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The pyrolysis of diallyl and reactions of allyl radicals with l-butene. Ruzicka, Dalibor Josef 1958

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T H E  P Y R O L Y S I S  OF  D I A L L Y L  A N D R E A C T I O N S  OF  W I T H  A L L Y L  R A D I C A L S  1 - B U T E N E  by  DALIBOR J . RDZICKA cand. mag. (University of Oslo) 1956  A THESIS SUBMITTED IN PARTIAL FIIEJFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE  i n the Department of Chemistry  We accept t h i s thesis as conforming to the required standard  THE UNIVERSITY  OF BRITISH COLUMBIA  June, 1958  ABSTRACT  A study has been made of the pyrolysis of 1,5-hexadiene ( d i a l l y l ) and of the reactions of the a l l y l r a d i c a l s thus produced with 1-butene. The pyrolysis of 1,5-hexadiene was found to be f i r s t order and appears to proceed mainly by free r a d i c a l reactions i n the temperature range 460 5209 C.  However, molecular rearrangement i n the formation of cyclo-  hexene and benzene are possible.  The o v e r - a l l a c t i v a t i o n energy f o r the 8.5  process was found t o be 31.3 kcal/mole f o r an A factor of 10 *  —1 min"* .  The main products among the gaseous compounds were methane, ethane, ethylene, propylene, and 1-butene.  The main l i q u i d products were cyclopentene, cyclo-  pentadiene, 1-hexene and benzene.  Evidence was obtained f o r hydrogen  abstraction by a l l y l and addition of the a l l y l r a d i c a l to o l e f i n i c double bonds.  Activation energies f o r the formation of methane, ethane, propane,  ethylene, propylene and 1-butene were determined. Addition of approximately 5 i° by volume of 1,5-hexadiene was found to increase the rate of the pyrolysis of 1-butene about 6 - 7  times at 506° C.  The formation of C^-cyclic products and methane was found to be proportional 1/2 to ( d i a l l y l )  .  This provides k i n e t i c evidence that a l l y l r a d i c a l s are  involved d i r e c t l y i n the formation of these products i n the sensitized decomposition and also that the primary step i n the pyrolysis of d i a l l y l involves the formation of two a l l y l r a d i c a l s .  Mechanism f o r the pyrolysis  of d i a l l y l and f o r the reactions of a l l y l with 1-butene are discussed. A b r i e f study has also been made of the pyrolysis of 1-butene i n the presence of acetaldehyde.  The thermal decomposition of 1-butene sensitized  by addition of 25 i by volume of acetaldehyde was carried out at 477° C. a  -iii-  The decomposition of acetaldehyde was found to be inhibited by the presence of 1-butene by a factor of more than two.  1-butene, which  practically does not decompose at the above temperature by itself, decomposes appreciably in the presence of acetaldehyde.  In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available f o r reference and study.  I  further agree that permission f o r extensive copying of t h i s thesis f o r scholarly purposes may be granted by the Head of my Department or by h i s representative.  I t i s understood that copying or publication of t h i s  thesis f o r f i n a n c i a l gain s h a l l not be allowed without my written permission.  Department of Chemistry The University of B r i t i s h Columbia Vancouver 8, Canada June 27,  1958  -iv-  ACKNOWLEDGEMENTS  This investigation was carried out under the d i r e c t i o n of Dr. W.A. Bryce to whom the author i s greatly indebted. Thanks are due to Mr. S.A. Ryce f o r the use of h i s gas chromatographic apparatus as w e l l as f o r many valuable discussions. Mr. E.W.C. Clarke carried out the mass spectrometric analyses which was g r a t e f u l l y appreciated. The author i s also indebted t o the Defence Research Board f o r f i n a n c i a l assistance during the course of t h i s work and to the B r i t i s h Columbia Sugar Refining Company Limited f o r a graduate scholarship (1957 1958).  -V-  TABLE  OF  C O H I E H S Page  DiTRODUCTION  1  GENERAL  1  HISTORICAL  2  EXPERIMENTAL  4  PYROLYSIS APPARATUS  4  METHOD OP PYROLYSIS  6  ANALYSES OP THE REACTION PRODUCTS BY GAS CHROMATOGRAPHY  7  Apparatus  7  Separation of the products  9  Product I d e n t i f i c a t i o n  10  Quantitative determination of the products  12  Qualitative and quantitative determination of hydrogen 13 Analytical r e s u l t s RESULTS  14 17  I . PYROLYSIS OP 1,5-HEXADIENE  17  Order of the reaction  17  Pressure increase i n reaction system  17  Rate and o v e r - a l l energy of a c t i v a t i o n f o r the pyrolysis of 1,5-Hexadiene  17  Rates of formation and over-all activation energies f o r the l i g h t hydrocarbons  19  Temperature and time dependence i n the formation of products I I . PYROLYSIS OP 1-BUTENE IN THE PRESENCE OP DIALLYL Introduction  26 32 32  -vi-  Page  III. If.  Results for the decomposition of 1-butene sensitized by d i a l l y l  52  PYROLYSIS OP n-BDTANE IN THE PRESENCE OP 1,5-HEXADIENE  57  PYROLYSIS OP 1-BUTENE IN THE PRESENCE OP ACETALDEHYDE  DISCUSSION  59 45  PRECISION OP THE QUANTITATIVE DETERMINATIONS OP GAS CHROMATOGRAPHY UNCERTAINTY IN THE VALUES OP THE KINETIC RATE CONSTANTS  43 44  THE OVER-ALL ACTIVATION ENERGY POR THE DECOMPOSITION OP 1,5-HEXADIENE MECHANISM OP THE PYROLYSIS OP 1,5-HEXADIENE  46 43  General  48  Reactions of the a l l y l radical  49  (1) Hydrogen abstraction by a l l y l  49  (2) Addition of a l l y l to double bond  51  (5) Combination of a l l y l with other radicals  55  The fate of the 1,5-hexadienyl radical  54  (1) Rearrangement  54  (2) Decomposition  55  (5) Combination with other radicals  55  Formation and reactions of other radicals  55  Mechanism for the formation of the pyrolysis products  57  MECHANISM OP THE THERMAL DECOMPOSITION OP 1-BUTENE SENSITIZED BY DIALLYL  60  (1) Addition to the double bond  60  (2) H-abstraction  61  DECOMPOSITION OP n-BUTANE SENSITIZED BY DIALLYL  62  -vii-  Page BIBLIOGRAPHY  64  APPENDIX MASS SPECTROMETRY IDENTIFICATION OF SOME OF THE PRODUCTS  66  -viii-  TABLES Page I. II. III. IV. V. VI. VII. VIII.  IX.  Products of 1,5-Hexadiene Pyrolysis (Mole Various Temperatures "  at 15  Rate Constants for the Over-all Decomposition of 1,5-Hexadiene  20  Rate Constants for the Overfall Decomposition of 1,5-Hexadiene for Different Reaction Times  20  Rate Constants (min - 1 ) for the Formation of the Light Products  23  Energies of Activation and Frequency Factors for the Formation of the Light Products  23  Decomposition Products of 1,5-Hexadiene Pyrolysis as a Function of Time at 5 0 1 ° C.  31  Composition of the Reaction Mixture for Sensitized Decomposition of 1-Butene with 1,5-Hexadiene  35  Main Products of Pyrolysis of n-Butane and Sensitized Pyrolysis of n-Butane hy Addition of Appr. 5 i° by Volume of 1,5-Hexadiene at 506° C. Reaction Time 5 Minutes.  38  Percentage of Decomposition of Acetaldehyde and 1-Butene at 4 7 7 ° C.  40  -ix-  FIGURES Page l a . Pyrolysis Apparatus  5  l b . Circuit Diagram of Furnace Heaters  5  2a. Gas Chromatography Apparatus  8  2b. Charcoal Trap System  8  2c. U-Tube Trap  8  3.  Gas Chromatographic Separation of the Pyrolysis Products  11  4.  Pressure Change vs. Time for 1,5-Hexadiene  18  5.  Arrhenius Plot for Decomposition of 1,5-Hexadiene  21  6.  Arrhenius Plots for the Light Saturated Products i n the Pyrolysis of 1,5-Hexadiene  24  Arrhenius Plots for the Light Unsaturated Products i n the Pyrolysis of 1,5-Hexadiene  25  Light Products of the Pyrolysis of 1,5-Hexadiene vs. Temperature. Reaction Time 5 Minutes  27  7. 8.  9a and 9b. Hydrogen and the Heavy Products of the Pyrolysis of 1,5-Hexadiene vs. Temperature. Reaction Time 5 Minutes.  28  10.  Products of the Pyrolysis of 1,5-Hexadiene vs. Time at 501» C.  29  11.  Heavy Products of the Pyrolysis of 1,5-Hexadiene vs. Time at 501« C.  50  Pressure Change vs. Time at 506° C. 200 mm.  33  12. 15.  I n i t i a l Pressure  Products of Sensitized Decomposition of 1-Butene vs. 36  14.  Extent of Decomposition at 477° C. I n i t i a l Pressure 200 mm. I. Acetaldehyde + Helium (l:jj) II. Acetali + 1-Butene (l:5)  I  I N T R O D U C T I O N  -1-  I N T R O D P C T I O N  GENERAL Previous work on the thermal decomposition of 1-butene (4,12,19) provided very good evidence that the primary step i n the process involves formation of methyl and a l l y l radicals.  By studying the decomposition of  1-butene sensitized by methyl radicals from mercury dimethyl (lO) i t was further established that the methyl radicals play an important role i n the mechanism of the subsequent decomposition. It i s to be expected that the a l l y l radical formed i n the primary step would also participate i n the mechanism of the decomposition.  Little is  known about the reactions of the a l l y l radical i n hydrocarbon systems and therefore a study was undertaken of the fate of a l l y l i n the pyrolysis of 1-butene by sensitizing the decomposition with the a l l y l r a d i c a l . Mercury d i a l l y l and other metal-allyl compounds would be suitable sources of a l l y l radicals for the investigations. available, nor are they mentioned i n the l i t e r a t u r e .  However, they are not It was therefore  necessary to use a hydrocarbon as a source of a l l y l . Lossing and co-workers (12) have observed a l l y l radicals to be the main radical products of the pyrolysis of 1,5-hexadiene ( d i a l l y l ) i n the temperature range 690 - 8 9 0 ° C i n a mass-spectrometer with a flow reactor. The decomposition of 1,5-hexadiene i n a static system at  temperatures  around 500° C should thus most probably involve generation of a l l y l radicals i n the primary step.  