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

Basicity constants, p K a , 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 p K a 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 NH 2 ‐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.