Substituent effects of positive poles in aromatic substitution. Part III. Nature of the deactivating effect of the trimethylammonio-group

Abstract

The rates of meta-nitration of the ions Ph[CH₂]_nNMe₃⁺, where n= 0, 1, 2, or 3, in aqueous sulphuric acid accord with an electrostatic treatment of the interaction between the nitrogen pole and the charge on the ring in the transition state. The variation of the enthalpy and entropy of activation with the number of methylene groups is also consistent with this approach, much of the change in reactivity deriving from the entropy term. In less polar media, the relative rates of nitration of the salts Ph[CH₂]_nNMe₃⁺ClO₄^– and benzene vary with the conditions in a way suggesting preferential reaction through ‘onium’ perchlorate ion-pairs. Relative to benzene, the reactivity of such ion-pairs to nitration in acetic anhydride can exceed the reactivity of the corresponding free ions to nitration in aqueous sulphuric acid.