Inductive and field effects in aromatic substitution. Part IX. Assessment of results

Abstract

Earlier work on rates of nitration of the ions Ph[CH₂]_nX (X = NR₃, SR₂ PR₃, etc.) is analysed in terms of the non-conjugative and conjugative interaction of [CH₂]_nX with the aromatic ring. Partial rate factors for meta- substitution are taken as a measure of the non-conjugative interaction. The variation of the deactivation at the meta-position with the distance of the pole from the ring is shown to be more consistent with the operation of a field effect than with that of an inductive effect. Apparent exceptions occur when the methylene chain is U shaped for the deactivation is then less than expected. From this and studies with bridged substrates, it is concluded that the transmission of the field effect occurs almost entirely through the molecular cavity but, unlike the inductive effect, is not restricted to the direction of chemical bonds with in that cavity. In the ions Ph[CH₂]_nX, there is no simple relationship between the orientation of substitution and the deactivation at the meta-position. The additional factor involved is identified as conjugative interaction and an approximate linear free energy relationship is derived relating the para : meta ratio to the σ°_R value for the substituent. This relationship is used to provide information on the relative importance of conjugative and non-conjugative interaction, the comparison of nitrogen and phosphorus poles, and the possible effects of the conformation of the CH₂NH₃ substituent on the deactivation of the ring.