The Pyrolysis of n-Propyl-Benzene and the Heat of Formation of Ethyl Radical

The investigation of the pyrolysis of n‐propyl‐benzene by the ``toluene carrier'' technique revealed that this compound decomposes initially into benzyl and ethyl radicals: $${\mathrm{C}}_6{\mathrm{H}}_5·{\mathrm{CH}}_2·{\mathrm{C}}_2{\mathrm{H}}_5\to {\mathrm{C}}_6{\mathrm{H}}_5·{\mathrm{CH}}_2\mathrm{·+}{\mathrm{C}}_2{\mathrm{H}}_5.$$ Ethyl radicals decompose rapidly into ethylene and hydrogen atoms. The hydrogen atoms reacting with toluene produce H₂ or CH₄. The rate of the initial dissociation process can be measured by the rate of formation of C₂ hydrocarbons. It was shown that the decomposition is a homogeneous, first‐order gas reaction. The activation energy was estimated at 57.5 kcal/mole, the frequency factor at 3.10¹² sec⁻¹. It appears that the activation energy is equal to D(C₆H₅·CH₂–C₂H₅). Since the heat of formation of benzyl radicals is known, the estimation of D(C₆H₅·CH₂–C₂H₅) leads to the value for the heat of formation of ethyl radicals, i.e., $$\Delta {\mathrm{H}}_f({\mathrm{C}}_2{\mathrm{H}}_5\text{)≈22\hspace{0.17em}}\mathrm{kcal/mole}.$$ The latter value makes it possible to evaluate various relevant bond dissociation energies, i.e., D(C₂H₅–H) ≈94±4 kcal/mole.