One‐scale basicities of diaminobenzenes and diaminonaphthalenes: from aniline to proton sponge

Abstract

Basicity constants, pKa, for a wide range of mono‐protonated diaminobenzenes and diaminonaphthalenes, including dimethylamino derivatives were for the first time uniformly measured in 20% aqueous ethanol (29 compounds) and 80% aqueous dioxane (39 compounds) spanning from aniline to 1,8‐bis(dimethylamino)naphthalene (‘proton sponge’). The dioxane system proved to be more versatile and because of better solubility of N‐alkylated polyaminoarenes allowed to add to the same scale some superbasic bis(dialkylamino)‐, tetrakis(dialkylamino)‐, and hexakis(dialkylamino)naphthalenes, thus extending the scale for almost 10 pKa units, revealing possible limits of basicity changes in aromatic amines. The basicity of reference bases, pyridine and triethylamine, was also measured in these solvent systems. A group of N‐alkylated compounds was found to be less basic in aqueous dioxane when compared with their NH2‐analogs. This anomaly was not observed in aqueous ethanol. Other basicity trends and correlations between different basicity scales were also discussed. Copyright © 2016 John Wiley & Sons, Ltd.