The reactivities of alpha methylenic groups towards free radicals: II. Steric effects in alkylbenzenes

The retardation method 1) described previously has been applied to a number of polyalkylbenzenes. The reactivities of the alpha methylenic *) groups towards hydrogen abstraction by trichloromethyl radicals were compared with those of monoalkylbenzenes. Compounds having two alkyl groups in adjacent positions ‐ with the exception of methylated benzenes ‐ were found to be far less reactive than expected on the basis of additivity. In these cases, scale drawings as well as model considerations show that coplanarity of the three remaining valencies at the alpha methylenic carbon atom is markedly interfered with by steric repulsions. With other isomers, reactivity appeared to be nearly additive. The results are interpreted in terms of steric hindrance of resonance: geometrical arguments are put forward to show that this hindrance occurs not only in the final state — i.e. in the benzyl‐type radical formed upon hydrogen abstraction ‐ but also in the transition state. The extremely low reactivity found for the anthracene‐maleic anhydride addition product is attributed to the fact that the bridge prevents formation of a planar radical.