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Kinetic studies on the pryolysis of pentenel Woods, Sally Anne 1953

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KINETIC STUDIES ON THE PYROLYSIS OF PENTENE-1 by SALLY ANNE WOODS A THESIS SUBMITTED IN PARTIAL FULFILMENT 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 standard required from candidates f or the degree of MASTER OF SCIENCE Members of the Department of Chemistry THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1953 ABSTRACT The thermal decomposition of pentene-1 i n a s t a t i c system has been investigated over a tem-perature range of V70 to 530°C. and a pressure range of 50 to 250 mm. The decomposition was a homogeneous f i r s t - o r d e r reaction with an average o v e r a l l a c t i v a t i o n energy of 52 kcal./mole. The reaction rate was retarded by propylene and by ine r t gases, but was unaffected by n i t r i c oxide. Free r a d i c a l s from lead t e t r a e t h y l produced an acceleration. The a c t i v a t i o n energy exhibits a s l i g h t increase with increasing i n i t i a l pressure of pentene. Evidence i s presented for a composite reaction mechanism involving both a f r e e - r a d i c a l chain process and a d i r e c t intramolecular re-arrangement . This inves t i g a t i o n was carried out under the supervision of Dr. W. A. Bryce to whom the author i s greatly indebted. CONTENTS Page INTRODUCTION 1 Thermal decomposition of hydrocarbons . . 1 Rice mechanisms 3 Survey of the l i t e r a t u r e 5 Decomposition of lower o l e f i n s . . . 6 Decomposition of the pentenes . . . 12 Basis of the present inves t i g a t i o n . . . 18 EXPERIMENTAL 20 Reagents 20 Description of the apparatus 21 Description of a t y p i c a l experimental run 26 General form of the pressure-time curves 28 Dependence of the rate on the i n i t i a l pentene pressure 29 E f f e c t of increased surface-to-volume r a t i o i n reaction vessel 30 E f f e c t of addition of in e r t gases . . . . 31 E f f e c t of addition of n i t r i c oxide . . . 32 E f f e c t of addition of propylene . . . . 33 Effect of addition of inert gases on rate of maximally-inhibited decomposition . 35 Effect of addition of lead t e t r a e t h y l . . 35 Dependence of the rate of decomposition on temperature . . . . . 36 Effect of addition of in e r t gases on ac t i v a t i o n energy 39 i i Page Effect of addition of freon on a c t i v a t i o n energy * f l Effect of addition of propylene on a c t i v a t i o n energy h2 Effect of addition of inert gases on a c t i v a t i o n energy of maximally-inhibited reaction . . If 3 Effect of addition of n i t r i c oxide on ac t i v a t i o n energy Summary of the experimental r e s u l t s . . . . M+ DISCUSSION . . . . . . . . . . i . kS Nature of the primary a c t i v a t i o n process . . h& Dependence of a c t i v a t i o n energy on i n i t i a l pentene pressure 59 REFERENCES 63 INTRODUCTION !£he thermal decomposition reactions of hydrocarbons are of considerable interest from a theoretical standpoint, since a study of their mechanisms yields valuable information on the fundamental nature of chemical change. In the whole paraffin series, decomposition reactions involve the breaking and forma-tion of only two types of linkage; only three types are in-volved with olefins. The nature of the problem is therefore not unduly complicated. However, the mechanism of olefin decompositions is not well understood, In spite of the fact that such thermal decompositions have been the subject of a considerable amount of study. thermal Decomposition of Hydrocarbons Although the kinetics of the thermal decompositions of a variety of complex organic molecules have received much study, mechanisms are not, in a l l cases, fully established. It seems evident however,' that each such reaction involves one or both of two primary acts of decomposition: an intramolecular re-arrangement to stable products or a split into free radicals, which initiate further decomposition by a chain process. When both mechanisms operate simultaneously in a given reaction, their relative importance depends upon two factors: ( a ) the relative activation energies and steric factors, and (b) the chain length. On the basis of much experimental evidence a composite reaction mechanism involving both types 2 of reaction has "been assigned to the pyrolysis of normal paraf-f i n hydrocarbons (1). Evidence for the p a r t i c i p a t i o n of free r a d i c a l s i n such reactions comes from experiments on sensitized decompositions at temperatures f a r below those at which the normal decomposi-tions occur. Radicals from ethylene oxide induce the decom-po s i t i o n of propane and of n-butane ( 2 ) ; methyl r a d i c a l s from decomposing azomethane induce decomposition of ethane and propane ( 3 ) ; and again the addition of 1% of mercury dimethyl to n-butane at 525°C. causes the decomposition of twenty equi-valents of butane 0+), Such observations indicate that free r a d i c a l s produced by the decomposition of the s e n s i t i z e r can react with the hydrocarbon, causing i t s decomposition by a f r e e - r a d i c a l chain process. Thus i t i s established that r a d i c a l s can cause chain decomposition of p a r a f f i n s , although i t does not necessarily follow that a chain process occurs i n the normal pyrolysis of the substance under consideration. N i t r i c oxide has been found a very e f f e c t i v e substance for i n h i b i t i n g chain r e a c t i o n s ( 5 ) • N i t r i c oxide, i t s e l f a free r a d i c a l , combines with other r a d i c a l s , so removing them from reaction systems. Very small amounts of n i t r i c oxide have been found to i n h i b i t reaction rates greatly; by removing one r a d i -c a l , a molecule of n i t r i c oxide prevents the chain decomposi-t i o n of many molecules. By means of n i t r i c oxide i n h i b i t i o n , conclusive evidence has been provided f o r the operation of a f r e e - r a d i c a l chain mechanism i n p a r a f f i n decompositions ( 6 , 7 ) . 3 As increasing amounts of n i t r i c oxide are added to a reaction system, the rate of decomposition decreases rap i d l y to a con-stant f r a c t i o n of i t s o r i g i n a l value; subsequent additions produce no further i n h i b i t i o n ; a l l chains appear to be sup-pressed. Certain other i n h i b i t o r s , such as propylene, have been shown to have a s i m i l a r e f f e c t . The re s i d u a l reaction i s believed to represent a non-chain molecular rearrangement. Nevertheless, considerable uncertainty s t i l l remains as to the precise nature of t h i s part of the reaction. Work with I s o t o p i c a l l y l a b e l l e d compounds indicates that the maximally in h i b i t e d reaction s t i l l involves f r e e - r a d i c a l chains (8). The plot of pressure increase against time f o r the un-i n h i b i t e d decomposition of a normal p a r a f f i n shows a pronounced curvature near the o r i g i n (9),-^but afterwards approximates to a straight l i n e . A n a l y t i c a l r e s u l t s show that, i n most instances, o l e f i n s constitute an appreciable percentage of the reaction products. Hence the shape of the curve has been ex-plained as due to I n h i b i t i o n by these unsaturates during the i n i t i a l stages of the reaction. The p a r t i c i p a t i o n of f r e e - r a d i c a l chains i n such thermal decomposition reactions was greatly c l a r i f i e d by Rice (1G), who devised mechanisms for organic decompositions. These Rice mechanisms form the basis of our understanding of f r e e - r a d i c a l chain reactions. Rice Mechanisms Mechanisms for the decomposition of a wide variety of organic compounds have been proposed by Rice and Herzfeld ( 11 ) . On the basis of the detection of free r a d i c a l s i n such decern^ p o s i t i o n reactions, Rice has suggested that these r a d i c a l s play a v i t a l r o l e i n the reaction mechanisms. In general, the proposed steps are as follows: M 1 ^ M 2 T- % R l + M i -> * i H f *2 . R 2 R x -f M3 R]_ - j - R 2 Mi,.. The i n i t i a l step involves the rupture of a bond i n molecule M^ , y i e l d i n g a smaller molecule, M2, and a r a d i c a l , R]_. A chain process follows. Each step i n the chain involves the abstraction of a hydrogen atom from the parent hydrocarbon, y i e l d i n g an a l k y l r a d i c a l and a molecule, R^H. The large r a d i c a l s , R2, are assumed to decompose r e a d i l y . Chain termina-t i o n takes place by r a d i c a l recombination, with formation of a stable molecule, . The mechanisms devised f o r the pyrolysis of p a r a f f i n hydrocarbons appear to give a f a i r l y s a t i s f a c t o r y interpreta-t i o n of the complicated chemical changes involved, even when applied to paraffins as high as the octanes ( 1 2 ) . Normal pentane w i l l serve as an example: the i n i t i a t i n g step i s assumed to be a breakdown of the parent hydrocarbon to two r a d i c a l s : C^H 1 2->CH 3» -f CB^CB^CIL^CE?* ; The large a l k y l r a d i c a l , assumed to be unstable, decomposes: 5 GH^CHgCHgCB^* -^CRy 4 - CH^CH r GH2 or CH3CH2CH2CH2 • -> CH3GH2 • 4- GH^ = CHg ; the chain process then proceeds by steps of the following type: G^H^2 4- R — R H -f~ CH^CI^CIL^CH/jCJE^ * R z CEy, C 2H 5» ; the large a l k y l r a d i c a l so formed now decomposes to stable products and lower r a d i c a l s which perpetuate the chain. By suitable choices of the various p o s s i b i l i t i e s f o r the chain-perpetuating and chain-terminating steps, the observed f i r s t -order o v e r a l l rate can be explained. In addition, a r b i t r a r y assignment of a c t i v a t i o n energies to the various steps can lead to an o v e r a l l a c t i v a t i o n energy i n agreement with the low ex-perimental value; that i s , a value f a r smaller than the strength of the C-G bond broken i n the i n i t i a l step. Survey of the Literature A survey of the l i t e r a t u r e dealing with the pyrolysis of o l e f i n s reveals that nearly a l l the information available on the k i n e t i c s of such reactions has been accumulated i n the l a s t t h i r t y years. During t h i s period much work has been done i n order to gain a better understanding of the thermal reactions -both decomposition and. polymerization - of o l e f i n s . Experi-ments have, for the most part, dealt c h i e f l y with the lower o l e f i n s . Dynamic methods have usually been employed; that i s , the experiments have been done i n flow systems, using short 6 contact times. A wide variety of temperatures and pressures have been used, and i n few cases have comparable experimental conditions been employed. Decomposition of Lower Olefins P r i o r to 1925 very l i t t l e information of a precise nature was available on o l e f i n decompositions. I t was known that at about 750°G. the pyrolysis of an o l e f i n yielded acetylene, a considerable part of which polymerized to benzene (13)• One of the e a r l i e s t investigations was made by Noyes (ih) f who passed isobutylene through a glass tube heated to "low redness," and i d e n t i f i e d ethylene,propylene, butadiene, methane, hydrogen, benzene, toluene, and naphthalene i n the reaction products. Ipatiev (15) passed isobutylene over alumina at 500°C, obtaining propylene, hydrogen, and methane. At law temperatures the chief reaction of the lower o l e f i n s was found to be polymerization, but at higher temperatures the process was found to be more complex, consisting of both de-composition and polymerization. Frey and Smith (16), working at 575°C. with a flow system, showed the high-temperature reaction of ethylene to be homo-geneous i n s i l i c a vessels, and i d e n t i f i e d methane, ethane, hydrogen, and higher o l e f i n s i n the products. They also decomposed propylene under s i m i l a r conditions, y i e l d i n g butene and higher hydrocarbons together with large amounts of methane and ethylene. Isobutylene, heated i n a flow system, was shown by Hurd and Spence (17) to be much more stable than isobutane under s i m i l a r conditions, a behaviour attributed by these workers to the greater strength of the C-C single bond i n isobutylene to the corresponding bond i n the saturated compound. At tempera-tures above 600°C. they found the isobutylene p y r o l y s i s to be