompose. It has been assumed by l a i d l e r et a l (46-49) that the surface 263 acta l i k e NO as an i n h i b i t o r , an assumption whioh may be v a l i d when the net ef f e c t of surface i s a reduction i n the rate. However surfaces can gave an effect on both i n i t i a t i o n and termination. I f the S/V r a t i o i s increased the rate w i l l be decreased or i n -creased, depending upon whether i n i t i a t i o n or termination i s favored by the surfaoe. One can generalize for a l l of the isomeric hexanes, by stat-ing that i n h i b i t i o n may take place i n both phases: by r a d i c a l capture i n the gas phase and by suppression of i n i t i a t i o n on the surface of the vessel. N i t r i c oxide changes the product d i s t r i b u t -ion s i g n i f i c a n t l y , a f f e c t i n g e s p e c i a l l y those products which come from competing processes. For example, i t reduces the formation of CgHg and CgHg while i t increases the formation of CgH4 and of CgHg to a s l i g h t extent. 1) The mechanism of the i n h i b i t e d decomposition of n-hexane N i t r i c oxide, i t s e l f a r a d i c a l , i s ohemisorbed on the quartz surface. C o l l i s i o n s between NO and a l k y l r a dicals such as CHg and C2H5 are assumed to be more probable on the w a l l than i n the gas phase. The formation of CHgNO and CgHgNO w i l l lead to the desorp-t l o n of NO. Nitroso methane i s unstable and i t s decomposition i s accelerated by NO (1E8). The mechanism proposed for the i n h i b i t e d reaction i s similar to that discussed for the uninhibited reac-t i o n , except i t i s assumed for n-hexane that heterogeneous i n i t -i ations are suppressed completely by NO and that a l l termination reactions are affected by NO. I f we assume that the reaction chain i s short and that the l i m i t i n g i n i t i a l rate, which for n-hexane i s 264 very small, i s dependent on the rate of homogeneous i n i t i a t i o n only, we can write t R j ) u n i n h i b . _ <* ^ Ri^hom. + ^ R i ^ h e t , ( i ) ( R i ) i n h i b . ( Ri)hom. where <*• i s a faotor ind i c a t i n g the extant to which the homogeneous reaction occurs. The observed value of t h i s r a t i o i s approximately 9 (Fig. 42). Thus (Ri)het. — * ^ Ri^hom. • Expressing the rate constants for homogeneous and heterogeneous i n i t i a t i o n s as W . == A e - % e t . ^ T and assuming similar values for A we obtain in \ -Ehom/RT . -Ehet/ R T . Q  < Ri)uninhib _ « A e ' »n + A e »n\ 9 ( R i ) i n h i b . A Q-Ehom/ RT # n where n i s the concentration of n-hexane i n the gas phase i n molecules/cm , and n 1 i s the surface concentration of adsorbed molecules i n molecules/cm 2. Therefore Ehom-lhet <* = 9 - e . n^ ( 2 ) Data on the heats of adsorption of hydrocarbons on quartz are not available. The heat of adsorption of n-hexane on ferrous and f e r r i c oxide has been found to be about 15 kcal/mole (129). A similar value i s assumed for quartz. This value can be assigned to the energy difference Ehom~ Ehet* T J n a e r t h e conditions of the present study n = 4.5 x 1 0 1 7 molec/cm3. Assuming a c o l l i s i o n diameter for £65 n-hexane (130) of 4 x 10~8om and that the surface i s covered completely, the surfaoe concentration w i l l have a maximum value of 1 0 " molecules/cm 2. The S/V r a t i o i s about unity and thus for complete coverage of the surface we have _ 2 x 10" 3 n ~ However, the high temperature and the disorder i n adsorption w i l l probably re s u l t i n inoomplete coverage. Using the approximate values for % o m " " E h e t a n ^ f o r n'/n we calculate from equation (2) that oc > 0. I f ot - o then n'/n - 10" 4. Thus i f 10$ of the surfaoe i s covered the contribution of homogeneous reaction to the surface w i l l be eliminated. However, a small l i m i t i n g rate e x i s t s which indicates a s l i g h t p a r t i c i p a t i o n of homogeneous i n i t i a t i o n . Under these circumstances i t i s expected that the active centers w i l l constitute 10$ of the surface and that the rate of heterogeneous i n i t i a t i o n w i l l be almost 9 times that of homogeneous i n i t i a t i o n . The contention that heterogeneous i n i t i a t i o n i s completely suppressed by NO i s supported by the observation that the amounts of products vary l i n e a r l y with time i n the f u l l y i n h i b i t e d decom-posi t i o n of n-hexane , indi c a t i n g that s e l f - i n h i b i t i o n does not occur, i n contrast with the uninhibited decompositions. N i t r i c oxide appears to have no effect upon the main mode of decomposition since i t does not affect the amounts of CgHg and l-C4Hg. For very