The development of an improved kinetic flow technique and its application to the pyrolysis of methyl bromide

THE DEVELOPMENT OF AN IMPROVED KINETIC

FLOW TECHNIQUE AND ITS APPLICATION TO THE

PYROLYSIS OF METHYL BROMIDE

Gordon Robert Woolley

A Thesis Submitted for the Degree of PhD

at the

University of St Andrews

1965

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AND ITS APPLICATION TO THE PYROLYSIS OP METHYL EROÎALDE

being a Thesis presented by

GORDON ROBERT WOOLLEY, B ,S o.,

to the

UNIVERSITY OP ST. ANDREWS

in a p p lic a tio n f o r the

DEGREE OF DOCTOR OF PHILOSOPHY.

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( i l )

DBOIARATION

I hereby d e c la re th a t the follow ing T hesis i s a record of r e s u lts of experim ents c a rrie d out by me, th a t i t is my own com position, and th a t i t has not p rev io u sly been presented in a p p lic a tio n fo r a H igher Degree,

The experim ents were c a rrie d out in the Chem istry Research L aboratories of S t. S a lv a to r’s C ollege,

CBEmiPEIATB

( i v )

UmVERSCTY CARBEE

I entered th e U n iv ersity of S t. Andrews in October, 1955 and graduated in 1959 w ith F ir s t C lass Honours in Chem istry. Since October, 19&2 I have been an A ssista n t L ecturer in the Department of C hem istry, S t. S a lv a to r’s C ollege, S t. Andrews.

The work d escribed in th is Thesis was c a rrie d out under the d ire c tio n o f Dr. C. Horrex during the period

ACKITOffLBDGEMEIMT

I should lik e to record my g ra titu d e to my su p erv iso r, Dr- 0 . Horrex, fo r th e h elp , in te re s t and encouragement he has given throughout th e p r a c tic a l and th e o re tic a l asp ects

of th is work.

I am indebted to th e T ru stees of th e Carnegie T rust fo r the S c o ttish U n iv e rs itie s f o r a Research S cholarship f o r the period 1959 to I 962, I wish to record my indebtedness to the la te P ro fesso r John Read, F .R .S ., and to P ro fesso r J . I . G. Gadogan f o r research f a c i l i t i e s during th e period of the research .

I should a lso lik e to thank Dr. David C alv ert fo r h is w illin g advice and f o r many h e lp fu l d iscu ssio n s, Mr. T. N orris Mr. A, IfcGhee and Mr. M. Z<^hewski f o r h elp w ith the con­

' "I: ' ’7; ~: : ' ‘V« - ,

(v i)

COîTTEKTS

D eolaration C e r tif ic a te

U n iv ersity C areer Ac knowledgements 0 en ten te

L ist o f I I lu s tr a tio n 8 L ist of Tables

( i i ) ( i l l ) (iv ) (v) (t 1.) (ix ) (x l) IMRODÜCTIQIT

1. Bond D isso c iatio n E nergies Toluene as a r a d ic a l acceptor R adical re a c tio n s

2. 5*

4. H alide pyrolyses w ith p a r tic u la r referen ce to bromides

5# Previous d eterm inations of D(CH^--Br)

APPARATUS AND EXPBRIIÆENTAL PROCEDURE 1. D escrip tio n of th e apparatus 2. In je c tio n o f re a c ta n ts

3 . P u rific a tio n of re a c ta n ts

4# R eactant in je c tio n c a lib r a tio n 5. C a lib ra tio n of flow c a p illa r ie s 6. C irc u la tio n pump flow ra te

7. P ressure g ra d ie n t in th e flow system 8. The I’h s s Spectrom eter

9. E arly experim ents

10. A pparatus m odifications

( v ii)

APPARATUS Am EXPERIMENTAL PREGEDURB (Oont’d .) 11. F a rth e r c a lib ra tio n s

12. T ypical rim procedure

13. T ests f o r surface re a c tio n

60 62 65

DESCRIPTION OP EXPERIMENTS PERFORMED AND EXPERIMENTAL RESUIffS

1. Introduc t ory

2. Seasoning of furnace

3. E ffe c t o f v a ria tio n of methyl bromide p a r tia l p ressure

4. E ffe c t of v a ria tio n of c o n ta c t tim e 5# E ffe c t of v a ria tio n of toluene

p a r tia l pressure

6. E ffe c t o f v a ria tio n of c a r r i e r gas p ressu re 7* E ffe c t of tem perature v a ria tio n

(unpacked iU m ace)

8, E ffe c t of tem perature v a ria tio n (packed fu rn ace)

68 69 72 73 76 78 80 81 DISCUSSION

A. The em pirical ra te equation, ev alu atio n of the 83 ra te co n stan ts and th e ir tem perature dependence

1 The r a te equation 83

2. Toluene dependence of r a te law 86 3. E ffe c t of v a ria tio n of co n tact time and c a r r ie r 08

gas pressure

4 . E valuation of k^, k and k^ and th e ir 89 tem perature dependence

B. R eaction sequences and m echanistic c o n sid eratio n s 94

C. A nalysis of the r a te equation 100

D. General assessm ent of the method 109

( v i i i )

APPENDICES

I C a lcu latio n of co n tact tim es 115

I I Gas d is trib u tio n d ata I I 7

I I I Seasoning r a te data I I 9

IV V a ria tio n of p a r tia l pressure w ith co n tact tim e 121 V Temperature c o e ffic ie n t d ata (unpacked fu rn ace) 123 VI Temperature c o e ffic ie n t d ata (packed furnace) 125 V II Rate of m olecular d iffu s io n to furnace w all 126 V III An assessm ent of e rro rs and e rro r lim its 128 IX E ffe c t of c o n tact time v a ria tio n and removal of 130

toluene dependence

X Furnace seasoning 135

XI Plov/ co n d itio n s and design c h a ra c te ris tic s 136

( ix )

LIST OP ILLUSTRATIONS AND GRAPHS

1. Flow system Fao ing page38

2, Furnace tem perature p ro file ₃₉

5. R eactant in je c tio n 40

4. Diaphram valve 40

5. Flow system gas c a lib r a tio n 40

6. Toluene in je c tio n c a lib ra tio n 42 7. Methyl bromide in je c tio n c a lib ra tio n 43 8. Flow c a p illa ry c a lib r a tio n ₄₄ 9. Flovf c a p illa ry c a lib r a tio n 45

10, C irc u la tio n pump flow r a te 45

11. C irc u la tio n pump e ffic ie n c y 45 12a. P ressure g rad ien t in flow system 46 12b. Pressure g rad ien t in flow system 46 13. Mass spectrom eter e l e c tr ic a l requirem ents 47 14. O utline of tra p c u rre n t s ta b ilis e d

filam en t supply 50

15. E.H.T. v a ria tio n c o n tro ls 51

16. P o te n tia ls on the ion gun p la te s 52 17. V a ria tio n of peak h e ig h t w ith re p e lle r-b o x

voltage 52

18. #iS8 spectrom eter re s o lu tio n 52

19. Peak shape 52

20. Typical e a rly run d ata ₅₄

21. V ariatio n of leak r a tio w ith argon pressure 58

22. Sampling system 58

23. S ta in le ss s te e l valve 59

24. C o lle c tio n tra p 61

25. Gas d is tr ib u tio n (tem perature v a ria tio n ) 62 26. Gas d is tr ib u tio n (pressu re v a ria tio n ) 62

(x)

27* Mass spectrom eter response to methyl bromide 62

28. T ypical experim ental data 65

29. Gas d is trib u tio n (tem perature v a ria tio n ) 67 30. Gas d is tr ib u tio n (pressure v a ria tio n ) 67

31. Seasoning r a te s of furnaces JO

32. Exam ination of furnace co atin g 7I 35. V a ria tio n of methyl bromide p ressure ’J2

34. V a ria tio n of c o n tact tim e 73 35. V a ria tio n of toluene/m ethyl bromide r a tio 76 36. V a ria tio n of toluene pressure 77 37. V a ria tio n of c a r r ie r gas pressure 78 38. Temperature v a ria tio n (unpacked furnace) 80 39- Temperature v a ria tio n (packed furnace) 82 40. V a ria tio n of argon and toluene p ressu res 83 41. C o rre la tio n betiveen graphs in fig u re 40 86 42. Toluene and methyl bromide pressure d ata 86 43» Combined d ata of f i r s t order co n stan ts 87

