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

Abstract

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: R.Br→R·+BrR.Br→olefin+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 CH2:CH. CH2·+C6H5. CH3→CH2:CH. CH3+C6H5CH2·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.

H- and D-atom formation from the pyrolysis of C6H5CH2Br and C6H5CD2Br: Implications for high-temperature benzyl decomposition

“…The C-Br bond strengths are 50.5 ± 2 kcal mole À1 [28], and thermal decomposition of the benzyl-bromides are then expected to be nearly instantaneous for the conditions of the present experiments [11]. Jones et al [11] have also demonstrated that benzyl bromide decomposition is a simple C-Br bond fission with no competing reactions, and, therefore, with these precursors, benzyl-radicals are instantaneously formed with concentrations equal to that of the benzyl-bromides used for each experiment.…”

Reactions of Allyl Radicals With Olefins

“…A quantitative separation was achieved in the analysis of the reaction mixture for a single experiment, using a squalane-pelletex colunln a t 0' C. The initial diallyl concentration in this experinlent was 6.7%. This ratio is presumably a measure of the relative rates of reactions [3] and [4].…”

The Thermal Decomposition of Phenyl Methyl Sulphide

“…The pj.rolysis was studied in a conventional vacuum flow system similar to those employed in previous investigations using the toluene carrier technique (5). The toluene pressure was varied between 5.7 and 10.1 mm Hg.…”