The C–H Bond Energy in Toluene and Xylenes

Abstract

The pyrolysis of toluene and the xylenes was investigated by a new technique, which made it possible to study these reactions for degree of decomposition ranging from 0.01 percent up to a few percent. It was shown that these reactions are homogeneous, unimolecular reactions. It was found that the weakest bond in these molecules is the C–H bond in the methyl group. The bond energy of that C–H bond was found to be 77.5 kcal. for toluene and meta‐xylene, 75 kcal. for para‐xylene and 74 kcal. for ortho‐xylene. These data indicate that the resonance energy of the benzyl radical is 24.5 kcal. and that the hyper‐conjugation in para‐xylene decreases the C–H bond energy in the methyl group of this molecule by 2.5–3 kcal.