44- E x trap o la tio n fo r a and 90

45- Data fo r 0.4 and 0.8 mm. toluene 92

46, A rrhenius p lo t of k^ 92

47- A rrhenius p lo t of k 92

48. Toluene v a ria tio n fo r lin e d furnace 101

49- Comparison w ith o th er work 104

50. Removal of toluene dependence from 133 co n ta c t time

LIST OF TABLES

I V ariatio n of methyl bromide p a r tia l p ressu re 72

I I C ontact time v a ria tio n 74

I I I V a ria tio n of toluene p a r tia l pressure 77 IV V ariatio n of c a r r ie r gas pressure 79 V Temperature v a ria tio n (unpacked fu rn ace) 123 VI Temperature v a ria tio n (packed furnace) 125

VII Data fo r a 91

V III Data f o r and k^ 95

INTRODUCTION

In v e stig a tio n s of th e ra te s of décom position of organic h a lid e s have played an im portant p a rt in th e c re a tio n of rece n t l i s t s of hand stre n g th s in organic compounds, and of h eats of f ornmtion of ra d ic a ls . Such d ata are very u se fu l fo r our understanding of the p rin c ip le s of m olecular r e a c tiv ity and i t is im portant th a t these l i s t s of values s h a ll he as accu rate as p o ssib le . The work des- n r i b ^ in th is th e s is was undertaken w ith th e aim of improving

one p a r tic u la r technique of in v e stig a tio n and te s tin g the method on an example of im portance.

The p a rtic u la r method chosen f o r study was th e toluene ra d ic a l accep to r technique where the follow ing re a c tio n s are said to occur

RX ^ R- + X- (slow) . . . . . . . . ( l) R- + PbOR^ ) PhOHg- t RH ( f a s t) ... (2) X----+ PhOBL---> PhD Eg- + EX ( f a s t ) ...,.( 3 ) RPhCHg--- > dibenzyl ( f a s t) . . . . (4)

In order to reduce the p o s s ib ility of sid e re a c tio n s as much as p o ssib le , the usu al recommended procedure i s to choose operating co n d itio n s which give only sm all percentages of dec omp os i t don of RX. By u sin g a flow technique, adequate q u a n titie s of one compound are u s u a lly c o lle c te d fo r a n a ly s is . When bromo compounds have been pyrolysed, the hydrogen bromide formed has been determ ined and

the proof of f i r s t order c h a ra c te r bas not been convincing in

many c a se s. For some compounds the tem perature c o e ffic ie n t of such f i r s t order c o n stan ts has been stu d ied and en erg ies of a c tiv a tio n deduced. In o th e r oases i t has been assumed th a t the A rrhenius

13 -1

tem perature independent fa c to r should be 10 sec and the values of v e lo c ity c o n sta n ts a t a sin g le tem perature have y ield ed en erg ies of a c tiv a tio n through use of the equation

log^O^cCseo”’’ ) = 13- (e/4 .5 7 T ).

The deduced energies of a c tiv a tio n have been equated to th e en th a lp ie s of re a c tio n ( l ) , leading to the re la tio n s h ip

E «=• AH f o r re a c tio n ( l )

« A % ( R - ) + AHp ( X - ) -A H ^ (RX)

I t is c le a r th a t the d ata on s e rie s of compounds such as PhCHg-X and OE^-X should y ie ld c o n s is te n t v a lu e s of AE^ fo r the

PhCHg- and ra d ica ls. This has not always been found. In

some cases the methods of a n a ly sis of the d ata may have been a t fa u lt, Mearns(l9) in th is Department, has shown, in a study of the decom position of Ph^^OEgBr, th a t Szwaro 's value of E is too lov/ and h is use of a co n stan t A f a c to r is not ju s tif ie d .

s-- I ' . . : s-- : . ;

(a) the p o s s ib ility of an aly sin g sim ultaneously fo r re a c ta n ts and products emerging from a flow type re a c to r, obtaining d ata which can be follow ed w ith tim e in order to assess the seasoning

( i f any) of th e re a c to r surface |

(b) th e opportunity to base ra te c o n stan ts on re a c ta n t con­ c e n tra tio n , r a th e r than on product co n cen tratio n s and an assumed s t oicbiom etry ;

(c) the p o s s ib ility of amassing a s u b s ta n tia l q u a n tity of a n a ly tic a l d a ta in a reasonable time while re ta in in g the power of a flow technique to study the i n i t i a l stages of a re a c tio n .

In o rd er to prove th e technique vfhich was developed, the work was co n cen trated on the decom position of methyl bromide. The

only p y ro ly tic work on th is compound has been done by Szwaro who considers the re a c tio n complex under h is co n d itio n s and p re fe rs not to a tta c h any weight to the a c tiv a tio n energy found experimen­ t a l l y , although i t ag rees w ith the value deduced from r e lia b le thermochemioal data#

in the re p re se n ta tio n of the energies of chemical bondsr The f i r s t ; and more u se fu l, is the libund d is s o c ia tio n energy (b,D.E. ) or simply d is s o c ia tio n energy. This is defined as the energy d iffe re n c e between th e parent molecule (in i t s eq u ilib riu m con­ fig u ra tio n ) and the two fragm ents (in th e ir eq u ilib riu m ground

s ta te co n fig u ratio n s) a f te r breaking the bond. Thus the bond d iss o c ia tio n energy of a bond AB - C is th e change in energy

AE^ fo r the re a c tio n

AB —— C AB— 4- (C —

occurring in the id e a l ^ s s ta te a t absolute zero . The s t r i c t d e fin itio n i s th e re fo re where the su p e rsc rip t re fe rs to products in th e ir ground s ta te s and the su b sc rip t to the zerotli v ib ra tio n a l le v e l (23) . O ccasionally AH^^cy of the re a c tio n i s used in ste a d (80), but fre q u en tly lite r a tu r e values are not c le a rly s ta te d .

o

For a d is s o c ia tio n reactio n » AC^ is g en erally p o sitiv e but sm all and so ^ . The d iffe re n c e r a re ly exceeds

1 k.cal/m ole (fo r example (H - H) *= 103.24 k .cal/m ole and AHg^gO^ fo r Hg—) H- + H* is 104.18 k .cal/m o le) and fre q u e n tly the accuracy o f the AS° 0^ value does not ju s tif y the c o rre c tio n to ab solute zero.

Thus the BDB i s equal to the d ifferen ce between the h e a ts of form ation of the fragm ents and the parent m olecule,

D(AB-O) « (AB-) + AH f(O -)— AH^ (ABG).

The heat of re a c tio n i s th e sum of the B.D.E’s of bonds formed minus the sum of B.D .E’s of bonds broken. Thus f o r the r e ­ a c tio n AB 4- 0 —> A t BO ÿ AH ^ B (B -G ) - B (A -B ).

The h eat of atom ization (Qa) is eqüal to the sum of a l l the d is s o c ia tio n energies involved as the m olecule i s degraded

Stepwise in to sep arate atoms.

In a unimols)Gular decom position in which two ra d ic a ls or atoms are formed the a c tiv a tio n energy fo r the process w ill be very clo se i f not equal to the B.D,E. This assumes th a t the re ­

verse re a c tio n , the recom bination of the two atoms o r r a d ic a ls , has zero o r very sm all a c tiv a tio n energy. In th e determ ination

of th is a c tiv a tio n energy i t i s necessary to in h ib it any p o te n tia l ohain re a c tio n s involving th e products otherw ise the value obtained cannot be re la te d to th e B.B.E. For most exothermic ra d ie al-m olecule re a c tio n s, the a c tiv a tio n energy

is a ls o very sm all.

F in a lly it"sh o u ld be pointed out th a t the d is s o c ia tio n energy of a bond depends upon th e groups attach ed to the atoms c a rry ­ ing the broken bond. The G — C sin g le bond d is s o c ia tio n energy in ethane is 84 k .cal/m ole but in hexaphanyle thane i t is

a molecule such th a t the sum is equal to th e h eat of atom ization of th e m olecule. For th is term to be of value i t is necessary to assume the constancy of bond en erg ies from one molecule to an o th er. Thus th e bond energy, in methyl c h lo rid e may be estim ated from th e heat of atom ization of GH^Cl and a loiowledge o f E(G—H) 5 th is l a t t e r value would be tsxen as one q u arte r of th e heat of atom ization of methane.

Many d iscrep an cies in such methods have come about as a re s u lt of the u n c e rta in ty of the la te n t h e a t of sublim ation o f carbon.

