ACIDITIES OF CARBOXAMIDES, HYDROXAMIC ACIDS, CARBOHYDRAZIDES, BENZENESULFONAMIDES, AND BENZENESULFONOHYDRAZIDES IN DMSO SOLUTION

A comparison of acidities of six series of analogous oxygen, nitrogen, and carbon acids in dimethyl sulfoxide (DMSO) solution and the gas phase has shown that the element effect usually causes nitrogen acids to be more acidic than their carbon acid counterparts by an average of 17 ± 5 kcal/mol, and oxygen acids to be more acidic than their nitrogen counterparts by a like amount. A much smaller difference was observed between the NH acidities of carboxamides and the CH acidities of ketones (1–2 kcal/mol in DMSO and 7–8 kcal/mol in the gas phase). Equilibrium acidities in DMSO for a number of substituted benzamides, acetamides, N-phenylacetamides, acetohydroxamic acids, benzohydroxamic acids, carbohydrazides, and benzenesulfonamides are reported. Aceto-and benzohydroxamic acids were found to be 9.8 and 10.1 pKHA units more acidic in DMSO, respectively, than acetamide and benzamide. In each instance the effect of N-alkylation decreased the acidity more than did O-alkylation, which indicates that the parents are NH, rather than OH, acids in DMSO. Conclusive supporting evidence for the NH acid assignment was provided by the observation that the N-alkylhydroxamic acids exhibited strong homo-H-bonding, whereas the parent acids and their O-alkyl derivatives did not. Oxidation potentials of hydroxamate anions in DMSO are close to those of O-alkylhydroxamate ions, confirming that their conjugate acids are NH acids, but in MeOH they are close to those of N-alkylhydroxamate ions showing that their conjugate acids can act as OH acids in hydroxylic solvents. The N-alkyl- and O-alkylhydroxamic acids exhibited much stronger chelating power toward K+, Na+, and Li+ ions than did the parent acids. © 1990, American Chemical Society. All rights reserved.