high amounts of NO, acceleration has been observed by other workers, presumably because of the reaction RH-NO »R+HN0 266 The p a r t i c i p a t i o n of suoh processes appears to he i n s i g n i f i c a n t i n the present study because of the low NO pressure used. The effect of n i t r i c oxide on termination reactions i s assumed to be of major importance. As we have seen before the two main radicals which propagate the decomposition are CHg and CgH5 while the p a r t i c i p a t i o n of n-CgH7 and H i s considered to be i n s i g n i f i -cant. We have assumed that chemisorbed NO can capture these r a d i -cals more e a s i l y than NO i n the gas phase. The reactions concerned SI1© CHgNO » CHgNO (1) CgHg-*- NO »C 2H 5NO (2a) =>C2H4+HN0 (2b) These compounds may have very small l i f e t i m e s and w i l l decompose or react with other radicals either heterogeneously or homogene-ously CHgNO-CHg >C2H5-rN0 (3) CHgNO- CgHg >CH4 ->C2H4* NO (4) CgHgNO + CHg > C 2H 4 -rCH4 + NO (5) C 2H gN0i- CgHg » C 2H 4+ CgHg-NO (6) The HNO molecule can also react with CHg and CgHg HNO - CHg ^CH4^N0 HN0+C2Hg >C 2H 6 +N0 For short propagation chains, reactions (2a) and (2b) can account for most of the increase i n the y i e l d of C 2H 4 with increasing NO and the decrease i n the y i e l d of CgH6. Reactions (2a) and (2b) compete with H-abstraction by CgHg. However, the increased amount of C 2H 4 with NO pressure can be accounted for by reactions (4), 267 (5) and (6) which explains the observations that the increase i n the y i e l d of G^ H^  i s not equal to the decrease of GgH^ with NO. This could be expected from reaction (2b). However, reactions (1) (3) and (5) may have minor importance because of the i n s t a b i l i t y of CHgNO. For higher s/V r a t i o s , and for fixed NO pressure, more NO would be adsorbed and therefore the reactions ( l ) and (2) w i l l be reduced. This indicates that the y i e l d s of C 2H 6 and CH4 w i l l be favored at higher S/V r a t i o s i n accordance with the results obtained. Methyl radicals can recombine homogeneously or hetero-geneously to form ethane. Therefore i f the active centers are occupied by NO, the concentration of CHg radicals w i l l be a l i t t l e higher i n the gas phase. This fact w i l l cause a small increase i n the y i e l d of CH4 with NO. The small consumption of NO whioh was observed may be attributed to the decomposition of either CHgNO or CgHgNO into products which were not detected. It i s of interest to compare the results of the present study with some of the recent t h e o r e t i c a l approaches on i n h i b i t i o n . The complexity of the mechanisms,of course, does not permit any accurate quantitative treatments. Therefore some severe assump-tions w i l l be made. By following Norrish and Pratt's scheme (95) as we have seen i n Chapter I P vRL -,RS (1) R S - P »M-R L . (2) RL V01*R S (3) Rj, »end-products (4) Rj.-t-NO * RNO -i Ox (5) 868 Ox-NO > Rs -«-products (6) Ox •> products (7) The kinetics of pyrolysis of n-hexane w i l l not d i f f e r markedly from those for n-pentane and thus the same rate constants can he used i n c a l c u l a t i o n of the rate of n-hexane pyrolysis as were used by Norrish and Pratt i n c a l c u l a t i o n on n-pentane (95), except for; the rate constant for the primary d i s s o c i a t i o n whioh should he d i f f e r e n t . The rate constants used by Norrish and Pratt do not refer s t r i c t l y to n-pentyl r a d i c a l s but to smaller r a d i c a l s whose kinetic parameters are known. I f t h i s assumption i s v a l i d , t h i s k i n e t i c treatment should also be applicable to the i n h i b i t e d pyrolysis of n-hexane. For fc^, a frequency factor of I O 1 7 has been used for n-^butane, This value, i f correct, should also be v a l i d for n-hexane. Thus for n-hexane we obtain, using the value calculated e a r l i e r for the primary reaction, k x - I O 1 7 e"78R^° - 8.5 x lO-Sgeo" 1 at 520°C The corresponding value used by Norrish and Pratt for n-pentane was 1.78 x 10~ osec at 540 C. The other values are those used by Norrish and Pratt for n-pentane. k 3/k 4 - 17.6 ^3/^5 - 8.2 x 10~6mole co" 1 6 -7 k e/k 7 - 5.4 x 10 cc mole k4/k5 - 1.26 x 10 mole c c " 1 The various NO pressures used i n the present study i n mole co*"1 were inserted i n the expression 269 - 3 k l f . t k gk 7/k 5k 6^(Vs 5) [NO] + [NO] 2 I ( k 4 k 7 A 5 k 6 ) + [ ( k 7 / k 6 ) - (k 4 / k 5 ) ] f N O ] j and using the appropriate rate constants, the rates obtained for various NO pressures at a n-hexane pressure of 100 mm. The r e s u l t s are given i n Table 49. Table 49 - Decomposition rates i n min""*" for 100 mm of