The Acidity of Weak NH Acids: Expanding the pK _a Scale in Acetonitrile

Nitrogen heterocycles and aromatic amines are well-known and widely used compounds that are usually not seen as acids, although in organic solvents like acetonitrile (MeCN) or dimethyl sulfoxide (DMSO) their acidic properties can be observed. In this work, the acidities (pK _a values) of 37 such weak NH acids were determined in acetonitrile and presented together with computational gas-phase acidities and literature pK _a values in DMSO. In the course of the work the weakest acids range (from pK _a 29 upward) of the MeCN acidity scale has been revised and expanded to around 33.5 by adding 31 compounds in that specific region and the span of experimental pK _a values in MeCN is now more than 30 orders of magnitude. The relations between the structure and acidity of a selection of the studied compounds have been investigated in MeCN and DMSO. The measured pK _a values in MeCN and the gathered pK _a values in DMSO were used for a correlation analysis between the two solvents and for assessing the quality of pK _a conversion equations. A number of pK _a values have been predicted in MeCN from pK _a values in DMSO via the correlation analysis and pK _a conversion equations.