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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 t h e s i s as conforming t o the standard required from candidates f o r the degree of MASTER OF SCIENCE  Members of the Department o f 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 i n v e s t i g a t e d over a temperature range of V70 t o 530°C. and a pressure range of 50 t o 250 mm.  The decomposition was a  homogeneous f i r s t - o r d e r r e a c t i o n w i t h 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  r e a c t i o n r a t e was retarded by propylene and by i n e 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 e x h i b i t s a  s l i g h t increase w i t h i n c r e a s i n g i n i t i a l pressure of pentene.  Evidence i s presented f o r a composite  r e a c t i o n mechanism i n v o l v i n g both a f r e e - r a d i c a l chain process and a d i r e c t i n t r a m o l e c u l a r r e arrangement .  This i n v e s t i g a t i o n was  carried  out under the supervision  of  Dr. W. A. Bryce t o whom the author i s g r e a t l y indebted.  CONTENTS Page 1  INTRODUCTION 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 . . . Decomposition of the pentenes B a s i s of the present i n v e s t i g a t i o n  6  ...  12  ...  18 20  EXPERIMENTAL Reagents  20  D e s c r i p t i o n of the apparatus  21  D e s c r i p t i o n 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 30  r a t i o i n reaction vessel E f f e c t of a d d i t i o n of i n e r t gases . . . .  31  E f f e c t of a d d i t i o n of n i t r i c oxide  ...  32  ....  33  E f f e c t of a d d i t i o n of propylene  E f f e c t of a d d i t i o n of i n e r t gases on rate .  35  E f f e c t of a d d i t i o n of lead t e t r a e t h y l . .  35  of maximally-inhibited  decomposition  Dependence of the rate of decomposition on temperature ..... E f f e c t of a d d i t i o n of i n e r t gases on a c t i v a t i o n energy  36 39  ii  Page E f f e c t of a d d i t i o n of f r e o n on a c t i v a t i o n energy E f f e c t of a d d i t i o n of propylene on a c t i v a t i o n energy  *fl h2  E f f e c t of a d d i t i o n of i n e r t gases on a c t i v a t i o n energy of maximally-inhibited reaction . . If 3 E f f e c t of a d d i t i o n o f n i t r i c oxide on a c t i v a t i o n energy Summary of the experimental r e s u l t s DISCUSSION  ....  . . . . . . . . . . i.  Nature of the primary a c t i v a t i o n process . . Dependence of a c t i v a t i o n energy on i n i t i a l pentene pressure REFERENCES  M+ kS h& 59 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 formation of only two types of linkage; only three types are i n volved with olefins.  The nature of the problem i s therefore  not unduly complicated. However, the mechanism of olefin decompositions i s 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, i n a l l cases, f u l l y 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 i n 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 r e a c t i o n has "been assigned t o the p y r o l y s i s of normal paraff i n hydrocarbons  (1).  Evidence f o r 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 s i n such r e a c t i o n s comes from experiments on s e n s i t i z e d decompositions at temperatures f a r below those at which the normal t i o n s occur.  decomposi-  R a d i c a l s from ethylene oxide induce the decom-  p o 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 a d d i t i o n of 1% of mercury d i m e t h y l to n-butane at 525°C. causes the decomposition of twenty equiv a l e n t s of butane 0+),  Such observations i n d i c a t e that f r e e  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 w i t h 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 e s t a b l i s h e d 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 n e c e s s a r i l y f o l l o w that a chain process occurs i n the normal p y r o l y s i s of the substance under c o n s i d e r a t i o n . N i t r i c oxide has been found a very e f f e c t i v e substance f o r 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 f r e e  r a d i c a l , combines w i t h other r a d i c a l s , so removing them from r e a c t i o n systems.  Very small amounts of n i t r i c oxide have been  found t o i n h i b i t r e a c t i o n r a t e s g r e a t l y ; by removing one r a d i c a l , a molecule of n i t r i c oxide prevents the chain decomposit 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 o p e r a t i o n of a f r e e - r a d i c a l c h a i n mechanism i n p a r a f f i n decompositions ( 6 , 7 ) .  3 As i n c r e a s i n g amounts of n i t r i c oxide are added t o a r e a c t i o n system, the r a t e of decomposition decreases r a p i d l y t o a constant f r a c t i o n of i t s o r i g i n a l v a l u e ; subsequent produce no f u r t h e r i n h i b i t i o n ; a l l pressed.  additions  chains appear t o be sup-  C e r t a i n other i n h i b i t o r s , such as propylene, have  been shown t o have a s i m i l a r e f f e c t .  The r e s i d u a l r e a c t i o n i s  believed t o represent a non-chain molecular rearrangement. Nevertheless, considerable u n c e r t a i n t y s t i l l remains as t o the p r e c i s e nature of t h i s p a r t of the r e a c t i o n .  Work w i t h  I s o t o p i c a l l y l a b e l l e d compounds i n d i c a t e s that the maximally i n h i b i t e d r e a c t i o n s t i l l i n v o l v e s f r e e - r a d i c a l chains ( 8 ) . The p l o t of pressure increase against time f o r the uni 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 t o a straight l i n e .  A n a l y t i c a l r e s u l t s show t h a t , i n most  i n s t a n c e s , o l e f i n s c o n s t i t u t e an appreciable percentage of the r e a c t i o n products.  Hence the shape of the curve has been ex-  p l a i n e d as due t o 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 r e a c t i o n . 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 r e a c t i o n s was g r e a t l y c l a r i f i e d by Rice (1G), who devised mechanisms f o r organic decompositions.  These Rice  mechanisms form the b a s i s of our understanding of f r e e - r a d i c a l chain r e a c t i o n s . Rice Mechanisms Mechanisms f o r the decomposition of a wide v a r i e t y of  organic compounds have been proposed by Rice and H e r z f e l d ( 1 1 ) . On the b a s i s o f the d e t e c t i o n of f r e e r a d i c a l s i n such decern^ p o s i t i o n r e a c t i o n s , 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 r e a c t i o n mechanisms. In general, the proposed steps are as f o l l o w s : M  R  l +  1^ 2 M  -j-  %  i -> * i f *2  M  H  .R R]_  T-  R  R  2  x  -f M3  Mi,..  2  The i n i t i a l step i n v o l v e s the rupture of a bond i n molecule M^, y i e l d i n g a smaller molecule, M , and a r a d i c a l , R]_. A 2  c h a i n process f o l l o w s .  Each step i n the chain i n v o l v e s the  a b s t r a c t i o n 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 l a r g e  r a d i c a l s , R , are assumed t o decompose r e a d i l y . 2  Chain termina-  t i o n takes place by r a d i c a l recombination, w i t h formation of a s t a b l e molecule,  .  The mechanisms devised f o r the p y r o l y s i s o f p a r a f f i n hydrocarbons appear t o give a f a i r l y s a t i s f a c t o r y i n t e r p r e t a t i o n o f the complicated chemical changes i n v o l v e d , even when applied t o p a r a f f i n s as h i g h 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 t o be a breakdown of the parent hydrocarbon t o two radicals: C ^ H - > C H » -f CB^CB^CIL^CE?* ; 1 2  3  The l a r g e a l k y l r a d i c a l , assumed t o be unstable, decomposes:  5 GH^CHgCHgCB^* - ^ C R y or  4 - CH^CH r GH  2  CH CH CH CH • -> CH3GH2 • 4- GH^ = CHg ; 3  2  2  2  the chain process then proceeds by steps of the f o l l o w i n g type: G^H^  2  4- R — R H  -f~ CH^CI^CIL^CH/jCJE^ *  R z CEy, C H » ; 2  5  the l a r g e a l k y l r a d i c a l so formed now decomposes t o s t a b l e products and lower r a d i c a l s which perpetuate the chain.  By  s u i t a b l e choices of the various p o s s i b i l i t i e s f o r the c h a i n perpetuating and chain-terminating steps, the observed f i r s t order o v e r a l l r a t e can be explained.  In addition, arbitrary  assignment of a c t i v a t i o n energies t o 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 w i t h the low experimental 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 L i t e r a t u r e A survey of the l i t e r a t u r e d e a l i n g w i t h the p y r o l y s i s of o l e f i n s r e v e a l s that n e a r l y a l l the information a v a i l a b l e on the k i n e t i c s of such r e a c t i o n s 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 t o g a i n a b e t t e r understanding both decomposition  o f the thermal r e a c t i o n s -  and. polymerization - of o l e f i n s .  Experi-  ments have, f o r the most p a r t , d e a l t c h i e f l y w i t h the lower olefins.  Dynamic methods have u s u a l l y been employed; t h a t i s ,  the experiments have been done i n flow systems, using short  6 contact times. A wide v a r i e t y of temperatures and pressures have been used, and i n few cases have comparable experimental c o n d i t i o n s been employed. Decomposition of Lower O l e f i n s P r i o r t o 1925 very l i t t l e information of a p r e c i s e nature was a v a i l a b l e on o l e f i n decompositions.  I t was known that a t  about 750°G. the p y r o l y s i s of an o l e f i n y i e l d e d acetylene, a considerable part of which polymerized t o benzene (13)• of the e a r l i e s t i n v e s t i g a t i o n s was made by Noyes (ih)  f  One  who  passed isobutylene through a glass tube heated t o "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 r e a c t i o n products.  I p a t i e v (15) passed isobutylene over  alumina a t 500°C, o b t a i n i n g propylene, hydrogen, and methane. At law temperatures the c h i e f r e a c t i o n o f the lower o l e f i n s was found t o be p o l y m e r i z a t i o n , but at higher temperatures the process was found t o be more complex, c o n s i s t i n g of both decomposition and p o l y m e r i z a t i o n . Frey and Smith (16), working a t 575°C. w i t h a f l o w system, showed the high-temperature r e a c t i o n o f ethylene t o be homogeneous i n s i l i c a v e s s e l s , 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 a l s o decomposed propylene under s i m i l a r c o n d i t i o n s , y i e l d i n g butene and higher hydrocarbons together w i t h l a r g e amounts of methane and ethylene. Isobutylene, heated i n a f l o w system, was shown by Hurd  and Spence (17) to be much more s t a b l e than isobutane under s i m i l a r c o n d i t i o n s , a behaviour a t t r i b u t e d by these workers t o the greater strength of the C-C the corresponding  s i n g l e bond i n isobutylene t o  bond i n the saturated compound.  At tempera-  t u r e s 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 r e a c t i o n r a t e was unchanged by increase of the surface-to-volume  ratio  of the r e a c t i o n tube or by d i l u t i o n w i t h n i t r o g e n or hydrogen. S i m i l a r conclusions were reached by Kurd and Meinert experiments  (18) from  w i t h propylene i n a pyrex flow system at tempera-  t u r e s above 5 2 5 ° G .  The propylene decomposition was, however,  s l i g h t l y slower i n the presence of n i t r o g e n , which appeared t o l e s s e n the formation of l i q u i d products by d i l u t i n g the  primary  r e a c t i o n products, h i n d e r i n g t h e i r p o l y m e r i z a t i o n . Rice has proposed a f r e e - r a d i c a l chain mechanism f o r o l e f i n decompositions (10).  