homogeneous and independent of concentration since the reaction rate was unchanged by increase of the surface-to-volume r a t i o of the reaction tube or by d i l u t i o n with nitrogen or hydrogen. Similar conclusions were reached by Kurd and Meinert (18) from experiments with propylene i n a pyrex flow system at tempera-tures above 525°G. The propylene decomposition was, however, s l i g h t l y slower i n the presence of nitrogen, which appeared to lessen the formation of l i q u i d products by d i l u t i n g the primary reaction products, hindering t h e i r polymerization. Rice has proposed a f r e e - r a d i c a l chain mechanism for o l e f i n decompositions (10). Here, a number of complications arose, which were not encountered i n devising the mechanisms of p a r a f f i n decompositionst (a) The double bond exerts a strong influence on the r e a c t i v i t y of the adjacent H atoms: the cA -H atoms are rendered i n a c t i v e , whereas the j3 -H atoms are activated. Rice has therefore assumed that i n a reaction involving the abstraction of a hydrogen atom from an o l e f i n molecule, the attack of a free r a d i c a l w i l l be almost exclusiv-ely at the j3 -H atoms, (b) A free r a d i c a l i s able to react with an unsaturated hydrocarbon not only by abstracting H atoms, but also by adding to one of the doubly-bound carbon 8 atoms. In proposing a chain mechanism for the process, Rice has neglected such addition reactions, assuming them to be of importance only at lower temperatures. For an unsaturated r a d i c a l Rice has assumed the following type of behaviour'. Consider, as an example, the proposed re-action between propylene and a methyl r a d i c a l : CH • -I- CH^CH = C H 2 ^ CH^ + -CB^CH Z CH 2 . The methyl r a d i c a l abstracts a j3 -H atom to y i e l d methane and an a l l y l r a d i c a l . The l a t t e r , because of a resonance e f f e c t , •CH2-CH Z CH 2 CH 2 - CH - CH 2 • , i s considered much more stable than an ordinary free r a d i c a l ; hence, the decomposition: •CH2CH = CH 2 CH 2 = C = CH 2 + H-seems u n l i k e l y . This conclusion i s substantiated by the fact that allene has not been detected among the products of o l e f i n i c decompositions. Since the reaction of a l l y l r a d i c a l s with surrounding o l e f i n molecules regenerates a l l y l r a d i c a l s , these are assumed to disappear only by c o l l i s i o n with one an-other. The d i a l l y l so formed i s presumed to decompose, y i e l d -ing the observed o i l y products. The presence of unsaturated r a d i c a l s also introduces the p o s s i b i l i t y of isomerization of the o l e f i n s . Gn the basis of these assumptions Rice has postulated a fundamental difference between the decomposition mechanisms of the lower and higher members of the o l e f i n s e r i es. The decom-positions of propylene and the butylenes are presumed to 9 involve no chain cycle other than i n i t i a t i o n : i n i t i a l rupture of a |3 C-G bond i s followed by r a d i c a l abstraction of H atoms from o l e f i n molecules, producing a l l y l - t y p e r a d i c a l s which disappear by combination with one another. For higher o l e f i n s , a chain mechanism i s proposed, y i e l d i n g p a r a f f i n hydrocarbons and conjugated d i o l e f i n s . Thus Rice has assumed that, because of t h e i r unsaturated nature,olefins decompose d i f f e r e n t l y than do most other organic compounds. Eglof f and Wilson (19), i n considering the mechanisms f o r the thermal reactions of gaseous hydrocarbons, regarded ethy-lene as the basic material, since above a certain temperature the reaction products of any hydrocarbon are e s s e n t i a l l y those of ethylene at that temperature. They reasoned that the thermal reactions of higher hydrocarbons must be studied below 750°C. i n order that they be characterized by differences due to the nature and s t a b i l i t y of the p a r t i c u l a r molecules con-cerned. Since, at the i n i t i a l temperatures of reaction, o l e f i n s were found to polymerize to nonaromatic.substances, i t was concluded that points of unsaturation are conducive to polymerization. The pyrolysis of ethylene, propene, and the three butenes was studied by Tropsch, Parrish, and Egloff (20) i n a flow system, at temperatures (1100 to llf00oC.) considerably higher than those used i n previous investigations. They observedthat, -as the experimental conditions were gradually made more severe, the volume contraction r e s u l t i n g from polymerization gradually 10 became masked by expansion due to decomposition, u n t i l only a volume increase remained. By a consideration of the a c t i v a t i o n energies of the two competing processes, they inferred that decomposition does not precede polymerization under severe conditions, but that polymerization i s the primary step, the polymer so formed being unstable under the experimental con-d i t i o n s , and decomposing to the observed gaseous products. Rice and Haynes (21) pyrolyzed isobutylene i n a high-temperature flow system designed to eliminate the formation of o i l y and t a r r y material. Propylene, methane, hydrogen, ethy-lene, ethane, acetylene, allene, and methyl acetylene were found. On the basis of t h e i r r e s u l t s these investigators suggested a f r e e - r a d i c a l mechanism involving rather short chains due to the formation of appreciable amounts of propy-lene by some non-chain reaction such as the following: GHt Z CHp -f- (CHJ 2G - GH —> CH CM = CH 4- .CH^CHp. . 5 J ^ 3 * GH^ They also suggested that the methyl acetylene i s formed from allene by a r a d i c a l chain mechanism. Isobutylene was also pyrolyzed at high temperatures by Szwarc ( 2 2 ) , who found the reaction to be homogeneous and f i r s t - o r d e r , and proposed a chain mechanism to explain the low a c t i v a t i o n energy found. In s i m i l a r experiments with propylene, the same author (23) demonstrated the homogeneity and f i r s t - o r d e r character of the decomposition and proposed a unimolecular mechanism i n which the rate determining step involves the breaking of the G-C 11 ' bond, followed by a sequence of rapid reactions between the rad i c a l s so created. Rice and "Wall (2lf), decomposing isobutylene with short contact times at 851 to 900°G.at pressures between 50 and 200 mm. and obtained i d e n t i c a l products by carrying out the re-action both i n a quartz tube and i n a stai n l e s s s t e e l tube. The f i r s t use of a s t a t i c system, such as that used i n the present inv e s t i g a t i o n , was made by Ingold and Stubbs (25) i n studying the thermal decomposition of propylene. Operating over a pressure range of 50 to 500 mm. and a temperature range of 570 to 650°C. they showed the decomposition to be a homo-geneous f i r s t - o r d e r reaction with an a c t i v a t i o n energy of 57.1 k c a l . per mole, the decomposition products being mainly methane, ethylene, hydrogen, a condensable intermediate which subsequently decomposed, and carbon. They concluded that over t h i s temperature range propylene decomposes mainly by a molecu-l a r rearrangement reaction, the i n h i b i t i n g action of propylene i t s e l f preventing the propagation of chains. I t was suggested that at higher temperatures, such as those used by Szwarc (23), i t i s possible that a r a d i c a l mechanism, with or without the propagation of chains, may predominate due to the i n s t a b i l i t y of the a l l y l r a d i c a l . The picture of lower o l e f i n pyrolysis thus appears to be none too clear at the present time. A great variety of experi-mental conditions have been employed by di f f e r e n t workers so that c o r r e l a t i o n proves somewhat d i f f i c u l t . The reactions 12 appear to be homogeneous and f i r s t - o r d e r . At low temperatures polymerization predominates, accompanied by a volume contrac-t i o n ; as the temperature i s raised a point i s reached where there i s no observable pressure change, indi c a t i n g that the two competing processes of polymerization and decomposition are o f f s e t t i n g one another; at higher temperatures decomposi-t i o n predominates. Both f r e e - r a d i c a l chain mechanisms and molecular rearrangement mechanisms have been proposed to account for the observed reaction products. I t i s possible that d i f f e r e n t mechanisms may operate under d i f f e r e n t condi-tions of temperature and_pressure. Decomposition of the Pentenes As was the case f o r the lower o l e f i n s , the pyr o l y s i s of the pentenes has been studied mainly by flow methods. Norris and Reuter (26) heated -a- pentene-2 under a variety of conditions i n a flow system at 6G0°C. Rough analyses gave evidence f o r the formation of methane, butene, butadiene, propylene, ethylene, and higher straight-chain hydrocarbons, but f o r no branched-chain products. A comparison of the thermal behaviour of c e r t a i n pentanes and pentenes was made by Morris and Thomson (27). The decompositions were performed at or near the cracking temperatures of the hydrocarbons, thus y i e l d i n g the products f i r s t formed. For both pentanes and pentenes a certain temperature was noted at which the hydro-carbon pyrolysis began. For a pentene an a d d i t i o n a l s i g n i f i -cant temperature was observed above which the rate of expansion 13 either remained constant or decreased as the temperature was raised. Such reversal temperatures, which occurred, f o r the three pentenes studied, 53 to 65°C. higher than the tempera-tures at which the hydrocarbons began to decompose, were attributed to simultaneous pyrolysis and polymerization. Hurd and Goldsby (28), i n the pyr o l y s i s of pentene-1 and pentene-2 at 550 to 600°C. i n a flow system, established that isomeric unsaturated hydrocarbons were important reaction products, one-third to two-fifths of the t o t a l products con-s i s t i n g of isomeric pentylenes. Both s t a t i c and flow methods were used by Pease and Morton (29) i n a study of the pyrolysis of pentene-2. They showed that the reaction was homogeneous and monomolecular;at at temperatures between 500 and 600°C. Since the pressure-time curves rose perfectly regularly from the s t a r t to something short of 100$ pressure increase, they concluded that polymerization i s not an important primary reaction:. Chain mechanisms were proposed by Rice (10) for the decomposition of higher o l e f i n s . The decomposition mechanism suggested f o r pentene-1 was the following: i n i t i a t i o n occurs by s p l i t t i n g of the |3 G-C bond: G ^ H i g - > CH3CH2« 4- .CH2CHCH2. ; t h i s i s followed by reaction of the r a d i c a l s formed with pentene molecules: C 5 H 10 + C H 3 C V - ^ C 2 H 6 + CH3CH2CHGHCH2-Ik-G5H10 + •CH2GHCH2.