n-hexane at various NO pressures T - 520°C NO i n mm 0 1 2 4 8 10 20 , -10 -9 -9 -9 -9 -8 NO i n moleoo - 1 0 6x10 1.2x10 2.4x10 4.8x10 6x10 1.2x10 calculated rate i n min' 1 5.6 3.2 2.85 2.10 1.55 1.22 0.85 experimental rate i n m i n - 1 1.6 0.8 0.4 0.25 0.20 0.15 0.15 (present work)  The re s u l t s obtained from Norrish and Pratt's expression can f i t to a similar rate curve to that obtained experimentally for n-hex-ane.1 i f an appropriate value for k-j_ i s obtained. However, serious d i f f e r e n c e s / s t i l l e x i s t . For instance, the l i m i t i n g rate i s s t i l l decreasing even at an NO pressure of 20 mm. It i s also of interest to compare the i n h i b i t e d mechanisms proposed for n-hexane with that proposed by l a i d l e r et a l (133) for the r e l a t i v e l y complex p a r a f f i n , n-butane. Laidler does not consider the i n i t i a t i o n process and, of course, ignores hetero-geneity completely. His approach covers the f u l l y i n h i b i t e d decom-position. N i t r i c oxide i s assumed not to be consumed and i t i s 270 involved i n i n i t i a t i o n reactions of the type C4H10-»-N0 £4H9-*-HN0 N i t r i c oxide and HNO are involved i n termination processes similar to those proposed i n the present study. The steady state treatment of the mechanism outlined for n-hutane indioates that ^c^S.^' tiie rate of formation of CgH4, depends on NO, which i s also i n agree-ment with the present r e s u l t s . However, V CgHg depends only upon n-C 4H 1 Q. Thus CgHg i s not affected by NO. However, NO i s involved i n reactions such as CgHg ^ NO ^-»CgHgN0 whioh do not come into equilibrium; the concentration of CgHg i s reduced with increasing NO and therefore the y i e l d of C,gHg i s also reduced. Although i n the mechanism proposed by Laidler for n-butane the y i e l d of CgHg i s assumed to be independent of NO, his r e s u l t s show (133) the y i e l d of CgHg i s higher i n the unin-h i b i t e d than i n the in h i b i t e d reaction at the same extent of decomposition. Although surface effects have been found i n the previously mentioned work (133), Laidler considers that the effect of surface i s similar to that of the i n h i b i t o r , i n capturing chain-propagating r a d i c a l s . Therefore no heterogeneous i n i t i a t i o n i s considered i n the i n h i b i t e d decomposition of n-butane. 2. The mechanism of the in h i b i t e d pyrolysis of 2-methyl-pentane In the case of 2-methyl-pentane i t i s assumed that hetero-geneous i n i t i a t i o n i s not suppressed completely. The hydrocarbon i s more strongly adsorbed than n-hexane and therefore the adsorp-271 t i o n of NO ia reduced. However, we can a t i l l use the approximate r e l a t i o n ( R i W n h i b . ^ <X( R i ) h o m + (Rj)het (Ri) inhih. (Ri)hom A value of 8.5 ia obtained from the observed ratea. Therefore ( Ri>het. ~ ( 8 - 5 - ^ H R i ) h o m . TJaing the same approach as i n n-hexane and making the assumption that & H a ( i s > w i l l be the same as for n-hexane and that for 2-methyl-pentane the effective c o l l i s i o n a l diameter i a 4.54xl0~ 8, (130), the surface concentration for a monolayer i s about 5 x l 0 1 4 molecules/cm 2 . Thus ™ ™ hom-^het c* = 8.5 - e n where n_[_ ^ 10"° . Therefore for oc •= 0 we obtain n ^ 7.7 x 10~ 5 n This value corresponds to about 7fo surface coverage. This means that a smaller surface coverage by the hydrocarbon than i n n-hexane can rule out the homogeneous i n i t i a t i o n completely. Thus (Ri)het ^ 8 * 5 ( R i ) h o m The relationship between product d i s t r i b u t i o n and time i s also l i n e a r , (Chapter I I I ) , i n dicating the lack of s e l f - i n h i b i t i o n . N i t r i c oxide affects the formation of CH4, CgHg and C0H4 greatly and CgHg and CgHg s l i g h t l y . Is i s assumed that both NO and the surface are i n s i g n i f i c a n t i n the propagation processes. They ap-pear to affect termination reactions markedly. The termination w i l l be caused by the rad i c a l s CHg} C 2H 5 j i_C gH 7 and H atoms. Therefore, 272 NO can act i n both phases,capturing these radicals CHg + NO * CHgNO (1) C 2H 5+N0 »> C2H6N0 (2a) » C 2H 4 + HN0 (2b) i-C 3H ?