Very‐low‐pressure pyrolysis of ethylbenzene, isopropylbenzene, and tert‐butylbenzene

The thermal unimolecular decomposition of ethylbenzene, isopropylbenzene, and tert‐butylbenzene was studied using the very‐low‐pressure pyrolysis (VLPP) technique. Each reactant decomposed by way of β CC bond homolysis, producing methyl radicals and benzyl or benzylic‐type radicals. RRKM calculations show that the observed rate constants, when combined with thermochemical estimates, are consistent with the following high‐pressure rate expressions: {article}{empty}{document}log k( - 1) = 15.3 - (72.7/ θ){document} for ethylbenzene between 1053 and 1234 K, {article}{empty}{document}log k( - 1) = 15.8 - (71.3/ θ){document} for isopropylbenzene between 971 and 1151 K, and {article}{empty}{document}log k( - 1) = 15.9 - (69.1/ θ){document} for tert‐butylbenzene between 929 and 1157 K, where θ (kcal/mol) = 2.303RT. Resulting activation energies combined with heat capacity and heat of formation data led to the following dissociation enthalpies and enthalpies of formation at 298 K: DH° (øCH(CH3)CH3) = 73.8 kcal/mol, ΔHf° (øÇCH(CH3)) = 39.6 kcal/mol, DH° (øC(CH3)2CH3) = 72.9 kcal/mol, and ΔHf° (øÇ(CH3)2) = 32.4 kcal/mol. Derived high‐pressure rate constants are in good accord with results of lower temperature toluene‐ and aniline‐carrier experiments.