It was therefore chosen as the source of  a l l y l radicals for the present study.  -2-  A preliminary study of the pyrolysis of d i a l l y l i t s e l f was undertaken to gain some insight into the mechanism of i t s decomposition before using i t as a source of a l l y l radicals i n reactions with 1-butene.  HISTORICAL Very l i t t l e i s known about reactions of a l l y l radicals.  Mooney and  Ludlam (16) studied the pyrolysis of a l l y l bromide but radical mechanisms were not known at that time.  This pyrolysis has been studied recently  by Szwarc, Gosh and Sehon ( 2 3 » 2 4 ) using the toluene carrier technique. They ascribe the overall activation energy of 47»5 kcal to the dissociation of the a l l y l - B r bond i n the primary step.  Szwarz (22) has also obtained  excellent kinetic evidence for the formation of a l l y l and hydrogen i n the pyrolysis of propylene i n a flow system at 680-870° C. Taylor and Smith (25) mentioned that the resonance  characteristics  of the a l l y l radical result i n a weak a l l y l - H bond i n propylene.  Hence  i n hydrocarbon systems the resonance s t a b i l i t y of the a l l y l radical  (2,5)  would be expected to have a long l i f e and to disappear by reactions such as methyl + a l l y l — ^ 1-butene or by combination with i t s e l f to give d i allyl.  According to the authors the reaction: allyl  +  C5H6  . CgH^  may be involved i n the polymerization of propylene sensitized by the photolysis of mercury dimethyl, and i n the mercury-photosensitized reactions of propylene. Hydrogen abstraction by a l l y l has been demonstrated by several workers: Molera and Stubbs (15) have postulated that hydrogen abstraction by a l l y l takes place from isobutene i n the pyrolysis of isobutene.  -3-  Yery recently McNesby and Gordon (14) have studied the decomposition of cyclopentane i n presence of acetone and found: at 3 8 1 ° very l i t t l e hydrogen abstraction by a l l y l takes place, a l l y l combines with CH^ to give 1-butene. at 453° 1-butene i s s t i l l present i n the system, but hydrogen abstraction by a l l y l both from cyclopentane and from CD-COCD,  3  3  takes place* at 5 0 0 ° extensive abstraction of hydrogen by a l l y l takes place but also small amount of 1-butene i s formed. Some work has also been done on the reactions of a l l y l i c radicals i n solution ( l ) . The information to date on the reactivity of a l l y l radicals i n hydrocarbon systems can be summarized as follows: (1)  the resonance s t a b i l i t y of about 17 kcal/mole (average of values given by 2 and 5) leads to appreciable life-time of the a l l y l radical so that combination with other radicals may take place.  (2) abstraction of hydrogen at temperatures above about 450° C takes place. (3)  addition of a l l y l to o l e f i n i c double bonds may take place.  E X P E R I M E N T A L  -4-  E X P E R I M E N T A L  PYROLYSIS APPARATUS The pyrolysis of 1,5-hexadiene was carried out i n an all-glass static system ( f i g . l a ) .  V i s a quartz vessel of a volume of about  300 ml placed i n an e l e c t r i c furnace (p), A^ to A^ are reservoirs for the gases to be pyrolysed, P i s a sampling pipette with a metal-teflon tap to avoid adsorption i n stopcock grease.  M^ and Mg are manometers.  The furnace was heated by means of three heating elements connected as shown i n f i g . l b . e  l'  e  2' y e  w e r e  The potentials across the three heating elements  adjusted so that temperature gradient along the reaction  vessel did not exceed 1°C. Temperature measurements were made by means of two chromel-alumel thermocouples placed i n position a and b.  A rough calibration curve  "applied voltage vs. furnace temperature" was constructed and used for approximate temperature setting.  Pine temperature adjustments were  obtained by switching the resistance r on or off.  The heat capacity  of the system was increased by f i l l i n g the space between reaction vessel and furnace with sand.  The purpose of t h i s was to keep the temperature  constant for sufficiently long periods of time to carry out a reaction even when s t r i c t steady state between the furnace and the surroundings was not reached.  This technique proved satisfactory.  It was possible  to keep the furnace temperature constant during the pyrolysis experiments to within 19 C at 500" C. In order to prevent condensation i n the manometer M^and the adjacent c a p i l l a r i e s , this part of the system was heated e l e c t r i c a l l y to about 50 -  -5-  to vacuum  Fig, la  Pyrolysis Apparatus  Ampm.  Pig. lb  Circuit Diagram of Furnace Heaters  -6-  60B C when necessary.  METHOD OF PYROLYSIS When pyrolysis experiments are to be carried, out i n a static system there i s the problem of admitting the reactant with a certain i n i t i a l pressure i n the reaction vessel.  At the same time the starting point of  the reaction should be reasonably well defined. In order to satisfy both requirements, the volume of the system was increased by attachment of vessel A^. created.  A pre-expansion volume was thus  A calibration curve was constructed giving the dependence  between the pressure i n the pre-expansion volume (read on manometer M^) and the pressure i n the whole system (which i s equal to the i n i t i a l pressure i n the reaction vessel) at different  temperatures.  Before each experiment the system was pumped down to "black vacuum" which was tested with a Tesla c o i l .  From the calibration curve the  pressure i n the pre-expansion volume corresponding to the desired i n i t i a l pressure i n the reaction vessel was read.  The pre-expansion volume was  f i l l e d with the gas or mixture of gases to be pyrolysed.  In the case of  a mixture 5 minutes was considered to be sufficient for the gases to mix completely.  By opening the stopcock S^ the reactant was admitted into  the reaction vessel.  The moment when the mercury column i n manometer M^  settled at a certain value was taken as the beginning of the reaction and the i n i t i a l pressure was read.  During the course of the reaction the  pressure i n the reaction vessel was read at 1 minute intervals.  At the  end of the reaction the stopcock S^ was opened (S^ was closed) such that the products expanded freely into the evacuated sampling pipette.  -7-  ANALYSSS OF THE REACTIOH PRODUCTS BY GAS CHROMATOGRAPHY Apparatus The separation of the products of the pyrolysis of 1,5-hexadiene and of the a l l y l sensitized pyrolysis of 1-butene was effected by gas chromatography. available ( f i g .  A gas chromatograph constructed i n the laboratory was 2a).  With the exception of Hg analyses where nitrogen was employed, helium was used as the carrier gas i n a l l analyses. conductivity c e l l (18) was the detector.  A self-compensating thermal  Several gas chromatographic  columns with different packings were available.  The unbalance signal  from the thermal conductivity c e l l mounted i n a bridge c i r c u i t was recorded on a chart of a Speedomax recording potentiometer. The existing gas chromatograph had to be modified for the present analysis: a)  a second column furnace was b u i l t and attached to the apparatus so  that two chromatographic columns, one for l i g h t hydrocarbons and the other for heavier hydrocarbons, could now be connected i n p a r a l l e l .  This  enabled a quick switch from one column to the other by merely turning the stopcocks S^ and S^ ( f i g . b)  2a).  i t was found that the amount of sample admitted into the bypass by  free expansion from the sampling pipette was insufficient. pump was therefore attached to the admission system.  A Toepler  The efficiency of  the pump was such that i n the f i r s t pumping operation the pressure i n the admission volume was increased more than four times. c)  Moisture was shown to be not the only impurity i n the helium carrier  gas, and therefore the CaClg purifying tower alone was not sufficient. Two charcoal traps were f i t t e d to the apparatus as indicated i n f i g . 2b.  -8-  u p  a>  § s  —©-  2  W  o Columns with Heaters  Toepler Pump  P i g . 2a Gas Chromatography Apparatus  C7  ^  F i g . 2b  5  Charcoal Trap System F i g . 2c  U-Tube Trap  -9-  While one was being used as a p u r i f i e r , the other was regenerated byevacuating and heating to about 300° C.  Separation of the products Previous experience i n this laboratory (9) had shown that two columns were necessary to separate the components of a mixture of products with such widely varying boiling points as that obtained from the decomposition of 1-butene.  The l i g h t products, i . e . methane, ethane,  e t c . , up to 1-butene had been successfully resolved on an alumina column ( i . e . an adsorption-elution column).  Products of higher boiling points  than 1-butene were separated on a T r i C*esyl Phosphate (TCP) column ( i . e . a partition elution column).  Both columns had been operated with r i s i n g  column temperature to decrease the elution times. In the present work, however, i t appeared desirable, with a view to quantitative measurements, to use one column only.  