Contiguous interactions in α-substituted nitrogen acids and conjugate nitroanions: the role of mesomeric and polar components.

Abstract

¹ H, ¹⁹F, and ¹³C n.m.r. parameters of α-substituted sodium phenyl amides PhN^–X (VIII) are obtained in Me₂SO solution to study the interactions of the variable substituent X with the contiguous negatively charged nitrogen atom. Results probe the considerable structural reorganization undergone by the precusor nitrogen acids PhNHX (IV) upon deprotonation: the effect on ¹³C shifts exerted by the [2.2.1] cryptand provides evidence for ion pairing and/or aggregation of the salts. Correlative analyses with the n.m.r. data probe the effect of the substituent in PhN^–X: in addition to the monoparametric treatment in terms of the σ_C^– constants, DSP analysis with the σ_1B and σ_R^– constants reveals that charge transfer from the nitrogen atom to the phenyl ring is dominated by the mesomeric component of the effects exerted by the substituent X. Unlike the analogous acid–base pair of toluene carbon acids PhCH₂X and of benzyl carbanions PhCH^–X, no correlation whatsoever is found between C(p) of PhN^–X (VII) and Bordwell's acidities of PhNHX (IV) in Me₂SO. The DSP treatment of parent α-substituted nitrogen acids NH₂X and of their phenyl homologues reveals that the energetics of the deprotonation process is dominated by the polar–inductive component of the effect exerted by the substituent. The different sensitivities of charge release from nitrogen in nitroanions and of acidities of nitrogen acids are accounted for on plausible structural arrangements in these species. The pK_a of ammonia is extrapolated from the intersystem correlation between acidities of NH₂X and of PhNHX: the value (35.8) is in excellent accord with the value obtained (36.6) from the DSP treatment of NH₂X acidities.