—^,G3H6 -)- CH CHgCHCHGHg• ; and a chain reaction takes place: °5H10 + S->RH + CH^HgCHCHCHg • —^»RH f CHj : CHCH Z CHg -f C H y where R = CH^» This scheme predicts the formation of equal proportions of methane and butadiene, which should therefore constitute the main decomposition products. Both pentene-1 and pentene-2 were studied by Hurd, Goodyear, and Goldsby (30) i n a flow system at temperatures between 500 and 600°C. Pentene-2 was found to be the more stable. Analyses were made of the entire gaseous reaction products, showing the chief products from both to be methane, butene-1, propylene, ethane, and ethylene, together with small amounts of butene-2, butadiene, and hydrogen, and, i n the case of pentene-1, some propane. Isomerization of pentene-1 to pentene-2 and the reverse, at temperatures above 580°C. was established by precise d i s t i l l a t i o n . The non-formation of isopropylethylene from pentene-2 was taken as evidence against an a l l y l i c type of intramolecular wandering of the methyl r a d i c a l . This i s also assumed to exclude intramolecular wandering of the H r a d i c a l as the mechanism for the observed conversion. Mikhallov and Arbuzov (3D, i n pyrolyzlng pentene-1 and 15 pentene-2 i n a flow system i n the temperature range 500 to 700° G., found that the use of steam as a diluent tended to prevent polymerization. Gorin, Oblad, and Schmuck (32) investigated the pentene decompositions under conditions designed to promote maximum su r v i v a l of the d i o l e f i n products. A flow system was employed with nitrogen as a di l u e n t . At 800°C. the main reaction of pentene-1 was found to he a s p l i t t i n g to ethylene, propylene, butylene, and butadiene. Pentene-2 yielded mainly butadiene, with smaller amounts of ethylene and propylene. In the pyrolysis of mixtures of n-pentane and pentene-2, no selective cracking of the o l e f i n was observed. On the basis of these observations, Gorin proposed a modified Rice mechanism f o r the decomposition of the pentenes. Rice's f r e e - r a d i c a l chain mechanism f o r o l e f i n decomposition involved the fundamental assumption that the chain sequence reaction, wherein an a l k y l r a d i c a l reacts with an o l e f i n , takes place exclusively by removal of a j3 -H atom. This predicts that pentene-1 should decompose exclusively to methane and butadiene i n the primary process. Since Gorin's analyses showed that only 30$ of the pentene-1 decomposes i n t h i s way, and that the majority of the reaction involves s p l i t t i n g to l i g h t o l e f i n s , he suggested that the free r a d i c a l s do not react exclusively with the fh -hydrogens at high temperatures but also with the Y - and £ -hydrogens. The chain sequence predicted i s : CH 2 - CH* +- CH^CI^CH^CH " CHg-^CHg - CH 2 -f CH^-CH-CB^-CHZCHg 16 C H 3 - 6 H - C H 2 - C H Z CH 2 CH^CH I CH 2 -f- GHg Z CH« . Two further points of evidence indicate short chains: ^ r e l a -t i v e l y large amounts' of ethane were formed; by assuming the primary reaction to be,, exclusively, a s p l i t t i n g of the y3 C-C bond to y i e l d an a l l y l and an ethyl r a d i c a l , each of which subsequently reacts with pentene to produce ethane, a maximum value of 7 was calculated f o r the chain length; (b) ethylene was formed i n considerably greater amounts than was propylene. Gorin explains t h i s as follows: abstraction of a / -H from pentene-1 y i e l d s a r a d i c a l which must decompose to ethylene and an a l l y l r a d i c a l . The r e l a t i v e l y stable a l l y l r a d i c a l w i l l tend to dimerize or to combine with other r a d i c a l s rather than perpetuate the chain by combining with the pentene-1 to give propylene. Butenes are assumed to res u l t from the chain-stopping recombination of a methyl and an a l l y l r a d i c a l . The experimental r e s u l t s f o r pentene-2 were found i n better agree-ment with the Rice predictions, butadiene forming the p r i n c i p a l reaction product. The pyrolysis of pentene-2 and trimethylethylene by flow methods at 778 to 85G°C. and_high pressures i n the presence of steam was carried out by Hepp and Frey (33)• These i n v e s t i -gators also found butadiene to be the p r i n c i p a l reaction product from pentene-2, with lesser amounts of pentadiene, ethane, butene, ethylene, and propylene. In the case of trimethylethylene, formation of the a l l y l r a d i c a l i n the primary process seemed u n l i k e l y , i n d i c a t i n g the operation of 17 some mechanism other than that proposed by Gorin for the butene formation. The authors suggest addition of a hydrogen atom to one of the doubly-bound C atoms, with the formation of a high-energy r a d i c a l , decomposing to an o l e f i n and a smaller r a d i c a l . This proposal was substantiated by the detection of r e l a t i v e l y large amounts of hydrogen from the trimethylethylene and pentene-2 decompositions. For pentene-1 however such H atom addition would lead only to C 2 and hydrocarbons. Recently, a comparative study of the thermal decomposi-tions of several o l e f i n s i n a s t a t i c system was made by Molera and Stubbs ( 3 * 0. The hydrocarbons studied were butene-1 , pentene-1, hexene-1, heptene-1, butene-2, isobutene, 2-methylbutene-l, and 3-niethyrbutene-l. Analyses were made i n some cases. The decompositions were shown to be of the f i r s t order, and the a c t i v a t i o n energies were determined. A reaction mechanism was proposed f o r the isobutene decomposi-t i o n . In the case of pentene-1, the i n i t i a l part of the pressure increase-time curve was found to be a straight l i n e passing through the o r i g i n , with a decrease i n rate as the reaction came to an end. The a c t i v a t i o n energy, measured over the temperature range V30 to 530°C., was found to be 53.1 k c a l . per mole f o r 100 mm. pentene pressure, and 5^.6 k c a l . per mole for 300 mm. pressure. Addition of n i t r i c oxide produced no appreciable effect on the reaction rate; added propylene and ethylene both caused a s l i g h t decrease i n the rate. Ho analy-ses were made f o r the reaction products from pentene-1 . 18 From t h i s summary i t i s seen that, although several i n -vestigations have been made on the pyrolysis of pentenes, with the exception of the experiments of Pease and Morton ( 2 9 ) and those .of Molera and Stubbs ( 3 ^ ) , these experiments have been done In flow systems under conditions of temperature and pres-sure d i f f e r e n t from those used i n the present work. The pentene decompositions have been found homogeneous and f i r s t -order. At temperatures of about 800°C, the reaction products from pentene-1 are mainly l i g h t o l e f i n s with a lesser amount of butadiene, while pentene-2 y i e l d s mainly butadiene. High temperatures seem to favour d i o l e f i n i c products. At lower temperatures (500 to 600°C.) the chief products of both are methane, butene-1, propylene, ethane, and ethylene. Isomeri-zation takes place above 580°C. At low temperatures poly-merization predominates. Various reaction mechanisms have been proposed. Basis of the Present Investigation O l e f i n i c hydrocarbons e x h i b i t , on p y r o l y s i s , c e r t a i n i n t e r e s t i n g p e c u l i a r i t i e s not found i n the case of saturated organic compounds. Furthermore, the pyrolysis of a normal saturated p a r a f f i n y i e l d s an o l e f i n and a lower p a r a f f i n . Therefore, for a f u l l i n t e r p r e t a t i o n of a p a r a f f i n decomposi-t i o n , a knowledge of,the decomposition k i n e t i c s of the o l e f i n i c part of the product would be required. Secondary decomposition of the products has been shown to lead to a sigmoid type of pressure increase-time curve which, for analysis, e n t a i l s a 19 knowledge of a l l the products and t h e i r r e l a t i v e s t a b i l i t i e s (35). I t would be necessary to know, f o r example, i f the ethylene and propylene, which are found i n the reaction pro-ducts of most paraf f i n s , are produced d i r e c t l y from the par a f f i n s , or by the secondary decomposition of a higher o l e f i n . Accordingly, o l e f i n decompositions deserve study not only on t h e i r own merits, but also because of t h e i r p o t e n t i a l use i n further elucidating the mechanisms of p a r a f f i n decom-pos i t i o n s . Pentene-1 was selected f o r invest i g a t i o n i n the present study as i t i s a t y p i c a l "higher o l e f i n " of the type believed to be formed i n the pyrolysis of the higher p a r a f f i n s . The object of the work was to obtain a d d i t i o n a l information on the thermal s t a b i l i t y of t h i s compound and also on the mechanisms involved i n i t s p y r o l y s i s . 20 EXPERIMENTAL The thermal decomposition: .of pentene-1 was studied i n the gas phase. The decomposition was carried out i n a closed quartz reaction vessel, heated externally by a furnace, and the extent of the reaction was followed by observation of pressure changes, using a mercury c a p i l l a r y manometer. The experi-mental conditions were varied by changing the pressure and temperature of the gas, by adding i n e r t gases, i n h i b i t o r s , and a free-radical-producing substance to the reaction system, and by a l t e r i n g the surface-to-volume r a t i o of the reaction vessel. Reagents The pentene-1 used i n t h i s i n v e s t i g a t i o n was obtained from P h i l l i p s Petroleum Company, Special Products D i v i s i o n , B a r t l e s v i l l e , Oklohoma. Since t h i s material was specified as "Research Grade," i t was not subjected to further p u r i f i c a t i o n . Propylene, also "Research Grade," was obtained from the same source. The gas was condensed i n a l i q u i d nitrogen trap and pumped before admission to a storage bulb i n the system. "Reagent Grade" argon was obtained from The Matheson Company Incorporated, E. Rutherford, N. J . Nitrogen was obtained from the Canadian Liquid A i r Company and was specified as "Commercial Grade." N i t r i c oxide was prepared by the action of a s u l f u r i c acid solution of ferrous sulfate on sodium n i t r i t e (36). The gas was freed from carbon dioxide and higher oxides of nitrogen by 21 passage through 6 normal sodium hydroxide and a tube containing sodium hydroxide pellets.- I t was dried by passage through phosphorus pentoxide. Lead t e t r a e t h y l was obtained from the Imperial O i l Company i n the form of an approximately 10% solution i n a hydrocarbon solvent, containing ethylene d i c h l o r i d e , ethylene dibromide, and a dye. Owing to the extreme t o x i c i t y of lead t e t r a e t h y l no attempt was made to obtain t h i s compound i n a pure form. Monochlorotrifluoromethane was obtained from the Canadian Ice Machine Company. Description of the Apparatus The apparatus used i n t h i s i n v e s t i g a t i o n was an a l l - g l a s s s t a t i c system^ as shown i n F i g . 1. This consisted e s s e n t i a l l y of an externally heated quartz reaction vessel, A, connected to an evacuating system, N, a mercury manometer, B, f o r pres-sure measurement, storage vessels, C, D, E, F, G, and H, for reactants, and a sampling system, K. The quartz reaction vessel had a volume of approximately 200 ml. and an outside diameter of 55 mm. I t was connected to the evacuating system and mercury manometer by quartz tubing and a ground-glass j o i n t , J ^ . The P i c e i n wax used f o r the seal did not develop any leaks during several months of continuous heating. The pressure JLn the system was measured by a closed U-tube mercury manometer, B, connected;, to the reaction vessel by means of c a p i l l a r y tubing. Pressure readings were made with reference to a mirror scale graduated i n millimeters. 22 The system was evacuated by.a mercury d i f f u s i o n pump backed by a rotary o i l pump. A trap, T-j_, cooled i n dry i c e -acetone, was situated between the mercury pump and the rest of the system i n order t o prevent mercury vapour from d i f f u s i n g to the two g a l l e r i e s and the reaction vessel and also to pre-vent reaction vapours from reaching the pumping system. Through the upper and lower g a l l e r i e s , which could be evacuated separately or simultaneously, the reaction vessel was evacuated. A discharge tube, L, attached to the system, and capable of connection with a l l parts of the system, was used to indicate the attainment of a "black vacuum." Pentene-1, which i s a l i q u i d at room temperature and pressure, was stored i n a small bulb, C, attached to the lower g a l l e r y . The bulb was f i l l e d through a small-bore side-arm by suction from a vacuum i n the system above. Before use, the pentene was thoroughly frozen i n l i q u i d nitrogen and pumped to remove traces of air!. I t was v o l a t i l i z e d by warming the bulb i n a beaker of warm water. The lead t e t r a e t h y l solution was stored i n a small bulb, D, connected to the lower gal l e r y by a ground-glass j o i n t , «J"2. The gaseous materials, propylene, argon, nitrogen, n i t r i c oxide, and freon, were stored i n 2-l i t e r glass bulbs, E, F, G, H, and I, attached to the upper g a l l e r y . These were f i l l e d i n the following ways. To admit propylene, the cylinder containing the l i q u i d under pressure was connected to the apparatus at 0 with pressure tubing. With taps S-J^ Q and S -Q open, the upper g a l l e r y , connecting 23 tubes, and trap T 2 were evacuated. Trap T^ was cooled i n l i q u i d nitrogen. With taps Sg and S 1 Q closed, the reducing valve on the cylinder was opened, and s u f f i c i e n t propylene gas was allowed to enter and condense i n trap T 2. After thorough pumping of the condensed propylene, trap T 2 was warmed, and the propylene was vaporized into the evacuated storage globe. The other gases used were admitted i n a d i f f e r e n t manner. The cylinder (or, i n the case of n i t r i c oxide, the generator) containing the gas was attached to the system through the was 2-way stopcock by pressure tubing. A strong stream of gas/ allowed to f l u s h out the connecting tubing, and the upper g a l -l e r y and storage bulb were evacuated. The 2-way stopcock was then reversed, and the gas allowed to enter u n t i l the pressure, as registered by an open mercury manometer, M, attached to the upper g a l l e r y , was approximately one atmosphere. To prevent condensation of the lead t e t r a e t h y l or of the reaction products, the lower g a l l e r y and c a p i l l a r y connections were wound with Chromel resistance wire and could be heated e l e c t r i c a l l y . The stopcocks which were not subjected to heating were sealed with Apiezon M grease which was found to provide an excellent vacuum sea l . 