The reactivities of alpha methylenic groups towards free radicals: III. Steric effects in tetralin, indan and dihydroanthracene derivatives

The relative reactivities towards trichloromethyl radicals were determined for a number of tetralin, indan and dihydroanthracene homologues, using the retardation method described previously 1 ) 2 ). As in the case of alkylbenzenes 2 ), several of the hydrocarbons showed marked decreases in reactivity compared with the additive values. The effects are again attributed to steric interference with the most favourable transition state configuration. They were found to become increasingly important in the order (1‐methylindan, hexahydrofluorene), 1,4,7‐trimethylindan, (cumene, 1‐methyltetralin, 9,10‐dimethyl‐9,10‐dihydroanthracene), 9,10‐diisopropyl‐9,10‐dihydroanthracene, (o‐cymene, hexaethylbenzene, 1,2,4,5‐tetraisopropylbenzene). The magnitudes of the effects are discussed on the basis of the calculated distances between the centres of the interfering groups in the absence of steric repulsion. This leads to an estimate of the effective methyl radius of about 1.5 Å at angles of approach of about 80°, i.e. a value considerably smaller than the Van der Waals radius (2.0 Å).