This i s now f a i r l y w ell e s ta b lish e d a t 170 k .o a l/m o le (2 0 ,2 l). Such an elem entary treatmon+ h as obvious f a i l i n g s 5 n e v e rth e le ss, w ith c a re fu l u se , h eats of form ation of tiDOleoules can be c a lc u la te d to w ith in a few k ilo c a lo rie s of the observed value from a s e t of average

bond en erg ies (see fo r example, P itz e r ( 22) ) . Some of the d i f f i ­ c u ltie s of such schemes have been discussed by G o ttr e ll( 23).

More re c e n t re fin e d treatm ents of bond energy schemes have involved n e a re st neighbour c o rre c tio n s and the s ta te of h y b rid iza­ tio n of atom s(24), the use o f bond distance/bond energy r e la tio n s ( 25) , s te r ic c o rre c tio n s (26) and th e use of the valency s ta te s of the

atoms involved — the aim being to obtain an a d d itiv ity scheme. B ernstein(25) fo r in stan ce has developed a scheme allow ing the c a l­ c u la tio n of h e a ts of atom ization a t 298^K of alm ost a l l hydrocarbons

Perhaps one should m ention here a th ir d type o f bond energy r efer r ed to a s the coo rd in ate bond energy. This i s fo r the h e te -rolytio breaking o f a bond i . e . fo r th e r e a c tio n RY— —> R^ +

T his i s r e le v a n t to io n ic r e a c tio n s and may become im portant in th e gas phase as a r e s u lt o f M a cco ll’s work (se e th e s e c tio n on h a lid e d ecom p osition ). In s o lu tio n , where such a term might be o f v a lu e , so lv e n t in te r a c tio n in t e r f e r e s w ith the treatm en t.

To retu rn to bond d is s o c ia tio n e n e r g ie s , se v e r a l schemes have been proposed f o r th e c a lc u la tio n o f such q u a n titie s but th ese are o fte n e ith e r very com plex or in a ccu ra te. R ecen tly th e p u b lica tio n s o f E rrede(27, 28) g iv e a sim ple equ ation determ ined e m p ir ic a lly from published d a ta . The bond d is s o c ia tio n energy fo r a s e r ie s of 0 —^ C or C—X ""bonds i s g iv e n by D = fo r a bond

R. - R. 5 ; the _{^ J} € value of a group (A_A_A_)C- _{1 A d} i s r e la te d to th e su b s titu e n ts on th e carbon atom attach ed to th e bond in

q u estio n by

Ç = 0*43 4* 0 . 1 6 2 ( +

T his form ula h old s provided the does not have a c e n tr e of u n sa tu ra tio n a to one o f th e c en tr a l carbon atoms* The €' v a lu es

The values o alo u la te d by E rre d e 's method agree w ith o th er lite r a tu r e v alues to v /ith in + 1 - 2 k .o a l./m o le in most o ases.

The experim ental methods of in v e stig a tio n of bond d is s o c ia tio n energies v iz . therm al eq u ilib riu m , k in e tic s , e le c tro n impact and spectroscopic methods are summarized and d iscussed in th e lit e r a tu r e by C o ttre 11 (23), Seho.n and 3zwarc(29), 8zware (30), R eed(3l), Mortimer (32), S teacie(33),K ondratev(34) and Trotman-Dic kens on(3 5)* E lec tro n impact d ata are a v a ila b le from F ield and F r a n k lin ( ll) ,

Spectroscopic d ata of diatom ic m olecules are d iscussed in d e ta il by H erzberg(36) and by Gaydon(3?)* The appjicationU of spectroscopy

to polyatomic m olecules i s somewhat lim ite d . The d if f i c u lt ie s l i e in a knowledge of the p re c ise nature and degree of e x c ita tio n of th e fragm ents. However d a ta on th e sim pler polyatom ics has .been of use in supplementing r e s u lts from the o th e r methods.

Any fa c to r s which in flu e n c e th e heat o f form ation o f the r a d i­ c a ls or the parent m olecule w i l l be r e fle c te d in th e r ele v a n t bond d is s o c ia t io n energy. Changes in r a d ic a l resonance energy and the e f f e c t of changing io n ic ch a ra cter of the r ele v a n t bonds were sug­ g ested as main c o n tr ib u tin g fa c to r s by Baughan e t a l . (3 8 ). Szwaro(39) has d iscu ssed h is r e s u lt s on h alogenated m ethyl bromides in terms

o f th e s t e r ic rep u lsio n s in crea sin g as the atomic s iz e in c r e a se s. The C Br bond len g th s in the s e rie s Br ^ OX^— Br are about c o n sta n t and so the d ecrease in d is s o c ia tio n energy alo n g the

The resonance s ta b iliz a tio n of th e ra d ic a ls formed w ill increase w ith in c re asin g number of lialogen atoms, w ith decreasing sep aratio n

of the p o r b ita ls and w ith d ecreasin g e lee t r one gat iv&t y of the fqalogen atom s.

I t has been suggested th a t the sta b iù z a tio n of a ra d ic a l R- be measured by th e d iffe re n c e D(CH^— -H) — D(R—H). Then such r a d i­ c a ls as C 01^-,for example^ would have about 12 K .cal/m ole of to ta l

s ta b iliz a tio n .

Skinner(40) has discussed the c a lc u la tio n of Q^(R-) and then compares c a lc u la te d and experim ental values of D(R—X).

Id e a lly , i t is d e sira b le th a t d if fe re n t determ inations of d is ­ s o c ia tio n energies should a l l y ie ld ^ " c o n s iste n t h e a ts of forma­ tio n of r a d ic a ls .

2. Toluene as a ra d ic a l a c c e p to r.

Toluene, along w ith many other r a d ic a l a ccep to rs such a s propylene, n it r ic oxide and cy clo h exen e, has found numerous a p p lic a tio n s as a r a d ic a l scavenger in k in e tic s tu d ie s . I t r e a c ts very e f f i c i e n t l y w ith r a d ic a ls , p r ev en tin g th e development o f c h a in s, to produce the r e la t iv e ly sta b le benzyl r a d ic a l (PhGHg-) and i t has been used in th e d eterm in ation o f numerous bond str e n g th s, many of th e se by the tolu en e c a r r ie r techn iqu e (50, 55» 41) ^

energy of a c tiv a tio n fo r th e decom position of the ra d ic a l formed must also be g re a te r th an th a t f o r any re a c tio n involving to lu en e. At tem peratures where toluene decomposes i t is d e s ira b le th a t th e products of the main decom position d if f e r from those of toluene

decom position. (This was not th e case in th e work described in th is th e s is and was one of th e e a rly d if f i c u lt ie s which had to be- overcome by a change in technique# )

The method has proved u se fu l in h alid e stu d ie s e sp e c ia lly bromides where HBpi* is formed as an end product. Szwaro and Ghosh(42) have dem onstrated how to d istin g u ish between ra d ic a l decom position and HBr e lim in a tio n to y ie ld u n sa tu iated compounds. In r a d ica l z ea ctio n s fo r e v e iy RBr decomposing, one each o f HBr, RH and dibenzyl are formed w hilst dibenzyl does not appear in the elim in a tio n re a c tio n .

The bond d is s o c ia tio n energy of to lu en e, D(Ph.CH2 — H) has been the su b ject of rmch in v e s tig a tio n . Several p y ro ly tic stu d ie s

of toluene y ie ld vary in g v a lu e s. Szwaro’s value (43) of 77*5 k c a ^ o l had tended to become accepted but was c ritic is e d , by S teaoie e t a l (44) who found dim ethyldiphenyls in the products and th a t v a ria tio n s in co n tact tim e, pressure and surface/volum e r a tio a ffe c te d the r a te c o n sta n t. More re c e n tly Takahasi (45) found s im ila r v a ria tio n s and P rice (46) in a thorough a n a ly sis obtained a f i r s t order ra te constant f o r the decom position;

11

Anderson e t a l (47) usin g a therm al and photoohemical hrom ination of toluene obtained 89#5k*cal/rao3e a t room tem perature as an upper lim it, Benson and Buss(48), who review the v ario u s attem pts to determ ine B(PhCH2— H), tr ie d to measure fo r

Ph.GH^ t Brg PhGHg-Br + HBr a t 150% and obtained a value D(PhCÏÏ2— H) « 84 k.o al/m o le.