^N0 »i-tC3H7NO (3a) ?C3H6^HN0 (3b) H>N0 'HNO (4) Reaction (2b) accounts mainly for the increase of C 2H 4 and the decrease of C^H^ with increasing NO pressure. S i m i l a r l y (3b) accounts for the small increase i n CgHg and decrease i n CgHg with increasing NO pressure. Both reactions (2a) and (3a) can account for the observed consumption of NO, i n products which were not detected. Reaction (4) accounts for the sli g h t reduction of H 2 with NO pressure. Reactions (2), (3) and (4) are competing with the corresponding H-abstraction by CgHg, i-C 3H 7 and H atom. Nitroso methane i s assumed to be very unstable and therefore equi-librium can e a s i l y bp accomplished. The termination may be com-pleted by reactions such as CHgNO •+ CHg — >G2E6^- NO (5) CHgNO+ C 2H 5 - » CgHg - NO (6a) — - CH4 - NO (6b) CgHgNO + CHg —' CgHg +N0 (7a) *C 2H 4- CH 4- NO (7b) C2H5N0+ C 2H 5 -— C 2 H 4 - C 2H 6- NO (8) CHgNO + H - C^H4 - NO (9) C2H5N0+ H .- >C2H6+ NO (10) 273 HNO -H * Hg-+ NO (11) Higher S/V r a t i o favors CgHg and CgH8 which can he attributed to some of the above recombinations taking place on the w a l l such as (5), (6a), (7a), (10), etc. The p a r t i c i p a t i o n of reactions such as those given above cannot be examined without knowing t h e i r kinetic parameters which are unknown so far. Their p a r t i c i p a t i o n i s postulated to explain some of the a n a l y t i c a l r e s u l t s . 3. The mechanism of the inhibi t e d pyrolysis of 3-methyl-pentane This isomeric hexane appears to be adsorbed to the same ex-tent as NO. The observed r a t i o ( Rj)uninhib. _ ^ i W h i b . cannot be treated a r i t h m e t i c a l l y as i n the previous cases because too many constants are involved. In fact t R j ) u n i n h i b . _ <*(Rj)hom + t R i ) h e t (Ri)inhib. ( 3(Ri)hom-,(Ri) het where P i s a constant in d i c a t i n g the extent to which homogeneous i n i t i a t i o n participates to the in h i b i t e d rate. The r a t i o has an approximate value of 6, and therefore ( R i ) h e t c r {Ri]hom (1 -6f*) Two unknown quantities are involved so that a plausible solution cannot be obtained. What can be concluded i s that the heterogeneous i n i t i a t i o n w i l l have a maximum value given by ( Ri^het < 6 ( Ri)hom £74 N i t r i c oxide has a large effect on the yiel d s of CH4, CgH4 and CgHg. No s i g n i f i c a n t effect i s observed on a l l other products. Higher s/V ra t i o s reduce the y i e l d of CH4 and increase the y i e l d of C2Hg (Chapter I I I ) . No effe c t of surface and NO pressure i s observed on the main modes of the decomposition which y i e l d CgHg, i-C 4H 8, C^Hg-E and 2-methyl-butene-l. Again i t i s assumed that termination i s accomplished mainly by CHg and CgHg and to a less extent by H atoms. Thus CHg -v- NO > CHgNO (1) C 2H 5 -+ NO > G g H 5 N 0 ( 2 a ^  >CgH 4-HN0 (£b) H + NO =»HNO (3) The i n s t a b i l i t y of these compounds w i l l r esult i n further decom-position or i n reactions such as CHgNO CHg >CgHg 4 - NO (4) CHgNO-C2H5 -—*• CH4+ CgH 4- NO (5a) —^C 3H 8+ NO (5b) C 2H 5N0 - CHg -- v C 2H 4- CH4+ NO (6a) ^CgHg- NO (6b) W ° - C £ H 5 -^ C 2 H 4 ^ C 2 H 6 + N 0 (7) CHgNO -H — —•> CH4 - NO (8) CgHgNO + H -—» C2Hg + NO (9) HNO-1- H -—»Hg+- NO (10) Consumption of NO can be accounted for by reactions ( 1 ) and (2a) assuming further decomposition of the formed nitrosocompounds. 275 4 . The mechanism of the in h i b i t e d pyrolysis of neohexane Neohexane i s adsorbed more strongly than a l l other isomeric hexanes. Therefore one should expect that any heterogeneous i n h i -b i t i o n by NO be markedly reduced. The present results show that NO caused only a 50 - 60$ reduction i n rate at maximum i n h i b i t i o n . A similar r e s u l t was also observed i n neopentane (92). The mech-anism of the l i m i t i n g reaction at maximum i n h i b i t i o n w i l l pre-sumably be similar to that for n-hexane and w i l l l i k e l y be closer to the mechanism of the uninhibited reaction. The effect of NO may cause a small reduction i n heterogeneous process and a corres-ponding increase i n homogeneous process. Because the neohexane i s strongly adsorbed, the effect of the i n h i b i t o r may be much more on gas phase reactions than for the less-branched hydrocarbons. The ra t i o -( R i ) u n i n h i b . i a approximately 1.9 - 2.2 ( R i ) i n h i b . for neohexane pressures varying from 50 - 120 mm. which indicates that heterogeneous and homogeneous reactions contribute to both rates. Heterogeneous i n i t i a t i o n appears to be somewhat suppressed by NO, but s t i l l may participate to a s i g n i f i c a n t extent. The phenomenon of s e l f - i n h i b i t i o n i s obvious i n the v a r i a t i o n of products with time (Fig. 63) presumably due i n part to adsorp-t i o n of unsaturated products on the surface. N i t r i c oxide has a small effect on the y i e l d s of CH4, CgHg, C 2 H 4 and i-C 4H 8. A change i n the s/V r a t i o does not affect the y i e l d of CgHg but the yie l d s of C H 4 and CgHg are increased at high-er surface area. It i s assumed that termination i s affected by NO 