4s t h i s grease i s not e f f e c t i v e at high temperatures, heated stopcocks were sealed with Dow Corning " S i l i c o n e High Vacuum" stopcock grease which maintains i t s consistency up to 200°C., although does not give such an ef-f i c e n t seal as the Apiezon grease. 2h The reaction vessel was heated i n an e l e c t r i c furnace con-structed by P a t r i c k (37). The furnace, b u i l t from a c y l i n d r i -c a l quartz core, three inches i n diameter, was heated by alternating current i n three sections of Chromel resistance wire winding. I t was insulated with several inches of powdered asbestos, and the top opening was sealed with a mixture of powdered asbestos and alundum cement. The temperature of the reaction vessel was measured by two Chromel-Alumel thermocouples placed i n contact with the w a l l of the vessel, at top and bottom respectively. These were con-nected to the potentiometer c i r c u i t through a double-throw switch, to permit rapid consecutive reading. The thermo-couples were calibrated by use of the melting points of pure t i n , lead, zinc, and aluminum, and the t r a n s i t i o n temperature of potassium sulphate, which covered the lange of temperature used i n t h i s i n v e s t i g a t i o n . The temperature of the furnace was adjusted to give a constant reading over the length of the reaction vessel by means of three variable resistances, each connected i n series to one of the three furnace windings. These three sections were connected i n p a r a l l e l to the power supply. The temperature was controlled automatically by an elec-t r o n i c thermoregulator operating a relay. Closing of the relay shorted out a c o n t r o l l i n g resistance i n the power supply, so increasing the current to the furnace. A diagram of the furnace c i r c u i t i s shown i n F i g . 25 The thermoregulator, of the type developed by Coates (38), was constructed by Coope (39) • The c i r c u i t diagram i s shown i n F i g . 2... Operation of the instrument i s based on the revers-a l of phase of the out-of-balance e.m.f. of an a-c. bridge which occurs on passing from one side of balance to the other. The c i r c u i t i s composed of four main parts: an a-c. bridge, a c i r c u i t f o r amplifying the bridge output, a c i r c u i t f o r converting the bridge output to variable d-c. voltage, and a relay. The a-c. bridge consists of a centre-tapped transformer, T^, a resistance thermometer, R^, and a standard variable resistance, B^, which can be adjusted to balance the bridge at any desired temperature. The bridge output, e g^, i s amplified by and applied to the grid of V 2 as the much larger a l t e r -nating voltage e g 2 . An alternating voltage, e a 2 , i s applied to the anode of Y 2 which w i l l therefore pass current only during the ha l f cycles i n which e & 2 i s p o s i t i v e . Hence the magnitude of the anode current, i 2,depends on both the magni-tude and the phase of the a-c. grid voltage, « g 2» The anode icurrent generates a po t e n t i a l difference across R which i s smoothed by C^, R^, and G2, and applied to the grid of the output t r i o d e , as the d-c. voltage, e g 3 . The anode current of controls the furnace through the rela y . In the apparatus used, T^ was a Type 167-D 110 to 6 v. centre-tapped transformer. The anode supply to V 2 was pro-vided by the 110 v. a-c. mains. V., was a 6SJ7 pentode, V 2 a P i g . 3. Relay and. output c i r c u i t s c o n t r o l l i n g furnace temy -.rature • F i g . !+. Power supply for thertaoregulator. 26 6SF5 t r i o d e , and V^ a 6B1+G power output tr i o d e . The 350 v. d-e. anode supply to and was provided by a full-wave r e c t i f i e r and smoothing c i r c u i t shown i n F i g . h. In the powerpack, T 2 was a Thordardson T-13R13 transformer, and V^ . - a type 80 full-wave r e c t i f i e r . The variable resistance R 2 i n the a-c. bridge was a standard 0.1 to 1000 ohm decade d i a l resistance box. The Type M,molybdenum resistance thermometer used by Coope was found inadequate for the temperature range used i n the present i n v e s t i g a t i o n . A platinum resistance thermometer was constructed from 11 f t . of 0.05 i n . platinum wire of 30 ohms resistance. The wire was t i g h t l y coiled and wound on a t h i n mica s t r i p . The ends were silver-soldered to thi c k copper lead wires, which were fastened to an alundum rod and mounted inside the furnace. The relay c i r c u i t consisted of a Sunvic Type 602 vacuum relay with suitable series and shunting resistances. The instrument was able to control the furnace temperature to w i t h i n ~t 0.5°C. at the temperature of t h i s i n v e s t i g a t i o n . Description of a Typical Experimental Run The furnace was f i r s t adjusted to the desired temperature. Since several hours were required to heat the furnace from room temperature and to allow f o r the temperature d i s t r i b u t i o n along the length of the furnace to reach equilibrium, i t was maintained continuously at temperatures i n the required range. Before an experimental run the variable resistance i n the thermoregulator was adjusted to the appropriate temperature. 27 With the amplifier set at zero gain, the negative grid bias of was checked so the relay was at the t r i p point when the bridge was balanced. This relay t r i p point, about 16 ma., was indicated by a p i l o t l i g h t i n the relay output c i r c u i t . With the gain set at low s e n s i t i v i t y the furnace was allowed to warm up; the gain was then adjusted for maximum s e n s i t i v i t y , and the furnace temperature was allowed to reach equilibrium. The three rheostats c o n t r o l l i n g the temperature d i s t r i b u t i o n required separate adjustment f o r each temperature. The mercury d i f f u s i o n pump was used to evacuate the system to a "black vacuum." The pentene was cooled i n a bath of l i q u i d nitrogen and thoroughly pumped to remove traces of a i r . With taps and closed, and tap S-j^ open, the bulb contain-ing the pentene was immersed i n a beaker of warm water, from 50 to 1G0°C. depending on the i n i t i a l pressure required, and a few seconds were allowed for some of the pentene to vaporize. Tap was then opened cautiously and the pressure i n the re-action vessel, as indicated by the manometer, was allowed to increase s u f f i c i e n t l y . Taps and S-^ were then closed. At the completion of the f i l l i n g , the timer was started, and the course of the reaction was followed by pressure-time measure-ments made at regular i n t e r v a l s . Constant tapping of the manometer was found necessary i n order to prevent the mercury from s t i c k i n g i n the c a p i l l a r y tubing. At the conclusion of a reaction, taps and were opened to the pumps, and the system was thoroughly evacuated. 28 In an experiment involving a gaseous material, both the upper and lower g a l l e r i e s were f i r s t evacuated. Tap Sg was closed, the upper ga l l e r y was opened to the reaction vessel, and a s u f f i c i e n t pressure of the desired gas was allowed to enter. After reversing of tap S^, the pentene was v o l a t i l i z e d and admitted to the reaction vessel as described previously. General Form of the Pressure-time Curves Plots of pressure change vs. time for the decomposition of 150 mm. of pentene-1 at three d i f f e r e n t temperatures are shown i n F i g . 5t. At high temperatures the i n i t i a l portion of the curve i s a straight l i n e passing through the o r i g i n , followed by a gradual decrease i n rate as the reaction comes to an end. I f , i n such an experimental run, the i n i t i a l pressure increase was too rapid to observe the exact i n i t i a l pressure, t h i s value was obtained by extrapolation of the straight l i n e to zero time. Estimation of the i n i t i a l rate from the slope of t h i s straight l i n e portion of the curve thus presented_no d i f f i -c u l t y . At lower temperatures there i s at f i r s t a s l i g h t de-: crease of pressure, followed by a short period during which A p i s inappreciable. The curve then r i s e s to a maximum, and subsequently decreases as the reaction comes to an end. The rates f o r comparison purposes were therefore taken as the maximum slopes of such curves, the i n i t i a l rates obviously being useless as c r i t e r i a for comparison. Reproducibility of the rate curves was not found d i f f i c u l t to a t t a i n . To ensure the r e p r o d u c i b i l i t y of runs used i n the 60 50 « 2 C 5 3 0°C, 29 calculations, each was made i n duplicate or t r i p l i c a t e . Dependence of. the Rate on the I n i t i a l Pentene Pressure The dependence of the rate of pressure change on the i n i t i a l pentene pressure was investigated over a range of i n i t i a l pressures from 60 to 21+7 mm. In Table I are recorded the r e s u l t s for experiments conducted at 500°C. and various i n i t i a l pentene pressures. TABLE I Dependence of reaction rate on i n i t i a l pentene pressure at 500°C. I n i t i a l pentene pressure, mm. Reaction rate, mm./min. 60 3.80 100 6.03 111 7.6*+ 152 9.32 200 11.3 21+6 l>+.5 The Ap-time curves for these runs are shown i n F i g . 6 . The order was determined from the plot of log dP/dt vs. log P Q , shown i n F i g . 7 . Integration of the equation: dP/dt = k P Q n, y i e l d s : d log dP/dt = n d log P Q hence the slope of the curve gives the order of the reaction. The slope of the best l i n e through these points i s uni t y : i t Slops s 1 0.5 J I i I i i i i _ 1.7 1-8 1.9 ' 2.0 2.1 2.2 2.3 2.h l o g . P 0 F i r . 7. V a r i a t i o n of rate v i t h i n i t i a l pentene pressure, a.t £00 °C. 30 i s therefore concluded that the reaction i s f i r s t - o r d e r with respect to the pentene pressure. Rate constants were calculated from the f i r s t - o r d e r equation: dP/dt = k P Q and are given i n Table I I . They were found approximately con-stant over the range of pressures investigated. TABLE I I Reaction rates and rate constants f o r various i n i t i a l pentene pressures, at 500°C. I n i t i a l pressure, mm. Rate, mm./min. Rate constant, min." 1 x K r 60 3.80 6.3>+ 100 6 .03 6 .03 111 7.6*f 6.88 152 9.32 6.13 200 11.3 5 .65 2lf6 1M-.5 5.90 E f f e c t of Increased Surface-to-Volume Ratio i n Reaction Vessel A q u a l i t a t i v e estimation of the degree of heterogeneity of a gaseous reaction can often be determined by varying the surface-to-volume r a t i o i n the reaction vessel. The quartz reaction vessel was packed with short pieces of pyrex tubing. As no quartz tubing was available, the results of such surface increase can be of a q u a l i t a t i v e nature only. The ends of the pieces of tubing were fi r e - p o l i s h e d i n order 31 to avoid possible c a t a l y s i s by "active centers" at sharp edges. A 10-fold increase i n the surface-to-volume r a t i o was obtained i n t h i s way. In F i g . 8 are shown the A p-time curves for the decomposition of 110 mm. of pentene-1 at 5G0°C. i n both packed and unpacked reaction vessels. C l e a r l y the effect of increased surface i s very small. A s l i g h t decrease i n the i n i t i a l rate was shown, in d i c a t i n g the p o s s i b i l i t y of a small amount of chain termination on the sur-face. In addition, a s l i g h t lowering of the f i n a l pressure attained may have been due to adsorption of the products. From the r e s u l t s i t may be concluded that the reaction i s es-s e n t i a l l y homogeneous. Effect of Addition of Inert Gases In homogeneous gas-phase reactions involving a c t i v a t i o n of the reactant molecules by bimolecular c o l l i s i o n s , i t has often been found possible, by decreasing the p a r t i a l pressure of the reactant gas, to maintain the normal reaction rate by the ad-d i t i o n of some in e r t gas. Accordingly, experiments were carried out i n the presence, of both argon and nitrogen. F i g . 