So h iss 1er and Stevens on (49) using an electron impact method +

showed th a t th e appearance p o te n tia ls o f G^Hy from toluene and from dibenzyl led to a value of 77,0 fo r B(PhOH2— H), Trotraan- Dickenson e t a l,(5 0 ) v/arned of the u n c e rta in ty of e le c tro n impact

d ata because the benzyl p o sitiv e ion iso m erises(51>52) and they obtained values of 83.3 and 84,6 k.cal/m ole from pyrolyses o f e th y l benzene

and n-propylbenzene re sp e c tiv e ly and p referre d the l a t t e r value. They a ls o pointed out th a t Anderson e t a l , (47) used a value of zero fo r the a c tiv a tio n energy* of a benzyl ra d ic a l a tta c k in g a bromine mole­ cu le whereas in f a c t i t i s more iiloely to be about 5 k.oal/m oli,,

which would give a value of D(Ph G Eg— h)‘. = 85 k.oal/m ole* A value of 86,5 oan be deduced ( 50) from the work of Bus f ie ld and lv in ( 53 ) although th is may c a rry a la rg e e rro r.

B rickstook and P o p le (8 l) o a lo u la te d th e resonance energy o f benzyl a t 2 1 .9 k .c a l s / mole and compared i t w ith experim en tal v a lu es of

23,5

17,0

Smith (8) in an extensive exam ination of the decom position obtained a r a te co n stan t given by log^Q k(sec"^) « I 5. I - 8470o/2,3RT over the tem perature range 750 to 88OOG but does not f e e l ju s tif ie d in a ssig n in g th is a c tiv a tio n energy to th e bond d is s o c ia tio n process. Rhind(56) in an exam ination of th e p y ro ly is of ethylbenzene u sin g

toluene as a r a d ic a l accep to r was ab le to a ssig n a value of 84k/)a]s/ mole to th e ra te determ ining ste p f o r th e breakdown of to lu en e.

A resuné of se v e ra l determ inations i s shown in th e ta b le and i t w ill be seen th a t a value of about 84 k.oals/m ole f o r ^(PhOHg— R) would appear to be w ell e s ta b lish e d ,

D(PhCHo-H) Temp, range Method

k .c a l./m o le . % _______ u s e d _____________ R eference

77.0 - Appearance p o te n tia l ₄₉

77.5 680 - 850 Toluene c a r r ie r ₄₃

84.0 150 E quilibrium 48

84,0 476 " 785 F I0W& s ta tic 56

84,6 603 - 727 AHf(B2^ ) from Ph.Et 50

84,7 750 - 880 Flow system 8

85.0 640 - 870 Flow system 46

89.5 ( 82 - 132

13

In gas phase photd^tio stu d ie s of toluene and deuterotoluenes a t 60% ; C her(57) oonoludes th a t the r a te of a b s tra c tio n by methyl ra d ic a ls o f hydrogen atoms from the rin g i s 0,17 tim es the ra te of a b s tra c tio n from the sid e ch a in . He suggested th a t sid e ch ain and rin g a tta c k s proceeded through geom etrically d if fe re n t tr a n s itio n s ta te s . A b stractio n from the rin g re su lte d in a to l y l ra d ic a l which reac ted w ith toluene to foim a benzyl ra d ic a l, which in tu rn reacted w ith a methyl r a d ic a l to produce the main product ethylbenzene,

w hile the a b s tra c tio n from the side chain proceeded v ia a 3 -cen tre tr a n s itio n complex o f th e type suggested by Johnston and P a r r (58).

Burkley and R ebbert(59)> in experim ents aimed a t determ ining the rate of ÏÏ a b s tr a c tio n by m ethyl from prim aiy, secondaiy and t e r t ia r y p o s itio n s alpha to th e arom atic r in g , deduced the Ea v a lu es fo r such r e a c tio n s . T heir experim ents in vo lved the gas phase pho­ t o l y s i s o f ace to n e -to lu e n e m ixtures and th ey claim ed th a t m ethyl r a d ic a l a b s tr a c tio n from the r in g was of minor im portance.

B erezin e t a l , (60) have a lso studied th is a b s tra c tio n re a c tio n a t 6 0 9 6 % , u sin g t r i t i a t e d to lu en e. They showed th a t, a t 85°0, the ra te of a b s tra c tio n of tr itiu m from th e sid e ch ain is 156 tim es th e ra te of a b s tra c tio n of the para-hydrogen in the rin g . They

c a rb o n -tritiu m bond in th e methyl group was ra p id and accounted fo r

io of th e t o t a l methane.

In th e p y ro ly sis of toluene a t 750^0 using hydrogen as a c a r r ie r gas, WJeyer and B u rr(6 l) claim ed much sim p lified k in e tic s w ith only methane and benzene a s products in approxim ately equal amounts. To ex p lain th e ir r e s u lts th a t rin g a b s tra c tio n is the major e ffe c t they had to assume th a t one primary process was a s p l i t of tolueme in to Ph and follow ed by a b s tra c tio n from c a r r ie r hydrogen o r to lu e n e. They concluded t i n t both Ph** and Me“ a b stra c te d from th e rin g and not th e sid ech ain under t h e i r c o n d itio n s, Benson and Buss(48) had pointed out th a t entropy c o n sid e ra tio n s favour PhOH^ —>Ph-*K)H^- over PhGHj •^►PhGHg** + d esp ite the la rg e r a c tiv a tio n energy of th e former#

3. R adical re a c tio n s .

R eactions of ra d ic a ls a b s tra c tin g a hydrogen atom from

toluene proceed w ith a c tiv a tio n en erg ies of about 5 to 10 k .o als/m o le. The p a r tic u la r re a c tio n s re le v a n t to th e p resen t work are f o r methyl ra d ic a ls and fo r bromine atoms a tta c k in g toluene#

15

makes i t only of value in the tem perature range lOO—

A gome thane and a o e ty l peroxide have a ls o been used as therm al sources# The o th e r main source of methyl ra d ic a ls is p h o to ly sis of compounds which c o n tain a methyl group, fo r example, acetone, dim ethyl mercury, azomethane and sim ila r compounds.

Bata g e n e ra lly leased on th e ra te s of th e com petitive re a c tio n s :

2CH,- --- > Cg Hg (1)

CHj- + RH --- ^2 CH^ + R- (2) 1/2

One can deduce k^/k^ ' and hence determ ine

The value of k^ appears to he w ell e sta b lish e d as a r e s u lt of work by Gomer and Kistiakow8ky(62 ) u sin g the com bim tion ra te of

methyl ra d ic a ls from a r o ta tin g se c to r method of in te rm itte n t illu m in a tio n on acetone and dim ethyl mercuiy. A value fo r k^ of 10^^*^ exp (-C^700/RT) mol ^ oo# sec. ^ was deduced which was in agreement w ith the A f a c to r expected from c o llis io n theory, w ith a p ro b a b ility fa c to r o f u n ity and zero energy b a r r ie r . The

ra te co n stan t was however p ressu re dependent, f a llin g o ff w ith d ecreasing p ressu re. This is expected since under co n d itio n s

of eq u ilib riu m th e forward and rever&e r a te s of re a c tio n ( l) above must both vary to th e same ex te n t w ith pressure and k ^ i s a_^ J-unim olecular decom position which would e x h ib it such behaviour.

M iller and S teacie(8 3 ) suggest th a t a value of 1-2 k.oal/m ole fo r E is p o ssib le f o r the m ethyl ra d ic a l recom bination process. Some

second order processes fo r m ethyl a tta c k in g various su b stra te s

are quoted by F ro st and P earson(63). A value of Î of to 10~^ is req u ired fo r c o llis io n th e o ry in te rp re ta tio n . The a c tiv a tio n energieè deduced are about 10 k .ca ls/m o le.

Another method of fin d in g k^ involves the use of acetone-d^. The GDg ra d ic a ls form methane by:

+ CD^.C O.GB^— ^ GD^ + GB-.GO.OBg-and

k

GD,- + R H --- ^ G D % H +

R-3 3

I t follow s th a t kJC D , . CO . C D j

,— i " -_ I 2

^CDjH ÎÇ Tm ]

Experim ental c o n d itio n s are such as to keep|cD^.GO.ODj/RÏ^ e f f e c tiv e ly co n stan t and the r a te r a tio i s determ ined mass sp e ctro - me tr io a lly to give th e k^/k^ r a tio . Then v alues of k^ and k^/kg may be found by se p arate experim ents whence kg is a v a ila b le .

