The C–Br Bond Dissociation Energy in Benzyl Bromide and Allyl Bromide

The pyrolysis of organic bromides in a stream of toluene is described as a method for the determination of the C–Br bond dissociation energies. The technique used makes it possible to discriminate between two mechanisms of decomposition: $$\mathrm{R}.\mathrm{Br}\to \mathrm{R}\mathrm{·+}\mathrm{Br}$$ $$\mathrm{R}.\mathrm{Br}\to \mathrm{olefin}+\mathrm{HBr}.$$ It was found that both benzyl and allyl bromide decomposed according to mechanism (a), the bromine atoms reacting readily with toluene to give hydrobromic acid. The rate of the primary dissociation process was measured by the rate of formation of HBr. It was proved that the thermal decompositions of benzyl and allyl bromide were homogeneous gas reactions obeying first order kinetics. The activation energies were calculated at 50.5±2 kcal./mole and 47.5±2 kcal./mole, and identified with D(C₆H₅.CH₂–Br) and D(CH₂:CH.CH₂–Br), respectively. The fate of the allyl radical is discussed, and a rough estimate of the activation energy for the reaction $${\mathrm{CH}}_2:\mathrm{CH}.{\mathrm{CH}}_2\mathrm{·+}{\mathrm{C}}_6{\mathrm{H}}_5.{\mathrm{CH}}_3\to {\mathrm{CH}}_2:\mathrm{CH}.{\mathrm{CH}}_3+{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{CH}}_2·$$ leads to a value of 14–17 kcal./mole. The problem of ionic contribution in the C–Br bonds is discussed. The estimation of the dissociation energies of the C–Br bonds of various organic bromides is of great interest since the values for these dissociation energies, combined with the heats of formation of the relevant bromides in the gaseous state, make it possible to determine the heats of formation of various organic radicals. Furthermore, the latter data in conjunction with the heats of formation of hydrocarbons enable us to calculate the various C–H and C–C bond dissociation energies and the resonance energies of the relevant radicals.