Acid-Base Properties of Organic Solvents

The protonation equilibria of organic compounds commonly employed as dipolar nonhydroxylic solvents (including amides, nitriles, sulfones, and nitro compounds) have been measured at 25 C in aqueous sulfuric acid with an NMR technique. The definition of the protonation parameters pKBH+ and m* allows comparison of the relative basicities in water and in concentrated acids. An inversion of relative basicities is observed between dimethylformamide and hexamethylphosphoric triamide on going from dilute to concentrated solutions. The relative basicity scale for dipolar nonhydroxylic solvents in aqueous sulfuric acid is amides > dimethyl sulfoxide > acetone >> acetonitrile > sulfones > nitromethane. The last three functional groups are only 20-50% protonated in 100% sulfuric acid. Hexamethylphosphoric triamide is oxygen-protonated in aqueous acids, and in oleum mixtures it is largely diprotonated; diprotonation also occurs (to a lesser extent) for tetramethylurea.