Here, a number of complications  arose, which were not encountered i n d e v i s i n g the mechanisms of p a r a f f i n decompositionst  (a)  The double bond exerts a  strong i n f l u e n c e 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 a c t i v a t e d . Rice has t h e r e f o r e assumed t h a t i n a r e a c t i o n i n v o l v i n g the a b s t r a c t i o n of a hydrogen atom from an o l e f i n molecule, the a t t a c k of a f r e e r a d i c a l w i l l be almost e x c l u s i v e l y at the j3 -H atoms,  (b) A f r e e r a d i c a l i s able t o react  w i t h an unsaturated hydrocarbon not only by a b s t r a c t i n g H atoms, but a l s o by adding t o one of the doubly-bound carbon  8 atoms.  I n proposing a chain mechanism f o r the process, Rice  has neglected such a d d i t i o n r e a c t i o n s , assuming them t o be of importance only at lower temperatures. For an unsaturated r a d i c a l Rice has assumed the f o l l o w i n g type of behaviour'.  Consider, as an example, the proposed r e -  a c t i o n 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 a b s t r a c t s a j3 -H atom t o 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  •CH -CH Z CH 2  effect,  CH - CH - CH • ,  2  2  2  i s considered much more s t a b l e than an o r d i n a r y f r e e r a d i c a l ; hence, the decomposition: •CH CH = CH 2  seems u n l i k e l y .  2  CH  2  = C = CH  2  + H-  This c o n c l u s i o n i s s u b s t a n t i a t e d by the f a c t  that a l l e n e has not been detected among the products of o l e f i n i c decompositions.  Since the r e a c t i o n of a l l y l r a d i c a l s  w i t h 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 t o disappear only by c o l l i s i o n w i t h one another. ing  The d i a l l y l  so formed i s presumed t o decompose, y i e l d -  the observed o i l y products.  The presence of unsaturated  r a d i c a l s a l s o introduces the p o s s i b i l i t y of i s o m e r i z a t i o n of the o l e f i n s . Gn the b a s i s of these assumptions Rice has postulated a fundamental d i f f e r e n c e between the decomposition mechanisms of the lower and higher members of the o l e f i n s e r i e s .  The decom-  p o s i t i o n s of propylene and the butylenes are presumed t o  9  i n v o l v e no chain c y c l e 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 a b s t r a c t i o n 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 w i t h 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 t h a t , because  of t h e i r unsaturated n a t u r e , o l e f i n s decompose d i f f e r e n t l y than do most other organic compounds. E g l o f f and Wilson (19), i n considering the mechanisms f o r the thermal r e a c t i o n s of gaseous hydrocarbons, regarded ethylene as the b a s i c m a t e r i a l , since above a c e r t a i n temperature the r e a c t i o n products of any hydrocarbon are e s s e n t i a l l y those of ethylene a t that temperature.  They reasoned t h a t the  thermal r e a c t i o n s of higher hydrocarbons must be studied below 750°C. i n order that they be c h a r a c t e r i z e d by d i f f e r e n c e s 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 concerned.  S i n c e , at the i n i t i a l temperatures of r e a c t i o n ,  o l e f i n s were found t o polymerize t o nonaromatic.substances, i t was concluded t h a t p o i n t s of u n s a t u r a t i o n are conducive t o polymerization. The p y r o l y s i s of ethylene, propene, and the three butenes was studied by Tropsch, P a r r i s h , and E g l o f f (20) i n a f l o w system, a t temperatures (1100 t o llf00 C.) considerably higher o  than those used i n previous i n v e s t i g a t i o n s .  They observedthat, -  as the experimental conditions were g r a d u a l l y made more severe, the volume c o n t r a c t i o n r e s u l t i n g from p o l y m e r i z a t i o n g r a d u a l l y  10  became masked by expansion due t o decomposition, volume increase remained.  u n t i l only a  By a c o n s i d e r a t i o n of the a c t i v a t i o n  energies of the two competing processes, they i n f e r r e d that decomposition  does not precede p o l y m e r i z a t i o n under severe  c o n d i t i o n s , but that p o l y m e r i z a t i o n i s the primary step, the polymer so formed being unstable under the experimental conditions,  and decomposing t o the observed gaseous products.  Rice and Haynes (21)  pyrolyzed isobutylene i n a h i g h -  temperature flow system designed t o e l i m i n a t e the formation of o i l y and t a r r y m a t e r i a l .  Propylene, methane, hydrogen, ethy-  lene, ethane, acetylene, a l l e n e , found.  and methyl acetylene were  On the b a s i s of t h e i r r e s u l t s these i n v e s t i g a t o r s  suggested a f r e e - r a d i c a l mechanism i n v o l v i n g r a t h e r short chains due to the formation of appreciable amounts of propylene by some non-chain r e a c t i o n such as the f o l l o w i n g : GHt 5  Z CHp  -f- ( C H J G - GH — > J ^ 2  CH CM = CH 4- .CH^CHp. . 3 * GH^  They a l s o suggested that the methyl acetylene i s formed from a l l e n e by a r a d i c a l chain mechanism.  Isobutylene was a l s o  pyrolyzed at high temperatures by Szwarc (22),  who found the  r e a c t i o n t o be homogeneous and f i r s t - o r d e r , and proposed a chain mechanism t o e x p l a i n the low a c t i v a t i o n energy found. In s i m i l a r experiments w i t h 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 r a t e determining step i n v o l v e s the breaking of the  G-C  11 ' bond, followed by a sequence o f r a p i d r e a c t i o n s between the r a d i c a l s so created. R i c e and "Wall (2lf), decomposing isobutylene w i t h short contact times at 851 t o 900°G.at pressures between 50 and 200 mm. and obtained i d e n t i c a l products by c a r r y i n g out the r e a c t i o n both i n a quartz tube and i n a s t a i 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 t h a t used i n the present i n v 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 t o 500 mm. and a temperature  range  of 570 t o 650°C. they showed the decomposition t o be a homogeneous f i r s t - o r d e r r e a c t i o n w i t h 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 t h a t over  t h i s temperature range propylene decomposes mainly by a molecul a r rearrangement  r e a c t i o n , the i n h i b i t i n g a c t i o n of propylene  i t s e l f preventing the propagation o f chains.  I t was suggested  that a t higher temperatures, such as those used by Szwarc (23), i t i s p o s s i b l e that a r a d i c a l mechanism, w i t h or without the propagation of chains, may predominate due t o 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 p i c t u r e of lower o l e f i n p y r o l y s i s thus appears t o be none too c l e a r a t the present time.  A great v a r i e t y o f e x p e r i -  mental c o n d i t i o n s have been employed by d i f f e r e n t workers so t h a t c o r r e l a t i o n proves somewhat d i f f i c u l t .  The r e a c t i o n s  12 appear t o be homogeneous and f i r s t - o r d e r .  At low temperatures  p o l y m e r i z a t i o n predominates, accompanied by a volume contract i o n ; as the temperature i s r a i s e d a point i s reached where there i s no observable pressure change, i n d i c a t i n g that the two competing processes of p o l y m e r i z a t i o n and decomposition are o f f s e t t i n g one another; at higher temperatures decomposit 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 t o account f o r the observed r e a c t i o n products.  I t i s possible  t h a t d i f f e r e n t mechanisms may operate under d i f f e r e n t condit i o n s 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 p y r o l y s i s of the pentenes has been studied mainly by flow methods. N o r r i s and Reuter (26) heated -a- pentene-2 under a v a r i e t y 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 s t r a i g h t - c h a i n 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 c e r t a i n temperature was noted at which the hydrocarbon p y r o l y s i s 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 r a t e of expansion  13 e i t h e r remained constant or decreased as the temperature was raised.  Such r e v e r s a l temperatures, which occurred, f o r the  three pentenes studied, 53 t o 65°C. higher than the temperatures a t which the hydrocarbons began t o decompose, were a t t r i b u t e d t o simultaneous p y r o l y s i s and p o l y m e r i z a t i o n . Hurd and Goldsby (28), i n the p y r o l y s i s of pentene-1 and pentene-2 a t 550 t o 600°C. i n a f l o w system, e s t a b l i s h e d that isomeric unsaturated hydrocarbons were important r e a c t i o n products, one-third t o t w o - f i f t h s of the t o t a l products cons i s t i n g of isomeric pentylenes. Both s t a t i c and f l o w methods were used by Pease and Morton (29) i n a study o f the p y r o l y s i s of pentene-2.  They  showed that the r e a c t i o n was homogeneous and monomolecular;at at temperatures between 500 and 600°C.  Since the pressure-  time curves rose p e r f e c t l y r e g u l a r l y from the s t a r t t o something short of 100$ pressure increase, they concluded that p o l y m e r i z a t i o n i s not an important primary reaction:. Chain mechanisms were proposed by Rice (10) f o r the decomposition of higher o l e f i n s .  The decomposition mechanism  suggested f o r pentene-1 was the f o l l o w i n g :  i n i t i a t i o n occurs  by s p l i t t i n g o f the |3 G-C bond: G ^ H - > CH CH « 4- .CH CHCH . ; i g  3  2  2  2  t h i s i s followed by r e a c t i o n o f the r a d i c a l s formed w i t h pentene molecules: C  5 10 H  +  C H  3 V -^ 2 6 C  C  H  +  CH CH CHGHCH 3  2  2  IkG  5 10 H  +  •CH GHCH .—^,G H 2  2  3  6  -)- CH CHgCHCHGHg• ;  and a chain r e a c t i o n takes p l a c e : °5 10 + S - > R H + CH^HgCHCHCHg • H  —^»RH f  C H j : CHCH Z CHg -f C H y  where R = CH^» This scheme p r e d i c t s the formation of equal proportions of methane and butadiene, which should therefore c o n s t i t u t e the main decomposition products. Both pentene-1 and pentene-2 were studied by Hurd, Goodyear, and Goldsby (30) i n a f l o w system a t temperatures between 500 and 600°C. Pentene-2 was found t o be the more stable.  Analyses were made of the e n t i r e gaseous r e a c t i o n  products, showing the c h i e f products from both t o be methane, butene-1, propylene, ethane, and ethylene, together w i t h small amounts of butene-2, butadiene, and hydrogen, and, i n the case of pentene-1,  some propane.  Isomerization of pentene-1 t o  pentene-2 and t h e reverse, at temperatures above 580°C. was e s t a b l i s h e d by p r e c i s e 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 o f the methyl radical.  This i s a l s o assumed t o exclude intramolecular  wandering of the H r a d i c a l as the mechanism f o r the observed conversion. M i k h a l l o v and Arbuzov (3D,  i n p y r o l y z l n g pentene-1 and  15 pentene-2 i n a flow system i n the temperature range 500 t o 700° G., found that t h e use of steam as a d i l u e n t tended t o prevent polymerization. Gorin, Oblad, and Schmuck (32) i n v e s t i g a t e d the pentene decompositions under conditions designed t o promote maximum s u r v i v a l of the d i o l e f i n products.  A flow system was employed  w i t h n i t r o g e n as a d i l u e n t . At 800°C. the main r e a c t i o n o f pentene-1 was found t o he a s p l i t t i n g t o ethylene,  propylene,  butylene, and butadiene.  butadiene,  Pentene-2 y i e l d e d mainly  w i t h smaller amounts of ethylene and propylene.  I n the  p y r o l y s i s of mixtures of n-pentane and pentene-2, no s e l e c t i v e cracking of the o l e f i n was observed.  On the b a s i s o f 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 r e a c t i o n , wherein an a l k y l r a d i c a l r e a c t s w i t h an o l e f i n , takes place e x c l u s i v e l y by removal of a j3 -H atom.  