While the TCP  column had been found satisfactory to separate the products of boiling point higher than 1-butene (9), the l i g h t e r products were not satisf a c t o r i l y resolved on this column. An attempt was therefore made to separate a l l the products of the butene decomposition on a TCP column 12 feet long ( i . e . twice the length of the column previously used for this kind of work i n this laboratory). Fire brick was used as a support for the TCP to give a lower flow resistance (6) than that obtained with C e l i t e . The separation of the products was better on this column of double length.  A l l the l i g h t products were i n fact separated, except ethane  and ethylene.  The column was lengthened further to 18 feet ( i . e . three  -10-  times the original length) but even this was not sufficient to separate ethane and ethylene.  In the end i t was found that a 6 foot alumina  column was sufficient to separate a l l the products of the pyrolysis of 1,5-hexadiene i f i t was gradually heated to high enough temperature. It was found important to purge the column from high boiling compounds previous to each run.  A standard purging method was to heat the column to  210° C for about 30 minutes with the carrier gas flowing. The separation of the products of the hexadiene pyrolysis are shown i n the chromatogram ( f i g . 3)«  The l i g h t products are clearly separated.  Although two pairs of the products following 1-butene are not completely resolved, the separation was sufficient for identification of the products and for their quantitative estimate.  Product Identification A l l the light products were identified by comparison of present charts with those previously obtained i n this laboratory.  analysis  An indication of  the identity of the higher products was obtained by injection of a pure sample of a compound suspected to be present, together with the reaction products and thus obtajjiing re-inforcement of a particular peak.  Re-inforcement  of a particular peak i s , however, merely a necessary but not sufficient condition to prove the presence of a compound.  Positive identification was  effected with a mass spectrometer. The method adopted was to trap a particular peak. products was admitted onto the column as usual.  The mixture of the  The stopcock  i n f i g . 2a  was turned so that the gas stream was diverted from the flow meter d i r e c t l y into the atmosphere.  A U-tube with two stopcocks ( f i g . 2c) was cooled i n  0 O  5  J vj 10 22  80 Fig. 3  125  150  170  Gas Chromatographic Separation of the Pyrolysis Products  14 min.  12 200"C  -12-  l i q u i d nitrogen and kept ready to be connected to the outlet from the thermal conductivity c e l l *  As soon as the peak of the compound to be  trapped appeared on the chart, the U-tube was attached to the outlet and the gas stream allowed to pass through.  The trapped sample of the com-  pound concerned was then analyzed with a mass-spectrometer as described i n the Appendix.  Quantitative determination of the products It has been shown that the peak areas i n gas chromatograms are approximately proportional to the amount of the substance put through the column. (6)  This relationship was made use of i n the present quantitative  determinations of the pyrolysis products, although some modifications had to be made because the column was not operated at constant temperature. Also since internal standards for a l l the reaction products were not a v a i l able, approximations had to be used i n some cases to determine the products quant i t a t i v e l y . The procedure used was as follows:  the pure compounds which were  available were mixed i n the following proportions: Compound:  Percentage:  Benzene  11.9  1,5-hexadiene  13.9  cyclopentene  6.28  Butene-1  8.28  Propylene  48.0  Ethane  12.6  The above mixture was run through the gas chromatographic column  -13-  under exactly the same conditions as the mixture of reaction products and the peak areas thus obtained were related to the amount of the particular compound. The compounds for which no internal standards were available had to be determined quantitatively by an approximation.  RaisiHg; the temperature  causes the flow-rate of the carrier gas to decrease which i n turn influences the peak areas.  Therefore the variation of flow-rate of the carrier gas  with column temperature (or time, since the temperature r i s e as a function of time was well reproducible) was determined.  The thermal conductivity  of the compound to be determined was assumed to be equal to the thermal conductivity of the internal standard with the closest retention volume. Assuming that the peak areas were inversely proportional to flow rate of the carrier gas (6), correction was then made for the variation of the flow-rate between the peak of the internal standard and the peak of the compound to be determined.  The peak areas were measured with a plsjiimeter.  In general two correction factors were applied i n the quantitative determinations:  correction for the variation i n the c e l l current and i n  the c e l l temperature.  It was found that a c e l l current of around 200 ma.  •ef the peak area increased by approximately 1.7 # per ma. current increase. The peak area decreased by approximately 0.6 io when the c e l l temperature was increased by 1 ° C .  Qualitative and quantitative determination of hydrogen Although no hydrogen peak was obtained when analyzing for the products of the pyrolysis of 1,5-hexadiene using alumina column and He as carrier gas i t was suspected to be present because of the presence of certain other  -14-  compounds.  An activated charcoal column was used to separate hydrogen  from the other products.  Injection of a mixture of hydrogen and methane  indicated that these two gases are easily separated by the charcoal column at room temperature.  Analyzing for hydrogen with helium as carrier gas,  the sensitivity of the apparatus i s greatly reduced as the thermal conductivities of hydrogen and helium differ by a factor of 2 only. nitrogen was employed instead of helium. further s l i g h t l y modified;  Hence  The analytical method was  because of the low heat conductivity of nitrogen  (as compared to helium), less heat i s removed from the detection filaments i n the c e l l than when helium i s used as carrier gas. the filaments consequently increases.  The temperature of  Since the sensitivity of the detector  increases with temperature of the filaments, s t a b i l i t y of the base l i n e produced by the recorder was affected. from 200 ma. to 160 ma.  The c e l l current had to be decreased  The l a t t e r value was found sufficiently low to  maintain the base-line s t a b i l i t y .  Analytical Results The composition of the reaction mixture from the pyrolysis of 1,5hexadiene for 5 minutes at various temperatures i s given i n Table I.  All  i n i t i a l pressures (P. ..) were 70 mm. except for 506e C where P. . , was 100 mm. lint  nut  The figures show percentage of sample withdrawn from the reaction vessel.  -15-  Table I Products of 1,5-Hexadiene Pyrolysis (Mole jo) at Various Temperatures 5 minutes, P. . , — 70 mm ' init « G. Products  460  470.5  480  490  501  506  510  521  Hydrogen  1.0  2.0  3-3  4.5  6.3  6.5  7.0  7.8  methane  1.4  1.5  2.6  4.6  4.1  6.3  5.4  7.7  ethane  0.9  0.8  1.5  2.2  2.0  3.4  2.3  2.7  ethylene  4.7  4.2  6.7  9.2  9.2  9.2  8.4  10.3  propane  0.11  0.17  0.28  0.8  0.6  1.3  1.1  1.3  18.2  16.6  26.8  35.3  35.6  36.7  31.0  35.3  1-butene  3-2  2.9  4.4  5.7  5.3  5-3  4.5  5.1  cyclopentene  1.7  1.3  1.3  2.3  0.9  0.8  0.85  1.0  cyclopentadiene  3-2  2.6  4.5  5.4  3.9  3.1  2.6  2.5  1-hexene  1.7  . 1«5  2.4  2.7  2.2  1.6  1.9  2.2  1.4  1.6  2.1  0.7  1.0  0.6  0.7  43.5  36.2  16.8  11.2  5.3  4.2  3-2  3.2  2.4  2.9  4.6  7.7  6.4  8.7  6.9  8.8  73-4  74.1  76.8  93.7  82.5  89.1  75.8  88.6  propylene  cyclohexene & 1.4 methylcyclopentene 1,5-hexadiene benzene Total mole i»  From the above table i t can be seen that the analysis has accounted for approximately 70 - 90 % of the reaction products. several reasons for t h i s :  There may be  (l) Uncertainty i n the quantitative gas chroma-  tographic determinations (discussed i n a later chapter).  (2) The sensi-  t i v i t i e s for those products for which no internal standards were available  -16-  (approximately one half of the products) had to be estimated from the values for similar compounds.  (3) Some loss might possibly also be  accounted for by assuming that some of the heavier products are so strongly adsorbed to the column that desorption does not take place. From Table I i t can be seen—at least qualitatively—that the " t o t a l mole$", i . e . products accounted for, increases with increased temperature. It has been mentioned e a r l i e r that the characteristics of the pressurechange vs. time curves for different temperatures ( f i g . 4) indicate that more of the heavier products are formed at low than at high temperatures. If this i s the case the loss should be higher at lower temperatures than at higher temperatures. Although some doubt exists regarding the absolute values of a few of the concentrations given i n Table I, values for the compounds for which internal standards were available should be reasonably accurate.  For  the other compounds the relative concentrations at the different temperatures should be reliable and are of some considerable interest.  