9 shows the rate curves for the decomposition of 100 mm. of pentene both alone and i n the presence of 100 mm. of argon. A curve f o r the decomposition of 200 mm. of pentene at 530°C. i s included f o r comparison. F i g . 10 shows a s i m i l a r set of curves i n which nitrogen i s substituted f o r argon. In both cases, the presence of the i n e r t gas does not Time, rain. F i g , <c, A p - t i ' i e cvrves f o r 6econpor:-ition of 1 1 G of pentene-1 at 500°C. i n packed f>nu vnpurkye r-.tion vessel:- • < ^ r-1 h CT .< n -• ^ j O / ? i n i £ j J i n . F i g . 10. Sffact of N 2 on rate, ??.t 53G°C. 32 serve to maintain the high-pressure reaction rate. In f a c t , rates for the di l u t e d reactions of 100 mm. of pentene are somewhat lower than the normal value f or 100 mm. of pentene alone. This observation leads to the conclusion that the reaction i s not a simple c o l l i s i o n a l process, but rather that i t involves the intervention of some type of f r e e - r a d i c a l chain process. The observed decrease i n rate could then be att r i b u t a b l e to chain termination by r a d i c a l recombination i n the gas phase due to three-body c o l l i s i o n s with inert gas molecules. Since surface effects were found to be n e g l i g i b l e , any r a d i c a l recombinations must, indeed, take place i n the gas phase rather than at the surface of the reaction vessel. E f f e c t of Addition of N i t r i c Oxide A method frequently employed to test f o r the presence of f r e e - r a d i c a l chains i n a gas reaction i s to add small amounts of some substance capable of i n h i b i t i n g any such chains by re-action with the free r a d i c a l s . N i t r i c oxide has often been found an e f f e c t i v e substance f o r t h i s purpose. Experiments were done at 5l6°C. with 120 mm. of pentene. Consecutive runs were made with 1, 5, 11, and 50 mm. of n i t r i c oxide. A second set of observations was made with 5*+ mm« of pentene and 1, 6, 10, and 13 mm. of n i t r i c oxide respectively. A p-time curves f o r these runs are plotted i n F i g . 11 . I t i s observed that n i t r i c oxide has no appreciable effect on the reaction rates. This r e s u l t may be interpreted i n sev-e r a l ways: (a) that radical-chain processes are absent i n A ko I o + 0 m:ria m " ° A 6 * 7 12 0 mm, p ent e ne 0 min. NO -1 " " - A i j ti tr _ £ 11 " " - A 3 0 ^ A A Q A < * O ^ o I • A -A ^ ° ^ 7 5!* - i . penter ^ f +0 mm, UQ ne O 4 • . . . " i" -* 4 4 • 13 4 | 4 OL 0 1 2 3 s+ 1 L T i - i e . -.in. i g . 11. .,ff-«rt of NO on r a t e ;:t ;;16°C, 5 6 7 8 33 t h i s reaction; (b) that any i n h i b i t o r y action of n i t r i c oxide i s masked by a c a t a l y t i c e f f e c t ; that i s , that n i t r i c oxide 1 can s t a r t chains as w e l l as stopping them; or (c) that the presence of some other chain i n h i b i t o r , presumably a reaction product, i s more e f f i c i e n t i n combining with the free r a d i c a l s than i s n i t r i c oxide. In the l a t t e r case, any chain steps i n the normal reaction would be mostly those of chain termination. Effect of Addition of Propylene Another substance which has frequently been used as an i n h i b i t o r of f r e e - r a d i c a l chains i s propylene. Since propylene was found by previous investigators to constitute one of the decomposition products of pentene-1 (32, 3 * 0 , i t was of i n t e r -est to determine i f the presence of further amounts of t h i s substance would i n h i b i t the reaction r a t e . F i g . 12 shows Apr-time curves f o r an experiment conducted at 516°C. with an i n i t i a l pentene pressure of 110 mm., and for successive runs i n the presence of k, 6, and 2k mm. of propylene respectively. In F i g . 13 reaction rates are plotted as func-tions of the amount of propylene added. The r e s u l t s f o r three < d i f f e r e n t i n i t i a l pentene pressures are recorded i n Table I I I . Propylene was found to cause a decrease i n the observed reaction rates. Quite considerable amounts of propylene were necessary to cause appreciable i n h i b i t i o n . In each case, the f i r s t few millimeters added produced no change i n the reaction r a t e . As the p a r t i a l pressure of the propylene was increased, the rate gradually decreased to approximately 80$ of i t s 3^ i n i t i a l value. As i s shown by the long, f l a t minimum of the i n h i b i t i o n curves, further additions of propylene were without e f f e c t . The greater the i n i t i a l pentene pressure, the greater was the amount of propylene required to produce maximum i n -h i b i t i o n . TABLE I I I t Rates of pentene-1 decomposition maximally i n h i b i t e d by propy-lene, at 516 C. Pentene pressure, mm. Normal r a t e , mm./min. Propylene pres-sure for maxi-mum i n h i b i t i o n , mm. Rate of maxi-mally inhibited reaction, mm./min. 5.83 10 5.90 110 20 1 2 . 5 200 23.h ho 18.7 Since propylene alone, at t h i s temperature, polymerises at a n e g l i g i b l e rate (26), i t s effect on the pentene decom-po s i t i o n would seem to represent a true i n h i b i t i o n rather than an i l l u s o r y effect due to a pressure decrease superimposed on the normal increase. I f the propylene i s effect-ive i n supressing a l l the chains, the re s i d u a l reaction may represent a process of simple molecular rearrangement to products. However, i t i s possible that the maximally i n h i b i t e d reaction i s s t i l l a modified chain reaction. Such observations suggest the p a r t i c i p a t i o n of f r e e - r a d i c a l chains, repressible by propylene, i n the nor-mal decomposition. The function of propylene i s presumably 35 to terminate r a d i c a l chains "by combining with the chain car-r i e r s to form stable molecules or less active chain c a r r i e r s . E f f e c t of Addition of Inert Gases on Rate of Maximally- Inhibited Decomposition Since the decrease i n the normal reaction rate due to d i l u t i o n with i n e r t gases could possibly be due to suppression of a chain reaction by three-body c o l l i s i o n s i n the gas phase, i t was of i n t e r e s t to investigate the effect of such d i l u t i o n on the maximally Inhibited reaction. F i g s . 1*+ and 15 show rate o curves f o r the decomposition, at 516 C., of 100 mm. of pentene, in h i b i t e d by 20 mm. of propylene, i n the presence of 100 mm. of argon, and 100 mm. of nitrogen, respectively. The curves for the undiluted i n h i b i t e d reactions are included f o r com-parison. Neither argon nor nitrogen causes a decrease i n the rate of the i n h i b i t e d reaction. Rather, there i s , i n each case, a s l i g h t increase, possibly due to an increase i n the c o l l i s i o n rate of a non-chain part of the reaction. E f f e c t of Addition of Lead Tetraethvl A further means of detecting the presence of f r e e - r a d i c a l chains i s to add small amounts of an i n i t i a t o r , at temperature conditions under which the reactant gas i s normally stable. The i n i t i a t o r must be a substance known to decompose into free r a d i c a l s at the appropriate temperature. Metal a l k y l s are use-f u l f o r t h i s purpose. In an attempt to i n i t i a t e the decomposition of pente.ne-1 at low temperatures, lead t e t r a e t h y l was selected as a sensi-t i z e r , since t h i s substance i s known to decompose r e a d i l y at 30 36 350°C., y i e l d i n g ethyl r a d i c a l s (*+2). Pure lead t e t r a e t h y l was unobtainable: the mixture used was a solution of Pb(G 2H^)i f i n ethylene dibromide and ethylene d i c h l o r i d e . Results from sensitization experiments can therefore be of a q u a l i t a t i v e nature only. Experiments carried out at 350°C. led to a decrease i n pressure, i n d i c a t i n g that, at t h i s low temperature, free ethyl r a d i c a l s induce polymerization rather than decomposition of the pentene. , Similar runs were made at t emperatures at which pentene-1 normally decomposes at a measurable rate. F i g . 16 shows A p -time plots f or the decomposition of 110 mm. of pentene, i n the presence of 15 mm. of Pb(C 2H^)i +, at temperatures of ^70, 500, and 530°C. Curves f o r the normal decompositions are shown f o r comparison. At each temperature, an increase i n rate i s exhibited i n the presence of PbCC^H^)^. Free ethyl r a d i c a l s thus appear to catalyze the reaction. This behaviour suggests that free r a d i -cals can cause decomposition of pentene-1 to a small degree. However, since the exact nature of the Pb(C 2H^)^mixture used was unknown, no very d e f i n i t e conclusions can be drawn. Dependence of"the Rate of Decomposition on Temperature The a c t i v a t i o n energy of a reaction may be determined from a study of the temperature .dependence of the reaction r a t e . The dependence of the rate of decomposition on tempera-ture was investigated at various i n i t i a l pressures of pentene i i n the temperature range between 1+70 and 530°C. Table IV con-tains the r e s u l t s f o r the experiments involved. Two or three runs were made at each temperature f o r each i n i t i a l pentene pressure, and the rates l i s t e d are averages of the i n i t i a l rates, as calculated-from the maximum slopes of the Ap-time curves. TABLE IV Temperature dependence of the reaction rates and reaction rate constants at various i n i t i a l pentene pressures I n i t i a l pressure, mm. Temp., °C. 1/T x 1 0 3 dP/dT, mm./min. Rate constant,> sec.-i-x IO4" Log (k x 10*) 60 5 3 0 1.21+7 1 1 . 5 3 2 . 0 1 . 5 0 5 516 1.268 6.62 18.1+ 1 . 2 6 5 5 0 0 1 . 2 9 3 3 . 8 0 1 0 . 6 1 . 0 2 5 1*86 1 . 3 1 8 2 . 0 * * 5 . 6 6 0 . 7 5 3 1+70 1.3k8 0 . 9 1 0 2 . 5 3 0 . 1*03 1 1 0 5 3 0 1.21*7 2 5 . ^ 3 8 . 5 1 . 5 8 5 516 1.268 11+.8 22 .1+ 1 . 3 5 0 5 0 0 1 . 2 9 3 7.61+ 1 1 . 6 1.061* 1+86 1 . 3 1 8 1+.0I+ 6 . 1 1 0 . 7 8 6 k-70 1 . 3 W 1.82 2 . 7 6 0.1+1+1 1 5 0 5 3 0 1 .21*7 3 0 . 9 3 i*.l* 1 . 5 V 7 5 1 6 1.268 I8 . 3 20.1* 1 . 3 1 0 5 0 0 1 . 2 9 3 9 . 3 2 1 0 . 1 * 1 . 0 1 7 ' 1*86 1 .318 ' 5 . 0 0 5 . 5 5 0.7kh 1+70 1 . 3 ^ 8 2 . 1 8 2 .1*2 O.38I* 2 0 0 5 3 0 1.21+7 1+0.0 3 3 . ^ 1 . 5 2 1 * 516 1.268 23.1+ 1 9 . 5 I . 2 9 0 500 1 . 2 9 3 1 1 . 3 9M 0 . 9 7 ^ 1*86 1 .318 5 . 9 8 h.98 0 . 6 9 7 k-70 1 . 3 W 2 . 7 6 2 . 3 0 0 . 3 6 2 530 I.21+7 5 2 . 7 3 ^ . 6 1 . 5 5 1 ' 5 1 6 1.268 2 9 . 0 1 9 . 6 I . 2 9 2 500 1 . 2 9 3 11+.5 9 . 7 8 0 . 9 9 0 \86 1.318 7 . 8 5 5 . 3 0 0.721+ 1*70 1 . 3 ^ 8 3 . 6 3 2 .1*5 O.389 38 A plot of log k versus l/T for an i n i t i a l pentene pressure of 150 mm. i s shown i n F i g . 17. The l i n e a r i t y of the curve shows that the Arrhenius Equation i s v a l i d over t h i s range of temperatures; that i s , k = A e " E / R T . Since integration of t h i s equation y i e l d s the expression: In k = In A - E/RT, therefore the slope of the In k vs. l/T graph i s equal to -E/RT. The a c t i v a t i o n energies f o r each i n i t i a l pentene pres-sure were calculated from the slopes of the respective Arrhenius p l o t s ; the r e s u l t s are l i s t e d i n Table V. TABLE V A c t i v a t i o n energies f o r normal decomposition of pentene-1 Pentene pressure, mm. Slope A c t i v a t i o n energy, kcal./mole 60 10.95 50.2 110 11.23 51.5 150 11. ^ 7 52.6 200 11.61 53.2 2V7 11.60 53.2 From the rate constant at 500°C. the frequency factor, A, 11 -1 was calculated to be approximately 10 sec. . This value i s i n agreement with those normally found for such f i r s t - o r d e r decomposition reactions. In the a c t i v a t i o n energy values l i s t e d i n Table V, there 39 i s evident a s l i g h t increase corresponding to increase i n i n i t i a l pentene pressure. In F i g . 