276 i n the manner considered e a r l i e r . CH 3-N0 *CH3NC (1) CgHg-NO > C 2 H 5 N 0 ( 2 a^ —»C 2H 4-HN0 (2h) H -NO —» HNO (3) The large y i e l d s of CH4 and H g indicate that the concentrations of CH3 and H are quite high and thus further reactions of these ra d i c a l s can take place as follows CHg - CHgNO » C 2H 6 + N 0 (4) CHgNO-H » CH4 + NO (5) C 2H 5N0- CHg » C 2H 4-C 2H 6-^N0 (6a) » CgHg-NO (6h) C 2H 5N0-H »C 2H 6-N0 (7) C 2H 5N0^ C 2H 5 * C 2H 4- C 2H 6. NO (8) HNO — H i H 2 - H NO (9) The consumption of NO can he explained through reactions such as CH3 CH3 l 1 CH3C-+ NO > CH3-C-NO I 1 CH3 CH3 The t e r t i a r y butyl nitroso compound i s r e l a t i v e l y stable. S i m i l a r l y NO CH3«*C«-CH2CH3 - NO *CH3C-CH2CH3 CH3 CH3 In t h i s way the s l i g h t reduction i n the y i e l d of Hg.with increasing 277 NO pressure can be explained since reactions such as CH„ 1 CHg CHgC« ^H-CH 2:Cs are sources of Hg i CHg CHg formation. 5. The mechanism of the inhibit e d pyrolysis of diisopropyl Diisopropyl l i e s between 3-methyl-pentane and neohexane with respect to both extent of i n h i b i t i o n by NO and adsorption on quartz. The r a t i o ^ i " ^ ^ ^ ^ / ^ i ^ i n h i b . i a approximately 2.5 to 3 i n the pressure range 50 - 120 mm. The f u l l y i n h i b i t e d reaction i s assumed to be both hetero-geneous and homogeneous i n nature. However, the heterogeneity w i l l be less important than i t was i n neohexane. The v a r i a t i o n of y i e l d of products with time indicates that s e l f - i n h i b i t i o n occurs i n the f u l l y i n h i b i t e d decomposition. The termination reaction again may be affected by NO i n both phases. The chain-propagating r a d i c a l s are probably CHg, n-CgH7, i-C3H 7 and H atoms. Therefore the termination reaction w i l l include CH3+NO > CHgNO (1) i-CgHy +N0 > i-CgH7N0 (2a) " CgHg-t- HNO (2b) H-NO » HNO (3) Reaotion (2a) can account p a r t i a l l y for the consumption of NO, provided that the reverse reaction does not occur. The termination may also involve the following reactions. CHgNO +CHg ^C 2H 6-N0 (4) CHgNO ^  H > CH4 + NO (5) 278 i-C 3H 7N0 - CH2 *C3H6-t- CH4-»- NO (6) i-C 3H yN0- i-C 3H ? » CgHg- C3Hg+ NO (7) i-C3H?NO +H 'CgHg- NO (8) HNO^H 'H - NO (9) Consumption of NO can be attributed to the following reaction i n addition to reaction (2a). NO i CHgC-CH-CH3-NO CH3C-CHKJH3 CH3CH3 CH3CH3 which should be at least as stable as i-CgH^NO. Further pyrolysis of t h i s compound w i l l produce a variety of nitrogen-containing products. Summary A b r i e f comparison of the conclusions drawn from the present study with those which were used by other workers i n various t h e o r e t i c a l approaches to the problem of i n h i b i t i o n now seems appropriate. The rates obtained for the i n h i b i t e d pyrolyses of the various isomeric hexanes can be accounted for reasonably w e l l using the rate expression deduced by Norrish and Pratt (95). However, one has to make some major assumptions regarding the kinetic parameters involved. The consumption of n i t r i c oxide observed can be accounted for by the Norrish-Pratt mechanism but none of the nitrogen-con-ta i n i n g compounds postulated by t h i s mechanism have been detected, probably because of the a n a l y t i c a l methods used. This 'approach i s 279 therefore, consistent with some of the results obtained i n the present study. However, there are important differences between the present re s u l t s and those predicted by the Norrish-Pratt mechanism (95). No effe c t of surfaces i s considered by these authors and NO, as an i n h i b i t o r , i s considered to react solejL$£n the gas phase. Their mechanism does not predict the increases i n y i e l d of CH4 and C2H4 and the decrease i n y i e l d of CgHg with increasing NO pressures, which were observed i n the present study. They postulated that only large r a d i c a l s are affected by NO, and thus NO should markedly affect the yi e l d s of those products which are formed i n the main mode of decomposition of these r a d i c a l s . The present r e s u l t s i n -dicate that t h i s i s not the case. The yi e l d s of products l i k e CgHg and C^ Hg appear to be independent of the pressure of NO. It i s more d i f f i c u l t to reconcile the present result s with the mechanisms proposed by Laidler et a l (46-48,133). This 1 mech-anism: does not account for the progressive reduction i n rate with increasing NO pressure i n the p a r t i a l l y - i n h i b i t e d reaction but i s concerned only with the reduction i n rate at f u l l i n h i b i t i o n . They assume that i n the f u l l y i n h i b i t e d pyrolysis