On the  average about 85$ of the pyrolysis products have been accounted for. This should be sufficient to be able to make reasonable assumptions about the principal features of the mechanism of 1,5-hexadiene decomposition.  R E S U L T S  -17-  R E S U L T S  I. THE PYROLYSIS OF 1.5-HEXADIENE Order of the reaction The differential method was applied to determine the order of the pyrolysis of 1,5-Hexadiene.  The reaction was f i r s t order but rose to higher  values as extensive decomposition occurred. Pressure increase i n reaction system In f i g . 4 the relative pressure change i n the reaction vessel as a function of time i s given for temperatures between 4 6 0 ° and 5219 C. pressure dependence curves show a characteristic shape.  The  At lower temper-  atures the curvature is convex to the time coordinate while at higher temperatures the curvature i s concave to the time coordinate.  This  behaviour should indicate that more products of lower molecular weight than that of 1,5-hexadiene are formed at higher temperatures while r e l a t i v e l y more products with higher molecular weight than that of hexadiene are formed at lower temperatures.  The pressure change and the distribution of the  products w i l l be discussed  later.  An attempt was made to make an estimation of the overall activation energy from the i n i t i a l pressure change i n the above mentioned temperature range.  The relative pressure change after both l / 2 minute, 1 minute and  2 minutes of reaction was attempted, used for an Arrhenius plot but unsuccessfully.  Rate and overall energy of activation for the pyrolysis of 1.5-hexadiene As was mentioned above, the kinetic order of the hexadiene pyrolysis  Fig. 4  Pressure Change vs. Time for 1,5-Hexadiene  -19-  i s approximately f i r s t .  F i r s t order rate constants were therefore  calculated for reaction time 5 minutes at different temperatures and are given i n table I I . F i g . 5 i s the Arrhenius plot for the pyrolysis of 1,5-hexadiene. The activation energy obtained i s 31'3 kcal/mole and the frequency factor  8.5 found to be 10 *  —1 min  .  As a check on the constancy of the rate  constants with time several experiments were performed, the results of which are l i s t e d i n table III.  Using the values given i n the table  an activation energy of 32.6 kcal/mole was obtained for the decomposition of 1,5-hexadiene.  This i s approximately the same value as the one  obtained using the rate constants for reaction time of 5 minutes. Since the values at 1 and 3 minutes reaction time are considered more unreliable, both because of the high uncertainty i n the reaction time and i n the determination of the percentage of decomposition, only the values obtained for the reaction time of 5 minutes were used for the Arrhenius p l o t . Rates of formation and overall activation energies for the ligfot hydrocarbons In order to calculate the kinetic rate constants for the formation of the l i g h t products from methane to 1-butene i t was assumed that their formation i s dependent upon the f i r s t power of (hexadiene).  It has been  previously shown (9) that the above dependence exists i n the pyrolysis of 1-butene.  As the products of the pyrolysis of 1,5-hexadiene are similar  to those obtained i n the pyrolysis of 1-butene, i t was supposed that the mechanisms are very similar and the above assumption should be j u s t i f i e d . The mathematical expression for the rate constants for the formation  -20-  Table II Rate Constants for the Over-all Decomposition of 1,5-Hexadiene Reaction time 5 minutes. l/T  Temp. »C.  P. . . = 70 mm.  fo decomp.  k (min"*'')  -In k  f  «K.  x 10~ 5  460 733  1.365  50.5  0.705  0.141  1.960  470.5 743-5  1.345  61.7  0.960  0.192  1.73  480 753  1.328  69.5  1.19  0.238  1.44  490 763  1.311  79.0  1.56  0.312  1.17  501 774  1.291  89.6  2.27  0.454  0.790  510 783  1.278  92.6  2.60  0.520  0.653  p  Table III Rate Constants for the Over-all Decomposition of 1,5-Hexadiene for Different Reaction Times Temp.  k (min - 1 )  k  -log10l  1 min.  3 min.  5 min.  510  0.539  0.543  0.520  0.534  +0.273  501  0.554  0.545  0.454  0.517  +0.287  490  0.307  0.275  0.312  0.298  +0.526  9C.  -215  Pig. 5  -22-  of the products was derived as follows: p  = (product) P p^ = (hexadiene)  k = rate const, for formation of p. P k^ = " " " decomposition of hexadiene  Pho= ^ h e x a d i e n e ) i n i t i a l  p dt  d p  =  k  dt  d  P~ P  Pp  =  =  ph  P  ^  I  K  where  ph = p k o e ' V  G  -k. t P h o ee Thi dt _ k  J P  Jj> P  h 0  (1 " e"V)  *h No approximation can be used since k^t i s neither small nor large. Prom the above expression the rate constant for the formation of a product can be obtained: P,  \o  1 - e-V  By means of the above expression the rate constants were calculated and are given i n Table 17. Pigs. 6 and 7 are the Arrhenius plots for the formation of the saturated and unsaturated l i g h t products respectively.  Table V gives the energies  of activation and the frequency factors obtained from the above graphs.  -25-  Table IV Rate Constants (min - 1 ) for the Formation of the Light Products 460" Methane  480<»  4.24 * 10.6 x 10"° x 10~  5  4900  5010  5100  19.7 , x 10"°  27.0 x 10"0  44.0 , x 10"  65.9 .  15.0  18.5  22.7  Ethane  3.05  6.58  9.28  Propane  0.336  1.17  3-38  4.0  9.15  11.4 87.8  27.7  39-5  60.8  68.1  Propylene 55.6  111.2  150.0  235.0  257.0  18.1  24.1  34.8  56.6  1-Butene  9.65  x 10~  0  14.5  Ethylene  5210  501 45.2  Table V Energies of Activation and Frequency Factors for the Formation of the Light Products Product  E  £1  (kcal/mole)  A (min""')  Methane  50.1  io  Ethane  59.1  IO9'1  Propane  65.4  IO16'1  Ethylene  52.8  IO8'9  Propylene  56.0  io '  5  1-Butene  55.9  io *  0  1 2  8  8  '  6  -24-  -2.500 1.250 1  1.300 1.350 x 10" l/T F i g . 7 Arrhenius Plots f o r the l i g h t unsaturated Products i n the Pyrolysis of 1,5-Hexadiene 3  -26-  Temperature and time dependence i n the formation of products Table I [from quantitative determination of the products] was used to construct the graphs i n figures 8, 9a, and 9b giving the concentrations of the different products at different temperatures after a reaction time of 5 minutes.  It i s seen that the concentrations of the saturated pro-  ducts increases with temperature while the concentrations of the unsaturated products increase to a certain point and then gradually decrease.  This  fact can be explained by decomposition (presumably attack by free radicals present i n the system) of the unsaturated products formed.  More attention  w i l l be paid to this fact i n a later chapter. Similarly the dependence of the concentration of the products at 5 0 1 ° C on the reaction time was plotted i n f i g a . 10 andv11 using the values given i n table V I .  Also i n this case there i s an indication that the unsaturated  products, both acyclic and c y c l i c , undergo decomposition i n later stages of the reaction.  -27-  40 Mole fo  30 O Methane % Propylene A Ethane A  1,5-Hexadiene  • Propane 20  • Ethylene V 1-Butene  10  460  470  Fig.8  480  490  500  510  520 C  Temperature  Light Products of the Pyrolysis of 1,5-Hexadiene vs. Temperature Reaction Time 5 Minutes  -28-  Mole <fo  10  A  Benzene  O  Hydrogen  •  Cyclohexene & Methylcyclopentene  460  470  -1  480  1  490  L  500  510  520 C  Temperature  P i g . 9a  Mole fc 10 _  O Cyclopentene # Cyclopentadiene A 1-Hexene  460  470  480  490  500  510  520 C  Temperature  F i g . 9b F i g . 9a and Fig. 9b Hydrogen and the heavy Products of the Pyrolysis of 1,5-Hexadiene vs. Temperature Reaction Time 5 Minutes  -29-  -30-  -31-  Table VI Decomposition products of 1,5-Hexadiene pyrolysis as a function of time at 5 0 1 ° C.  Product  1 minute  Methane  1.2 $>  3-2 1»  4.1 %  6.0  Ethane  0.40  1.1  2.0  2.5  Ethylene  4.2  7.3  9.2  9.2  Propane  0.0  0.26  0.60  1.0  Propylene  3 minutes  5 minutes  10 minutes  i»  15.5  28.9  35.6  33-0  1-Butene  2.5  4-4  5.3  4.9  Syclopentene  2.0  1.6  0.87  0.75  Cyclopentadiene  2.1  3-1  3-9  2.6  1-Hexene  1.4  2.1  2.2  1.8  Cyclohexene & methylcyclopentene  0.93  1.1  0.68  0.76  10.7  5.3  3-2  6.4  6.9  1,5-Hexadiene Benzene  35.9 1.4  5.3  -32II. PYROLYSIS OF 1-BUTENE IN THE PRESENCE OF DIALLYL Introduction It has been shown (4) that the primary s p l i t i n the pyrolysis of 1-butene i n the temperature range round  500°  C occurs i n the following  way: CH 2 = CH - CH 2 - CH 5  > CH 2 = CH - CHg. + ^CHj  [l]  It has also been shown that the addition of methyl radicals increases the rate of pyrolysis of 1-butene, increasing the amount of the light products. From that i t was. deduced that methyl radicals play an important part even i n the unsensitized thermal decomposition of 1-butene and that i t s pyrolysis most probably proceeds by free radical chain reactions. In the present work the fate of the a l l y l radical i n the pyrolysis of 1-butene was taken up.  The concentration of a l l y l radicals was increased  by addition of 1,5-hexadiene, which on decomposition yields a l l y l s i n the primary step.  The condition that a sensitizer should decompose much more  readily than the substrate i s satisfied i n the case of 1,5-hexadiene, the difference of activation energies of 1-butene and 1,5-hexadiene being almost 35 kcal/mole.  The product distribution for varying amounts of the  sensitizer was determined by gas chromatography.  