17

Data on CH^- + PhCH^— ^ GH^ +

Ph.GHg-k. (oo mol ^ see ^

6 . 8 + o .e )x 10^ ^ (2.3 + 0*4) X 10^ °

E ,(k .o a l/ mole) 1 2 + 2

13.03 + O..27 a

8.5 5.6 7.5

7 .4 + 0.3

Reference 64 65 57 57 66 67 68 59

a Average of toluene a tta c k by se v eral CH^ sources»

b Side ch ain a b s tra c tio n of H-o Ring a b s tra c tiH-o n H-of

ÏÏ-Some of the fe a tu re s of methyl ra d ic a l a tta c k on toluene have been discussed in th e proceeding se c tio n . The ra te of a b s tra c tio n

of H- from th e side ch ain appears to predominate over rin g a b s tra c tio n in th e gas phase (59)- Some evidence (69, 70) has been proposed fo r rin g a b s tra c tio n based on experim ents w ith toluene a-d^ but these are in s o lu tio n and probably the rin g

a b s tra c tio n occurs a f t e r rin g a d d itio n since methyl ra d ic a ls are known to add to the rin g in the liq u id phase (70).

energies-C o n sid eratio n of th e re a c tio n s kc

Br- + RH ---^— > HBr + R- (5) suggests th a t a value of d(r—H) may he deduced from the r e la tio n

D(R-H) = AH. + D(H—Br)

« (E^ - B_ ) + B(H - Br ) .

Since the h eats of form ation of H, Br " and HBr are w ell e s ta b lis h e d we have an accu rate knowledge of B(H - B r), th u s allow ing the d e te r­ m ination of B(R-H), once and E ^ a re knovm. T rot man-Bic kens on e t a l . (71) l i s t a c tiv a tio n en erg ies fo r the forw ard re a c tio n in ( 5) f o r se v e ra l hydrocarbons. These values were as expected. The value fo r however is le ss w ell known; T rot man-Bio kens on has estim ated a value o f th is q u a n tity , basing h is c a lc u la tio n s on th e Polanyi

r e la tio n E^ = a AH + C, but he p o in ts out th a t the estim ated value may be dubious and he emphasized the need f o r more experim ental

evidence on allcyl r a d ic a l re a c tio n s w ith liydrogen bromide. Since the method of study was one of com petitive r a te s , th e r e s u lts of T rot man-Bio kens on were re la te d to the ra te of brom ination of methyl bromide. T h eir r e s u lts on th e brom ination of methane were not su f­ f ic ie n tly accu rate to allow the methane re a c tio n to be used as a standard of re fe re n c e .

19

f o r th e photochem ical and therm al brom ination of to lu en e. In fra red analyses showed th a t the re a c tio n y ield ed m ainly benzyl bromide and hydrogen brom ide. The photochem ical re a c tio n was stu d ied in th e range 8 2 —>150^0 and the therm al re a c tio n a t 166% . An a c t i ­ v a tio n energy of 7.2 k .cal/m o le was assigned to

Br. + Ph.CH- Ph.CHg- + HBr

The E fo r the re v e rse of th is re a c tio n was estim ated a t _a

5 k .cals/m o le based on th e tem perature dependerce of HBr in h ib itio n in th e photochem ical re a c tio n .

B ata on th e r e la tiv e e ffic ie n c ie s of Br^, argon and COg as th ir d bodies in th e bromine atom recom bim tion re a c tio n was given by Givens and W illa rd (75 ) . The ta b le below gives some re le v a n t d ata on bromine atom re a c tio n s .

E

R eaction (k .calV m o le) Br- + Pl£!H,->HBr +

Ph.CHg-HBr + Ph.OHg— »PhCH- + Br-Br- + CH. —ÿOH_- + HHr ₄

- 3

B r- + CH. —>CH,- + HBr _{4 5} OH,- + HBr-^CH. + Br- _{3 4}

Br- + CH,Br -»OHgBr- + HBr

7.2 + 0.6

>

+

17.8 + 0,4 18.3

about 2

15.6

1.0

Temp range (%_)

80 - -5>130

80 -# 1 3 0 150 -*230

100

150-4230 150 -^2 3 0

E eferenoe

47 47 72, 75

71 72, 75

75

I t should be pointed out th a t the assignm ent of 5 k .cals/m o le a c tiv a tio n energy to the re a c tio n :

by Anderson e t a l , (47) leads to a high value of 89#5 fo r the G - H bond d is s o c ia tio n energy in to lu e n e. A value higher than 5 k.cal/m ole would b rin g the bond stre n g th more in lin e w ith lite r a tu r e v alu es.

The r e s u lts of Van A rtsd alen and c o lla b o ra to rs on brom ination ra te s of hydrocarbons have been suspect because they quote ra th e r high A fa c to rs ; fo r example, Benson and Buss(76) suggested th a t

th e ir assum ptions of a rap id attainm ent of steady s ta te co n cen tratio n s of bromine atoms were in v a lid f o r the more re a c tiv e of th e compounds used. They a lso suggested th a t the re a c tio n s were p a r tly heteæogenous

but th ere was not evidence f o r t h i s .

S everal r e la tio n s have been proposed to c o rre la te a c tiv a tio n en erg ies of processes of the above types. Probably the b est knovm is H irsch feld er* s ru le (77) f o r a bim oleoular process involving atoms or radicals^w hich s ta te s that,w hen w ritte n in the exothermic

d ire c tio n , the a c tiv a tio n energy fo r the re a c tio n is about 5*5/^ of th e energy of the bond being broken. For th e endothermie d ire c tio n the becomes plus the AE of the re a c tio n . The value 5*5 was found by sem iem pirical c a lc u la tio n .

21

TrotmarL-I)ickenson(78) showed th a t good agreement can be expected between th e observed a c tiv a tio n en erg ies f o r hydrogen

a b s tra c tio n by m ethyl from the lower members of th e p a ra ffin s e rie s and th e deduced from P o la n y i's r e la tio n , E^ « 0 .4 9 [B(0—H) — 74,5]

E y rin g 's r e la tio n , E « aAH + C, where a and 0 a re c o n sta n ts fo r a s e rie s of re a c tio n s , was shown by B u tler and Polanyi to hold fo r

sodium atoms re a c tin g w ith a lk y l h a lid e s . Szabo(79) quoted a

r e la tio n E " ^ D ^ tro k e n ) " “ "j®ô(fornBd) homogeneous gas re a c tio n s which took in to account the stre n g th s of bonds broken and formed where a is ag ain co n sta n t f o r a given type o f re a c tio n .

4 , H alide pyrolyses w ith p a r tic u la r referen ce to bromides

On p y ro ly sis, monochloro and monobromoalkanes y ie ld halogen acid and an o le fin . In the case of the chloddes a ra d ic a l ch ain process is absent and th e mechanism involves th e unim olecular e l i ­ m ination of hydrogen c h lo rid e . The bromides can d isp lay th ree re a c tio n mechanisms : ( l) a ra d ic a l chain p ro cess, (2) a ra d ic a l nonchain process and (3) unim olecular e lim in a tio n of hydrogen bromide. P y ro ly sis o f th e io d id es produces io d in e , an o le fin and the corresponding p a ra ffin — th e mechanisms in these cases are

le s s w ell e s ta b lish e d , although i t has been shown th a t unim olecular elim in a tio n of hydrogen iodide and iodine c a ta ly se d decom position are both f e a s ib le .

compounds u sin g the toluene c a r r ie r tech n iq u e, Benson a lso has rep o rted on h a lid e p y ro ly ses; h is work appears to he p a r tic u la r ly ap p lied to io d id es ( I I 5 ) , Szabo(4l) gives a b r ie f survey of such work (see a ls o (8 5 )), Some re le v a n t d ata on bromides is ta b u la te d belov;. A d d itio n al referen ce s are quoted in those c ite d .

In most cases v e sse l co n d itio n in g was n ecessary to o b tain rep ro d u cib le d ata and to avoid ch ain decom position a t the w alls.

To o b ta in inform ation on the ra d ic a l ch ain present in the p y ro ly sis of e th y l bromide, p h o to ly sis experim ents were performed

in the tem perature range 150-300° and a r a te equation

tJL

— d [E tB r]/d t = C onst. Iq [EtBr] was deduced ( I 04). The quantum y ie ld was high and th e f i r s t o rd er ra te co n stan t gave an a c tiv a tio n energy of 10.5 k .c a ls/m o le . At low p ressu res a second order re a c tio n became im portant. Blades e t a l .( l 0 5 ) , in an isotope e f f e c t in v e s ti­ g atio n , dem onstrated th a t the in h ib ite d decom position of e th y l

bromide was p rim arily a m olecular process and th a t th e r a t e c o n tro llin g ste p involved a C-H bond f is s io n .