NO i s involved i n i n i t i a t i o n reactions and i s regenerated again by decomposition of HNO or reaction of HNO with r a d i c a l s . This means that no sig-n i f i c a n t consumption of NO i s expected. This prediction does not agree with the present r e s u l t s . Furthermore, these authors assume that the effect of the surface i s negli g i b l e and that i n i t i a t i o n takes place homogeneously. In their expression for the in h i b i t e d 280 rate the pressure of NO required for maximum i n h i b i t i o n i s related to the hydrocarbon pressure through a c o l l e c t i o n of rate constants. In the present study, t h i s NO pressure i s found to be e s s e n t i a l l y independent of both the nature and the pressure of hydrocarbon. However, the present results are, i n general, consistent with the above mechanism as far as uninhibited reactions are concerned, although the mechanisms for the isomeric hexanes are obviously much more complex than those proposed by l a i d l e r and his coworkers for the l i g h t p araffins. The approach used by Eyring et a l (67) i s similar i n some respects to that proposed by Norrish and Pratt (95). In the mech-anism of Eyring and his coworkers the effect of NO on the product d i s t r i b u t i o n can be predicted q u a l i t a t i v e l y through the capture of f3 or Y r a d i c a l s by NO. Some consumption of N^ i s allowed for i n this mechanism but they appear to assume that such reactions are i n s i g n i f i c a n t i n influencing the o v e r a l l rate. Norrish and Pratt assumed that reactions i n which NO i s consumed have a marked effect i n reducing the rate of pyro l y s i s . The reduction i n rates with i n -creasing pressure of NO calculated by these workers i s similar to that calculated by Norrish and Pr a t t . A further c r i t i s i s m of the mechanism of Eyring et a l i s the retention of the concept of a molecular re s i d u a l reaction. No surface effects are considered; the NO as an i n h i b i t o r i s supposed to act solely i n the gas phase, an assumption which does not agree with the results of the present study. 281 The present r e s u l t s provide some support for the mechanism of i n h i b i t i o n proposed by Voevodskii (91). Reduction i n rate of pyrolysis i s observed with increasing surface area. Heterogeneous reactions are considered to be important i n the present study, i n agreement with the ideas proposed by Voevodskii. 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Sagert.N.H. and Laidler,K.J.:Can.J.Chem.,41,848(1963) 288 APPENDIX I Calculation of ac t i v a t i o n energies and frequency factors 1 ) n-hexane Table 50 - Rate constants for variotxs presstires and temperatures x 102min~l Pressure mm T - 793°K T -783°K T"763°K 0 - 4 0 2.25 1.30 0.45 40 - 60 2.25 1.40 0.49 60 - 70 2.20 1.40 0.48 70 - 80 2.20 1.42 0.48 80 - 90 2.18 1.42 0.47 90 2.18 1 o 4 2 0.47 100 2.15 1.39 0.46 110 2.10 1.35 0.46 120 2.10 1.35 0.46 The energy of ac t i v a t i o n was calculated from the Arrhenius equation S A - R ( T I V t 2 - t I > 2 - 3 i o g ( k 2 A i ) Substituting the values reported i n the table 50, we obtain the value3 given i n the following table Table 51 - Overall energies of ac t i v a t i o n i n ikcal/mole; for various i n i t i a l pressures of n-hexane Pressure mm 793°K,783°K 793°K,763°K 783°K,763°K 0 - 4 0 64 65 65 40 - 60 58 61 61 60 - 80 56 60 62 90 55 61 61 100 54.5 62 60 120 55 61 60 Calculation of frequency factors Substituting E A into the equation 289 log fcl zz. l o S A - E log e we obtain log A - log k-i + E log e For T •= 793°K and pressure 0-40 we obtain log A - log 2.25 x 10" 2 6.4 x 10 S x 0.434 + 1.98x793 -2 4-0.352+ 17.6 - 15.952 Using a l l rate constants given i n Table 50 we obtain the following table. Table 52 - log A for various i n i t i a l pressures of n-hexane pressure mm 793°K 783°K 7 63°K Average 0 - 4 0 15.952 16.113 16.144 16.002 40 - 60 14.352 15.146 14.990 14.810 60 - 80 13.742 15.150 14.970 14.610 90 13.440 15.150 14.970 14.505 100 13.332 14.843 14.862 14.320 120 13.323 14.843 14.862 14.300 2 ) 2-methyl-pentane Table 53 - Rate constants for various pressures and temperatures x 10 2 min-1 pressure mm T 793°K T 783°K T 773°K 0 - 2 0 2.20 1.33 0.80 • :. 30 2.40 1.33 0.80 40 - 60 2.30 1.45 0.90 60 - 80 2.20 1.40 0.85 80 - 90 2.20 1.40 0.83 90 -100 2.18 1.40 0.83 120 2.00 1.30 0.80 From the Arrhenius equation we obtain the energies of ac t i v a t i o n which are tabulated as follows. 