Studies of the variations in bond dissociation energies of aromatic compounds. I. Mono-bromo-aryles

Investigation of the pyrolyses of bromobenzene, $\beta $-bromonaphthalene, $\alpha $-bromonaphthalene, 9-bromophenanthrene and 9-bromoanthracene in the presence of an excess of toluene has shown that reaction (1) Ar.Br $\rightarrow $ Ar$\boldsymbol{\cdot}$+Br (1) is the primary and rate-determining step of the pyrolysis. The progress of reaction was measured by the rate of formation of hydrogen bromide, and it was shown that this rate obeys first-order kinetics. The following values were obtained for the activation energies and frequency factors of unimolecular decompositions represented by equation (1): $ \matrix\format\l\kern.8em&\c\kern.8em&\c \\ \quad \quad \text{compound} & E\, (\text{kcal}/\text{mole}) & 10^{-13}\nu \, (\sec ^{-1}) \\ \text{bromobenzene} & 70\cdot 9 & 2 \\ \beta \text{-bromonaphthalene} & 70\cdot 0 & \quad 1\cdot 5 \\ \alpha \text{-bromonaphthalene} & 70\cdot 9 & \quad 3\cdot 5 \\ 9\text{-bromophenanthrene} & 67\cdot 7 & 1 \\ 9\text{-bromoanthracene} & 65\cdot 6 & \quad 1\cdot 5 \endmatrix $ Assuming that recombination of bromine atoms with aromatic radicals does not involve any activation energy we conclude that the determined activation energies correspond to the respective C$\chembond{1,0} $Br bond dissociation energies. The effect of molecular structure on the C$\chembond{1,0} $Br bond dissociation energy is discussed. The heat of formation of the phenyl radical is determined, and this result is used for calculating the various Ph$\chembond{1,0} $X bond dissociation energies.