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

The ionization equilibria of dicarboxylic acids measured by K_1/K_2 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 K_1/K_2 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 K_1/K_2 of dicarboxylic acids by Kolthoff and coworkers with dimethyl sulfoxide (DMSO) as solvent. In extending these measurements, we have determined K_1/K_2 for 3,4-furandicarboxylic (1) and ,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 K_1/K_2 values for these acids in DMSO of 1 × 10^(15) and 4 × 10^(16) respectively. In contrast, the corresponding value for succinic acid is about 10^8.

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