Results for the decomposition of 1-butene sensitized by d i a l l y l A preliminary run was made by decomposing 1-butene by i t s e l f at  5069 c and  the pressure change i n the system studied.  Then approximately  5 i<> by volume of 1,5-hexadiene was mixed with 1-butene and pyrolysis was carried out under the same conditions (total pressure: 12 gives the pressure increase with time i n both cases.  200 mm).  Fig.  The slopes of  - 33  AP  -  (mm) 30  slope = 3.0 mm/min  P i g . 12 Pressure Change vs. Time at 506°C. I n i t i a l Pressure 200 mm  -34-  tangents drawn to these curves should, at least i n the early stages of the reaction, give an indication of the relative rates.  Thus the ratio  of the rates "sensitized to unsensitized" at the reaction time of 1 minute was found to be 6.5.  This indicates that increase i n concentration of  a l l y l does increase the decomposition of 1-butene. A series of pyrolyses with different concentrations of the sensitizer was carried out.  The purpose of this was to relate the formation of the  products to the concentration of a l l y l , which i n turn should be proportional l/2 to (hexadiene) ' symmetrical.  i f the primary s p l i t i n the 1,5-hexadiene pyrolysis i s  Table VII gives the concentrations of the products for the  different concentrations of the sensitizer. In f i g . 13 the dependence of the formation of methane, cyclopentene, cyclopentadiene, cyclohexene and methylcyclopentene on the concentration of d i a l l y l i s given for reaction time of 5 min.  It i s seen that the  rates of formation of the above products are roughly proportional to (diallyl)37'2.  -35-  Table VII Composition of the Reaction Mixture for Sensitized "Decomposition of 1-Butene with 1.5-Hexadiene 5 min. at 506a <j.  Product  0  Methane  i»  i»  6.7  io  9.2  i>  11.4  3-1 $  5.5  1.9  4.4  4.4  5.0  5.5  5.7  Ethane  0.7  2.0  2.4  2.4  2.6  2.8  Ethylene  1.2  2.8  5.5  4.2  4.2  5.2  Propylene  2.9  6.3  9.0  9.0  9.9  11.5  52.2  51.4  55-6  51.1  52.6  29.8  0.7  0.9  1.5  1.1  1.4  1.2  Cyclopentadiene 1.0  1.8  2.0  2.0  2.2  2.4  0.0  0.5  0.7  0.5  0.9  1.0  Cyclohexene & 0.0 Me-cyclopentene  0.5  0.4  0.4  0.5  0.6  Benzene  0.6  0.9  0.7  0.9  1.5  1-Butene Cyclopentene  1-Hexene  0.0  PRODUCTS OF SENSITIZED DECOMPOSITION OF 1-BUTENE VS.  {% DIALLYL)  Fig.  13  -37-  III. PYROLYSIS OF n-BUTANE IN THE PRESENCE OF 1.5-HEXADIENE In order to compare the reactivity of a l l y l with an olefin and a paraffin hydrocarbon, the sensitized decomposition of n-butane was studied i n one experiment. n-Butane was decomposed by i t s e l f and i n presence of appr. 5 i° by volume 1,5-hexadiene at 506° C.  The products of the pyrolysis of n-butane  had previously been determined ( 2 l ) .  The pyrolysis of n-butane by i t s e l f  at 506" C gave methane, ethane, ethylene and propylene as the principal products for a reaction time of 5 minutes (Hg was not analyzed f o r ) . The sensitization of the pyrolysis of n-butane with 1,5-hexadiene increases the amounts of products present when n-butane i s decomposed by i t s e l f and i n addition 1-butene and one compound with higher molecular weight than 1-butene are formed.  The retention volume of the above  compound appeared to be equal to the retention volume of 1-pentene and i t i s most probable that the compound concerned actually i s 1-pentene. This may be expected since both a l l y l and ethyl radicals are present i n the system and upon combination would y i e l d 1-pentene. Table VIII gives the products of the pyrolysis of n-butane and sensitized pyrolysis of n-butane by 1,5-hexadiene. From the ratio sensitized/non-sensitized i t can be seen that the formation of unsaturated products i s favoured by the increase of concentration of a l l y l .  -58-  Table VIII  by Addition of Appr. 5 $ by Volume of 1,5-Hexadiene at 506° C. Time 5 Minutes.  Non-sensitized  Reaction  P. . . = 200 mm. mit  Sensitized  Sensitized Non-sensitized  * Methane  2.0  4.9  2.5  Ethane  0.9  2.0  2.2  Ethylene  0.9  3-4  3-8  Propylene 1-Butene 1-Pentene Benzene  ?* negligible  ?*  5-5  1.7 0.85 negligible  * Propylene and n-butane are not completely resolved on the alumina column so that the absolute values could not be obtained. Since the propylene peaks were extremely sharp, the ratio of the peak heights was taken as a measure of the re1. amount of propylene i n the two runs.  -39-  IV. PYROLYSIS OF 1-BUTENE IN THE PRESENCE OF ACETALDEHYDE It has been shown (4) that methyl radicals play an important role i n thermal decomposition of 1-butene and that methyl radicals generated by pyrolysis of mercury dimethyl sensitize the decomposition of 1-butene. In order to show the effect of methyl radicals obtained from a source other than mercury dimethyl on 1-butene, and i n order to get acquainted with the general method of pyrolysis of gases, pyrolysis of 1-butene i n presence of acetaldehyde was carried out. It i s considered as well established that the thermal decomposition of acetaldehyde i s a free radical chain process (11,17)•  Methyl radicals  are formed i n this reaction and have been detected by several workers (ll,17).  It would be therefore expected that reactions between these  and 1-butene would take place i n analogy with the work mentioned above. From a pressure v s . temperature plot i t was found that 1-butene starts to decompose at about 480° C  At 477° C there i s essentially no  decomposition of 1-butene while acetaldehyde was found to decompose appreciably at 440 - 450° C.  The temperature of 477° C was therefore  chosen as a suitable temperature and two series of experiments were performed: I. II.  acetaldehyde and helium i n molar ratio 1 : 3 acetaldehyde and 1-butene i n molar ratio 1 : 3 *  The t o t a l i n i t i a l pressure was 200 mm. i n both cases. The degree of decomposition of acetaldehyde was determined by taking the total amount of methane and carbon monoxide (20) as a measure of the decomposition.  Percentage of 1-butene decomposed was estimated by gas  chromatography from the amount of 1-butene which remained i n the reaction  -40-  vessel after each reaction was completed. The analytical results of the above pyrolyses are given i n Table IX. Table IX Percentage of Decomposition of Acetaldehyde and 1-Butene at 477° C. 1 minute  3 minutes S e r i e s  Acetaldehyde  8.2  11.8 S e r i e s  Acetaldehyde  3.2  1-Butene  8  5 minutes I -  24  II  4.8 11  7.9 19  The above experimental results are plotted i n f i g . 14.  It can be  seen that while the i n i t i a l rate of decomposition of acetaldehyde i n presence of helium i s 6.0 $ min"*''", i t i s only 2.8 $ min 1 when 1-butene i s present.  Thus acetaldehyde decomposes more than twice as fast i n the  former case.  Suppression of the decomposition of acetaldehyde i n later  stages of the reaction i n presence of 1-butene seems to be s t i l l more pronounced.  1-Butene, which practically does not decompose at the temperature  of the experiments by i t s e l f , decomposes quite appreciably when acetaldehyde i s present. Since the thermal decomposition of acetaldehyde i s known to be a free radical chain process, the decrease i n rate of i t s decomposition i n presence of 1-butene must be therefore caused by decrease i n the steady state concentration of the free radicals which are the chain c a r r i e r s .  Thus  -41-  Decomp. (*) 30  slope =  min~* Acetaldehyde(I)  20  L  A ^ _ . "~ "l-Butene(ll)  slope = 2.8#tnin* 10 cetaldehyde ( I I )  5 min. F i g . 14  Time  Extent of Decomposition at 477 C. I n i t i a l Pressure 200 mm I. Acetaldehyde + Helium  (l :  3)  I I . Acetaldehyde + 1-Butene ( l : 3)  -42-  reactions of the free radicals formed must have taken place with 1-butene. Also, i f 1-butene was inert to attack by methyl or/and other radicals, more decomposition of acetaldehyde would rather be expected i n the presence of 1-butene than of helium.  Molecules of 1-butene (being poly-  atomic molecules with a high number of degrees of freedom) would be expected to enable a far better energy transfer during the pyrolysis. The nature of the products formed when 1-butene i s attacked by the above free radicals was not determined i n these preliminary experiments.  D I S C U S S I O N  -43-  D I S C I S S ION  PRECISION OF TBE QUANTITATIVE DETERMINATIONS BY GAS CHROMATOGRAPHY There are several factors influencing the reproducibility of quantitative gas chromatographic determinations (6).  The main ones have already  been mentioned when the quantitative determination of the products was described:  flow-rate of the carrier gas, column temperature (one of the  factors determining the flow-rate), thermal conductivity,, c e l l current and c e l l temperature.  In addition to these variations i n room temperature  could produce variation i n the amount of the gaseous sample admitted into the by-pass of the admission system.  There i s also uncertainty i n deter-  mining the peak area with a planimeter. The following are the estimated or assumed maxima of uncertainties i n the above parameters: 1)  uncertainty i n flow-rate (including the column temperature)  - 3 fo  2)  uncertainty i n c e l l current (after correction was applied)  - 0.8 fo  3)  uncertainty i n c e l l temperature (after correction was applied)  negligible  4)  uncertainty i n room temperature  negligible  5)  uncertainty i n planimeter measurements  - 2 $ - 5.8 <?o~6 <fo  In addition to the above uncertainties there might be a number of factors leading to systematic errors which might have increased the maximum of the total error.  -44-  UNCERTAINTY IN THE VALUES OF THE KINETIC HATE CONSTANTS The mathematical expression for the f i r s t order rate constant i s : [2] where t is. the reaction time (min i n our case), p Q i n i t i a l pressure of reactant, p pressure of reactant at time t .  I f a i s the degree ofifccom-  The kinetic constant i s i n this case dependent upon two variables the uncertainty i n which w i l l determine the total uncertainty of the value of the rate constant. The maximum uncertainty i n the degree of decomposition, a, i s , i n the best case, equal to the uncertainty i n the quantitative gas chromatographic determinations.  In cases where l i t t l e decomposition took place  the peak areas being subtracted from each other are of the same order of magnitude and consequently their difference w i l l be of a higher degree of uncertainty than the peak areas themselves.  Since normally two to  four parallel analysis-runs were made and the results averaged, i t should be reasonable to take the uncertainty i n a equal to the uncertainty i n the peak areas, or - 6 °?o. The reaction time was 5 minutes i n a l l the runs the data from which were used to calculate the rate constants.  The d i f f i c u l t y i n estimating  the starting- and end-point of the reaction was found to lead to a maximum uncertainty i n the reaction, time of i 5 The variation i n the rate constant with t and a i s given by:  -45-  and the maximum relative uncertainty by: lAti . l*vl - ^ " K l n JAklmax = - _t  1  ^  +  1 , 1 t  W  Putting:  +  +  T^  l n  (1-)  1*! '""'max  [4]  a = -|-  I  U t l  max  =  1 2  t  =  5 (min)  max  =  1  A  |Aa|  one gets:  |Ao max 1  k  =  1-a T  +  (Akl^  =  6 100  X  5  x  +  ?  100  "**  ^  k The above considerations were made on the assumption that there was no variation i n the furnace temperature during a run.  As was mentioned  i n the paragraph "Pyrolysis Apparatus", the furnace temperature (assumed to be equal to the temperature at which the reaction takes place) was kept constant to within - 1 « C.  Considering the variation of temperature one  obtains the following:  „ - a k = A e RT E - a E RT  and  l Ak  k  max  =  a R  E  '  1  1  —  1 — T2  IA TI  1  1  max  C5]  -46-  E  Putting:  a  32 000 cal/mole  R  2 cal/mole-deg  T  773° K 1» K  i t i s obtained: Ak max Considering uncertainty i n both t , a, and T, the max. uncertainty i n k i s then - 10 f>.  Consequently,. most probably no more than two figures  i n the rate constants obtained are significant.  THE OVER-ALL ACTIVATION ENERGY FOR THE DECOMPOSITION OF 1.5-BEXADIENE It was mentioned i n a previous section that the attempt to determine the over-all activation energy for the pyrolysis of 1,5-hexadiene from the i n i t i a l pressure changes at different temperatures was unsuccessful. Although this method of determining the over-all activation energy had successfully been applied i n the case of 1-butene (9) the negative result i n the present work should not be surprising.  A large number of products  i s formed i n the pyrolysis and there i s a considerable variation with temperature of the distribution with respect to their molecular weights (see Table I and Figs. 8, 9a and 9b).  Consequently, the pressure change  produced i n the pyrolyses at different temperatures cannot be assumed as being proportional to the extent of the reaction i n this system. Furthermore, the i n i t i a l pressure of the reactant i n the reaction vessel was rather low (approx. 70 mm) so that the i n i t i a l pressure changes involved were correspondingly small and the values obtained therefore uncertain.  -47-  The pressure change can be used as a measure of the extent of a reaction taking place i n the gas-phase only i f the distribution of the products i s more or less constant with respect to temperature and i f the pressures involved can be measured with sufficient accuracy. The determination of the over-all activation energy from the gas chromatographic analyses yielded the value 31 «3 kcal/mole.  The d i s -  sociation energy for the central bond i n 1,5-hexadiene had been estimated (22) to be around 42 kcal/mole.  Taking the heat of formation of the a l l y l  radical as 3 2 . 3 kcal/mole ( 1 3 ) and the heat of formation of d i a l l y l as 20.6 kcal/mole (calculated from the heat of hydrogenation, -60.5 kcal/mole ( 3 ) , and the heat of formation of n^hexane, -39.96 kcal/mole (8))  i t can be  calculated that the heat of the reaction: CH 2 = CH - CH 2 - CH 2 - CH = CH 2 —> 2 CHg = CH-CHg.  [6]  i s 44 kcal/mole. The value obtained for the over-all activation energy i s considerably lower than both of the above values.  This fact indicates that reactions  other than reaction [6] must take place, such as hydrogen abstraction by a l l y l radical and a l l y l addition to double bonds i n the system. Although the activation energies for these two processes are not known, i t i s certain that they are far below the bond dissociation energy for the central bond i n d i a l l y l .  Thus the over-all value of 31*3 kcal/mole should  not be surprising. Conversely, the fact that the over-all activation energy found i s considerably lower than the heat of reaction [6] could be used as an evidence that free radical processes take place. It can be shown (7, p. 232) that for a mechanism involving a single  -48-  chain carrier with second-order termination, as follows initiation  =» R +  propagation  R +  ....  termination  R+ R  » R + 5»  i f the activation energy for termination i s non-zero or i f chain termination i s second order (as above) the apparent (over-all) activation energy may be significantly less than the activation energy for the i n i t i a t i o n step.  MECHANISM OF THE PYROLYSIS OF 1.5-HEXADIENE General Three p o s s i b i l i t i e s exist for the mechanism of pyrolysis of  1,5-  hexadiene: (a) molecular mechanism (b) free radical mechanism (c) combined molecular and free radical mechanism. It was seen that a large number of both aliphatic and c y c l i c products i s formed i n the pyrolysis.  Since such a large spectrum of products  could not be expected i f the mechanism was purely molecular, p o s s i b i l i t y (a) should thus be ruled out. It has been shown (l2) that at high temperatures (690 - 8 9 0 ° C) 1,5-hexadiene decomposes by s p l i t t i n g the central bond: CH 2 = CH - CH 2 - CH 2 - CH = CH 2  > 2 CH = CH - CE^ 2  [6]  The results obtained i n the present work also show that s p l i t of the central bond must have taken place.  If the primary step i n the pyrolysis at  present conditions involved exclusively reaction [6], a pure free radical chain mechanism could be postulated.  -49-  On the other hand, cyclohexene, benzene and hydrogen have been found among the reaction products.  As i t i s shown i n the following chapter,  the formation of cyclohexene and benzene can be explained on the basis of a free radical mechanism.  But the presence of these products might equally  well indicate that the following molecular rearrangement followed by dehydrogenation also takes place to some extent: CH,  •CH,  At this stage i t can therefore be concluded that p o s s i b i l i t y (c) i s most likely.  Reactions of the a l l y l radical (l) Hydrogen abstraction by a l l y l Since the decomposition of 1,5-hexadiene probably involves formation of two a l l y l radicals i n the primary step, the a l l y l radical must play an important part i n the over-all reaction mechanism. Because of the resonance energy of 17 kcal/mole (2,5) the a l l y l radical would be expected to be rather unreactive i n thermal reactions.  However,  i t can be shown from thermochemical data (26) that the weakest bond i n a l l y l (a C - H bond) has a strength of about 68 k c a l .  The life-time of a l l y l  should thus be long enough to build up an appreciable steady state concentration and permit various reactions involving a l l y l to take place. McNesby and Gordon have recently investigated the thermal decomposition  -50-  of cyclopentane-dg and cyclopentane-acetone-dg mixtures (14) and have shown that reactions of a l l y l radicals are involved.  According to the  authors appreciable H-abstraction, both from cyclopentane and from CDjCOCD^ takes place at 453° and that the degree of H-abstraction i s s t i l l higher at  5000 c. The main product of the pyrolysis of 1,5-hexadiene i s propylene. This suggests that H-abstraction by a l l y l takes place readily i n the present system.  It would be expected to take place mainly by the reaction:  CH2=CH-CH2. + CH2=CH-CH2-CH2-Cft=CH2  >-CH2=CH-CH5 + CH2=sCH-CH-CH2-CH=CH2  since a resonance stabilized radical i s formed.  It has the following two  contributing forms: CH2=CH-CH-CH2-CH=CH2 *  > .CH2-CH=CH-CH2-CH=CH2  Hydrogen abstraction by a l l y l would probably also take place from the products of the decomposition which, upon hydrogen abstraction, would y i e l d resonance stabilized radicals. The following products would be expected to undergo such reactions: 1-butene cyclopentene cyclopentadiene 1-hexene cyclohexene methyl-cyclopentene Abstraction from propylene would also take place, but i t would only produce back propylene and a l l y l . The side reactions mentioned would be negligible i n the early stages of the decomposition, but may be quite extensive i n the l a t e r stages when  -51-  the concentrations of the products concerned are appreciable.  