T h i s p r e d i c t s t h a t pentene-1 should  decompose e x c l u s i v e l y t o methane and butadiene i n the primary process.  Since Gorin's analyses showed that only 30$ o f the  pentene-1 decomposes i n t h i s way, and that the majority of the r e a c t i o n i n v o l v e s s p l i t t i n g t o l i g h t o l e f i n s , he suggested that the f r e e r a d i c a l s do not react e x c l u s i v e l y w i t h the fh -hydrogens a t high temperatures but a l s o w i t h the Y - and £ -hydrogens. CH  2  The chain sequence predicted i s :  - CH* +- CH^CI^CH^CH " CHg-^CHg - CH -f CH^-CH-CB^-CHZCHg 2  16 CH -6H-CH -CH 3  2  Z CH 2  CH^CH  I  CH  2  -f- GHg Z  .  CH«  Two f u r t h e r p o i n t s of evidence i n d i c a t e short chains:  ^rela-  t i v e l y l a r g e amounts' of ethane were formed; by assuming the primary r e a c t i o n t o be,, e x c l u s i v e l y , a s p l i t t i n g of the y3 C-C bond t o y i e l d an a l l y l and an e t h y l r a d i c a l , each of which subsequently  r e a c t s w i t h pentene t o produce ethane, a maximum  value of 7 was c a l c u l a t e d f o r the chain l e n g t h ; (b) ethylene was formed i n considerably greater amounts than was Gorin e x p l a i n s t h i s as f o l l o w s :  propylene.  a b s t r a c t i o n of a / -H from  pentene-1 y i e l d s a r a d i c a l which must decompose t o 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 s t a b l e a l l y l r a d i c a l w i l l  tend t o dimerize or t o combine w i t h other r a d i c a l s rather than perpetuate the chain by combining w i t h the pentene-1 t o give propylene.  Butenes are assumed t o r e s 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 b e t t e r agreement w i t h the Rice p r e d i c t i o n s , butadiene forming the p r i n c i p a l r e a c t i o n product. The p y r o l y s i s of pentene-2 and t r i m e t h y l e t h y l e n e by flow methods at 778 t o 85G°C. and_high pressures i n the presence of steam was c a r r i e d out by Hepp and Frey (33)•  These i n v e s t i -  gators a l s o found butadiene t o be the p r i n c i p a l r e a c t i o n product from pentene-2, w i t h l e s s e r amounts of pentadiene, ethane, butene, ethylene, and propylene.  I n the case of  t r i m e t h y l e t h y l e n e , 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 f o r the butene formation.  The authors suggest a d d i t i o n of a hydrogen atom t o  one o f the doubly-bound  C atoms, w i t h the formation of a h i g h -  energy r a d i c a l , decomposing t o an o l e f i n and a smaller r a d i c a l . This proposal was substantiated by the d e t e c t i o n of r e l a t i v e l y l a r g e amounts of hydrogen from the t r i m e t h y l e t h y l e n e and pentene-2 decompositions.  For pentene-1 however such H atom  a d d i t i o n would lead only t o C  2  and  hydrocarbons.  Recently, a comparative study of the thermal decomposit i o n s of s e v e r a l 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 . pentene-1,  The hydrocarbons studied were butene-1,  hexene-1, heptene-1,  butene-2, isobutene,  2-methylbutene-l, and 3-niethyrbutene-l. i n some cases.  Analyses were made  The decompositions were shown t o 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  r e a c t i o n mechanism was proposed f o r the isobutene decomposition.  I n the case of pentene-1,  the i n i t i a l part o f the  pressure increase-time curve was found t o be a s t r a i g h t l i n e passing through the o r i g i n , w i t h a decrease i n r a t e as the r e a c t i o n came t o an end.  The a c t i v a t i o n energy, measured over  the temperature range V30 t o 530°C., was found t o be 5 3 . 1 k c a l . per mole f o r 100 mm. pentene pressure, and 5^.6 k c a l . per mole f o r 300 mm. pressure. A d d i t i o n of n i t r i c oxide produced no appreciable e f f e c t on the r e a c t i o n r a t e ; added propylene and ethylene both caused a s l i g h t decrease i n the r a t e . ses were made f o r the r e a c t i o n products from  Ho analy-  pentene-1.  18 From t h i s summary i t i s seen t h a t , although s e v e r a l i n v e s t i g a t i o n s have been made on the p y r o l y s i s of pentenes, w i t h the exception of the experiments of Pease and Morton ( 2 9 ) and those .of Molera and Stubbs ( 3 ^ ) , these experiments have been done I n flow systems under c o n d i t i o n s of temperature and pressure 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 order.  first-  At temperatures of about 800°C, the r e a c t i o n products  from pentene-1 are mainly l i g h t o l e f i n s w i t h a l e s s e r amount of butadiene, w h i l e pentene-2 y i e l d s mainly butadiene. temperatures seem to favour d i o l e f i n i c products.  High  At lower  temperatures (500 t o 600°C.) the c h i e f products of both are methane, butene-1, propylene, ethane, and ethylene. Isomeriz a t i o n takes place above 580°C. At low temperatures p o l y m e r i z a t i o n predominates.  Various r e a c t i o n mechanisms have  been proposed. B a s i s of the Present I n v e s t i g a t i o n 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 p y r o l y s i s 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, f o r 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 decomposit 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 r e q u i r e d . Secondary decomposition of the products has been  shown to lead t o a sigmoid type of  pressure increase-time curve which, f o r a n a l y s i s , 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 t o know, f o r example, i f the  ethylene and propylene, which are found i n the r e a c t i o n products of most p a r a f f i n s , are produced d i r e c t l y from the p a r a f f i n s , or by the secondary decomposition of a higher olefin.  Accordingly,  o l e f i n decompositions deserve study not  only on t h e i r own merits, but a l s o because of t h e i r p o t e n t i a l use i n f u r t h e r e l u c i d a t i n g the mechanisms of p a r a f f i n decompositions. Pentene-1 was selected f o r i n v e s t 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 p y r o l y s i s of the higher p a r a f f i n s .  The  object of the work was t o 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 a l s o 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 c a r r i e d out i n a closed  quartz r e a c t i o n v e s s e l , heated e x t e r n a l l y by a furnace, and the extent of the r e a c t i o n was followed by observation of pressure changes, using a mercury c a p i l l a r y manometer.  The e x p e r i -  mental c o n d i t i o n s were v a r i e d by changing the pressure and temperature  of t h e gas, by adding i n e r t gases, i n h i b i t o r s , and  a f r e e - r a d i c a l - p r o d u c i n g substance t o the r e a c t i o n system, and by a l t e r i n g the surface-to-volume r a t i o of the r e a c t i o n v e s s e l . 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, S p e c i a l 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 m a t e r i a l was s p e c i f i e d as  "Research Grade," i t was not subjected t o f u r t h e r p u r i f i c a t i o n . Propylene, a l s o "Research Grade," was obtained from the same source.  The gas was condensed i n a l i q u i d n i t r o g e n t r a p  and pumped before admission t o 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 L i q u i d A i r Company and was s p e c i f i e d as "Commercial Grade." N i t r i c oxide was prepared by the a c t i o n of a s u l f u r i c a c i d s o l u t i o n of f e r r o u s s u l f a t e on sodium n i t r i t e ( 3 6 ) . The gas was freed from carbon d i o x i d e and higher oxides of n i t r o g e n by  21 passage through 6 normal sodium hydroxide and a tube containing sodium hydroxide p e l l e t s . -  I t was d r i e d by passage through  phosphorus pentoxide. Lead t e t r a e t h y l was obtained from the I m p e r i a l O i l Company i n the form of an approximately 10% s o l u t i o n i n a hydrocarbon solvent, c o n t a i n i n g ethylene d i c h l o r i d e , ethylene dibromide, and a dye.  Owing t o 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 t o o b t a i n t h i s compound i n a pure form. Monochlorotrifluoromethane was obtained from the Canadian Ice Machine Company. D e s c r i p t i o n 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.  T h i s consisted e s s e n t i a l l y  of an e x t e r n a l l y heated quartz r e a c t i o n v e s s e l , A,  connected  t o an evacuating system, N, a mercury manometer, B, f o r pressure measurement, storage v e s s e l s , C, D, E, F, G, and H, f o r r e a c t a n t s , and a sampling system, K.  The quartz r e a c t i o n  v e s s e l had a volume of approximately 200 ml. and an outside diameter of 55 mm.  I t was connected t o the evacuating system  and mercury manometer by quartz tubing and a ground-glass joint, J^.  The P i c e i n wax used f o r the s e a l d i d not develop  any l e a k s d u r i n g s e v e r a l months of continuous h e a t i n g .  The  pressure JLn the system was measured by a closed U-tube mercury manometer, B, connected;, t o the r e a c t i o n v e s s e l by means of c a p i l l a r y tubing.  Pressure readings were made w i t h reference  t o a m i r r o r s c a l e graduated i n m i l l i m e t e r s .  22 The system was evacuated by.a mercury d i f f u s i o n pump backed by a r o t a r y o i l pump. A t r a p , T-j_, cooled i n dry i c e acetone, was s i t u a t e d between the mercury pump and the r e s t 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 r e a c t i o n v e s s e l and a l s o t o prevent r e a c t i o n 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 r e a c t i o n v e s s e l was evacuated. A discharge tube, L, attached t o the system, and capable of connection w i t h a l l p a r t s of the system, was used t o i n d i c a t e 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 t o the lower gallery.  The bulb was f i l l e d through a small-bore side-arm by  s u c t i o n from a vacuum i n the system above.  Before use, the  pentene was thoroughly f r o z e n i n l i q u i d n i t r o g e n and pumped t o remove t r a c e s 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 s o l u t i o n was  stored i n a small bulb, D, connected t o the lower g a l l e r y by a ground-glass j o i n t , «J". 2  The gaseous m a t e r i a l s , propylene,  argon, n i t r o g e n , n i t r i c oxide, and f r e o n , were stored i n 2l i t e r glass bulbs, E, F, G, H, and I , attached t o the upper gallery.  These were f i l l e d i n the f o l l o w i n g ways.  To admit  propylene, the c y l i n d e r containing the l i q u i d under pressure was connected t o the apparatus at 0 w i t h pressure t u b i n g . With taps S-J^Q and S - Q open, the upper g a l l e r y , connecting  23 tubes, and t r a p T  2  were evacuated.  l i q u i d n i t r o g e n . With taps Sg and S  Trap T^ was cooled i n 1 Q  c l o s e d , the reducing  valve on the c y l i n d e r was opened, and s u f f i c i e n t propylene gas was allowed t o enter and condense i n t r a p T . 2  pumping of the condensed propylene, trap T  2  A f t e r thorough  was warmed, and  the propylene was vaporized i n t o the evacuated storage globe. The other gases used were admitted i n a d i f f e r e n t manner. The c y l i n d e r (or, i n the case of n i t r i c oxide, the generator) containing the gas was attached t o the system through the was 2-way stopcock  by pressure t u b i n g . A strong stream of gas/  allowed t o f l u s h out the connecting t u b i n g , 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 t o enter u n t i l the pressure, as r e g i s t e r e d by an open mercury manometer, M, attached t o 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 r e a c t i o n products, the lower g a l l e r y and c a p i l l a r y connections were wound w i t h Chromel r e s i s t a n c e wire and could be heated electrically. The stopcocks which were not subjected to heating were sealed w i t h Apiezon M grease which was found t o provide an e x c e l l e n t vacuum s e a 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 w i t h Dow  Corning  " S i l i c o n e High Vacuum" stopcock grease which maintains i t s consistency up t o 200°C., although does not give such an e f f i c e n t s e a l as the Apiezon grease.  