290 Table 54 - Overall energy of a c t i v a t i o n i n kcal/mole for various i n i t i a l pressures of 2-methyl-pentane pressure mm 7930K,783°K 783°K,773°K 793°K,773°K 0 $.30 63.0 61.5 62.5 30 - 60 62.0 58.0 59.5 60 - 90 56.0 61.5 58.5 100 55.5 62.5 58.0 120 54.0 58.0 56.0 Frequency factors Using the method outlined above we obtain the following table Table 55 - Log A for various i n i t i a l pressures of 2-methyl-pentane pressure mm 793°K 783°K 773°K Average 0 - 3 0 15.742 15.322 15.403 15.450 30 - 60 15.562 14.361 14.754 15.520 60 - 90 13.962 15.346 14.228 14.860 100 13.938 15.546 14. 118 14.520 120 13.200 14.314 13.704 13.780 3" ) 3-methyl-pentane Rate constants for 3-methyl-pentane are given i n the following table Table 56 - Rate constants for various pressures and temperatures x 10 2 min~l pressure mm 7 93°K 783°K 773°K 763°K 0 - 3 0 1.87 1.10 0.63 0.38 30 - 50 2.55 1.60 1.00 0.60 50 - 60 2.70 1.60 1.05 0.60 60 - 70 2.70 1.70 1.10 0.72 70 - 80 2.60 1.70 1.10 0.66 80 - 90 2.60 1.60 1.10 0. 66 100 2.50 1.60 1.05 0.66 120 2.40 1.55 1.00 0.63 Acti v a t i o n energies,calculated as was done previously, are given i n the following table. 891 Table 57 - Overall energies of a c t i v a t i o n i n kcal/mole for various pressures of 3-methyl-pentane pressure mm 793,783 °K 783,773 °K 773,763 °K 793,773 °k 793,763 °k 783,763 °k 0 - 3 0 65.5 67.0 63.5 66.0 65.0 63.0 - 40 58.0 57.0 61.0 59.0 58.5 59.0 50 62.0 52.0 62.0 59.0 60.5 59.0 60 - 80 59.0 54.0 57.0 55.5 54.0 53.0 90 59.5 49.0 55.0 54. 0 55.0 53.0 100 57.0. 51.0 57.0 54. 5 53.0 53.0 120 58.0 52.0 55.0 55.5 51.0 51.0 Frequency factors Frequenoy factors calculated as above are given i n the f o l -lowing table. Table 58 - Log A for various i n i t i a l pressures of 3-methyl-pentane. pressure mm 793°K 783°K 773°K 763°K Average 0 - 3 0 16.10 16.44 15.50 15.98 15.72 40 14.31 14.02 15.02 14.08 14.38 50 15.22 12.72 14.98 14.95 14.68 60 - 80 14.45 13.11 14.04 12.98 13.68 90 14.61 11.80 14.14 14.12 13.78 100 14.12 13.35 13.98 12.98 13.59 120 14.28 12.60 13.50 12.30 13.19 4 ) 8,8-dimethyl-butane (neohexane) Rate constants are given i n the following table. Table 59 - Rate constants for various pressures and temperatures x 10 2 min" 1 pressure mm T^793°K T-783°K Tr773°K T ~ 763°K 0 - 30 8.00 1.30 0.80 0.50 30 - 50 8.10 1.40 0.90 0.60 50 - 80 8.15 1.50 1.00 0.70 80 - 100 8.10 1.4.5 1.08 0.68 100 - 180 8.06 1.40 1. 00 0.68 180 - 150 1.98 1.38 0.95 0.65 292 Energies of aotivation calculated as above are given i n the f o l -lowing table. Table 60 - Overall a c t i v a t i o n energies i n kcal/mole for various i n i t i a l pressures of neohexane pressure mm 793,783 °K 783,773 °K 773,763 OK 793,773 OK 793,763 °K 783,763 °K 0 - 3 0 54 58 55 57 56 57 30 - 50 51 53 49 52 50 50.5 50 - 80 47 50.5 44 48 51 46 80 -100 48 47 47 44 50 46 100-120 48 47 46 44 48 43 120-150 47 45 44 45 48 43 Frequency factors calculated as previously are given i n the f o l -owing table. Table 61 - Log A for various i n i t i a l pressures of neohexane pressure mm 793QK 783°K 773°K 763°K Average 0 - 3 0 13.01 14.11 13.20 13.30 13.41 30 - 50 12.32 12.45 12.55 12.38 12.42 50 - 80 11.50 12.18 11.50 12.45 11.92 80 -100 11.60 11.16 11.10 12.20 11.52 HBO-120 11.50 11.15 11.015 11.44 11.34 120-150 10.90 10.74 10.38 11.40 10.85 5 ) 2,3-dimethyl-butane (diisopropyl) Rate constants for diisopropyl are given i n the following table. Table 62 - Rate constants for various pressures add temperatures x IO 2 min "1 pressure mm T ^  793°K T^783°K T = 773°K T = 763°K 0 - 30 4.60 3.10 2.00 1.25 30 - 50 4.70 3.20 2.10 1.28 50 - 80 4.40 3.15 2.15 1.35 80 -100 4.30 3.00 2.10 1.30 100 - 130 4.20 2.95 2.06 1.25 130 - 150 4.10 2.85 2.04 1.23 393 Energies of aotivation for a l l possible combinations of temperature are tabulated as follows. Table 63 - Overall energies of a c t i v a t i o n i n Iceal/mole for various i n i t i a l pressures of diisopropyl pressure 793,783 783,773 773,763 793,773 793,763 783,763 mm °K °K °K °K °K °K 0 - 3 0 49.0 5E.0 51.0 50.5 53.0 53.0 30 - 50 48.5 50.5 56.0 48.0 53.0 53.0 50 - 8© 43.0 46.0 53.0 45.0 48.0 50.0 80 -100 45.5 49.0 52.0 44.0 48.5 49.0 100 -130 44.5 43.0 53.0 45.0 49.0 49.5 130 -150 44.0 43.0 53.0 43.0 48.5 50.0 Frequency factors calculated as above are given i n the following table. Table 64 - Log A for various i n i t i a l pressures of diisopropyl pressure mm 793°K 783°K 773°K 763°K Average 0 - 3 0 13.11 13.09 IS.60 13.10 12.98 30 - 50 13.13 13.12 13.92 13.13 13.33 50 - 80 10.54 12.40 13.22 11.73 11.95 80 -100 11.13 12.35 12.20 11.70 11.80 100 -130 11.12 12.36 12.24 11.65 11.80 130 -150 11.10 12.34 12.21 11.68 11559 294 APPENDIX I I Calculation of the D(C-C) fs for the primary decomposition i n isomeric hexanes. 