18 the plot of a c t i v a t i o n energy as a f inaction of pentene pressure yi e l d s a curve that i s almost l i n e a r . I t was of interest to investigate t h i s v a r i a t i o n of a c t i v a t i o n energies. A possible explanation of the observed trend i n the values i s the following: a higher a c t i v a t i o n energy at higher pressures may be due to a process of c o l l i s i o n a l deactivation of activated molecules. When a pentene. molecule enters into a c o l l i s i o n , i t gains a c e r t a i n amount of energy, which i t subsequently d i s t r i b u t e s among i t s various i n t e r n a l degrees of freedom. Further favourable c o l -l i s i o n s fransfer more energy, and when the amount of energy necessary f o r decomposition has been l o c a l i z e d to a s p e c i f i c bond, t h i s bond w i l l break. Now i t i s possible that, during t h i s i n t e r n a l r e d i s t r i b u t i o n of energy, a higher pressure, and a correspondingly higher c o l l i s i o n rate, may cause a greater proportion of unfavourable c o l l i s i o n s , leading to deactivation. Thus, i n order f o r an activated molecule, at high pressures, to possess s u f f i c i e n t energy f o r decomposition i t must gain, i n favourable c o l l i s i o n s , larger amounts of energy than i t would require at lower pressures. The corresponding increase i n the "height of the p o t e n t i a l b a r r i e r f o r the decomposition reaction would be evidenced i n a somewhat greater value of the a c t i v a -t i o n energy. Ef f e c t of Addition of Inert Gases on A c t i v a t i o n Energy Assumption of the above mechanism leads to the prediction 1+0 that the presence of an i n e r t gas w i l l produce the same type of c o l l i s i o n a l e f f e c t . This prediction Involves the further assumption that a l l molecules, regardless of complexity, are equally capable of trans f e r r i n g energy. In an e f f o r t to test the v a l i d i t y of t h i s idea, experiments were done i n which the p a r t i a l pressure of pentene was decreased, the t o t a l pressure being maintained by the addition of inert gases. In Table VI are l i s t e d the r e s u l t s for two sets of runs i n which 100 mm. of pentene was decomposed i n the presence of 100 mm. of argon and 100 mm. of nitrogen respectively. Results f o r 100 and 200 mm. of pentene alone are included for comparison. These runs were made i n the temperature range of 1+70 to 530°C. TABLE VI A c t i v a t i o n energies f o r decomposition of pentene-1 i n the presence of i n e r t gases Total pressure, mm. Pentene pressure, mm. Argon pressure, mm. Nitrogen pressure, mm. Ac t i v a t i o n energy, kcal./mole 200 100 100 0 50.6 ,: 200 100 0 100 5 i .o 100 100 0 0 51.2 200 200 0 0 53.2 C l e a r l y , increase of pressure due to the presence of either Argon or nitrogen does not produce any s i g n i f i c a n t i n -crease i n the a c t i v a t i o n energy. E f f e c t of Addition of Freon on A c t i v a t i o n Energy Since the molecular weight of nitrogen, 28, and the atomic t weight of argon, kO, are both les s than the molecular weight of pentene, 70, and, furthermore, the number of degrees of freedom for both these molecules i s considerably less than for pentene, i t was hoped to obtain, more sa t i s f a c t o r y r e s u l t s with the use of a heavier and more complex molecule, which would s t i l l be i n e r t at the temperatures required. Fluorocarbons are known to be both extremely stable to heat and chemically i n e r t . As no pure fluorocarbons were av a i l a b l e , a freon, monochlorotrifluoromethane, was used, with the hope that i t would prove s u f f i c i e n t l y i n e r t f o r the purpose. 100 mm. of the freon gas showed no s i g n i f i c a n t pressure increase when sub-jected, f o r over an hour, to the temperatures used i n the pentene decomposition. Experiments were therefore done with 100 mm. of pentene i n the presence of 100 mm. of freon. The reaction showed a con-siderable increase i n rate, amounting to approximately a 51$ increase above the normal value. The a c t i v a t i o n energy was found to be 53.5 k c a l . However, samples of the reaction mix-tures, taken a f t e r ten minutes, gave positive t e s t s f o r halogen with a l k a l i and a l c o h o l i c s i l v e r n i t r a t e . Since the freon gas alone, when tested s i m i l a r l y , did not give p o s i t i v e test f o r halogen, i t was concluded that some methyl chloride, methyl f l u o r i d e , or other a l k y l halide had been formed i n the reaction, and hence that the freon used was not i n e r t at these temperatures 1+2 i n the presence of pentene. Effect of Addition of Propylene on A c t i v a t i o n Energy Since propylene has been found to i n h i b i t the pentene decomposition, presumably by a chain terminating mechanism, the resi d u a l reaction may be regarded as a process of molecular re-arrangement. I t was of interest to investigate the a c t i v a t i o n energy of t h i s residual reaction i n order to determine whether there s t i l l existed a v a r i a t i o n of a c t i v a t i o n energy with pres-sure. The a c t i v a t i o n energies f o r the maximally in h i b i t e d de-compositions were investigated at three d i f f e r e n t i n i t i a l pressures over the temperature range from 1+70 to 530°G. The r e s u l t s are shown i n Table V I I . TABLE VII i A c t i v a t i o n energies of maximally in h i b i t e d decomposition Total pressure, mm. Pentene pressure, mm. Propylene pressure, mm. A c t i v a t i o n energy, kcal./mole 6*+ 9* 10 110 21+ 50.7 250 i 200 50 52.0 The values f o r the a c t i v a t i o n energy of the res i d u a l reaction are very close to those of the t o t a l reaction, being approxi-mately 1. k c a l . lower. In F i g . 1 9 i s shown a plot of a c t i v a t i o n energy as a function of i n i t i a l pentene pressure. Again, an increase i n pressure corresponds to an increase i n a c t i v a t i o n o 50 l o o • 150 200 r5o • P r e s s u r e p e n t e n e 5 mm. ? i . Z . 1 9 . Dependence of a c t i v a t i o n en-.r^y of — r i . - ^ l l y i n h i b i t e d r e a c t i o n on i n i t i a l pentene p r e s s u r e . *3 energy. Hence t h i s effect i s not peculiar to the uninhibited part of the reaction alone. The close s i m i l a r i t y between the a c t i v a t i o n energy values fo r the in h i b i t e d reaction and f o r the t o t a l reaction indicates that the two processes are able to proceed with almost equal ease. Ef f e c t of Addition of Inert Gases on A c t i v a t i o n Energy of  Maximally-Inhibited Reaction Since the maximally i n h i b i t e d decomposition was found to exhibit an increase i n a c t i v a t i o n energy with increased i n i t i a l pressure of. hydrocarbon, the a b i l i t y of in e r t gases to maintain the high a c t i v a t i o n energy value of the residual reaction was investigated. Two sets of experiments were done over the range of temperatures from *H70 to 530°C. with pentene inhibi t e d with propylene, and i n the presence of argon and of nitrogen respectively. Results obtained are shown i n Table V I I I . Values f or the inh i b i t e d decomposition of 200 mm. of pentene-1, i n the absence of in e r t gas, are shown f o r comparison. TABLE VIII A c t i v a t i o n energies of maximally i n h i b i t e d decomposition diluted with i n e r t gases Total pressure, mm. Pentene pressure, mm. Propylene pressure, mm. Argon pressure, mm. Nitrogen pressure, " mm. Act i v a t i o n energy, Kcal./mole 220 100 20 100 0 50.2 220 100 20 0 100 50.5 250 100 50 0 0 52.0 As was found i n the case of the uninhibited decomposition, neither argon nor nitrogen i s e f f e c t i v e i n increasing the value of the a c t i v a t i o n energy of the residual reaction. Effect of Addition of N i t r i c Oxide on A c t i v a t i o n Energy Although n i t r i c oxide was found i n e f f e c t i v e i n i n h i b i t i n g the pentene decomposition, i t was thought possible that i t s presence might affect the value of the a c t i v a t i o n energy. Ac-cordingly the temperature dependence of the decomposition of pentene i n the presence of n i t r i c oxide was investigated at three d i f f e r e n t i n i t i a l pressures. Results f o r these experi-ments are given i n Table IX. TABLE IX A c t i v a t i o n energy of decomposition i n presence of n i t r i c oxide Total Pentene N i t r i c oxide A c t i v a t i o n pressure, pressure, pressure, energy, mm. mm. mm. kcal./mole 60 50 10 50.k 230 200 30 53. h C l e a r l y , the addition of n i t r i c oxide has no appreciable effect on the value of the a c t i v a t i o n energy. The increase with i n -creased pressure i s again observed. Summary of the Experimental Results The main r e s u l t s obtained i n t h i s i n v e s t i g a t i o n may be summarized as follows: (1) Pentene-1 decomposes at a measurable rate at temperatures \5 above *+70°C. The pre ssiare-time curves at t h i s temperature show an i n i t i a l decrease, followed by a rapid increase as the rate builds up to i t s maximum value. At somewhat higher tem-peratures there i s only an increase i n pressure, i n d i c a t i n g that at the high temperatures polymerization i s not an im-portant primary process. (2) The reaction rate i s of the f i r s t order with respect to the i n i t i a l pentene pressure. At 5QQ°C, the reaction rate constant i s approximately 9.061 m i n f l , (3) Increase of the surface-to-volume r a t i o causes no appre-ciable a l t e r a t i o n i n the rate of the reaction. The reaction i s therefore e s s e n t i a l l y homogeneous. (*+) Addition of argon and of nitrogen causes a s l i g h t de-crease i n the rate of the reaction, the effect being more pronounced i n the case of nitrogen. This phenomenon suggests that the reaction i s not a simple c o l l i s i o n a l a c t i v a t i o n process; rather, the operation of a f r e e - r a d i c a l chain mechanism i s indicated, wherein the in e r t gas molecules are able to function as t h i r d bodies, favouring r a d i c a l recombina-t i o n . (5) N i t r i c oxide has no effect on the rate of the reaction. This behaviour suggests three p o s s i b i l i t i e s : (a) the absence of free r a d i c a l s ; (b) a c a t a l y t i c effect of n i t r i c oxide, equal and opposite to i t s i n h i b i t o r y action; (c) the presence of some reaction product, such as 1+6 • i propylene, which i s able to combine with r a d i c a l s more e f f e c t i v e l y than i s n i t r i c oxide. This l a t t e r a l t e r n a t i v e suggests extremely short reaction chains, the majority of the f r e e - r a d i c a l reactions involved being chain terminating steps. (6) Added propylene causes a 20% reduction i n the rate of the reaction. Beyond a certain amount, further addition of propy-lene does not affect the rate. This phenomenon suggests the operation of a f r e e - r a d i c a l chain mechanism, repressible by propylene. The r e s i d u a l reaction may or may not involve chains. (7) The presence of i n e r t gases i n the maximally i n h i b i t e d reaction causes a s l i g h t increase i n the rate. This behaviour suggests that the r e s i d u a l reaction, i n the presence of propy-lene, does not involve f r e e - r a d i c a l chains. (8) The presence of lead t e t r a e t h y l , at low temperatures, i n -duces polymerization of pentene-1. At temperatures at which the normal decomposition proceeds at a measurable rate, small amounts of lead t e t r a e t h y l accelerate the decomposition. I t would appear probable therefore that both the low-temperature polymerization and the high-temperature decomposition are processes involving free r a d i c a l s . ( 9 ) The dependence of the rate of the reaction upon tempera-ture i s i n accordance with the Arrhenius Equation. The aetiva t i o n energy f o r the normal reaction i s approximately 5 2 k c a l . k7 per mole. The frequency factor, at 500°C, i s approximately 1011 secT 1 .  (10) Increase i n the i n i t i a l pentene pressure i s accompanied by an increase i n a c t i v a t i o n energy, amounting to between 1 and 2 k c a l . per mole for a pressure increase of 100 mm. (11) The energy of a c t i v a t i o n i s unaffected by the addition of nitrogen or argon to the reaction system. (12) The presence of CF^Gl considerably increased both the rate of pressure change and the a c t i v a t i o n energy f o r the process. P o s i t i v e evidence for the presence of an a l k y l halide i n the reaction mixture indicates that the freon i s not i n e r t i n the presence of pentene-1 at the temperatures employed. (13) Addition of propylene produces no appreciable change i n the values of the a c t i v a t i o n energy. The maximally in h i b i t e d reaction exhibits an increase of activation,energy with i n -crease of pressure. The mechanism causing the pressure-dependence of the a c t i v a t i o n energy would therefore appear to be also operative i n the r e s i d u a l reaction, and not a property of the repressible portion of a chain mechanism. (Ik) Addition of i n e r t gases to the maximally i n h i b i t e d re-action produce_s no appreciable effect on the a c t i v a t i o n energy. (15) Addition of n i t r i c oxide produces no appreciable effect on the a c t i v a t i o n energy. 