The CH ₂ : CH.CH ₂ —CH ₃ bond dissociation energy and the heat of formation of the allyl radical

The pyrolysis of butene-1 was investigated by a flow technique, toluene being used as a carrier gas. It was found that butene-1 decomposed into allyl and methyl radicals according to the equation CH$_{2}$: CH.CH$_{2}$$\chembond{1,0} $CH$_{3}\rightarrow $ CH$_{2}$: CH.CH$_{2}$$\cdot $+CH$_{3}$$\cdot $ Methyl radicals were removed by reaction with toluene giving methane and benzyl radicals. The rate of the initial decomposition was measured by the rate of formation of methane. The decomposition was found to be a homogeneous first order gas reaction. The activation energy was calculated at 61$\cdot $5 kcal./mole and it was identified with the CH$_{2}$: CH.CH$_{2}$$\chembond{1,0} $CH$_{3}$ bond dissociation energy. Taking D(CH$_{2}$: CH.CH$_{2}$$\chembond{1,0} $CH$_{3}$) at 61$\cdot $5 kcal./mole we calculated from thermochemical data D(CH$_{2}$: CH.CH$_{2}$$\chembond{1,0} $H) at 76$\cdot $5 kcal./mole and the heat of formation of allyl radical at +30 kcal./mole. The fate of allyl radicals is discussed and the thermal stability of these is compared with that of benzyl radicals.