1) n-hexane —* n-C 4H 9 -CgHg (lh) ~>2n-CgH7 (lc) Reactions (lh) and (lc) have been found to have d i s s o c i a t i o n energies of 78 and 76 kcal/mole (34,118). The fact that C 3H 8 appears i n negl i g i b l e quantities does not prove that (lc) has a low p r o b a b i l i t y of occuring, as Hinshelwood and his coworkers assume (131). It may simply be a consequence of the i n s t a b i l i t y of n-C 3H 7. The d i s s o c i a t i o n enerrgy for reaction (la) can be calculated as follows. n-0 6H 1 4 n-O gH n+ CHg (la) D(C-C)r AH f(CH g) + AH f(n-C 5H i ; L)- aH f(n-CgE 1 4) ( l ) The heat of formation of n-CgH^ can be calculated from available thermochemical data as follows. n-C 5H 1 2 n-CgHi:L+ H D(C-H) -92-94kcal/mole (34) n"°5 H12 n- C4 H9-*- C H 3 D(C-C)=78 kcal/mole (34) Prom the l a s t two equations we obtain D(C-C)~ &H f(CH 3 ) i - AH f(n-C 4H 9)- &H f(n-C 5H 1 2)= 78 kcal/mole D(C-H) z AH f(H) 4.iiH f(n-C| 5H 1 1)- ^ Hf(n-CgH ) - 93 kcal/mole By using the values (34) AH f(CH 3) - 31 kcal/mole & H f ( C 2 H 5 ) r 2 6 kcal/mole S95 & H f (H) = 5£ koal/mole and &H f(n-C 4H 9) ^  IE koal/mole &H f(n-CgH 7) -18.E kcal/mole & H f ( i-CgH 7) =18. Skcal/mole we obtain from the above equations: /iHf(n-CgH.^) - 6.5 kcal/mole This value can be used i n (1) or i n the following set of equations. n"°6 H14 — ^ n " C 5 H l l " + G H 3 D(C-C) - ? (la) n-C 4H 9 H-C2H5 D(C-C) - 78 kcal/mole (lb) So we obtain D(C-C) l a= 78 kcal/mole 8) 8-methyl-pentane CHgCHCHgCHgCHg CHg--»- CHgCHCHgCHg* (8a) CHg CHg -=> CHg -+ CHg CHCHgCHgCHg (Sb) C2Hg+ CHgCH*-CHg» (So) ^ 3 -» n-C 3H 7 +-i-CgH7 (Ed) Reaction (Ed) has a diss o c i a t i o n energy of 76 kcal/mole ( 3 4 ) . Assuming that A Hf(t-butyl) =- 4.5 kcal/mole (34) and supposing that A H f (CHgCHCHg.) ~ &H f (CHgC-CHg) we can obtain the d i s s o c i -CH3 0 H 3 ation energy of (So), using the same method as was done i n n-hex-ane. Thus ^(C-Cjgg" 76 kcal/mole. For reaction (Sa) the di s s o c i a t i o n energy can be calculated from the equations CHgCH-CHgCHg CHgCHCHg* +- CHg D(C-C) -76 kcal/mole (118) CHg CHg CHg CH-CHg CHg* + H D(C-H)~91 koal/mole t CH, 296 Therefore A.Hf(CH3CHCH2CH2«) = 0. However t h i s value i s a l i t t l e low because the used D(C-C) of 76 kcal/mole obtained from idide data i s suspected to be low. Thus a more reasonable value for & H f (CHg—CHCHrjCHrj* ) would be 2 - 3 kcal/mole. This r a d i c a l appears CH3 i n reaction (2a). Therefore, using the same method used i n n-hexane we obtain D(C-C) 2 a-- 72 - 75 kcal/mole The di s s o c i a t i o n energy for reaction (2b) can be calculated i n a similar way. The heat of formation of CH2CHCH2CH2CH3 i s unknown. Using the hypothesis (34) that D(C-H) . -D(C-H) -prim s 8 c 2 to 3 kcal/mole we obtain n-C 5H 1 2 —^ CH3CHCH2CH2CH3 4-H D( C-H) - 89 kcal/mole n-C4Hg CH3 D(C-C) =78 kcal/mole Thus &H f(CH 3CHCH 2CH 2CH 3) -2.5 kcal/mole There fore D(C-C) 2 b - 7 9 kcal/mole 3) 3-methyl-pentane CH3CH2CHCH2CH3 —^ CH3 ~f-CH3CHgCHCH2* CH, CH3 ->C 2H 5 -»-CH3CH2CHCH2 —^ CHg CH3CH2CHCH2CH3 (3a) (3b) (So) Reaction (3c) i s similar to (2b). Therefore D(C-C) 3 o ~ 79 kcal/mole 297 Reaction (3a) i s comparable to (2a). Thus D(C-C) 3 a~ 72-75 kcal/mole To calculate D(C-C),,.. , we follow the same method outlined i n n-o b hexane using reaction (3b) and (3c). The value &Hf(CHgCHCHgCHgCHg) s 3 2.5 kcal/mole i s also assigned for AH f (CH3CH2CHCH2CH3). There fore D(C-C) 2 b- 79 kcal/mole 4) Neohexane CH, CH^ C H 3 - C - C H 2 C H 2 _^ CHgC-CH 2 - - r C H 3 (4a) >CH3 CHg C H 3 -> CH3C» * C 2H 5 (4b) CH3 CHgCCHgCHj + C H 3 (4c) CH* o DtC-C)^ i s given i n the l i t e r a t u r e as 73 kcal/mole (132), a value which i s probably low. For reaction (4a) the heat of for-mation of the involved r a d i c a l i s calculated as follows. CH3 CH3 l i CH3C-<3H^ —^ CH3C-CH2. -rH D(C-H) » 96 kcal/mole (118) CH3 CH 3 CH3 CH3-C»+ CH3 D(C-C)-74 kcal/mole (132) CH3 CH3 Thus /\H f (CHg-CCHg-) - 6 kcal/mole. Using t h i s value i n (4a) and CHa 298 (4b) , as i t was explained, i n n-hexane we obtain D(C-C) 4 - 80 koal/mole The d i s s o c i a t i o n energy for reaction (4c) can be calculated as follows. From the equations CHg-CH-CHgCHg CHg6"-CHgCHg * H D(C-H)_89 kcal/mole CHg CHg -> i-CgH 7+C 2H 5 D(C-C):=75 kcal/mole we obtain t £s.Hf (CHgCCHgCHg) - 1 kcal/mole . Using t h i s value and CHg reactions (4b) and (4c) we obtain D(C-C) 4 c = 75 kcal/mole 5) Diisopropyl CHgCHCHCHg CH CHCHCHg ^  CH3 (5a) 2i-C 2H 7 (5b) D ( C - C ) 5 V - 66.5 kcal/mole (132) From the equation CHg CH-CHgCHg CHgCH-CH-CHg + H D( C-H)- 91 kcal/mole CH,, CH, i-C 2H 7+ C 2H 5 D(C-C) -75 kcal/mole (118) we obtain &Hf(CHgCHfiHCHg)^ 8.2 kcal/mole CHg Using t h i s value with equation (5a) and (5b) we obtain D(C-C) 5 ar 70 kcal/mole 

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