Evidence  for attack of the above products by a l l y l (and possibly other) radicals  is  indicated i n f i g s . 10 and 11 where the concentration vs. time graphs for a l l the products l i s t e d above show a maximum. There i s also a drop i n concentrations of these products with increased temperature at constant reaction time (see f i g s . 8, 9a and 9b).  In the  case of a l l y l radical, which i n general reacts slower than the nonstabilized alkyl radicals, the extent of hydrogen abstraction would be expected to be quite temperature-dependent.  Drop i n concentrations of  certain products from a certain temperature on may also be a good indication of increased hydrogen abstraction by a l l y l r a d i c a l .  (2) Addition of a l l y l to double bond Addition of a l l y l radical to the double bond of olefins has not previously been reported.  It appears, however, that such an addition must  have taken place i n the present system.  The formation of cyclopentene,  cyclopentadiene and methyl-cyclopentene i s d i f f i c u l t to explain i n any other way.  In the present system there are two principal ways of addition  to 1,5-hexadiene  possible:  (a) addition to the terminal C-atom i n d i a l l y l :  CH,  2  CH CE CH,2 .CH  CH-CH2-CH2-CB>*CH2  \CH-CH -CH -CH=CH 2  CH,  2  2  2  -52-  This large radical would l i k e l y be too unreactive to abstract hydrogen. It seems rather more probable that i t s fate is the following: ,CH„  \  CH,  CH„  .CH-  -RH  -*  r  .CH, CH.  CH-CH2-CH2-CHaCH2  CH^  3H  CH-CH 2 .  +  .cH^jj^  CH—CH  CH  +RH - R  + RH - R  +H  Cfl 2  CH 2  + .CH 2 -CH 2 -CH=£H 2  CH:  /  CH-CH0-CH0-C&=CH0 , 2 2 2  + RH - R  1-butene  propylene  (b) addition to the non-terminal double bond i n d i a l l y l : CH2=CH-CH2.  +  CH2=CH-CH2-CH2-CHaCH2 CH2-CH2-CH=CH2  CH2-CH2-CH=CH2 ,CH CH,  / C  H  2  CH=—CH This radical i s identical with the one obtained above.  Addition of a l l y l  to both double bonds of 1,5-hexadiene would y i e l d products impossible to detect by the present method of analysis.  Since the concentration of  propylene i s appreciable from the early stages of the reaction, addition of a l l y l radical to propylene has to be considered:  CH2=CH-CH2. + CH2aCH-CH5  >  CBg  .CH-CHj  CH=  CH 2  CH, 0H2  CIKH,  ,CH 3 t!H=CH Addition to the non-terminal carbon atom would lead to the same products: CH2=*CH-CH2. + CH2=CH-CH5  •  .CH^ CH. 2  CH-CH. I 3  CH  CH 2  .CH. same products  <  CHg .CH  CH-CIL^ CH 2  (3) Combination of a l l y l with other radicals Apart from hydrogen abstraction and addition to the double bond the fate of the a l l y l radical may be combination with other radicals present. As w i l l be indicated i n a subsequent chapter, hydrogen atoms areassumed to be present i n the system.  Thus combination of an a l l y l with a hydrogen  atom would y i e l d a propylene molecule.  Combination with a methyl would  give a 1-butene molecule and recombination of two a l l y l s would produce the parent, 1,5-hexadiene.  -54-  The fate of the 1.5-hexadienyl radical Since both the a l l y l radical and unreacted 1,5-hexadiene are present i n quite high concentrations i n the beginning of the pyrolysis, the abstraction of hydrogen from hexadiene by a l l y l must be quite extensive. The rate of formation of 1,5-hexadienyl radical must consequently be rather high. It seems that the hexadienyl radical formed may mainly disappear i n three different ways: (l) Rearrangement:  CH  I2 CH  2  It may appear surprising that no cyclohexadiene, which would be expected to 1 be the intermediate i n the dehydrogenation process, has been found among the reaction products.  The reason may be that the radical  obtained upon hydrogen abstraction from cyclohexadiene has a high degree of resonance s t a b i l i z a t i o n .  Hydrogen abstraction from cyclohexadiene would  thus readily take place because of the high resonance energy of the radical formed.  Abstraction of hydrogen from this radical would be far more pro-  bable than hydrogen abstraction by, the r a d i c a l .  -55-  (2) Decomposition There are several ways i n which the 1,5-hexadienyl radical could be thought to decompose.  It appears, however, that such decompositions  would also necessarily lead to compounds l i k e butadiene and allene which have not been found among the. products.  The resonance energy of the  radical i s probably sufficiently high that the life-time i s appreciable and the radical rather undergoes re-arrangement as mentioned above. (3) Combination with other radicals The third p o s s i b i l i t y for the disappearance of the 1,5-hexadienyl radical may be combination with other radicals to y i e l d products which may be the ones that were suspected to suffer irreversible adsorption to the alumina column.  Since the steady state concentration of the 1,5-hexadienyl  radical i s presumably quite high, the t o t a l amount of such addition products might have been appreciable.  Formation and reactions of other radicals Hydrogen atoms: formation:  CH^H-CHg R  > allyl + H  (22)  > M+H  where M i s an unsaturated molecule and R may be cyclopentenyl or cyclohexenyl. reactions:  unsaturated product + H —?  saturated or p a r t i a l l y saturated product  R + H  RH  CH2=CH2 + 2H  > C^g  CH2=CH-CH5 + 2H 1,5-hexadiene + 2H  * CH -CH -CH 5  2  5  1-hexene  -56-  Methyl: formation:  CH2=CH-CHj  • CH2=CH. + CH^  -*rv  0" 3 cs  CH^ reactions:  CH-CH2 / 3 • CH—CH2  -  +  -  c  h  3  > f ^ l + CH, L J 3  GHj + RH —> CH 4 + R CH, + CH 2 -—>C-H,. 3 3 2o CHj + a l l y l  > 1-butene  Possibly addition to the double bonds of 1,5-hexadiene and some of the products giving compounds which have been adsorbed irreversibly to the alumina column. Vinyl: formation:  CH2=CH-CH^ CH2=aCH-CH2-CH2.  reactions:  CH2=CH. + RH  -  CH^aCH. + .CH^ * CH2=CH. + CH^CHg * CH2=CH2 + R  possibly also addition to the double bonds. Butenyl: formation:  CH„ !H„  CH CH-CH„ CH„-CH=CH«  CH=CH reactions: possibly  CHgaCH-CHg-CHg. + RH CH2=CH-CH2-CH2.  ^ CHg  JE  CH=CH ^ 1-butene + R CH2=CH. + CH2=CH2  + .CH2-CH2-CH=CH2  -57-  The c y c l i c radicals: The cyclopentenyl and cyclohexenyl radicals probably expel to y i e l d unsaturated compounds.  H-atoms  H-abstraction by the radicals also takes  place to some extent to produce cyclopentene and cyclohexene respectively.  Mechanism for the formation of the pyrolysis products Summarizing the discussion of a l l the above mentioned elementary processes the mechanism of the pyrolysis of 1,5-hexadiene could be represented by the following scheme: A. Molecular rearrangement  dehydrogenation'  B. Free radical reactions Formation of: (l) methane:  .CK"5 + RH  > CH 4 + R  (2) ethylene:  GH_=CH + RH  (3) ethane:  CH 5 + CHj-  * CH0=CH_ + R >- CgHg  CHgsCSHg + 2H (4) propylene:  5- C 2 H 6  CH_sCH-CH„. + RH 2  2  2  3  * CH0=CH-CH, + R . e.  (5) propane:  CH.=CH-CH, + 2H — * CH,-CH 0 -CH,  (6) 1-butene:  eH2=CH-CH2. + .QH5 —  3  CH =GH-CH -CH . + RH 2  (7) cyclopentene:  2  2  allyl + diallyl  2  5  3  •> 1-butene *  1-butene  -> addition radical  -58-  (8) cyclopentadiene: (9) 1-hexene:  +  *<0'  R  +  +H  m  CH2=CH-CH2-CH2-CH=CH2 + 2H •  CH2=CH-CH-CHg- CH 2 -CH 5  (io) cyclohexene: . CH-CaaCH-CH2-CH=CH2  RH  ( l l ) Me-cyclopentene: allyl + diallyl  CH2.  » addition radical  allyl  RH (12) hexadiene:  2 allyl  > hexadiene  (13) benzene: it  (14) hydrogen:  H +H H + RH  I+H  2R  -> H, -?H 2 + R  The postulated mechanism of the pyrolysis of 1,5-hexadiene was mainly based upon the qualitative analyses of the products and upon t h e i r concentrations at different reaction times at 5010 C.  Some of the mechanisms  for the formation of some of the products are rather speculative.  However,  a large variety of products i s formed i n the pyrolysis and the system i s thus very complex and d i f f i c u l t to deal with.  Nevertheless, the postulated  mechanism should give an over-all picture of the reactions which may be expected i n the system. It would be desirable to isolate some of the elenehtary reactions assumed to take place i n the system and obtain definite proof of their occurrence.  -59-  The most interesting feature of the mechanism i s probably the addition of a l l y l radical to olefinic double bonds. membered c y c l i c products results.  Formation of f i v e -  Since the formation of these products  could not easily be explained i n any other way, they should be sufficient evidence for addition reactions of the a l l y l radical with the 1,5-hexadiene. Thermal decompositions taken to a much lower conversion than was the case i n the present study might have simplified the mechanism of the reaction.  However, such low-conversion pyrolyses would have involved  much shorter reaction times i n the temperature range studied and the reproducibility with the present system would consequently have been extremely poor. The d i f f i c u l t y caused by the uncertainty i n the reaction times could be overcome by decreasing the temperature of the pyrolyses and use of longer reaction times.  However, i n both cases there remains the problem of  determining the percentage of conversion.  Comparing peak areas on the  chromatogram which are of the same order of magnitude leads necessarily to very serious errors and i s thus not usable. It i s realized that i t would have been desirable for the sake of comparison to have carried out pyrolyses at the different temperatures to approximately the same degree of conversion rather than use of constant reaction time.  