2h  The r e a c t i o n v e s s e l was heated i n an e l e c t r i c furnace constructed 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  a l t e r n a t i n g current i n three s e c t i o n s of Chromel r e s i s t a n c e wire winding.  I t was i n s u l a t e d w i t h s e v e r a l inches of powdered  asbestos, and the top opening was sealed w i t h a mixture of powdered asbestos and alundum cement. The temperature of the r e a c t i o n v e s s e l was measured by two Chromel-Alumel thermocouples placed i n contact w i t h the w a l l of the v e s s e l , at top and bottom r e s p e c t i v e l y .  These were con-  nected t o the potentiometer c i r c u i t through a double-throw switch, t o permit r a p i d consecutive reading.  The thermo-  couples were c a l i b r a t e d by use of the melting p o i n t s of pure t i n , l e a d , z i n c , and aluminum, and the t r a n s i t i o n of potassium sulphate, which covered the lange of  temperature 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 t o give a constant reading over the l e n g t h of the r e a c t i o n v e s s e l by means of three v a r i a b l e r e s i s t a n c e s , each connected t o one of the three furnace windings. were connected  i n series  These three s e c t i o n s  i n p a r a l l e l t o the power supply.  The temperature was c o n t r o l l e d a u t o m a t i c a l l y by an e l e c t r o n i c thermoregulator operating a r e l a y .  C l o s i n g of the  r e l a y shorted out a c o n t r o l l i n g r e s i s t a n c e i n the power supply, so i n c r e a s i n g the current t o the furnace. furnace c i r c u i t i s shown i n F i g .  A diagram of the  25 The thermoregulator, of the type developed by Coates (38), was constructed by Coope (39) • i n F i g . 2...  The c i r c u i t diagram i s shown  Operation of the instrument i s based on the r e v e r s -  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 t o the other. The c i r c u i t i s composed o f f o u r main p a r t s :  an a-c. b r i d g e ,  a c i r c u i t f o r a m p l i f y i n g the bridge output, a c i r c u i t f o r converting the bridge output t o v a r i a b l e d-c. v o l t a g e , and a relay. The a-c. bridge c o n s i s t s of a centre-tapped transformer, T^, a r e s i s t a n c e thermometer, R^, and a standard v a r i a b l e r e s i s t a n c e , B^, which can be adjusted t o balance the bridge a t any d e s i r e d temperature. by  The bridge output, e ^ , i s a m p l i f i e d g  and a p p l i e d t o the g r i d of V  n a t i n g voltage e  g 2  to the anode of Y  2  .  2  as the much l a r g e r a l t e r -  An a l t e r n a t i n g voltage, e  a 2  , i s applied  which w i l l t h e r e f o r e pass current only  during the h a l f c y c l e s i n which e  & 2  i s positive.  Hence the  magnitude of the anode current, i ,depends on both the magni2  tude and t h e phase of the a-c. g r i d v o l t a g e , « »  The anode  icurrent generates a p o t e n t i a l d i f f e r e n c e across R  which i s  g2  smoothed by C^, R^, and G , and a p p l i e d t o the g r i d of the 2  output t r i o d e , of  as the d-c. v o l t a g e , e  g 3  .  The anode current  c o n t r o l s the furnace through the r e l a y . In t h e apparatus used, T^ was a Type 167-D 110 t o 6 v.  centre-tapped transformer.  The anode supply t o V  vided by the 110 v. a-c. mains.  2  was pro-  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 f o r thertaoregulator.  26 6SF5 t r i o d e , and V^ a 6B+G power output t r i o d e . 1  d-e. anode supply t o  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. powerpack, T  2  The 350 v.  I n the  was a Thordardson T-13R13 transformer, and V^.  - a type 80 f u l l - w a v e r e c t i f i e r .  The v a r i a b l e r e s i s t a n c e R i n 2  the a-c. bridge was a standard 0.1 t o 1000 ohm decade d i a l r e s i s t a n c e box.  The Type M,molybdenum r e s i s t a n c e thermometer  used by Coope was found inadequate f o r the temperature used i n the present i n v e s t i g a t i o n .  range  A platinum r e s i s t a n c e  thermometer was constructed from 11 f t . of 0.05 i n . platinum w i r e o f 30 ohms r e s i s t a n c e . The wire was t i g h t l y c o i l e d and wound on a t h i n mica s t r i p .  The ends were s i l v e r - s o l d e r e d t o  t h i c k copper lead w i r e s , which were fastened t o an alundum rod and mounted i n s i d e the furnace. The r e l a y c i r c u i t consisted of a Sunvic Type 602 vacuum r e l a y w i t h s u i t a b l e s e r i e s and shunting r e s i s t a n c e s . The instrument was a b l e t o c o n t r o l the furnace to w i t h i n ~t 0.5°C. a t the temperature  temperature  of t h i s i n v e s t i g a t i o n .  D e s c r i p t i o n of a T y p i c a l Experimental Run The furnace was f i r s t adjusted t o the d e s i r e d temperature. Since s e v e r a l hours were required t o heat the furnace from room temperature  and t o a l l o w f o r the temperature  distribution  along the l e n g t h o f the furnace t o reach e q u i l i b r i u m , i t was maintained continuously a t temperatures  i n the required range.  Before an experimental r u n the v a r i a b l e r e s i s t a n c e i n t h e thermoregulator was adjusted t o the appropriate  temperature.  27  With the a m p l i f i e r set a t zero g a i n , the negative g r i d b i a s of was checked so t h e r e l a y was at the t r i p point when the bridge was balanced.  This r e l a y t r i p p o i n t , about 16 ma., was  i n d i c a t e d by a p i l o t l i g h t i n the r e l a y output c i r c u i t .  With  the g a i n set a t low s e n s i t i v i t y the furnace was allowed t o warm up; the g a i n was then adjusted f o r maximum s e n s i t i v i t y , and the furnace temperature was allowed t o reach e q u i l i b r i u m . The three r h e o s t a t s c o n t r o l l i n g the temperature  distribution  required separate adjustment f o r each temperature. The mercury d i f f u s i o n pump was used t o evacuate the system to a "black vacuum." The pentene was cooled i n a bath of l i q u i d n i t r o g e n and thoroughly pumped t o remove t r a c e s of a i r . With taps ing  and  c l o s e d , and tap S-j^ open, the bulb c o n t a i n -  the pentene was immersed i n a beaker of warm water, from  50 t o 1G0°C. depending on the i n i t i a l pressure r e q u i r e d , and a few seconds were allowed f o r some of the pentene t o v a p o r i z e . Tap  was then opened c a u t i o u s l y and the pressure i n the r e -  a c t i o n v e s s e l , as i n d i c a t e d by the manometer, was allowed t o increase s u f f i c i e n t l y .  Taps  and S-^ were then c l o s e d . At  the completion of the f i l l i n g , the timer was s t a r t e d , and the course of the r e a c t i o n was followed by pressure-time measurements made a t r e g u l a r i n t e r v a l s .  Constant tapping of the  manometer was found necessary i n order t o prevent the mercury from s t i c k i n g i n the c a p i l l a r y t u b i n g . At the c o n c l u s i o n of a r e a c t i o n , taps  and  were opened t o the pumps, and the  system was thoroughly evacuated.  28 In an experiment i n v o l v i n g a gaseous m a t e r i a l , both the upper and lower g a l l e r i e s were f i r s t evacuated.  Tap Sg was  c l o s e d , the upper g a l l e r y was opened t o the r e a c t i o n v e s s e l , and a s u f f i c i e n t pressure o f the d e s i r e d gas was allowed t o enter.  A f t e r r e v e r s i n g of tap S^, the pentene was v o l a t i l i z e d  and admitted t o the r e a c t i o n v e s s e l as described p r e v i o u s l y . General Form of the Pressure-time Curves P l o t s of pressure change vs. time f o r the decomposition of 150 mm. of pentene-1 at three d i f f e r e n t temperatures a r e shown i n F i g . 5 . At h i g h temperatures the i n i t i a l p o r t i o n o f the t  curve i s a s t r a i g h t l i n e passing through the o r i g i n , followed by a gradual decrease i n r a t e as the r e a c t i o n comes t o an end. I f , i n such an experimental run, the i n i t i a l pressure increase was too r a p i d t o observe the exact i n i t i a l pressure, t h i s value was obtained by e x t r a p o l a t i o n of the s t r a i g h t l i n e t o zero time.  E s t i m a t i o n of the i n i t i a l r a t e from the slope of t h i s  s t r a i g h t l i n e p o r t i o n o f the curve thus presented_no culty.  diffi-  At lower temperatures there i s at f i r s t a s l i g h t de-:  crease o f pressure, followed by a short period during which Ap  i s i n a p p r e c i a b l e . The curve then r i s e s t o a maximum, and  subsequently decreases as the r e a c t i o n comes t o an end.  The  r a t e s f o r comparison purposes were t h e r e f o r e taken as the maximum slopes of such curves, the i n i t i a l r a t e s obviously being u s e l e s s as c r i t e r i a f o r comparison. R e p r o d u c i b i l i t y of the r a t e 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  530°C,  60  50  «2C  29  c a l c u l a t i o n s , each was made i n d u p l i c a t e 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 r a t e o f pressure change on t h e i n i t i a l pentene pressure was i n v e s t i g a t e d over a range o f i n i t i a l pressures from 60 t o 21+7 mm.  I n Table I are recorded  the r e s u l t s f o r experiments  a t 500°C. and v a r i o u s  conducted  i n i t i a l pentene pressures. TABLE I Dependence o f r e a c t i o n r a t e on i n i t i a l pentene pressure at 500°C. I n i t i a l pentene pressure, mm.  Reaction r a t e , 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 f o r these runs are shown i n F i g . 6 . order was determined shown i n F i g . 7 .  The  from the p l o t of l o g dP/dt v s . l o g P , Q  I n t e g r a t i o n of the equation: dP/dt = k P , n  Q  yields: d l o g dP/dt = d log P  n  Q  hence the slope of the curve gives the order of the r e a c t i o n . The slope of the best l i n e through these p o i n t s i s u n i t y : i t  Slops  0.5 J 1.7  I  i  1-8  1.9  I  ' 2.0  s  1  i  i  i  2.1  2.2  2.3  log. P  0  F i r . 7. V a r i a t i o n of r a t e v i t h i n i t i a l pentene p r e s s u r e , a.t £00 °C.  i_ 2.h  30  i s t h e r e f o r e concluded t h a t the r e a c t i o n i s f i r s t - o r d e r w i t h respect t o the pentene pressure. Rate constants were c a l c u l a t e d 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 constant over the range of pressures i n v e s t i g a t e d . TABLE I I Reaction r a t e s and r a t e constants f o r various i n i t i a l pentene pressures, a t 500°C.  I n i t i a l pressure, mm.  Rate, mm./min.  Rate constant, min." x K r 1  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 R a t i o i n Reaction V e s s e l A q u a l i t a t i v e e s t i m a t i o n of the degree of heterogeneity of a gaseous r e a c t i o n can o f t e n be determined by varying the surface-to-volume  r a t i o i n the r e a c t i o n v e s s e l .  The quartz r e a c t i o n v e s s e l was packed w i t h short pieces of pyrex t u b i n g .  As no quartz tubing was a v a i l a b l e , the r e s u l t s  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 f i r e - p o l i s h e d i n order  31 t o avoid p o s s i b l e c a t a l y s i s by " a c t i v e c e n t e r s " 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.  I n F i g . 8 are shown the A p - t i m e curves f o r the  decomposition of 110  mm.  of pentene-1 at 5G0°C. i n both packed  and unpacked r e a c t i o n v e s s e l s . C l e a r l y the e f f e c t of increased surface i s very s m a l l .  