## Abstract

The 'toluene-carrier' technique has been used for the determination of the C$\chembond{1,0} $Br bond dissociation energies in the substituted benzyl bromides: p-, m- and o-xylyl bromides; p-, m- and o-chlorobenzyl bromides; p- and m-bromobenzyl bromides; p- and m-nitrobenzyl bromides; and p- and m-nitrilebenzyl bromides. The rate-determining step of the decompositions of all these compounds is represented by the unimolecular dissociation processes (s) Ph$_{3}$.CH$_{2}$.Br$\rightarrow $Ph$_{3}$.CH$_{2}$$\cdot $+Br, where Ph$_{s}$.CH$_{2}$.Br refers to the substituted benzyl bromide. Assuming that the frequency factor of the decomposition of each benzyl bromide is equal to the frequency factor of reaction (u) Ph.CH$_{2}$Br$\rightarrow $Ph.CH$_{2}$+Br, the differences in activation energies between E$_{u}$ and E$_{s}$ were calculated using the relation E$_{u}$-E$_{s}$ = RT ln (k$_{s}$/k$_{u}$); k$_{s}$ and k$_{u}$ denote the unimolecular rate constants of reactions (s) and (u) respectively. Since E$_{s}$ and E$_{u}$ are equal to the C$\chembond{1,0} $Br bond dissociation energies in the substituted benzyl bromides and benzyl bromide itself, equation (I) yields the differences, $\Delta $D's, between D(Ph.CH$_{2}$$\chembond{1,0} $Br) and the values for D(Ph$_{s}$.CH$_{2}$$\chembond{1,0} $Br). The calculated differences in the C$\chembond{1,0} $Br bond dissociation energies are listed below: $ \matrix\format\c\kern.8em&\c\kern.8em&\c\kern.8em&\c \\ \text{substituted} & \Delta D & \text{substituted} & \Delta D \\ \text{benzyl bromides} & (\text{kcal}./\text{mole}) & \text{benzyl bromides} & (\text{kcal.}/\text{mole}) \\ & & & \\ o\text{-chloro} & 0\cdot 9 & m\text{-methyl} & 0\cdot 0 \\ m\text{-chloro} & 0\cdot 1 & p\text{-methyl} & 1\cdot 4 \\ p\text{-chloro} & 0\cdot 4 & m\text{-nitro} & 2\cdot 1 \\ m\text{-bromo} & 0\cdot 3 & p\text{-nitro} & 1\cdot 1 \\ p\text{-bromo} & 0\cdot 3 & m\text{-nitrile} & 1\cdot 4 \\ o\text{-methyl} & 2\cdot 0 & p\text{-nitrile} & 0\cdot 7 \endmatrix $ The significance of these findings is discussed, and the effect of substitution on a bond energy is contrasted with the effect of ionic reactions.

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