The C—Br bond dissociation energy in halogenated bromomethanes

The pyrolyses of methyl bromide and of the halogenated bromomethanes, CH$_{2}$Cl.Br, CH$_{2}$Br$_{2}$,CHCl$_{2}$.Br, CHBr$_{3}$, CF$_{3}$Br, CCl$_{3}$, Br and CBr$_{4}$, have been investigated by the 'toluene-carrier' technique. It has been shown that all these decompositions were initiated by the unimolecular process RBr$\rightarrow $ R + Br. 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 C$_{6}$H$_{5}$.CH$_{3}$+Br$\rightarrow $C$_{6}$H$_{5}$.CH$_{2}\bullet $+HBr. Hence, the rate of the unimolecular process (1) has been measured by the rate of formation of HBr. The C$\chembond{1,0} $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 $\times $ 10$^{13}$ exp (-D/RT). The reason for choosing this particular value of 2 $\times $ 10$^{13}$ sec.$^{-1}$ for the frequency factor of these reactions is discussed. The values obtained for the C$\chembond{1,0} $Br bond dissociation energies in the investigated bromomethanes are: $ \matrix\format\c\kern.8em&\c\kern.8em&\c\kern.8em&\c \\ & D(\text{C} \chembond{1,0} \text{Br}) & & D(\text{C}\chembond{1,0} \text{Br}) \\ \text{compound} & (\text{kcal}./\text{mole}) & \text{compound} & (\text{kcal}./\text{mole}) \\ & & & \\ \text{CH}_{3}\text{Br} & (67\cdot 5) & \text{CHBr}_{3} & 55\cdot 5 \\ \text{CH}_{2}\text{ClBr} & 61\cdot 0 & \text{CF}_{3}\text{Br} & 64\cdot 5 \\ \text{CH}_{2}\text{Br}_{2} & 62\cdot 5 & \text{CCl}_{3}\text{Br} & 49\cdot 0 \\ \text{CHCl}_{2}\text{Br} & 53\cdot 5 & \text{CBr}_{4} & 49\cdot 0 \endmatrix $ The possible factors responsible for the variation of the C$\chembond{1,0} $Br bond dissociation energy in these compounds have been pointed out.