A  s l i g h t decrease i n the i n i t i a l r a t e was shown, i n 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 c h a i n t e r m i n a t i o n on the surface.  I n a d d i t i o n , a s l i g h t lowering of the f i n a l pressure  a t t a i n e d may have been due t o a d s o r p t i o n of the products. From the r e s u l t s i t may be concluded t h a t the r e a c t i o n i s ess e n t i a l l y homogeneous. E f f e c t of A d d i t i o n of I n e r t Gases In homogeneous gas-phase r e a c t i o n s i n v o l v i n g a c t i v a t i o n of the r e a c t a n t molecules by b i m o l e c u l a r c o l l i s i o n s , i t has o f t e n been found p o s s i b l e , by decreasing the p a r t i a l pressure of the reactant gas, t o maintain the normal r e a c t i o n r a t e by the add i t i o n of some i n e r t gas. A c c o r d i n g l y , experiments were c a r r i e d out i n the presence, of both argon and n i t r o g e n .  F i g . 9 shows the r a t e curves f o r  the decomposition of 100 mm.  of pentene both alone and i n the  presence of 100 mm. of 200 mm.  of argon.  A curve f o r the decomposition  of pentene a t 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 n i t r o g e n i s s u b s t i t u t e d f o r argon. I n both cases, the presence of the i n e r t gas does not  Time, F i g , <, A p - t i ' i e cvrves f o r 6econpor:-ition p a c k e d f>nu v n p u r k y e r-.tion vessel:- • c  rain. of 1 1 G  o f pentene-1 at  500°C.  in  <  ^  r-  1  h  -•  ^  F i g . 10.  CT  j ?ini£j  S f f a c t of N  .<  n  O  /  Jin. 2  on rate, ??.t 53G°C.  32 serve t o maintain the high-pressure r e a c t i o n r a t e .  In fact,  r a t e s f o r the d i l u t e d r e a c t i o n s of 100 mm. of pentene are somewhat lower than the normal value f o r 100 mm. o f pentene alone.  This observation leads t o the c o n c l u s i o n that the  r e a c t i o n i s not a simple c o l l i s i o n a l process, but r a t h e r that i t i n v o l v e s the i n t e r v e n t i o n of some type o f f r e e - r a d i c a l chain process.  The observed decrease i n r a t e could then be  a t t r i b u t a b l e t o chain t e r m i n a t i o n by r a d i c a l  recombination  i n the gas phase due t o three-body c o l l i s i o n s w i t h i n e r t gas molecules.  Since surface e f f e c t s were found t o 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 r a t h e r than a t the surface of the r e a c t i o n v e s s e l . E f f e c t of A d d i t i o n o f N i t r i c Oxide A method f r e q u e n t l y employed t o t e s t f o r the presence of f r e e - r a d i c a l chains i n a gas r e a c t i o n i s t o add small amounts of some substance capable o f i n h i b i t i n g any such chains by r e a c t i o n w i t h the f r e e r a d i c a l s .  N i t r i c oxide has o f t e n been  found an e f f e c t i v e substance f o r t h i s purpose. Experiments were done a t 5l6°C. w i t h 120 mm. of pentene. Consecutive oxide.  runs were made w i t h 1, 5, 11, and 50 mm. of n i t r i c  A second s e t of observations was made w i t h 5*+ mm« of  pentene and 1, 6, 1 0 , and 13 mm. of n i t r i c oxide r e s p e c t i v e l y . A p-time curves f o r these runs are p l o t t e d i n F i g . 1 1 . I t i s observed that n i t r i c oxide has no appreciable e f f e c t on the r e a c t i o n r a t e s . e r a l ways:  This r e s u l t may be i n t e r p r e t e d i n sev-  (a) t h a t r a d i c a l - c h a i n processes are absent i n  A *  ko I  6 12 0 mm, p e n t e ne NO + 0 m0 : r i amin. m  7  o  1 ij  A  11  A  "  " "  -  °  ti  tr  _  "  "  -  A £  A  A  30^  A Q  <  * ^ •  4  4  • 4  4  ^  . . . "  A  -A  ^  °  I  Oo ^  p e n t e rne  5!* - i .  7  +0 mm, UQ  f  i"  •  O  -*  13  |4 OL  0  1  2  1  3  L  s+  T i - i e . -.in. ig.  11.  .,ff-«rt  o f NO  on r a t e  5 ;:t ;;16°C,  6  7  8  33  t h i s r e a c t i o n ; (b) that any i n h i b i t o r y a c t i o n 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 , t h a t 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) t h a t the presence of some other chain i n h i b i t o r , presumably a r e a c t i o n product, i s more e f f i c i e n t i n combining w i t h the f r e e 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 c h a i n steps i n  the normal r e a c t i o n would be mostly those of chain t e r m i n a t i o n . E f f e c t of A d d i t i o n of Propylene Another substance which has f r e q u e n t l y 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 i n v e s t i g a t o r s t o c o n s t i t u t e one of the decomposition products of pentene-1 (32,  3 * 0 , i t was of i n t e r -  est t o determine i f the presence of f u r t h e r amounts of t h i s substance would i n h i b i t the r e a c t i o n r a t e . F i g . 12 shows Apr-time curves f o r an experiment  conducted  at 516°C. w i t h an i n i t i a l pentene pressure of 110 mm., successive runs i n the presence of k, 6, and 2k mm. respectively.  and f o r  of propylene  I n F i g . 13 r e a c t i o n r a t e s are p l o t t e d as func-  t i o n s 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 t o cause a decrease i n the observed reaction rates.  Quite considerable amounts of propylene were  necessary t o cause a p p r e c i a b l e i n h i b i t i o n .  In each case, the  f i r s t few m i l l i m e t e r s added produced no change i n the r e a c t i o n rate.  As the p a r t i a l pressure of the propylene was increased,  the r a t e g r a d u a l l y decreased t o 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, f u r t h e r a d d i t i o n s o f propylene were without effect.  The greater the i n i t i a l pentene pressure, the greater  was the amount of propylene required t o produce maximum i n hibition. TABLE I I I t  Rates of pentene-1 decomposition maximally i n h i b i t e d by propylene, at 516 C. Pentene pressure, mm.  Normal r a t e , mm./min.  Propylene pres- Rate of maxisure f o r maxi- mally i n h i b i t e d mum i n h i b i t i o n , r e a c t i o n , mm. mm./min.  5.83 110 200  23.h  10  5.90  20  12.5  ho  18.7  Since propylene alone, a t t h i s temperature,  polymerises  at a n e g l i g i b l e r a t e (26), i t s e f f e c t on the pentene decomp o s i t i o n would seem t o represent a t r u e i n h i b i t i o n r a t h e r than an i l l u s o r y e f f e c t due t o a pressure decrease superimposed on the normal i n c r e a s e . I f the propylene i s effect-ive i n supressing a l l the chains, the r e s i d u a l r e a c t i o n may represent a process of simple molecular rearrangement t o products.  However, i t i s p o s s i b l e  that the maximally i n h i b i t e d r e a c t i o n i s s t i l l a modified chain r e a c t i o n .  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, r e p r e s s i b l e by propylene, i n the normal decomposition.  The f u n c t i o n of propylene i s presumably  35  t o terminate r a d i c a l chains "by combining w i t h the chain carr i e r s t o form s t a b l e molecules or l e s s a c t i v e chain c a r r i e r s . E f f e c t of A d d i t i o n of Inert Gases on Rate of MaximallyI n h i b i t e d Decomposition Since the decrease i n the normal r e a c t i o n r a t e due t o d i l u t i o n w i t h i n e r t gases could p o s s i b l y be due t o suppression of a chain r e a c t i o n 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 t o i n v e s t i g a t e the e f f e c t of such d i l u t i o n F i g s . 1*+ and 15 show r a t e  on the maximally I n h i b i t e d r e a c t i o n . o  curves f o r the decomposition, at 516 C., of 100 mm. of pentene, i n 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 n i t r o g e n , r e s p e c t i v e l y .  The curves  f o r the u n d i l u t e d i n h i b i t e d r e a c t i o n s are included f o r comparison.  Neither argon nor n i t r o g e n causes a decrease i n the  r a t e of the i n h i b i t e d r e a c t i o n .  Rather, there i s , i n each  case, a s l i g h t increase, p o s s i b l y due t o an increase i n the c o l l i s i o n r a t e of a non-chain part of the r e a c t i o n . E f f e c t of A d d i t i o n of Lead T e t r a e t h v l A f u r t h e r means of d e t e c t i n g the presence of f r e e - r a d i c a l chains i s t o add small amounts of an i n i t i a t o r , at temperature c o n d i t i o n s under which the reactant gas i s normally s t a b l e . The i n i t i a t o r must be a substance known t o decompose i n t o f r e e r a d i c a l s a t 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 t o 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 s e n s i t i z e r , since t h i s substance i s known t o decompose r e a d i l y a t  30  36 350°C., y i e l d i n g e t h y l 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 s o l u t i o n of Pb(G H^)i 2  i n ethylene dibromide and ethylene d i c h l o r i d e .  f  R e s u l t s from  sensitization experiments can t h e r e f o r e be of a q u a l i t a t i v e nature o n l y . Experiments c a r r i e d out at 350°C. l e d t o a decrease i n pressure, i n d i c a t i n g t h a t , at t h i s low temperature, f r e e e t h y l r a d i c a l s induce p o l y m e r i z a t i o n r a t h e r than decomposition of the pentene. , S i m i l a r runs were made at t emperatures at which pentene-1 normally decomposes at a measurable r a t e .  F i g . 16 shows A p -  time p l o t s f o r the decomposition of 110 mm. presence of 15 mm. and 530°C.  of pentene, i n the  of Pb(C H^)i , at temperatures of ^70, 2  +  500,  Curves f o r the normal decompositions are shown f o r  comparison. At each temperature, an increase i n r a t e i s e x h i b i t e d i n the presence of PbCC^H^)^. c a t a l y z e the r e a c t i o n .  Free e t h y l r a d i c a l s thus appear t o  This behaviour suggests that f r e e r a d i -  c a l s can cause decomposition of pentene-1 t o a small degree. However, since the exact nature of the Pb(C H^)^mixture used 2  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 r e a c t i o n may be determined from a study of the temperature .dependence of the r e a c t i o n rate.  The dependence of the r a t e of decomposition on tempera-  t u r e was i n v e s t i g a t e d at v a r i o u s 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-  t a i n s the r e s u l t s f o r the experiments i n v o l v e d .  Two or three  runs were made a t each temperature f o r each i n i t i a l pentene pressure, and the r a t e s l i s t e d are averages o f the i n i t i a l r a t e s , as c a l c u l a t e d - f r o m the maximum slopes of t h e Ap-time curves. TABLE IV Temperature dependence o f the r e a c t i o n r a t e s and r e a c t i o n r a t e constants a t various i n i t i a l pentene pressures Initial pressure, Temp., 1/T x 1 0 mm. °C. 60  530 516 500  1*86 1+70 110  150  200  1.21+7  1.268  1.293 1.318 1.3k8  530 516 500 1+86 k-70  1.21*7  530 516 500 1*86 1+70  1.21*7  530 516 500 1*86 k-70  1.21+7  530 '516 500 \86 1*70  1.268 1.293 1.318 1.3W  1.268 1.293 1.318 1.3^8  1.268 1.293 1.318 1.3W  I.21+7  1.268  1.293 1.318 1.3^8  '  3  Rate dP/dT, constant,> Log (k x 10*) mm./min. sec.-i-x IO " 4  11.5 6.62 3.80 2.0** 0.910  32.0 18.1+ 10.6 5.66 2.53  1.505 1.265 1.025 0.753 0.1*03  25.^ 11+.8 7.61+ 1+.0I+ 1.82  38.5 22.1+ 11.6 6.11 2.76  1.585 1.350  1.061*  30.9 I8.3 9.32 5.00 2.18  3i*.l* 20.1* 10.1* 5.55 2.1*2  1.5V7 1.310 1.017 0.7kh O.38I*  1+0.0 23.1+ 11.3 5.98 2.76  33.^ 19.5 9M h.98 2.30  1.521* I.290 0.97^ 0.697 0.362  52.7 29.0 11+.5 7.85 3.63  3^.6 19.6 9.78 5.30 2.1*5  1.551 I.292 0.990 0.721+ O.389  0.786 0.1+1+1  '  38 A p l o t of l o g k versus l/T f o r 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 t h a t the Arrhenius Equation i s v a l i d over t h i s range of temperatures; t h a t i s , k = A "  E / R T  e  .  