Quantification of ion-pairing effects on the nucleophilic reactivities of benzoyl- and phenyl-substituted carbanions in dimethylsulfoxide.

Second-order rate constants for the reactions of acceptor-substituted phenacyl (PhCOCH(-) Acc) and benzyl anions (PhCH(-) Acc) with diarylcarbenium ions and quinone methides (reference electrophiles) have been determined in dimethylsulfoxide (DMSO) solution at 20 °C. By studying the kinetics in the presence of variable concentrations of potassium, sodium and lithium salts (up to 10(-2)  mol L(-1) ), the influence of ion-pairing on the reaction rates was examined. As the concentration of K(+) did not have any influence on the rate constants at carbanion concentrations in the range of 10(-4) -10(-3)  mol L(-1) , the acquired rate constants could be assigned to the reactivities of the free carbanions. The counter ion effects increase, however, in the series K(+)