The Acidity of Weak NH Acids: Expanding the pKa 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 (pKa values) of 37 such weak NH acids were determined in acetonitrile and presented together with computational gas-phase acidities and literature pKa values in DMSO. In the course of the work the weakest acids range (from pKa 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 pKa 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 pKa values in MeCN and the gathered pKa values in DMSO were used for a correlation analysis between the two solvents and for assessing the quality of pKa conversion equations. A number of pKa values have been predicted in MeCN from pKa values in DMSO via the correlation analysis and pKa conversion equations.