carrier gas

8/8/2019 Carrier Gas

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NOTES

S O M E A S P E CT S O F T H E T O L U E N E P Y R O LY S I S'

Th e extensive use of the toluene carrier gas techn ique by Szwarc (7 , 8) an dco-workers has attracted wide interest (1, 2 , 4, 6) in the mechanism of thepyrolysis of toluene in the past few years. T he present note is concerned wit hthe reactions of hydrogen at oms with toluene-d3 and of benzyl radicals wit hfi-fluorotoluene.

R E A C T I O N O F H Y D R O G E N A T O M S W I T H T O L U E N EAccording to Szwarc, radicals in general react with toluene in the following

manner:R'+ CsH5CH3 --t R H + CsH5CH2' [1I2CsH5CH2' 4 C14H14 [ z

He found it necessary, however, to propose an additional mechanism forthe reaction of hydrogen atoms with toIuene since methane as well a s hydrogenis a product of the pyrolysis of toluene. Two radically different mechanismsmight be proposed for the production of metha ne in this instance.

Since the rat io of H2 to CH4 in the products is independen t of temperat ure,it follows tha t eithe r E3 = E6or E.L= E 5 .Szararc reasoned th at the activatio nenergy for the over-all decomposition (77.5 kcal./mole) was too low for E6and hence assumed that mechanism I was responsible for the methane pro-duction. Recent studies on the kinetics of t he pyrolysis ( 2 ) and on the photo-brornination (1) of toluene suggest th at th is activation energy may not bereliable and hence th at nlechanism I1 cannot be ruled out by this argument.

Szwarc (5 ) has produced hydrogen atom s by the pyrolysis of n-propylbenzene in the presence of toluene and found both hydrogen and methane inthe products in roughly the same proportion as in the toluene pyrolysis.

' I s sued as N.R .C . N o . 5418.?Na tiona l Research Counc il of C anad a Posldoclorale Fellow 1962-1964.


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BLADES AND STEACII


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I i E A C T IO N O F B E N Z Y L R A D I C A LS W I T H p - F L U O R O T O L U E N ETh e relative unreactivity of the benzyl radical is the basis of the so-called

"toluene carrier gas technique" for studying free radical decomposition re-actions. In a high mole ratio of toluene to decomposing conlpound, a radicalwill tend to react preferentially with toluene to give RH and a benzyl radical(reaction [I]) thereby converting a reactive radical i nto a inolecule and aninert radical. Under the correct conditions of concentration and tem perature,these benzyl radicals will dimerize to bibenz~ll.Since the technique has beenused a t temperatures where bibenzyl would be expected to decompose rapidlyhowever, it would be possible for abstraction reactions of benzyl to conlpetewith the dimerization process. T o test this, some studies on the relative ratesof the reactions

CsH5CH2'+ p-CsH4FCH3+ C6H5CH3+ C6H4FCHn' [g ]and 2CsHbCH2' -+ C14H14 [g lhave been attempted by pyrolyzing bibenzyl in the presence of an excess ofp-fluorotoluene.

Th e apparatus for the pyrolysis of bibenzyl was similar to th at referred toabove. The bibenzyl was introduced by passage of the p-fluorotoluene over ita t 110" C. The major products were m ix t~ ~r esf fluor inated bibenzyls whichwere pumped free of p-fluorotoluene a t 0" C. Quanti ties of hydrogen (8070)and inethane (20%) were produced varying from 1 to 5% of the bibenzylpassed through the reactor.Analysis of t he bibenzyl fraction was attempted from its infrared absorptionspectrum in CS? solution. Unfor tunate ly, different absorption bands gaveradically different analyses and the conclusion was reached that monofluoro-bibenzyl was also a product along with the di-p-fluorobibenzyl. Analyses werethereby limited to conlbustion of the mixture and th is is unsatisfactory forkinetic data. A typical run at 994" I


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BLADES AN D SI'EACIE: TOLCENE PYROLYSIS 1145the decompositions of toluene and n-propyl benzene. The evidence is consistentwith the following mechanism:

H ' + C6H5CH3 --tCsHs+ CH3'CH,' + CGH5CH3+ CH4 + CtjH5CHz'.

Szwarc used the activation energy for the deconlposition of toluene todecide on the mechanism; confirnmation of th e mechanism does not , however,prove the correctness of this activation energy nor does it cast dou bt on thehigller value obtained by the photobronlination of toluene (1, 3) . I t is worthyof note, however, tha t this latter is a maximum value and t ha t the true valueis less by the act ivation energy of the reaction

CsH5CH2' + Br2--t CsH5CHzBr+ Br'and there is no a priori reason for supposing that this is zero.

Evidence has also been presented suggesting that radicals may abstracthydrogen atoms from the ring as well as the side chain in toluene but there isno indication as to how serious a complication this might be in the toluenecarrier gas technique. Presumably the resulting tolyl radicals would disappearrapidly by reaction with toluene.

CsH4CH3' + CsHsCH3 --t CsHSCH3+ CsH5CH2'Under t he conditions of the toluene carrier gas technique in the neighborhoodof 1000"K., abstract ion by benzyl radicals has been shown to compete with

combination. Thi s implies that, when this technique is used in th is temperatu reregion, complications must be anticipated from reactions of the type

CsH5CH2' + RH --t CsH5CH3+ R'where RH is the decomposing molecule, especially when the R-H bond is weak.

Horrex and Miles (4) have studied the pyrolysis of bibenzyl and arrived a tthe rat e expression

k = 2 X 109 e -48 . 000 /RT sec.-lfor the decomposition into two benzyl radicals. Under the experimental con-ditions used in the above experiment this would predict only about 2%decomposition of the bibenzyl. Since a minimum of 35y0 was decomposed, itis suggested t ha t .the above rate expression is in error owing to the neglect ofthe recombination step in their mechanism.1. ANDERSEN. . C.. SCHERAGA.. A.. and VANARTSDALEN. . R. I. Chem. Phvs. 21: 1258.1953.2. BLADES,., BLADES,. T., and STEACIE,. W. R. Can. J. Chem. 32: 298. 1954.3. ECKSTEIN,. H., SCHERAGA,. A , , and VANARTSDALEN,E. R. J . Chem. Phys. 22: 28.1954.4. HORRES,C. and MILES,S. E. D~SCIISS~OIISaraday Soc. 10: 187. 1951.5. LEIGH,. H. and SZWARC,. J. Chem. Phys. 20: 403. 1952.6. SCHISSLER,. 0. and STEVENSON,. P. J . Chem. Phys. 22: 151. 1954.7. SZWARC,. J . Chem. Phys. 16: 128. 1948.8. SLWARC,. Chem. Revs. 47: 75. 1950; see also J. Chem. Phys. 1950-54.9. TROTMAN-DICKENSON,. F. and STEACIE,. W. R. J. Chem. Phys. 19: 329. 1951.

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