Nucleophilic reactivities of carbanions in water: The unique behavior of the malodinitrile anion

The kinetics of the reactions of nine carbanions 1 a-i, each stabilized by two acyl, ester, or cyano groups, with benzhydrylium ions in water were investigated photometrically at 20 degreesC. Because the competing reactions of the benzhydrylium ions with water and hydroxide ions are generally slower, the second-order rate constants of the reactions of the benzhydrylium ions with the carbanions can be determined with high precision. The rate constants thus obtained can be described by the Ritchie equation, log(k/k(0)) = N+ (eq 1), which allows us to calculate Ritchie N+ parameters for a series of stabilized carbanions, for example, malonate, acetoacetate, malodinitrile, etc., and compare them with those of other n-nucleophiles in water (hydroxide, amines, azide, thiolates, etc.). Because the Ritchie relationship (eq 1) is a special case of the more general relationship log k = s(N + E) (eq 4), the reactivity parameters N and s for the carbanions 1a-i can also be calculated and compared with the nucleophilic reactivities of a large variety of n-, pi(-), and sigma-nucleophiles, including reactivities of carbanions in dimethyl sulfoxide. While the acyl and ester substituted carbanions are approximately 3 orders of magnitude less reactive in water than in dimethyl sulfoxide, the malodinitrile anion (1i) shows almost the same reactivity in both solvents. Correlations between the nucleophilic reactivities of carbanions with the pK(a) values of the corresponding CH acids reveal that the malodinitrile anion (1i) is considerably more nucleophilic than was expected on the basis of its pK(a) value. This deviation is assigned to the exceptionally low Marcus intrinsic barriers of the reactions of the malodinitrile anion (1i).