The C—Br bond dissociation energy in halogenated bromomethanes

The pyrolyses of methyl bromide and of the halogenated bromomethanes, CH2CI. Br, CH2Br2, CHCl2.Br, CHBr3, CF3Br, CCI3. Br and CBr4, have been investigated by the ‘toluene-carrier' technique. It has been shown that all these decompositions were initiated by the unimolecular process RBr → R + Br. (1) Since all these decompositions were carried out in the presence of an excess of toluene, the bromine atoms produced in process (1) were readily removed by the fast reaction C6H5.CH3 + Br → C6H5. CH2 • + HBr. Hence, the rate of the unimolecular process (1) has been measured by the rate of formation of HBr. The C—Br bond dissociation energies were assumed to be equal to the activation energies of the relevant unimolecular dissociation processes. These were calculated by using the expression k ═ 2 x 1013 exp (-D/RT). The reason for choosing this particular value of 2 x 1013 sec.-1 for the frequency factor of these reactions is discussed. The values obtained for the C—Br bond dissociation energies in the investigated bromomethanes are: D(C—Br) D(C—Br) compound (kcal./mole) compound (kcal./mole) CH3Br (67.5) CHBr3 55.5 CH2CIBr 61.0 CF3Br 64.5 CH2Br2 62.5 CCI3Br 49.0 CHCl2Br 53.5 CBr4 49.0 The possible factors responsible for the variation of the C—Br bond dissociation energy in these compounds have been pointed out.