Steric enhancement of resonance

Abstract

The rate constants and the activation energies for the alkaline hydrolysis of some substituted ethyl benzoates furnish evidence for the phenomenon of steric enhancement of resonance. For 3-substituted-4-alkoxybenzoic esters the rate constants calculated on the basis of the principle of additivity of substituent effects are appreciably higher than the observed values, indicating that the 3-substituent does not sterically inhibit the resonance interaction of the alkoxy and ester groups but actually enhances it. The cause of this steric enhancement of resonance is explained. The rates of quaternization of some N,N-dimethylanilines with methyl iodide have also been determined. The observed rate constants of 4-alkoxy-3-methyl-N,N-dimethylanilines are significantly higher than the values predicted on the basis of additivity of group effects. Thus there is enhanced resonance interaction of the alkoxy group with the dimethylamino group increasing the nucleophilicity of the dimethylamino group. A similar steric enhancement of resonance is observed in the case of 3-methyl-4-methylthio-N,N-dimethylaniline also.