Since i n t e g r a t i o n of t h i s equation y i e l d s the expression: In k = I n A -  E/RT,  therefore the slope of the I n 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 pressure were c a l c u l a t e d from the slopes of the r e s p e c t i v e 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 A c t i v a t i o n energy, kcal./mole  Pentene pressure, mm.  Slope  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 r a t e constant at 500°C. the frequency f a c t o r , A, 11 -1 was c a l c u l a t e d t o be approximately 10  sec.  .  This value i s  i n agreement w i t h those normally found f o r such f i r s t - o r d e r decomposition r e a c t i o n s . 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 p l o t of a c t i v a t i o n  energy as a finaction of pentene pressure y i e l d s a curve t h a t i s almost l i n e a r .  I t was of i n t e r e s t t o i n v e s t i g a t e 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 p o s s i b l e explanation of  the observed trend i n the values i s the f o l l o w i n g :  a higher  a c t i v a t i o n energy at higher pressures may be due t o a process of c o l l i s i o n a l d e a c t i v a t i o n of a c t i v a t e d molecules.  When a  pentene. molecule enters i n t o 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 f r a n s f e r 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 p o s s i b l e t h a t , 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 r a t e , may cause a greater p r o p o r t i o n of unfavourable c o l l i s i o n s , l e a d i n g t o d e a c t i v a t i o n . Thus, i n order f o r an a c t i v a t e d molecule, at high pressures, t o 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 ,  l a r g e r amounts of energy than i t would  r e q u i r e 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 r e a c t i o n would be evidenced i n a somewhat greater value of the a c t i v a t i o n energy. E f f e c t of A d d i t i o n of I n e r t Gases on A c t i v a t i o n Energy Assumption of the above mechanism leads t o the p r e d i c t i o n  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 .  T h i s p r e d i c t i o n Involves the f u r t h e r  assumption that a l l molecules, regardless of complexity, are e q u a l l y capable of t r a n s f e r r i n g energy.  I n an e f f o r t t o t e s t  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 a d d i t i o n of i n e r t gases.  In Table VI  are l i s t e d the r e s u l t s f o r two sets of runs i n which 100 of pentene was decomposed i n the presence of 100 mm. and 100 mm. 200 mm.  of n i t r o g e n r e s p e c t i v e l y .  mm.  of argon  Results f o r 100 and  of pentene alone are included f o r comparison.  These  runs were made i n the temperature range of 1+70 t o 530°C. TABLE V I 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.  200  100  100  0  50.6  200  100  0  100  5i.o  100  100  0  0  51.2  200  200  0  0  53.2  ,:  Nitrogen A c t i v a t i o n pressure, energy, mm. kcal./mole  C l e a r l y , increase of pressure due t o the presence of e i t h e r Argon or n i t r o g e n 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 A d d i t i o n of Freon on A c t i v a t i o n Energy Since the molecular weight of n i t r o g e n , 28, and the atomic t  weight of argon, kO, are both l e s s than the molecular weight of pentene, 70, and, furthermore, the number of degrees of freedom f o r both these molecules i s considerably l e s s than f o r pentene, i t was hoped t o obtain, more s a t i s f a c t o r y r e s u l t s w i t h 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 r e q u i r e d .  Fluorocarbons  are known t o be both extremely s t a b l e t o heat and chemically inert.  As no pure fluorocarbons were a v a i l a b l e , a f r e o n ,  monochlorotrifluoromethane, was used, w i t h the hope t h a t 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  f r e o n gas showed no s i g n i f i c a n t pressure increase when subj e c t e d , f o r over an hour, t o the temperatures used i n the pentene decomposition. Experiments were t h e r e f o r e done w i t h 100 mm. the presence of 100 mm.  of f r e o n .  of pentene i n  The r e a c t i o n showed a con-  s i d e r a b l e increase i n r a t e , amounting t o approximately a 51$ increase above the normal v a l u e . The a c t i v a t i o n energy was found t o be 53.5 k c a l .  However, samples of the r e a c t i o n mix-  t u r e s , taken a f t e r t e n minutes, gave p o s i t i v e t e s t s f o r halogen w i t h 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 f r e o n gas  alone, when tested s i m i l a r l y , d i d not give p o s i t i v e t e s t f o r halogen, i t was concluded that some methyl c h l o r i d e , methyl f l u o r i d e , or other a l k y l h a l i d e had been formed i n the reaction, and hence that the f r e o n used was not i n e r t at these temperatures  1+2 i n the presence of pentene. E f f e c t of A d d i t i o n 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 t e r m i n a t i n g mechanism, the r e s i d u a l r e a c t i o n may be regarded as a process of molecular r e arrangement.  I t was of i n t e r e s t t o i n v e s t i g a t e the a c t i v a t i o n  energy of t h i s r e s i d u a l r e a c t i o n i n order t o 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 w i t h pressure. The a c t i v a t i o n energies f o r the maximally i n h i b i t e d decompositions were i n v e s t i g a t e d  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 t o 530°G.  The  r e s u l t s are shown i n Table V I I . TABLE V I I i  A c t i v a t i o n energies of maximally i n h i b i t e d decomposition Total pressure, mm.  Pentene pressure, mm.  6*+  250  i  9*  Propylene pressure, mm.  Activation energy, kcal./mole  10  110  21+  50.7  200  50  52.0  The values f o r the a c t i v a t i o n energy of the r e s i d u a l  reaction  are very close t o those of the t o t a l r e a c t i o n , being a p p r o x i mately 1. k c a l . lower.  I n F i g . 1 9 i s shown a p l o t of a c t i v a t i o n  energy as a f u n c t i o n of i n i t i a l pentene pressure.  Again, an  increase i n pressure corresponds t o an increase i n a c t i v a t i o n  o ?i.Z. 19. inhibited  50  loo • 150 Pressure pentene  200 5  r5o  •  mm.  D e p e n d e n c e o f a c t i v a t i o n en-.r^y o f — r i . - ^ l l y r e a c t i o n on i n i t i a l p e n t e n e p r e s s u r e .  *3 energy.  Hence t h i s e f f e c t i s not p e c u l i a r t o the u n i n h i b i t e d  part o f the r e a c t i o n alone. The c l o s e s i m i l a r i t y between the a c t i v a t i o n energy values f o r the i n h i b i t e d r e a c t i o n and f o r the t o t a l r e a c t i o n i n d i c a t e s that the two processes are able t o proceed w i t h almost equal ease. E f f e c t of A d d i t i o n 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 t o e x h i b i t an increase i n a c t i v a t i o n energy w i t h increased i n i t i a l pressure of. hydrocarbon, the a b i l i t y of i n e r t gases t o maintain the h i g h a c t i v a t i o n energy value of the r e s i d u a l r e a c t i o n was investigated.  Two sets of experiments were done over the  range of temperatures from *H70 t o 530°C. w i t h pentene i n h i b i t e d w i t h propylene, and i n the presence of argon and of n i t r o g e n respectively.  R e s u l t s obtained are shown i n Table V I I I .  Values f o r the i n h i b i t e d decomposition of 200 mm. of pentene-1, i n the absence of i n e r t gas, are shown f o r comparison. TABLE V I I I A c t i v a t i o n energies of maximally i n h i b i t e d decomposition diluted w i t h i n e r t gases Propylene Pentene Total pressure, pressure, pressure, mm. mm. mm.  Argon Nitrogen A c t i v a t i o n pressure, pressure, energy, mm. " mm. 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 u n i n h i b i t e d  decomposition,  n e i t h e r argon nor n i t r o g e n i s e f f e c t i v e i n i n c r e a s i n g the value of the a c t i v a t i o n energy of the r e s i d u a l  reaction.  E f f e c t of A d d i t i o n 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 p o s s i b l e that i t s presence might a f f e c t 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 i n v e s t i g a t e d three d i f f e r e n t i n i t i a l pressures.  at  R e s u l t s f o r these e x p e r i -  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 pressure, mm.  Pentene N i t r i c oxide A c t i v a t i o n pressure, pressure, energy, mm. mm. kcal./mole  60  50  10  50.k  230  200  30  53. h  C l e a r l y , the a d d i t i o n of n i t r i c oxide has no appreciable e f f e c t on the value of the a c t i v a t i o n energy.  The increase w i t h i n -  creased pressure i s again observed. Summary of the Experimental R e s u l t s 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 f o l l o w s : (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 r a p i d increase as the r a t e b u i l d s up t o 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 a t the h i g h temperatures p o l y m e r i z a t i o n i s not an important primary process. (2)  The r e a c t i o n r a t e i s o f the f i r s t order w i t h respect t o  the i n i t i a l pentene pressure.  A t 5QQ°C, the r e a c t i o n r a t e  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-  c i a b l e a l t e r a t i o n i n the r a t e of the r e a c t i o n . The r e a c t i o n i s t h e r e f o r e e s s e n t i a l l y homogeneous. (*+) A d d i t i o n o f argon and of n i t r o g e n causes a s l i g h t decrease i n the r a t e of the r e a c t i o n , the e f f e c t being more pronounced i n the case of n i t r o g e n .  This phenomenon suggests  that the r e a c t i o n 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; r a t h e r , the operation of a f r e e - r a d i c a l chain mechanism i s i n d i c a t e d , wherein the i n e r t gas molecules are able t o f u n c t i o n as t h i r d bodies, favouring r a d i c a l recombination. (5) N i t r i c oxide has no e f f e c t on the r a t e of the r e a c t i o n . This behaviour suggests three p o s s i b i l i t i e s : (a)  the absence of f r e e r a d i c a l s ;  (b)  a c a t a l y t i c e f f e c t of n i t r i c oxide, equal and opposite t o i t s i n h i b i t o r y a c t i o n ;  (c)  the presence of some r e a c t i o n product, such as  1+6  • i  propylene, which i s able t o combine w i t h 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 r e a c t i o n chains, the m a j o r i t y of the f r e e - r a d i c a l r e a c t i o n s involved being chain terminating steps. (6)  Added propylene causes a 20% r e d u c t i o n i n the r a t e of the  reaction.  Beyond a c e r t a i n amount, f u r t h e r a d d i t i o n of propy-  lene does not a f f e c t the r a t e .  This phenomenon suggests the  operation of a f r e e - r a d i c a l chain mechanism, r e p r e s s i b l e by propylene.  The r e s i d u a l r e a c t i o n may  or may not i n v o l v e  chains. (7)  The presence of i n e r t gases i n the maximally i n h i b i t e d  r e a c t i o n causes a s l i g h t increase i n the r a t e .  This behaviour  suggests that the r e s i d u a l r e a c t i o n , i n the presence of propyl e n e , does not i n v o l v e 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 p o l y m e r i z a t i o n of pentene-1.  At temperatures at which  the normal decomposition proceeds at a measurable r a t e , small amounts of lead t e t r a e t h y l accelerate the decomposition.  It  would appear probable t h e r e f o r e that both the low-temperature p o l y m e r i z a t i o n and the high-temperature decomposition are processes i n v o l v i n g f r e e r a d i c a l s . (9)  The dependence of the r a t e of the r e a c t i o n upon tempera-  t u r e i s i n accordance w i t h the Arrhenius Equation. t i o n energy f o r the normal r e a c t i o n i s approximately  The a e t i v a 5  2  kcal.  k7 The frequency f a c t o r , at 500°C, i s approximately  per mole. 10  11  (10)  s e c T .. 