Effects of N-alkylation and NN-dialkylation on the pKa of anilinium and nitroanilinium ions

Abstract

In marked contrast with aniline or p-toluidine, whose N-methyl and NN-dimethyl derivatives are stronger bases in water than the parent amines, N-methyl- and NN-dimethyl-4-nitroaniline are weaker bases than 4-nitroaniline. In going from the N-methyl to the higher N-alkyl or from the NN-dimethyl to the NN-diethyl derivatives, however, effects on pK_a are in the inductive order and roughly parallel in the aniline, p-toluidine, and 4-nitroaniline series. N-Monoalkylation leads to similar changes in pK_a with 2-nitroaniline and 5-hydroxy-2-nitroaniline as are observed with 4-nitroaniline. These observations are rationalized in terms of inductive effects, solvation effects involving the free aromatic amines and the ammonium ions, and effects deriving from differing hybridization on the amine nitrogens.