Kale and M aocoll(l06) provided fu rth e r evidence f o r a unim olecular decom position Qf isopropyl bromide. The p y ro ly sis was c a rrie d out

a t low p ressu res (0.5 to 48 mm Hg. ) to v e rify th e Lindemann th eo ry . They found th e ir r a te co n stan ts gave a b e tte r f i t to the R ice- Ramspeiger theory th an to the Lindemann-Hinshelwocd theory.

c ompound î'echanism Temp(^C )

CgE^Br

n-C ^HyBr

Chain mechanism and u n i­ m olecular decom position

Homogeneous 1 st order i f in h ib ite d

Unim olecular elim in atio n 1.5 o rd er, homogeneous 1.5 order

510 -> 476

523 - 633 380 -» 430

467 -» 667 300 ->■ 380 350 -> 5 0 0

i-C 1st order

n-G^H^Br

Homogeneous

U nim olecular e lim in a tio n 1 st ord er up to 50/ dec.

i f îiax. inhib i t : 0n 1 st

510 ^ 550 570 4 :0

500 5!

te rt-U .y d_{4 •}

. -r 15;-û cnain

.‘US, 1st order 230 ^ 28C

iso-C y,H

tert-C c-H- -, dr ₅

U nim olecular elim in a tio n Homogeneous, 1st order if

max. in h ib itio n

Homogeneous, 1st o rder Unim olecular elim in a tio n

450

360 'K'd

ZZO >70

6 X 1 0 ^ ^ e x p ( - 4 6 . 4 / R T ) s e c ^

8 . 5 X 1 0 e x p ( - 5 2 . 2 / E G ? ) s e o “ ^

2 . 8 X 1 0 ^ 5 e x p ( - 5 3 . 9 / E T ) s e o “ ^

8 . 0 X 1 0 ^ ^ e x p ( 5 3 . 7 / E T ) s e o " ^

-2 . -2 9 X 1 0 ^ e x p ( - 3 3 . 8 / e T >

3 . 8 X 1 0 ^ e x p ( - 4 2 . 0 / E T ) s e c y

mm“ / 2

1 . 0 X 1 0 ^ ^ e x p ( - 5 0 . 7 / E T ) s e c " ^

4.17 X 10^^ exp (-47.8/E T )sec-1

4 .0 X 1 0 ^ 5 exp (-47.7/E T )seo"^

4.2 X 10^^ exp (-47.8/ET)seo~^

1.5 X 10^^ exp (-50.9/E T )seo"^

4.27 X 1 0 ^ exp (-43.8/E T )seo"^

1.1 X 10^3 exp (~45.5/ET)seo"^ 1.51 X 10^^ exp (-46.5/E T )seo” ^ 1.0 X 1 0 ^ exp (-42.0/E T )sec“ ^

3.2 X 10^^ exp (-41.5/E T )seo“ ^ 1.12 X 10^^ exp ( - 50. 4/R T)seo"^

Surface re a c tio n p resen t 86, 87 Isotope e ffe c t in v e stig a tio n 88

89

Shook tube method Retarded by propylene

E arly determ ination in presence of toluene V essel co n d itio n in g

necessary Toluene c a r r ie r Shook tube

In h ib itio n by o le fin io substance

No in h ib itio n by chain* breakers

No chain w ith cyolohexene

103 90 91 92 94,95 92 103 95 96 97 98 No in h ib itio n by chainbreakers; 99 4 centre tr a n s itio n s ta te

Shook tube method 100

101

3.98 X 10 exp (-40.5/H T)seo No in h ib itio n or a c c e le ra tio n 102 by cyolohexene e tc .

24

and corresponding a c tiv a tio n energies (ifeccoll and Thomas( 10?) f o r a s e rie s of compounds are given in th e ta b le s below.

n C-Br

V —XI

Prim ary Secondary T e rtia iy

Prim ary 1 170 32,000

Sec ondary _5-5 380 46,000

T e rtia ry 6.3 - 130,000

R ates are r e la tiv e to EtBr taken as u ti% a t 380%

C-H 0-B r

Primary Sec ondary T e rtia ry

Prim ary _55.9 47.8 42.2

Sec ondary 50.7 45.8 40.5

T e rtia ry 50.4 — 39.0

A c tiv a tio n en erg ies f o r HBr e lim in a tio n

There d id not appear to be a simple r e la tio n between the

EtBr Sec-C,E,Br

- ...2 7 .

T ert BuBr

% 2 r 55,9 47.8 42.2

D(R-Br) 67.2 67.6 65,8

D(E+Br~) 183*7 156.3 140.5

Rate r a tio 1 170 32,000

However, Maoooll and Thomas claimed a c le a r c o r r e la tio n between the a c tiv a tio n energy, f o r HBr elim ina.tion and the h e te ro ly tic

d is s o c ia tio n energy, D(R^Br~), fo r the process HBr — + Br" ,

a l l sp ecies being in the gas phase. The h e te ro ly tic bond d is s o c ia tio n en erg ies were c a lc u la te d from appearance p o te n tia l, io n iz a tio n

p o te n tia l and homolytic bond d is s o c ia tio n energy d a ta . Two u se fu l r e la tio n s which fo llo w from the gas phase ic n iz a tio n e n e rg e tic s diagram below a re ;

(a) D(R+X~) " A(E+) — I (X")

(b) D(E+X") = I(E- ) + D (R-X) — I (X“)

V

/iS. D(R-X)

4-T

l( x " )

E’^ +

X-E"^ + X'

A(R-^) I(E-) E(eV )

Jt

X-EX

26

Suoh c a lc u la tio n s led to h e a ts of h e te ro ly tic d is s o c ia tio n of around 200 k .c a ls/m o le , g re a tly in exceeds o f any observed a c tiv a tio n energy. la o c o ll(8 5 ) suggested th a t a value of about 150 k .cals/m o le may be su b stra cte d as th e oo*ulo.mbio energy of th e io n -p a ir tr a n s itio n s ta te , thus g iving a value s im ila r to th e observed a c tiv a tio n energies,

The io n -p a ir tr a n s itio n s ta te was f i r s t proposed by In g o ld (108), who suggested th e sequence

slow f a s t

>0 — ---) — 0 + / --- ) > 0 = o (

/ ] , S / I \ ^

H Br a Br- H - Br

fo r a unim olecular gas re a c tio n . This i s the same as the E l mechanism in p o la r so lv en ts except th a t the ions sta y as a p a ir.

The h e te ro ly tic tr a n s itio n s ta te , \ Q Q / 1

/ \ j Br*^- , was proposed in preference to th e homo ly tic fo u r

(

/ r * t

c e n tre tr a n s itio n s ta te (IO9 ), è* * - Br ® ‘^Lioh would involve the halogen atom, a p-hydrogen atom and the two carbon atoms to which th ese are bonded.

The p-hydrogen atom was suggested to have a ro le sim ila r to the p o la r so lv en t in th e 8^1 and El mechanisms in s o lu tio n re a c tio n s — th e ro le of the so lv en t being to s ta b iliz e th e p o la r tr a n s itio n s ta te .

In agreement w ith Ifeo o o il's c o rre la tio n between D(R'^X“ ) and

the hydrogen h alid e elim in a tio n a c tiv a tio n energy, Lossing e t a l.( llO ) have shown th a t, f o r a s e rie s of conjugated m olecules, a lin e a r

r e la tio n e x is ts betvæen D(RtX~) and the charge, c a lc u la te d by mole­ c u la r o r b ita l th eo ry , on the carbon atom form ally c a rry in g the p o s itiv e charge» M accoll has compared the gas phase e lim in a tio n ra te s w ith

^y l and E l ra te s in so lu tio n . He dem onstrated th a t th e se gas phase and s o lu tio n c o rre la tio n s could be extended to ire lu d e not only the very sim ple members but a lso a-m ethyl, a-ohloro and p-methyl sub­

s titu te d aliqrl h a lid e s and suggested th a t the id eas could be extended to o th er gas phase e lim in a tio n s. For example, o le fin form ing e l i ­ m ination from e s te r s bears a resemblance to E2 re a c tio n in so lu tio n , and hydrogen h alid e c a ta ly s is of the dehydration of alco h o ls may be the gas phase analogue of a c id c a ta ly s is in so lu tio n ,

A c o n d itio n f o r a re a c tio n to be ’q u a s i-h e te r o ly tic * is the presence of a p o la r group, S iixe suoh groups d i f f e r in t h e i r degree

of p o la rity the p o s s ib ility e x is ts of a g rad atio n in gas phase mechanisms from c om pletely h e te ro ly tic to com pletely homo ly ti c .