1  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 t o between 1 and 2 k c a l . per mole f o r a pressure increase of 100 (11)  mm.  The energy of a c t i v a t i o n i s unaffected by the a d d i t i o n  of n i t r o g e n or argon t o the r e a c t i o n system. (12)  The presence of CF^Gl considerably increased both the  r a t e 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 f o r the presence of an a l k y l  h a l i d e i n the r e a c t i o n mixture i n d i c a t e s t h a t the f r e o n i s not i n e r t i n the presence of pentene-1 at the temperatures employed. (13)  A d d i t i o n 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 i n h i b i t e d  r e a c t i o n e x h i b i t s an increase of activation,energy w i t h 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 t h e r e f o r e appear t o be a l s o operative i n the r e s i d u a l r e a c t i o n , and not a property of the r e p r e s s i b l e p o r t i o n of a chain mechanism. (Ik)  A d d i t i o n of i n e r t gases t o the maximally i n h i b i t e d r e -  a c t i o n produce_s no appreciable e f f e c t on the a c t i v a t i o n energy. (15)  A d d i t i o n of n i t r i c oxide produces no appreciable e f f e c t  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 a r i s e s i n p r e d i c t i n g the mechanism of a thermal decomposition r e a c t i o n concerns the fundamental nature of the primary a c t i v a t i o n process. act  The primary  i n the decomposition of any organic molecule may he e i t h e r  an i n t e r n a l rearrangement t o stable products or a bond rupture producing f r e e r a d i c a l s , which subsequently cause f u r t h e r decomposition by a chain mechanism.  I n any p a r t i c u l a r instance,  one of these mechanisms may be operative t o the v i r t u a l exc l u s i o n of the other, or the two processes may occur simultaneously i n p o t e n t i a l competition. In order t o decide upon the r e l a t i v e e f f e c t i v e n e s s of these two types of mechanism i n the thermal decomposition of pentene-1, i t w i l l be necessary t o 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 summarized as f o l l o w s : (a)  the presence of i n e r t gases i n the r e a c t i o n mixture , causes a decrease i n the r e a c t i o n r a t e ;  (b)  added propylene i n h i b i t s the r e a c t i o n , reducing the r a t e t o approximately 80$ of i t s o r i g i n a l value;  (c)  added f r e e r a d i c a l s a c c e l e r a t e the r e a c t i o n .  However, f u r t h e r experimental r e s u l t s do not appear t o 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 r e a c t i o n ;  (e)  the r e a c t i o n r a t e i s not s e n s i t i v e t o a change i n the surface-to-volume r a t i o i n the r e a c t i o n vessel.  These p o i n t s w i l l be considered i n t u r n . For a homogeneous decomposition r e a c t i o n , 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 rearrangements, the presence of i n e r t gases would be expected t o acc e l e r a t e the r a t e 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 t o prevent a f a l l i n g off i n rate.  I f , however, a chain mechanism i s  o p e r a t i v e , an i n e r t gas may a f f e c t the r a t e i n one of two p o s s i b l e 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 t o the surface of the r e a c t i o n v e s s e l , may be expected t o a c c e l e r a t e the r e a c t i o n ; (b) i f r a d i c a l s recombine homogeneously, an i n e r t gas should r e t a r d the r e a c t i o n 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 t o 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 t h a t may be present must be a homogeneous process..  I t has been shown t h a t  both argon and n i t r o g e n , e,ven when present i n l a r g e proportions,  50 f a i l t o cause any a c c e l e r a t i o n ; r a t h e r , they have been observed to exert a d e f i n i t e r e t a r d a t i o n on the r e a c t i o n r a t e .  Thus i t  may be i n f e r r e d t h a t the decomposition of pentene-1 i n v o l v e s a f r e e - r a d i c a l mechanism, i n which r e a c t i o n chains are t e r m i nated homogeneously, p a r t i a l l y , at l e a s t , by t e r n a r y c o l l i s i o n s . The e f f e c t of propylene w i l l next be considered.  In  a c t i n g as an i n h i b i t o r , a propylene molecule i s b e l i e v e d t o f u n c t i o n , e s s e n t i a l l y , as a t h i r d body f o r removing  radicals:  the propylene molecule r e p l a c e s a chain c a r r i e r by a more s t a b l e a l l y l - t y p e r a d i c a l , l e s s e f f i c i e n t f o r continuing the chain process: E -f CH^CH = CH -> RH -f 2  CH  2  CH _ =L  Z  CH » 2  Other unsaturated molecules may be expected t o act s i m i l a r l y ; f o r example, i t has been shown t h a t isobutene i n h i b i t s the decomposition of pentane (*+0) •  I n the pentene-1 decomposition,  the presence of a l a r g e amount of unsaturated m a t e r i a l , due both to the pentene i t s e l f and t o i t s decomposition products, may be expected t o cause appreciable i n h i b i t i o n i n the course of the normal r e a c t i o n .  Thus i t might be expected that a  l a r g e p r o p o r t i o n 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 r e p r e s s i o n of the r a d i c a l chains i s incomplete i n the  normal r e a c t i o n , the a d d i t i o n of i n c r e a s i n g amounts of propylene should supplement such i n h i b i t i o n t o the point of complete chain termination.  Such was indeed found t o be the case.  Propylene i n h i b i t i o n reduced the r a t e t o about Q0% of i t s  51  normal v a l u e .  Molera and Stubbs have a l s o 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 f r e e 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 t o be l i t t l e doubt t h a t a  metal a l k y l , . such as lead t e t r a e t h y l , decomposes w i t h the formation of f r e e 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). a l k y l s have f r e q u e n t l y been used as s e n s i t i z e r s .  Such metal Catalysis  of an organic decomposition by the presence of small amounts of such substances i s regarded as evidence f o r 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 t o a t t a c k reactant  molecules and a c c e l e r a t e t h e i r decomposition.. The observed c a t a l y s i s of the pentene-1 decomposition, then, by s m a l l amounts of lead t e t r a e t h y l , i n d i c a t e s that f r e e r a d i c a l s are able t o cause decomposition of the pentene.  Such i n d i c a t i o n s  do not, however, prove c o n c l u s i v e l y that chains are e f f e c t i v e i n the normal decomposition. On what would appear t o be the negative side of the argument f o r the a c t i o n of f r e e r a d i c a l s i n the pentene decomposit 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, t o i n t e r p r e t 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 n e c e s s a r i l y c o n t r a d i c t o r y , and a chain process may s t i l l provide a p l a u s i b l e  52 explanation of the r e a c t i o n mechanism. N i t r i c oxide produced no a l t e r a t i o n i n the decomposition rate.  Molera and Stubbs report the same r e s u l t  (3*+).  Assum-  ing the e f f e c t of a l l i n h i b i t o r s of chain r e a c t i o n s t o be e s s e n t i a l l y the same; t h a t i s , t o reduce the concentration of chain r a d i c a l s , i t would seem c o n t r a d i c t o r y 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 .  I n order t o provide  an argument i n favour of a chain mechanism, i t must be shown that the observed l a c k of i n h i b i t i o n does not c o n s t i t u t e proof of a non-chain r e a c t i o n : must be made:  one of t h e f o l l o w i n g  assumptions  (a) t h a t n i t r i c oxide can s t a r t chains as w e l l  as stop them i n pentene; (b) t h a t the r a d i c a l s present i n the pentene decomposition combine w i t h n i t r i c oxide too s l o w l y f o r appreciable i n h i b i t i o n ; o r , (c) that the i n h i b i t o r y e f f e c t due to the n e c e s s a r i l y h i g h concentration of o l e f i n s present i n the r e a c t i o n mixture w i l l swamp out any e f f e c t of n i t r i c oxide. The f i r s t o f these assumptions was adopted by Rice and P o l l y i n order t o account f o r 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 f o r the decomposition of acetaldehyde 0+2).  They proposed the  f o l l o w i n g scheme by which n i t r i c oxide can both s t a r t and stop chains: M  R  1  l  ~*  (I)  2Ri  +  NO —± HNO -f R  2  (II)  H-  M -± RjH + R  2  (III)  x  R  2  R  1  +  Mg  (IV)  53 ^4-  NO —> RjNO  (V) (VI)  R  2  (VII)  •+• NO -^B^NO  Rj^ "I  -  R  2  —^R-^~R  (VIII)  2  Decomposition was assumed t o occur w i t h chain t e r m i n a t i o n e i t h e r by steps (V) and ( V I ) , corresponding t o a low a c t i v a t i o n energy f o r step ( I V ) , or by steps (VII) and ( V I I I ) , w i t h a h i g h a c t i v a t i o n energy f o r step ( I V ) . I t i s p o s s i b l e t o assume a scheme of t h i s type f o r the pentene-1 decomposition: n i t r i c oxide may r e a c t w i t h 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 ( I V ) . According t o t h i s scheme, n i t r i c oxide a l s o f u n c t i o n s as an i n h i b i t o r by combining w i t h R^ and R . 2  I t i s p o s s i b l e , on t h i s b a s i s , t o  p r e d i c t no net e f f e c t of n i t r i c oxide on the observed  reaction  rate. Assumption (b), t h a t the r e a c t i o n between n i t r i c oxide and the r a d i c a l s present i s too slow t o produce appreciable I n h i b i t i o n , h a r d l y seems a probable e x p l a n a t i o n :  i t i s very  l i k e l y t h a t , i f r a d i c a l s are present at a l l , both methyl and e t h y l r a d i c a l s would be among these, and i n most other organic decomposition r e a c t i o n s i n v o l v i n g these r a d i c a l s , n i t r i c oxide has been found t o e x h i b i t very marked i n h i b i t o r y e f f e c t s  (1).  A c o n s i d e r a t i o n of the unsaturated nature of the decomposing 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 e s p e 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 l a r g e 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 t o terminate chains. Any i n h i b i t o r y e f f e c t s of n i t r i c oxide are completely masked.  The r e s u l t s of other  i n v e s t i g a t o r s bear out t h i s conclusion.  E l t e n t o n , from mass  spectrometric i n v e s t i g a t i o n s , concluded that propylene reacts more e a s i l y than does n i t r i c oxide w i t h methyl r a d i c a l s ( V } ) 5 at high temperatures he detected both methyl and a l l y l r a d i c a l s i n the decomposition of propylene.  Observations made by  Steacie and F o l k i n s 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 r e a c t i o n s proceed, the r a t e s approaching t h e i r normal values (M+).  These i n v e s t i g a t o r s have a t t r i b u t e d t h e i r f i n d -  ings t o the b u i l d i n g up w i t h 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 e f f e c t of n i t r i c oxide. Assumption (c) appears more convincing than ( a ) . I t i s r a t h e r improbable that the opposed a c c e l e r a t i n g and r e t a r d i n g a c t i o n s of n i t r i c oxide should e x a c t l y 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 r e a c t i o n . are  I f long chains  assumed, a l a r g e increase i n the surface-to-volume r a t i o  would be expected t o f a c i l i t a t e heterogeneous  radical  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 w h i l e a given number of hydrocarbon molecules decompose; hence, any competing chain-breaking process due t o i n h i b i t i o n on the surface w i l l be l e s s important. the chains are s h o r t .  