The additivity of methyl groups in electrophilic substitution

Abstract

The rates of detritiation at the ring positions in the polymethylbenzenes have been obtained for exchange at 70ºin anhydrous trifluoroacetic acid. The first-order rate constants are analysed in terms of the additivity principle, which is shown not to hold for this system; derived partial rate factors, describing the effect of ortho, meta, and para methyl groups on reactivity, decrease as the reactivity of the ring site increases. Much better agreement between observed and calculated results is obtained if the demand for stabilization made on the methyl groups in the transition state is assumed proportional to the activation free energy. This approach requires no more parameters than the additivity principle, but corrections for ortho-meta and meta-para buttressed methyl groups can give further improvement where required. The application of this "demand-dependent free energy relationship" to other systems, and the origin of the buttressing effects, are discussed.