It would be possible to predict the reaction time at a  certain temperature for a certain degree of conversion i f the reaction time at a different temperature for the same degree of conversion was known.  The following i s the method of calculation.  i s assumed.  F i r s t order process  -60-  If  a  then  ss  degree of conversion, k = l . In _1_ t 1-a  o  a  1  (Cx 1 t -Ba  [3]  RT  But we have also:  k = Ae  I f t^ i s the reaction time at T^ and tg i s the reaction time at T^ for i t i s obtained:  It^ _ t^  k5  t  2  ga _ RT e  =  JBa RT  -Ea l _ •Ea /^1  o r :  t2  =  a ss constant  tx e  R  T  l  1 j T  2  [8]  MECHANISM OF THE THERMAL DECOMPOSITION OF 1-BUTENE SENSITIZED BY DIALLYL As was mentioned i n a previous chapter a preliminary experiment clearly showed that addition of a small amount of d i a l l y l to 1-butene accelerates the pyrolysis of the l a t t e r .  The more detailed investigation involving  variation i n the d i a l l y l concentration shows that formation of a l l the products of 1-butene pyrolysis i s  accelerated.  There are two reactions possible between the a l l y l radical and 1-butene: (l) Addition to the double bond: CH„=CH-CH« + CH„-CH-CH„-CH,  CH,  -CH-CH--CH-.  \ // * E  N  CH  cyclopentene  CH 2  CH-CH 2 -CH 5  cyclopentadiene ethane ethylene hydrogen  CH,  CH„ CH  -61-  or: CH =CH-CH . + CH =aCH-CH -CH 2  2  2  2  5  •  .CH CH ^  CH  CH-CH" -CH  2  2  / CH  CH  CH  cyclopentadiene ethane ethylene hydrogen  2  CH-CH--CH,  0  I  cyclopentene  5  I  5  ca  5  ^  /  9  2  CH  (2) H-abstraction to produce the resonance stabilized butenyl radical:  CH =aCH-CH . + CH =CH-CH -CH 2  2  2  2  -> CH =CH-CH  5  2  +  5  CH =CH-CH-CH 2 • 3 0  2  • CH^CH—CH—CH^ 2  The butenyl radical may also add to the double bond of 1-butene: CH -CH-CH -CH,  I CH  2 . 0  2 3 CH-CH,  \ / 2  CH  3  »  CH,5  GH-CH -CH,  j 2 CH.  j 2 CH-CH,  0  ^ /  3  5  CH *  methylcyelopentene cyclopentene cyclopentadiene ethane ethylene methane hydrogen  -62-  or: CH-  CH-CH -CH_  CH 0  0  • c.  1  CH,-CH  d  y  CH-CH^-CH,  \ d.  CH. CH  c  •)  CH  CH^CH CH  •v  same products as above If the primary step i n the d i a l l y l decomposition i s mainly diallyl  > 2 allyl  and i f re-combination of two a l l y l radicals i s also taken into consideration, the following i s obtained: .(allvl)2 (diallyl) or  (allyl)  =  K  cc ( d i a l l y l ) 1 , 7 2  This means that the rate of formation of those products which result from attack of 1-butene by the a l l y l radical should be proportional to l/2 (diallyl)  .  In f i g . 13 i t can beseen that the formation of methane,  cyclopentadiene, cyclopentene, methylcyclopentene (and eventually but not necessarily cyclohexene) follows the above dependence within experimental error. It can therefore be concluded that the above postulated mechanism for the reactions of a l l y l with 1-butene i s correct.  Furthermore, this  should be an evidence that the primary step i n the 1,5-hexadiene decomposition i s mainly formation of two a l l y l  radicals.  DECOMPOSITION OF n-BUTANE SENSITIZED BY DIALLYL E a r l i e r work has shown that i t i s possible to sensitize the thermal  -63-  decomposition of n-butane by addition of small amounts of azomethane or ethylene oxide.  It appears that the a l l y l radical i s reactive enough  at 506° C to sensitize the pyrolysis of 1-butene.  It must be borne i n  mind, however, that i n the case of 1-butene the hydrogen abstraction by a l l y l i s eased by the formation of a resonance stabilized r a d i c a l .  No  stabilized radical could be expected i n the case of n-butane. Prom the results obtained there does not seem to be any doubt that the a l l y l radical i s capable of sensitizing the decomposition of n-butane. The rate of formation of the saturated products (methane and ethane) increased by a factor of 2 - 2.5 and the rates of formation of the unsaturated products increased by a factor of 3*5 - 4»  It was not intended  to postulate any mechanism for this sensitized decomposition, but i t can at least be concluded that hydrogen abstraction takes place: CH_=CH-CH0. + CH_CH0CB0CH, d  d  d  d  2  3  » CH0=CH-CH, + CH2CH0CH CH, d  2  3  2.  [9]  3  The activation energy for the decomposition of the radical formed i s probably lower than that for the decomposition of n-butane.  Therefore  an over-all acceleration i n the pyrolysis process results. The formation of 1-butene and 1-pentene i n the sensitized decomposition can be accounted for by: CH2=CH-CH2. + .CH 2  > CH2=CH-CH2-CH5  CH2=CH-CH2. + .CH 2 CH 5  > CHgaCH-CHg-CHg-CHj  It can be therefore concluded that the reactivity of the a l l y l radical at 506° C i s sufficient to abstract hydrogen from secondary carbon atom i n n-butane.  More generally i t might be concluded that the a l l y l radical  is capable of abstracting hydrogen even from compounds which upon abstraction do not y i e l d stabilized radicals.  B I B L I O G R A P H Y  -64-  B I B L I O G B A P H Y  1.  Bateman, L . , Gee, G . , Morris, A . L . and Watson, W.F., Disc. Faraday S o c , 10, 250 (1951)  2.  Bolland, J . L . , and Gee, Geoffrey, Trans. Farad. S o c , 42, 244 (1946)  3.  Branch, G . E . K . , and Calvin, Melvin, The Theory of Org. Chem., Prentice-Hall Inc., New York, 1947, p. 275  4.  Bryce, W.A., and Kebarle, Paul, Trans. Farad. S o c (in press)  5. 6. 7.  Coulson, C . A . , Proc. Roy. Soc., A 164. 383 Dimbat, M . , Porter, P . E . , Stross, F . H . , Anal. Chem., 28, 290 (1956) Frost, A . A . , and Pearson, R . G . , Kinetics and Mechanism, John Wiley & sons, New York, 1953, p. 231  8.  Handbook of Chem. and Physics, Chem. Rubber Publishing Co., 1956, p. 1772  9.  Kebarle, Paul, Thesis, B r i t i s h Columbia 1957  10.  Kebarle, Paul, and Bryce, W.A., Can. J . Chem., 35, 576 (1957)  11.  Letorfc, M . , Thesis, Paris, 1937  12.  Lossing, F . P . , Ingold, K . U . , and Henderson, I.H.S., J . Chem. Phys. 22, 621 (1954)  13.  McDowell, C . A . , Lossing, F . P . , Henderson, I.H.S., and Farmer, J.B. Can. J . Chem., 34, 345 (1956)  14.  McNesby, J . R . , and Gordon, A . S . , J . Amer. Chem. S o c , 79, 4593 (1957)  15.  Molera, M . J . , and Stubbs, F . J . , J . Chem. S o c , 381 (1952)  16.  Mooney, R . B . , and Ludlam, E . B . , P r o c Roy. S o c , Edinburgh 49, (1929), 160  17*  Rice, F . O . , Johnston, W.R., and Evering, B . L . , J . Amer. Chem. S o c , 54, 3529 (1932)  18.  Ryce, S . A . , Kebarle, Paul, And Bryce, W.A., Anal. Chem., 29, 1386 (1957)  19.  Sehon, A . K . , and Szwarc, M . , Proc. Roy. Soc. (London), A 202, 263 (1950)  -65-  20*  Steacie, E.W.R., Atomic and Free Radical Reactions, Reinhold Publishing Corporation, New York 1954, Vol I . , p. 206  21.  Steacie, E.W.R., and Puddington, I . E . , Can.J. Research, B 16, 176 (1938)  22.  Szwarc, M . , J . Chem. Phys., 17, 284 (1949)  23.  Szwarc, M . , and Ghosh, B . N . , J . Chem. Phys., 17, 744 (1949)  24.  Szwarc, M . , Ghosh, B . N . , and Sehon, A . H . , J . Chem. Phys., 18, 1142 (1950)  25.  Taylor, H . S . , and Smith, J . O . , J r . , J . Chem. Phys., 8, 543 (1940)  26.  Trotman-Dickenson, A . F . , Gas Kinetics, Butterworths Scientific Publications, London 1955, p. 156  A P P E N D I X  r66-  A P P E N D I X  MASS SPECTROMETRIC IDENTIPICATIOH OF SOME OF THE PRODUCTS As mentioned previously, i t was necessary to identify some of the products formed i n the pyrolysis of 1,5-Jiexadiene by means of a mass spectrometer.  Definite proofs have not been obtained i n a l l cases, but  very good evidence i n a l l cases.  The procedures used are described i n  each individual case. Cyclopentene;  sample of pure cyclopentene was run through the mass spectro-  meter and i t s spectrum compared with the unknown.  Definite proof was  obtained* Cyclopentadiene:  pure substance was not available.  From the value for the  retention volume on alumina column i t could be deduced that a C^ hydrocarbon was present.  The trapped sample was run at low electron energies and the  mass number of the parent peak was thus obtained to be 66* formula i s thus C^H^.  The empirical  Since -ynic products are highly improbable, i t must  be concluded that cyclopentadiene i s present. 1-Hexene:  pure sample was not available.  Presence of impurities made i t  d i f f i c u l t to compare the spectrum with that of 1-hexene published by the American Petroleum Institute.  Mass spectrometric analyses at low electron  energies showed that the parent peak had a mass number of 84*  Retention  volume on alumina column indicated that a Cg hydrocarbon was present. From the cyclic compounds cyclohexane i s the only one which might be considered.  But i t s retention volume was different from the retention volume  -67-  of the unknown. Conclusion:  Thus only a straight-chain hydrocarbon can be considered.  1-hexene.  Cyclohexene and methyl-cyclopentene: on the alumina column.  these two products were not resolved  Mass spectrometric analyses carried out at low  electron energies showed that there was only one parent peak and that was of mass number 82. CgH^Q.  Retention volume indicated a Cg hydrocarbon.  Thus  Comparison with the spectrum of pure cyclohexene showed that  cyclohexene might be present but that also some other hydrocarbon CgR^ was present.  Methyl-cyclopentene seemed to be the most probable.  

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