Ionization equilibria of dicarboxylic acids in dimethyl sulfoxide as studied by NMR

The ionization equilibria of dicarboxylic acids measured by K1/K2 are expected to be substantially influenced by stabilization of the monoanion through intramolecular hydrogen bonding. However, except in sterically favorable cases, such as with maleic, 3,4‐furandicarboxylic, 2,3‐bicyclo[1.2.2]heptadienedicarboxylic and diethylmalonic acids, the K1/K2 values for other dicarboxylic acids, where intramolecular hydrogen bonding of the monoanion might be expected, are relatively small at ∼100 in water. This research uses NMR to corroborate and extend the pioneering conductance measurements of K1/K2 of dicarboxylic acids by Kolthoff and coworkers with dimethyl sulfoxide (DMSO) as solvent. In extending these measurements, we have determined K1/K2 for 3,4‐furandicarboxylic (1) and 2,3‐bicyclo[1.2.2]heptadienedicarboxylic (2) acids, because these were predicted by McCoy to be especially sterically favorable for intramolecular hydrogen bonding in the form of their monoanions and, indeed, we have found K1/K2 values for these acids in DMSO of 1 × 1015 and 4 × 1016 respectively. In contrast, the corresponding value for succinic acid is about 108.