Cyanocarbon Acids: Direct Evidence That Their Ionization Is Not an Encounter-Controlled Process and Rationalization of the Unusual Solvent Isotope Effects

Article abstract of DOI:10.1021/ja00247a031

The rate of exchange of the acidic hydrogen of tert-butylmalononitrile was examined by using as a tracer, and the process was found not to be inhibited by hydronium ions in dilute aqueous hydrochloric acid solutions.This rules out the Swain-Grunwald mechanism for this reaction under these conditions.The bromination of malonitrile was investigated under conditions where reprotonation of the dicyanomethyl carbanion and its reaction with bromine occur at comparable rates, and the bromination reaction was found to have a specific rate twice that for reprotonation.Reprotonation therefore cannot be a diffusion-controlled process, and malonitrile is not a "normal" acid.The unusually large solvent kinetic isotope effects found for these cyanocarbon acid ionization reactions are explained by postulating that the transferring hydrogen and its positive charge are becoming associated with a solvent cluster rather than with a single water molecule.The thermodynamic acidity constant of malonitrile was determined to be 11.41 in aqueous solution at 25 deg C.