Herndo.n e t a l . ( l l l ) , a s a r e s u lt of p y ro ly tic work on secondaiy c h lo rid e s which d iffe re d g re a tly in th e ir so lv o ly tic r e a c tiv ity ,

suggested th a t the 'q u a s i-h e te ro ly tic ' mechanism was n o t c o rre c t in d e t a il but they then pointed out th a t the discovered p a r a lle l r e a c tiv itie s to so lv o ly tic re a c tio n s became d i f f i c u l t to ex p la in .

28.

is not a system in whioh the O -I bond s c is s io n i s ra te determ ining and veiy few in which i t is im portant. Organic iodide pyrolyses have been discussed by Benson (112, 115, 114); fo r a lk y l iodides two ra te lim itin g mechanisms a re operativ e :

(1) E l JH I + o le fin

(2) I + E l — > I + HI + o le fin

These are follow ed ty rapid re a c tio n between the a lk y l iodide and hydrogen iodide which m aintains the l a t t e r a t a low sta tio n a ry state# Mechanism (1) d escrib es the pyrolyses of iïîp l, E tI,

t-B u I, OH^COI, w hile (2) is th e mechanism fo r n -P rI, i-B uI, n-BuI . Sec-Bui and 1:2 diiodoethane involve both ( l ) and (2).

In i- P r I and n -P rI, alth o u g h both pyrolyse to y ie ld C^H^, C^Hg and I^ , th ey serve to i l l u s t r a t e the two general ra te law

s;--

a f

/at

- a

/at = k^

(b)

I t was found th a t allcyl iodides co n tain in g a prim ary iodine atom follow ed ra te law (b) while those w ith secondary or t e r t i a r y iodine atoms follow ed r a te law (a ).

The spontaneous e lim in a tio n o f HI by the iodides is an example of a genuine 4-oe n tre m olecular re a c tio n (compare Ife c c o il's theory*) This elim in a tio n i s most rap id when the I atom is attach ed to a

w ith an a c tiv a tio n energy equal to the endotherm iaity of the re a c tio n . Benson ( I I 5 ) suggested th a t the concerted e lim in a tio n re a c tio n

\ **_I

/

That the 4 ’-cen tre e lim in a tio n re a c tio n is f a s t e r fo r secondary and te r ti a r y io d id es, w hile the I atom a s s is te d e lim in a tio n is

f a s te r f o r prim ary io d id es lie s in the su p p o sitio n of a tr a n s itio n s ta te of the type s

^ 0 I J — H

/

H

With prim ary io d id e s, I a tta c k s the r e la tiv e ly weakly hound secondary or t e r t i a r y H atom, w hile f o r the secondary and t e r t i a r y iodides the I a tta c k would he on tte more stro n g ly hound p rin a iy hydrogen atom. For sec-B ui, a tta c k can occur on a secondary o r prim ary H atom and thus both processes compete.

Holmes and IVhcooll ( I I 6 ) have examined th e pyrolyses of isopropyl and 8- b u ty l io d id e s. These au th o rs, lik e Benson,found the isopropyl

iodide decom position to be a f i r s t o rd e r p ro cess, w ith an I 2 c a ta ly se d decom position of importance a t the lower tem peratures» The f i r s t

39

The therm al deoom positions of monoohloro^alkanes occur hy the unim oleoular e lim in a tio n of hydrogen c h lo rid e . For example,

1 -chloropropane shows a homogeneous f ir s b o rd er decom position to

propylene and B ]l(l0 9 ), This ’r u l e ', however, should not he extended to m u ltich lo ro alk an es; 1 .1 .2 -tric h lo ro e th a n e , fo r example, decomposes a t about 4^0^ to y ie ld vin y lid en e c h lo rid e , c is and tra n s d ic h lo ro -

ethylene and HÛ1; decom position in th e presence o f toluene has shown (11?) th a t a ra d ic a l chain and a unim olecular mechanism operate sim ultaneously

lane e t a l . ( l l 8 ) have pointed out th a t in the s e rie s CH^X, CgH^X, (CH^)g CÎÎX, (CH^)^ GX the d is s o c ia tio n energy decreases

re g u la rly i f X « ÏÏ, but i t i s a p p ro x im te ly co n stan t fo r th e f i r s t th ree members, i f X = 01 or Br, although lower f o r th e t e r t i a r y b u ty l h a lid e s . They have in te rp re te d t h i s constant d is s o c ia tio n energy as a balance between in c re a sin g s ta b ili ty of the lialid es along th e s e rie s and in c re a sin g s ta b iliz a tio n of the ra d ic a ls produced along the s e r ie s . The low v alues fo r (OH^)^X a re re a d ily understood since s te r ic e ffe c ts w ill lower th e s t a b i l i t y of these h a lid e s .

The C-F bond in OH^F i s estim ated to be about as stro n g as the C-H bonds in metliane. S everal bond d is s o c ia tio n energies of flu o ro - oarbons a re quoted by E rrede(28) and th e C-H bond stre n g th s in t r i ­

f lu or ome thane, pentafluoroethane and heptafluoropropane have been determ ined by P ritc h a rd and Thommarson(ll9)« The value of D(CF^-H)

Previous d eterm inations of D(CH^-Br)

There have been se v e ra l determ inations both d ir e c t and in d ire c t of th e bond d is s o c ia tio n energy in methyl bromide and r e ­ la te d h a lid e s . The value g e n e ra lly accepted f o r D (OH^-Br) is 67*5 k.cal/m ole determ ined by Sehon and Szv7arc(39)* In a to lu e n e c a r r ie r p y ro ly tic study on halogenated bromomethanes they found th e ir

ra te co n stan t dependent upon toluene pressure and so analysed th e ir d ata fo r tie se compounds u sin g the A rrhenius expression

k = A exp. (~E/RT), w ith a fix ed value of 2 x 1-0^^ sec~^ f o r A. This value of A was determ ined from work on the p y ro ly sis of methyl bromide tak in g B(CH^-Br) as 67.5 k .c a ls/m o le , th is l a t t e r value being com­ puted from w ell e s ta b lish e d thermoc hemic a l d a ta . The value of k used was th a t corresponding to the lov/est tem peratures w ith toluene p ressu res of about 10mm Hg.

The decom position of methyl bromide a t th re e d iffe re n t toluene p ressu res (5^ 11 and 20 mm Hg. ) gave th re e p a r a lle l s tr a ig h t lin e s

13 X

w ith frequency fa c to rs near to 10 sec and an a c tiv a tio n energy of 67 + 2 k .c a l/m o le . They considered th e agreement between th is and the value c a lc u la te d from the thermochemioal d a ta to be fo rtu ito u s . They stu d ied both chloro-and brom o-substituted methyl bromides and they considered th a t the deoom positions s ta rte d y/ith th e unim oleoular step HBr r> R - + Br- . Since both ra d ic a ls were then removed by

32

process was measured by the ra te of form ation of HBr. The pyrolyses were rep o rted to be e s s e n tia lly f i r s t order homogeneous processes and the ra te co n stan ts were found to be independent of the co n tact tim es. In only GCl^Br and GBr^ were th e ra te c o n stan ts unaffected by v a ria tio n of toluene p re ssu re . Since t h i s was the case th e tem perature independent fa c to r fo r CGl^Br decom position t e s d e te

r-13 -1

mined experim entally and was found to be 5 x 10 sec , in good agreement w ith the methyl bromide case. T his clo se agreement was Szwaro and Sehon's ju s t if i c a tio n f o r a co n stan t frequency fa c to r throughout the s e r ie s .