1+8 DISCUSSION Nature of the Primary A c t i v a t i o n Process The f i r s t question which arises i n predicting the mecha-nism of a thermal decomposition reaction concerns the funda-mental nature of the primary a c t i v a t i o n process. The primary act i n the decomposition of any organic molecule may he either an i n t e r n a l rearrangement to stable products or a bond rupture producing free r a d i c a l s , which subsequently cause further decomposition by a chain mechanism. In any p a r t i c u l a r instance, one of these mechanisms may be operative to the v i r t u a l ex-clusion of the other, or the two processes may occur simul-taneously i n p o t e n t i a l competition. In order to decide upon the r e l a t i v e effectiveness of these two types of mechanism i n the thermal decomposition of pentene-1, i t w i l l be necessary to consider the experimental evidence which has been accumulated both f o r and against the presence of r a d i c a l chains. The evidence i n favour of the p a r t i c i p a t i o n of a f r e e - r a d i c a l chain mechanism, may be sum-marized as follows: (a) the presence of i n e r t gases i n the reaction mixture , causes a decrease i n the reaction rate; (b) added propylene i n h i b i t s the reaction, reducing the rate to approximately 80$ of i t s o r i g i n a l value; (c) added free r a d i c a l s accelerate the reaction. However, further experimental r e s u l t s do not appear to support h9 these observations: (d) n i t r i c oxide i s incapable of causing i n h i b i t i o n of the reaction; (e) the reaction rate i s not sensitive to a change i n the surface-to-volume r a t i o i n the reaction vessel. These points w i l l be considered i n turn. For a homogeneous decomposition reaction, dependent upon c o l l i s i o n a l a c t i v a t i o n f o r causing d i r e c t molecular rearrange-ments, the presence of i n e r t gases would be expected to ac-celerate the rate of decomposition. That i s , i f the p a r t i a l pressure of reactant molecules were decreased, i n e r t gas molecules should maintain, by c o l l i s i o n s , the high-pressure energy d i s t r i b u t i o n among the molecules, so tending to prevent a f a l l i n g off i n rate. I f , however, a chain mechanism i s operative, an in e r t gas may aff e c t the rate i n one of two possible ways: (a) i f chains are terminated heterogeneously, the i n e r t gas, by impeding d i f f u s i o n of r a d i c a l s to the surface of the reaction vessel, may be expected to accelerate the reaction; (b) i f r a d i c a l s recombine homogeneously, an in e r t gas should retard the reaction by favouring recombination of r a d i c a l s at ternary c o l l i s i o n s . Now, since the surface has been shown to play no s i g n i f i c a n t part i n the decomposition of pentene-1, obviously recombination of any r a d i c a l s that may be present must be a homogeneous process.. I t has been shown that both argon and nitrogen, e,ven when present i n large proportions, 50 f a i l to cause any acceleration; rather, they have been observed to exert a d e f i n i t e retardation on the reaction rate. Thus i t may be inferred that the decomposition of pentene-1 involves a f r e e - r a d i c a l mechanism, i n which reaction chains are termi-nated homogeneously, p a r t i a l l y , at l e a s t , by ternary c o l l i s i o n s . The effect of propylene w i l l next be considered. In acting as an i n h i b i t o r , a propylene molecule i s believed to function, e s s e n t i a l l y , as a t h i r d body f o r removing r a d i c a l s : the propylene molecule replaces a chain c a r r i e r by a more stable a l l y l - t y p e r a d i c a l , less e f f i c i e n t f o r continuing the chain process: E -f CH^CH = CH 2 -> RH -f CH 2 CH = L_ Z CH2» Other unsaturated molecules may be expected to act s i m i l a r l y ; for example, i t has been shown that isobutene i n h i b i t s the decomposition of pentane (*+0) • In the pentene-1 decomposition, the presence of a large amount of unsaturated material, due both to the pentene i t s e l f and to i t s decomposition products, may be expected to cause appreciable i n h i b i t i o n i n the course of the normal reaction. Thus i t might be expected that a large proportion of any f r e e - r a d i c a l chains which may be i n i t i a t e d i n the primary process w i l l be terminated i n t h i s way. I f repression of the r a d i c a l chains i s incomplete i n the normal reaction, the addition of increasing amounts of propy-lene should supplement such i n h i b i t i o n to the point of complete chain termination. Such was indeed found to be the case. Propylene i n h i b i t i o n reduced the rate to about Q0% of i t s 5 1 normal value. Molera and Stubbs have also reported some i n -h i b i t i o n of the pentene-1 decomposition by propylene (3 * 0 . The t h i r d p o s i t i v e c r i t e r i o n f o r the presence of free-r a d i c a l chains i s the observed c a t a l y s i s of the decomposition by lead t e t r a e t h y l . There seems to be l i t t l e doubt that a metal alk y l , . such as lead t e t r a e t h y l , decomposes with the formation of free r a d i c a l s : where M i s the metal and R the a l k y l r a d i c a l 0+1). Such metal a l k y l s have frequently been used as s e n s i t i z e r s . Catalysis of an organic decomposition by the presence of small amounts of such substances i s regarded as evidence for a f r e e - r a d i c a l mechanism: the added r a d i c a l s are believed to attack reactant molecules and accelerate t h e i r decomposition.. The observed c a t a l y s i s of the pentene -1 decomposition, then, by small amounts of lead t e t r a e t h y l , indicates that free r a d i c a l s are able to cause decomposition of the pentene. Such indications do not, however, prove conclusively that chains are e f f e c t i v e i n the normal decomposition. On what would appear to be the negative side of the argu-ment for the action of free r a d i c a l s i n the pentene decomposi-t i o n , are the r e s u l t s from experiments on n i t r i c oxide i n h i b i -t i o n and from experiments on surface e f f e c t s . I t i s possible, however, to interpret the r e s u l t s of these experiments i n such a way that the observed r e s u l t s are not necessarily contra-d i c t o r y , and a chain process may s t i l l provide a plausible 52 explanation of the reaction mechanism. N i t r i c oxide produced no a l t e r a t i o n i n the decomposition r a t e . Molera and Stubbs report the same r e s u l t (3*+). Assum-ing the effect of a l l i n h i b i t o r s of chain reactions to be e s s e n t i a l l y the same; that i s , to reduce the concentration of chain r a d i c a l s , i t would seem contradictory that propylene should be capable of causing appreciable i n h i b i t i o n , whereas n i t r i c oxide i s completely i n e f f e c t i v e . In order to provide an argument i n favour of a chain mechanism, i t must be shown that the observed lack of i n h i b i t i o n does not constitute proof of a non-chain reaction: one of the following assumptions must be made: (a) that n i t r i c oxide can st a r t chains as w e l l as stop them i n pentene; (b) that the ra d i c a l s present i n the pentene decomposition combine with n i t r i c oxide too slowly for appreciable i n h i b i t i o n ; or, (c) that the i n h i b i t o r y effect due to the necessarily high concentration of o l e f i n s present i n the reaction mixture w i l l swamp out any effect of n i t r i c oxide. The f i r s t of these assumptions was adopted by Rice and P o l l y i n order to account f or s i m i l a r l y unusual i n h i b i t i o n r e s u l t s for the decomposition of acetaldehyde 0+2). They proposed the following scheme by which n i t r i c oxide can both st a r t and stop chains: M l ~* 2Ri (I) + NO —± HNO -f R 2 (II) R 1 H- Mx -± RjH + R 2 ( I I I ) R 2 R 1 + Mg (IV) 53 ^ 4 - NO —> RjNO R 2 •+• NO -^B^NO Rj^ "I- R 2 —^R-^~R2 (VIII) (VII) (VI) (V) Decomposition was assumed to occur with chain termination either by steps (V) and (VI), corresponding to a low a c t i v a -t i o n energy f o r step (IV), or by steps (VII) and (VIII), with a high a c t i v a t i o n energy f o r step (IV). I t i s possible to assume a scheme of t h i s type for the pentene-1 decomposition: n i t r i c oxide may react with a pentene molecule, y i e l d i n g HNO plus a r a d i c a l , R 2 , which decomposes, producing R^, a smaller r a d i c a l able to i n i t i a t e the chain step (IV). According to t h i s scheme, n i t r i c oxide also functions as an i n h i b i t o r by combining with R^ and R 2. I t i s possible, on t h i s basis, to predict no net effect of n i t r i c oxide on the observed reaction rate. Assumption (b), that the reaction between n i t r i c oxide and the r a d i c a l s present i s too slow to produce appreciable I n h i b i t i o n , hardly seems a probable explanation: i t i s very l i k e l y that, i f r a d i c a l s are present at a l l , both methyl and ethyl r a d i c a l s would be among these, and i n most other organic decomposition reactions involving these r a d i c a l s , n i t r i c oxide has been found to exhibit very marked i n h i b i t o r y effects ( 1 ) . A consideration of the unsaturated nature of the decom-posing pentene molecule and of the probable unsaturated nature 9* of i t s decomposition products, suggests that (c) i s the most reasonable assumption, more espe c i a l l y i n the l i g h t of the experimentally observed i n h i b i t i o n by propylene. Unsaturates, present i n large concentration, must f a r surpass n i t r i c oxide i n t h e i r a b i l i t y to terminate chains. Any i n h i b i t o r y effects of n i t r i c oxide are completely masked. The r e s u l t s of other investigators bear out t h i s conclusion. Eltenton, from mass spectrometric investigations, concluded that propylene reacts more e a s i l y than does n i t r i c oxide with methyl radicals(V})5 at high temperatures he detected both methyl and a l l y l r a d icals i n the decomposition of propylene. Observations made by Steacie and Folkins on the n i t r i c oxide i n h i b i t i o n of p a r a f f i n decompositions demonstrate that the i n h i b i t i o n f a l l s o f f as the reactions proceed, the rates approaching t h e i r normal values (M+). These investigators have attributed t h e i r f i n d -ings to the building up with time of o l e f i n concentration i n the products, the ensuing i n h i b i t i o n by products swamping out the effect of n i t r i c oxide. Assumption (c) appears more convincing than (a). I t i s rather improbable that the opposed accelerating and retarding actions of n i t r i c oxide should exactly counterbalance each other, y i e l d i n g no net e f f e c t . The second argument which seems to suggest a non-chain process i s the homegeneity of the reaction. I f long chains are assumed, a large increase i n the surface-to-volume r a t i o would be expected to f a c i l i t a t e heterogeneous r a d i c a l 55 recombinations. However, i f the chains are short, r e l a t i v e l y more homogeneous chain-breaking processes w i l l occur while a given number of hydrocarbon molecules decompose; hence, any competing chain-breaking process due to i n h i b i t i o n on the surface w i l l be less important. There are Indications that the chains are short. In the f i r s t place, quite considerable amounts of propylene are required i n order to bring about ap-preciable i n h i b i t i o n s t h i s f act suggests a large number of r e l a t i v e l y short chains rather than a few long ones, since the amount of i n h i b i t o r used to stop chains must l o g i c a l l y depend on the number of such chains present. Further, i f i t i s as-sumed that the maximally i n h i b i t e d reaction represents a state of a f f a i r s i n which a l l chains are cut down to t h e i r primary process, the average chain length may be calculated from the r a t i o of the normal to the in h i b i t e d r a t e . A low value of about 1 .25 i s obtained. This means eit h e r ; (a) that r a d i c a l s from every h primarily decomposing molecules cause the decom-po s i t i o n of 1 more; or, (b) that only one i n 100, say, of the primary processes y i e l d s r a d i c a l s , but that each of these causes the decomposition of about 25 molecules. In comparison with the extremely long chains found i n many organic decom-po s i t i o n reactions, even the second p o s s i b i l i t y does not lead to an extremely large value f o r the absolute chain length. Short chains were also postulated by Gorin on the basis of h i s a n a l y t i c a l r e s u l t s f o r the pentene-1 decomposition i n a flow system ( 3 2 ) . 