There are I n d i c a t i o n s that  I n the f i r s t p l a c e , quite considerable  amounts of propylene are required i n order t o b r i n g about appreciable inhibitions  t h i s f a c t suggests a l a r g e number of  r e l a t i v e l y short chains r a t h e r than a few long ones, since the amount of i n h i b i t o r used t o stop chains must l o g i c a l l y depend on the number of such chains present.  F u r t h e r , i f i t i s as-  sumed t h a t the maximally i n h i b i t e d r e a c t i o n represents a s t a t e of a f f a i r s i n which a l l chains are cut down t o t h e i r primary process, the average chain l e n g t h may be c a l c u l a t e d from the r a t i o of the normal t o the i n h i b i t e d r a t e . about 1.25 i s obtained.  T h i s means e i t h e r ;  A low value of (a) that r a d i c a l s  from every h p r i m a r i l y decomposing molecules cause the decomp o s i t i o n of 1 more; o r , (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 t h a t each of these causes the decomposition of about 25 molecules.  I n comparison  w i t h the extremely long chains found i n many organic decomp o s i t i o n r e a c t i o n s , even the second p o s s i b i l i t y does not lead to an extremely l a r g e value f o r the absolute chain l e n g t h . Short chains were a l s o postulated by Gorin on the b a s i s 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 c o n s i d e r a t i o n of the foregoing r e s u l t s therefore  ap-  pears to point t o a mechanism i n v o l v i n g a s p l i t of pentene molecules t o f r e e 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 w i t h 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 r e a c t i o n 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 r e a c t i o n c o n s i s t s of an intramolecular rearrangement process of low a c t i v a t i o n energy.  Although l a c k of f u r t h e r  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 maximall y i n h i b i t e d r e a c t i o n i s i t s e l f a modified chain r e a c t i o n , the f a c t that i n e r t gases exert an a c c e l e r a t i n g r a t h e r than a r e t a r d i n g e f f e c t on the r a t e of the i n h i b i t e d process, would suggest the absence of chains.  The f a c t that the a c t i v a t i o n  - energy values f o r the normal and the i n h i b i t e d r e a c t i o n s are very close probably renders the two competing mechanisms of about equal importance i n the o v e r a l l r e a c t i o n process, e x p l a i n s why they are able t o operate  and  simultaneously.  According t o the simple c o l l i s i o n theory, the r a t e constant may  be c a l c u l a t e d 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 f o r the r e a c t i o n by use of the equation: k * z e"  E / R T  .  The k obtained w i l l be the r a t e 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 c a l c u l a t e d 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 / c c . /sec,  was c a l c u l a t e d from the equation: z = 2n cr 2  2  irRT M  where n represents the number of molecules i n a u n i t 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.  T h i s y i e l d s a value f o r z of 1.7  c o l l i d i n g /cc./sec.  x 10  27  molecules  I n Table X the c a l c u l a t e d values of k are  compared w i t h those obtained experimentally.  Now, although  the a c t i v a t i o n energy f o r the i n h i b i t e d r e a c t i o n i s somewhat lower than that f o r the normal r e a c t i o n , i t i s seen from Table X that the observed r a t e 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 t o the a c t i vation. TABLE X Calculated and observed r a t e constants f o r normal and inhibited reactions 1  k from z e ~ / , sec."" E  1  Normal r e a c t i o n  5,2  x IO"  Maximally i n hibited reaction  l.h  x 10"  R T  k from a c t u a l r a t e , sec."" 1  8  9.h x IO'1* '  7  1.9  x 10"^  58  A c o n s i d e r a t i o n of the bond strengths involved imposes c e r t a i n r e s t r i c t i o n s on the proposed chain mechanism. ing t o Stevenson's r e s u l t s (^5), we may  Accord-  based on e l e c t r o n impact data, 77 k c a l . t o  a s s i g n a bond energy value of approximately  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 w i t h respect t o 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 r e a c t i o n i s thus  considerably lower than the energy necessary to d i s r u p t the weakest l i n k i n the molecule. important,  I f , then, a chain process i s  only e x c e p t i o n a l molecules, too rare t o a f f e c t the  mean a c t i v a t i o n energy, can give r a d i c a l s . r e l a t i v e l y few chains. important  This suggests  I n order that the chain process be  i n r e l a t i o n t o the competing i n t r a m o l e c u l a r mode of  decomposition, the chains must be of appreciable Now,  lengths.  i t has been observed that the a c t i v a t i o n energy f o r  the i n h i b i t e d r e a c t i o n i s somewhat lower than that f o r the o v e r a l l process. temperature.  I t f o l l o w s that i n h i b i t i o n must increase w i t h  Therefore the average chain l e n g t h must increase  w i t h temperature, suggesting that the chain r e a c t i o n i s more important at higher temperatures, when more thermal energy i s available. Thus i t would appear t h a t the decomposition of pentene-1 i s a complex process, i n v o l v i n g 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 r e a c t i o n products and by added  propylene,  together w i t h an i n t r a m o l e c u l a r rearrangement process of low a c t i v a t i o n energy, a l a r g e number of i n t e r n a l degrees of  59  freedom c o n t r i b u t i n g to the a c t i v a t i o n process.  At h i g h tem^,.  peratures the c o n t r i b u t i o n of the chain mechanism t o the o v e r a l l r e a c t i o n becomes more important. creases w i t h temperature:  The absolute chain l e n g t h i n -  probably very short chains predomi-  nate at lower temperatures, most of the chain steps being those of t e r m i n a t i o n , whereas at higher temperatures the chains are longer-lived.  Chain t e r m i n a t i o n takes place homogeneously  both by t e r n a r y c o l l i s i o n s , w i t h pentene molecules a c t i n g as t h i r d bodies, and by a d d i t i o n of r a d i c a l s to unsaturated molecules. Without detailed analyses of the r e a c t i o n products, no conclusions can be drawn as t o the exact mechanism of the decomposition.  I t i s t o be hoped t h a t f u t u r e mass spectro-  metric analyses w i l l be able t o e l u c i d a t e 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 t h a t the value of the a c t i v a t i o n energy exh i b i t e d at .small increase w i t h an increase i n the i n i t i a l pentene pressure.  Such a phenomenon i n the pentene-1 decomposi-  t i o n has a l s o been reported by Molera and Stubbs ( 3 6 ) .  These  authors, however, do not comment upon t h e i r f i n d i n g s .  Such a  trend of a c t i v a t i o n energies contrasts sharply, w i t h the behaviour of normal p a r a f f i n hydrocarbons, which e x h i b i t a l a r g e decrease of a c t i v a t i o n energy w i t h i n c r e a s i n g pressure. The increase of a c t i v a t i o n energy w i t h pressure found f o r the normal r e a c t i o n of pentene-1 has been shown t o apply a l s o  60  t o the maximally i n h i b i t e d r e a c t i o n .  Thus i t would appear  that the explanation f o r t h i s behaviour must l i e i n the a c t i v a t i o n mechanism of the molecular rearrangement process.  A  p o s s i b l e explanation f o r the phenomenon has been suggested i n the experimental s e c t i o n of t h i s t h e s i s (p.  ).  This sug-  g e s t i o n i n v o l v e s a mechanism of c o l l i s i o n a l d e a c t i v a t i o n : a c t i v a t e d 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 s e v e r a l squared terms and so i s not mobilized f o r 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 w i t h 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 t o t h i s  siphoning of energy by unfavourable c o l l i s i o n s , there w i l l f o l l o w an increase i n the energy t h a t 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 e f f e c t 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 w i t h high pressures maintained  by  the presence of f o r e i g n gases have f a i l e d to provide any supp o r t i n g evidence f o r t h i s hypothesis.  Their presence exerts  no appreciable e f f e c t on the observed values of the a c t i v a t i o n energy f o r e i t h e r the normal or the i n h i b i t e d r e a c t i o n .  It  may be, however, that argon and n i t r o g e n are not molecules of s u f f i c i e n t complexity f o r the purpose.  I t i s hoped that i n  f u t u r e work i t may be p o s s i b l e to f i n d a molecule comparable i n complexity t o 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 s e v e r a l more degrees of freedom, might be expected t o exert a c o l l i s i o n a l e f f e c t s i m i l a r t o that shown by pentene. Thus no d e f i n i t e conclusions can be drawn as t o the v a l i d i t y of the proposed c o l l i s i o n a l d e a c t i v a t i o n process.  In the  l i g h t of the r e s u l t s obtained w i t h n i t r o g e n and argon, i t would appear more probable t h a t the phenomenon a r i s e s 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 w i t h a s l i g h t l y  greater  a c t i v a t i o n energy than the r e s i d u a l r e a c t i o n , i t might be speculated that higher pressures favour a chain mechanism; that i s , t h a t 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 r e a c t i o n becomes of i n c r e a s i n g importance.  Were t h i s the case^ i t should  f o l l o w that r e a c t i o n s w i t h i n c r e a s i n g pentene pressure  should  demand i n c r e a s i n g percentages of propylene f o r i n h i b i t i o n . present observations  The  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 d e t a i l e d t o form the b a s i s of a d e c i s i o n of t h i s sort.  Presumably only a small e x t r a amount of propylene would  be required t o 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 t o the poor method a v a i l a b l e f o r 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 n e c e s s a r i l y be of s i g n i f i c a n c e . A small e r r o r i n the r e a c t i o n r a t e would lead to a l a r g e e r r o r  62  i n t h e slope of the Arrhenius p l o t , and a consequently e r r o r i n the value of the a c t i v a t i o n energy.  large  I f such small  e r r o r s i n the observed rate were c o n s i s t e n t , there could r e s u l t a c o n s i s t e n t but erroneous trend i n the c a l c u l a t e d a c t i v a t i o n energies.  For t h i s reason not too much s i g n i f i c a n c e should be  attached t o the present r e s u l t s of the dependence of a c t i v a t i o n energy on pentene pressure.  A small, c o s i s t e n t c o n t r i b u -  t i o n from some secondary r e a c t i o n could conceivably lead t o the phenomenon observed.  I n v e s t i g a t i o n s made over a f a r  greater pressure range would be necessary before any d e f i n i t e conclusions could be made as t o the genuine existence of an increase of a c t i v a t i o n energy w i t h i n c r e a s i n g 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. (If) Frey, F.E.  J.Chem.Phys. M-:608. 1936.  Ind.Eng.Chem. 26:198. 193*+.  (5) Hinshelwood, C.N.  The Kinetics of Chemical Change.  Clarendon Press, Oxford. 19 +9. 1  (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. A200:lf58. 1950.  Proc.Roy.Soc.(London)-  (10) Rice, F.O.'and Rice, K.K. The Aliphatic Free Radicals. 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