Electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part 14. Non-additivity of methyl substituent effects: the reactivity selectivity principle

Abstract

Rates of gas-phase elimination of acetic acid from polymethyl-substituted 1-arylethyl acetates, measured between 613.8 and 667.2 K, have shown that the effects of the methyl groups are not strictly additive in this reaction. The reactivity is greater than that calculated on the basis of the additivity principle, log k_rel.(obs.)– log k_rel.(calc.)/[log k_rel,(calc.)]² being approximately constant. This contrasts with the related electrophilic aromatic substitutions, in which the reactivities for polymethyl compounds are less than calculated from the additivity principle. The results confirm that the transition state for ester pyrolysis becomes more polar, the greater the electron supply to the α-carbon atom. Greater electron supply causes a shift in the transition state away from the ground state, whereas in electrophilic substitutions the transition state is shifted towards the ground state. These observations demonstrate the differences arising from the participation in the transition state of bond-breaking processes in the elimination as compared with bond-making processes in the substitution. As in solution reactions, the ratio of the reactivities of the 4- and 3-positions in o-xylene is less than that of the para- and ortho-positions in toluene, so this anomaly cannot arise from solvation effects.