56 A consideration of the foregoing r e s u l t s therefore ap-pears to point to a mechanism involving a s p l i t of pentene molecules to free r a d i c a l s , which subsequently i n i t i a t e short chains, terminated both by ternary c o l l i s i o n s i n the gas-phase and by combination with unsaturated molecules. The long, f l a t minima of the i n h i b i t i o n curves show that a d e f i n i t e f r a c t i o n •. of the reaction i s exempt from i n h i b i t i o n . Presumably t h i s r e s i d u a l reaction consists of an intramolecular rearrangement process of low a c t i v a t i o n energy. Although lack of further i n h i b i t i o n does not preclude the p o s s i b i l i t y that the maximal-l y i n h i b i t e d reaction i s i t s e l f a modified chain reaction, the fact that i n e r t gases exert an accelerating rather than a re-tarding effect on the rate of the i n h i b i t e d process, would suggest the absence of chains. The fact that the a c t i v a t i o n - energy values for the normal and the i n h i b i t e d reactions are very close probably renders the two competing mechanisms of about equal importance i n the o v e r a l l reaction process, and explains why they are able to operate simultaneously. According to the simple c o l l i s i o n theory, the rate con-stant may be calculated from the c o l l i s i o n number, z, and the observed energy of a c t i v a t i o n for the reaction by use of the equation: k * z e " E / R T . The k obtained w i l l be the rate constant expected from simple c o l l i s i o n a l a c t i v a t i o n i n two squared terms. Rate constants have been calculated i n t h i s way f o r both the normal and the 57 maximally i n h i b i t e d decomposition of 200 mm. of pentene-1 at 500°C. The value of z, the number of molecules c o l l i d i n g /cc. / s e c , was calculated from the equation: where n represents the number of molecules i n a unit volume, — 8 or the molecular diameter (5 x 10~ cm.), R'the gas constant/mole, T the absolute temperature, and M the molecular weight of pentene. This y i e l d s a value f o r z of 1.7 x 10 2 7 molecules c o l l i d i n g /cc./sec. In Table X the calculated values of k are compared with those obtained experimentally. Now, although the a c t i v a t i o n energy for the inh i b i t e d reaction i s somewhat lower than that f o r the normal reaction, i t i s seen from Table X that the observed rate constant i s s t i l l greater than would be expected from simple c o l l i s i o n a l a c t i v a t i o n i n two squared terms. Hence the i n t e r n a l rearrangement process must be one i n which many degrees of freedom contribute to the a c t i -vation. TABLE X Calculated and observed rate constants for normal and inh i b i t e d reactions 1 k from z e~ E/ R T, sec.""1 k from actual rate, sec.""1 Normal reaction Maximally i n -h i b i t e d reaction 5,2 x IO" 8 l.h x 10"7 9.h x IO'1* ' 1.9 x 10"^  z = 2n 2 cr 2 irRT M 58 A consideration of the bond strengths involved imposes ce r t a i n r e s t r i c t i o n s on the proposed chain mechanism. Accord-ing to Stevenson's r e s u l t s (^5), based on electron impact data, we may assign a bond energy value of approximately 77 k c a l . to the weakest bond i n the pentene molecule; that i s , to the CVC bond i n they3 p o s i t i o n with respect to the doubly-bound carbon atom. The a c t i v a t i o n energy of the o v e r a l l reaction i s thus considerably lower than the energy necessary to disrupt the weakest l i n k i n the molecule. I f , then, a chain process i s important, only exceptional molecules, too rare to affect the mean a c t i v a t i o n energy, can give r a d i c a l s . This suggests r e l a t i v e l y few chains. In order that the chain process be important i n r e l a t i o n to the competing intramolecular mode of decomposition, the chains must be of appreciable lengths. Now, i t has been observed that the a c t i v a t i o n energy for the i n h i b i t e d reaction i s somewhat lower than that f o r the o v e r a l l process. I t follows that i n h i b i t i o n must increase with temperature. Therefore the average chain length must increase with temperature, suggesting that the chain reaction i s more important at higher temperatures, when more thermal energy i s a v a i l a b l e . Thus i t would appear that the decomposition of pentene-1 i s a complex process, involving a f r e e - r a d i c a l chain mechanism, i n h i b i t a b l e both by reaction products and by added propylene, together with an intramolecular rearrangement process of low a c t i v a t i o n energy, a large number of i n t e r n a l degrees of 59 freedom contributing to the a c t i v a t i o n process. At high tem^,. peratures the contribution of the chain mechanism to the o v e r a l l reaction becomes more important. The absolute chain length i n -creases with temperature: probably very short chains predomi-nate at lower temperatures, most of the chain steps being those of termination, whereas at higher temperatures the chains are longer-lived. Chain termination takes place homogeneously both by ternary c o l l i s i o n s , with pentene molecules acting as t h i r d bodies, and by addition of r a d i c a l s to unsaturated molecules. Without detailed analyses of the reaction products, no conclusions can be drawn as to the exact mechanism of the decomposition. I t i s to be hoped that future mass spectro-metric analyses w i l l be able to elucidate more completely the complex k i n e t i c s of the thermal decomposition of pentene-1. Dependence of A c t i v a t i o n Energy on I n i t i a l Pentene Pressure I t was found that the value of the a c t i v a t i o n energy ex-h i b i t e d at .small increase with an increase i n the i n i t i a l pen-tene pressure. Such a phenomenon i n the pentene-1 decomposi-t i o n has also been reported by Molera and Stubbs ( 3 6 ) . These authors, however, do not comment upon t h e i r findings. Such a trend of a c t i v a t i o n energies contrasts sharply, with the be-haviour of normal p a r a f f i n hydrocarbons, which exhibit a large decrease of a c t i v a t i o n energy with increasing pressure. The increase of a c t i v a t i o n energy with pressure found f o r the normal reaction of pentene-1 has been shown to apply also 60 to the maximally i n h i b i t e d reaction. Thus i t would appear that the explanation for t h i s behaviour must l i e i n the a c t i -vation mechanism of the molecular rearrangement process. A possible explanation for the phenomenon has been suggested i n the experimental section of t h i s thesis (p. ). This sug-gestion involves a mechanism of c o l l i s i o n a l deactivation: activated molecules, whose energy of a c t i v a t i o n i s d i s t r i b u t e d i n several squared terms and so i s not mobilized for the rupture of s p e c i f i c bonds, are deactivated by c o l l i s i o n s with normal molecules at high pressures. That i s , the r e l a t i v e p r o b a b i l i t y of unfavourable to favourable c o l l i s i o n s must be assumed to be greater at higher pressures. Due to t h i s siphoning of energy by unfavourable c o l l i s i o n s , there w i l l follow an increase i n the energy that a pentene molecule must gain by favourable c o l l i s i o n s i n order to reach the top of the p o t e n t i a l energy b a r r i e r . Such an effect would be manifested i n a higher value of the a c t i v a t i o n energy at higher pressures. Experiments performed with high pressures maintained by the presence of foreign gases have f a i l e d to provide any sup-porting evidence for t h i s hypothesis. Their presence exerts no appreciable effect on the observed values of the a c t i v a t i o n energy f o r either the normal or the i n h i b i t e d reaction. I t may be, however, that argon and nitrogen are not molecules of s u f f i c i e n t complexity for the purpose. I t i s hoped that i n future work i t may be possible to f i n d a molecule comparable i n complexity to the pentene i t s e l f , yet s t i l l i n e r t at the J 61 temperatures of the pentene decomposition. Such a molecule, possessed of several more degrees of freedom, might be expected to exert a c o l l i s i o n a l e ffect s i m i l a r to that shown by pentene. Thus no d e f i n i t e conclusions can be drawn as to the v a l i d i t y of the proposed c o l l i s i o n a l deactivation process. In the l i g h t of the r e s u l t s obtained with nitrogen and argon, i t would appear more probable that the phenomenon arises from some inherent property of the mode of decomposition of the pentene molecule i t s e l f . Since the i n h i b i t e d part of the mechanism i s believed to proceed with a s l i g h t l y greater a c t i v a t i o n energy than the residual reaction, i t might be speculated that higher pressures favour a chain mechanism; that i s , that as the pentene pressure i s increased, the con-t r i b u t i o n of a chain mechanism to the composite reaction be-comes of increasing importance. Were t h i s the case^ i t should follow that reactions with increasing pentene pressure should demand increasing percentages of propylene f o r i n h i b i t i o n . The present observations on propylene i n h i b i t i o n , however, are not s u f f i c i e n t l y detailed to form the basis of a decision of t h i s s o r t . Presumably only a small extra amount of propylene would be required to lead to such a small change i n the a c t i v a t i o n energy. I t must be r e a l i z e d , however, that due to the poor method available for a c t i v a t i o n energy determination, a v a r i a t i o n of two or three k c a l . need not necessarily be of s i g n i f i c a n c e . A small error i n the reaction rate would lead to a large error 62 i n the slope of the Arrhenius pl o t , and a consequently large error i n the value of the a c t i v a t i o n energy. I f such small errors i n the observed rate were consistent, there could r e s u l t a consistent but erroneous trend i n the calculated a c t i v a t i o n energies. For t h i s reason not too much significance should be attached to the present r e s u l t s of the dependence of ac t i v a -t i o n energy on pentene pressure. A small, cosistent contribu-t i o n from some secondary reaction could conceivably lead to the phenomenon observed. Investigations made over a far greater pressure range would be necessary before any d e f i n i t e conclusions could be made as to the genuine existence of an increase of a c t i v a t i o n energy with increasing pressure. 63 REFERENCES (1) Steacie, E.W.R. Atomic and Free Radical Reactions. Reinhold Publishing Corporation, New York. 19*+6. (2) Echols, L.S. and Pease, R.N. J.Am.Chem.Soc. 58:1317. 1936. (3) Sickman, D.V. and Rice, O.K. J.Chem.Phys. M-:608. 1936. (If) Frey, F.E. Ind.Eng.Chem. 26:198. 193*+. (5) Hinshelwood, C.N. The Kinetics of Chemical Change. Clarendon Press, Oxford. 191+9. (6) Staveley, L.A.K. Proc.Roy.Soc.(London), A162:557. 1937. (7) Hobbs, J.E. and Hinshelwood, C.N. Proc.Roy.Soc.(London), (8) Wall, L.A. and Moore, W.J. J.Am.Chem.Soc. 73:28*f0. 1951. (9) Stubbs, F.J. and Hinshelwood, C.N. Proc.Roy.Soc.(London)-A200:lf58. 1950. (10) Rice, F.O.'and Rice, K.K. The Aliphatic Free Radicals. Johns Hopkins Press, Baltimore. 1935. (11) Rice, F.O. and Herzfeld, K.F. 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