The C–C Bond Energy in Ethylbenzene

Abstract

The pyrolysis of ethylbenzene in the presence of excess of toluene has been shown to be a unimolecular process, the rate‐determining step being $${\mathrm{C}}_6{\mathrm{H}}_5·{\mathrm{CH}}_2·{\mathrm{CH}}_3\to {\mathrm{C}}_6{\mathrm{H}}_5·{\mathrm{CH}}_2\mathrm{·+}{\mathrm{CH}}_3\mathrm{·.}$$ The methyl radicals were removed by a reaction with toluene, yielding CH₄ and benzyl radicals, and the quantity of methane obtained was a measure of the extent of the decomposition of ethylbenzene. The energy of activation and the frequency factor were estimated experimentally at 63.2 kcal./mole and 1.0·10¹³ sec.⁻¹, respectively. Taking the D(C–C)C₆H₅·C₂H₅ at 63.2 kcal./mole and using this value in conjunction with the value reported previously for D(C–H)C₆H₅·CH₃ and the relevant thermochemical data, we calculated the D(C–H)CH₄ at 102.9 kcal./mole. The results of the thermal decomposition of ethylbenzene in the absence of toluene have been investigated, but it has been found unsuitable for kinetics studies because of a complicating chain process. A short discussion on the frequency factors of these unimolecular reactions is given.