The g rad atio n in the s e rie s of G-Br bond d is s o c ia tio n en erg ies determ ined p y ro ly tic a lly showed good agreement w ith th e values

deduced from sodium flame re a c tio n s . Evans and P o lan y i(l2 9 ) re la te d th e v a ria tio n of tlie a c tiv a tio n energy fo r th e re a c tio n R-Br + Na — )• R- + Na Br in a s e rie s of kindred compounds to the v a ria tio n in bond d is s o c ia tio n en erg ies by the form ula AE « a AD. The p ro p o rtio n a lity c o n sta n t, a , was tak en t o be 0.27, md by

C ompound B(C-Br) computed from Na flame from p y ro ly sisD(C-Br) Other worker^

CH,Br₃ (6 7 .5 ) (67.5) See la te r

CHgC IBr 61.2 61.0

CBDlgBr _{54.5 55.5}

CG l_Br₃ 50.8 49.0 51, 0. 4^52.0 55.5

OHgBrg 58.6 62.5

CHBr^ 50.1 55.5

OBr^ 49 .0 4 5 0 .0

Data of various workers quoted by Sehon and Szwaro (59)*

The low v a lu e s, deduced from Na flame d a ta , f o r the poly bromo­ me thanes were explained by th e f a c t th a t the Evans-Polanyi treatm ent d id not allov/ fo r the a d d itio n a l resonance s ta b liz a tio n of the

tr a n s itio n s ta te R B r N a f o r molecules co n tain in g more than one id e n tic a l re a c tiv e s it e , and th is e f f e c t becomes more im portant as the number of 'ac tiv e * halogens in c re a se s.

The p a r tic u la r experim ental d e ta ils of th e methyl bromide

p y ro ly sis used by Sehon and Szwarc w ill be given in th e d isc u ssio n . However, i t may be noted here th a t in a d d itio n to la rg e d iffe re n c e s between the h a lid e s in th e ir behaviour to v a ria tio n of toluene

34

observed, GCl^Br, on the other hand, in an unpaoked v e sse l was ll^o and 2 , 5/ heterogeneous a t the low er and upper ends o f the tem perature range, re s p e c tiv e ly , and carbon tetrabrom ide showed 5 ^ h etero g en eity a t 695°K.

Other experim ental determ inations of D(CH^-Br) have involved e le c tro n impact methods and c a lc u la tio n s from appearance p o te n tia ls of the ap p ro p riate io n s. On the whole these show m oderately good agreement w ith th e p y ro ly sis work.

From d a ta on th e appearance p o te n tia ls of the p o sitiv e ions from methane and the monohalogenated methanes and using a value of 10, leV f o r th e io n iz a tio n p o te n tia l of GH^-(l22), Bianson and Smith (121) c a lc u la te d D(CH^-Br) ^ 5*1©V (=71.4 k .c a l./m o le ). They con­ sid ered th is v alu e to be an upper lim it, p o in tin g out th a t the

d iffe re n ce in energy between the G-X bond in the h a lid e and the C-H bond in methane should be re fle c te d in th e t o t a l d is s o c ia tio n energy. A ccordingly the energy of the GH^—Br bond should be 0,9.eV le s s than

the bond in methare and i t follow ed th a t the bond en erg ies in GH^Br

and GH^I should be 2.9eV(=66.8 k cals/m ole) and 2.3eV (=55,l k .cals/m o le) re sp e c tiv e ly .

H am ill(74) is shown fo r com parison.

Energy in k .o als/m o le Bond Lossing e t a l . Tsuda e t a l,

B(CH,-Br) 66.4 + 2 70.1

D(CH - I ) 50.7 ± 1.5 52.8

D(CH -C l) 80.5 ± 3 83 .9

D(CH -E) 107.0 + 12 104.9

D(OH,-H) 102.8 + 1.5 103.8

MsDowell and Cox(l25) give a value of 55.6 f o r th e methyl iodide bond d is s o c ia tio n energy^again based on e le c tro n impact d a ta , and a re c e n t k in e tic in v e s tig a tio n by Goy and P ritc h a rd (130) produced a value of 35*0 k.cal/m ole f o r the a c tiv a tio n energy of th e re a c tio n I 2 + CHj + I# This^ along w ith an a c tiv a tio n energy f o r the rev erse re a c tio n of 19.2 k .cal/m o le ( l3 l) , and a value f o r D (I-I) of 35.5 k.oal/m ole leads to D(GH^ - I) « 51.3 k .ca l/m o le. Lossing e t a l , were unable to suggest a reason fo r th e ir ap p aren tly low

value o f A(gH^*^‘)* They argued fo r th e exclusion of se v e ra l p la u sib le ex p lan atio n s.

Reed and Sneddon(2l) have estim ated the d is s o c ia tio n energies of se v e ra l CH^—X compounds. They claim ed good agreement w ith

previous workers but th e ir v alu e f o r D(GH^-Br) of 2.35eV .(= 54kna^/moL ) is low. T h eir methyl iodide value (2.30eV. ) is more in agreement

36

D(CH^-H) = 4 . 12eV. is low er than th a t p rese n tly accepted. Other values are given in the ta b le below, which may be compared w ith data discussed above.

Dis s o c ia tio n Energy

Bond eV k. c a l s / mol.

dCcHj-h) _{4.12 95,0}

d(ch, -c i) _{3.4 78.4}

D(CH,-Bi') _{2.33 55.7}

D(CHj-1) _{2.3 55.1}

DCOlgCH-H) _{3.46 79,7}

DCciCHg-CX) _{3.19 75.5}

D(OlgCH-Ol) _2,89 66.6

DCBiCHg-Br) 2.59 59.8

B(BrgCa-Br) 2.67 61.6

A thermoc he mical determ ination (l26) of the h eats o f form ation of mercury d ia lk y ls in a bomb c a lo rim e te r and use of thermoc hem ioal d ata (127, 128) allowed the c a lc u la tio n of tine h ea ts of form ation of the a lk y l h a lid e s . These are tab u lated belov/ alo n g w ith th e c a l­ c u la te d bond d is s o c ia tio n en e rg ies ( a C-H bond d is s o c ia tio n energy/"

H alide AHf (in k .o al/m o le) C—X bond d is s . 'En.

« V k ) - 8.6 67.8 k.oals/m ole

- 22.1 66.5 " "

- 2.3 53.4 " "

- 10.13 52.6 " "

A ta b le g iv in g th e various values fo r the bond d is s o c ia tio n energy in tiethyl bromide i s given below,

D(OH^"Br) k+oals/m ole Method Reference

4 71.5 E lectro n impact 121

70.1 E lectro n impact 74

67,8 The rmoc hem ical 126

67.4 Thermoo hemic a l 40

(67. 0) p y ro ly sis ₃₉

66.4 E lectro n im]%iot 124

53.7 E lectro n impact 21

4>

n

§4-> W

Î

♦rt ■

•H

r4 I—I •H

•H

r~t

iH

m A

cs

+» ♦H_a TJ

3O' 3

0 +>₀₁

I

oM

1 . D escrip tio n of th e apparatus.

This was a conventional type of flow system in which th e flow of gas c o n li be ad ju sted to give contact tim es in th e re a c tio n zone of from about 0*3 sec, up to about 5 se c s. I t i s shown diagram m atioally in f ig - 1 and was capable of evacuation to 10 mm Hg- pressure* The re a c ta n ts were in je c te d c o n tin u a lly , a t p o in t L, in to about 1 mm of argon c a r r ie r g as. C irc u la tio n was by a mercury d iffu s io n pump, P, which had a liq u id a i r

tra p on th e low p ressu re side and, on the high p ressu re side a liq u id a i r tra p follow ed by a mercury dem ister to prevent th e d iffu s io n of any mercury to o th e r p a rts of th e system

-V a ria tio n in co n tact tim e was obtained by c irc u la tio n of th e gases v ia a r e s tr ic tio n to flow c o n s titu te d by one, or a com bination of th i'ee, flow c a p illa r ie s . A fte r s u ita b le c a lib ra tio n , which i s d escrib ed l a t e r , observations of the McLeod gauge readings on e ith e r side of th ese flow c a p illa r ie s enabled th e flow r a te s in m oles/second to be c a lc u la te d and hence th e co n tact tim es deduced (see appendix l ) .

Large bulbs were p laced a t V (fig - 1) making th e flow system volume over 20 l i t r e s . This vfas to prevent an observable drop in p ressu re in th e system due to bleeding through th e m e tro sil

eter-FURNACE TEMPERATURE PROFILE

(a) Early smoothing at about 760 C

770

760

g 750

u 740

730

cms

10 20 _{30 40 50 60 70}

Distance along-furnace

(b) After smoothing at 709 C

710

700

k 690

0) 680

^ 670

cms 70

50 60

40

10 30

Distance along furnace

20