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+ + +, and the sensitivity of the carbanion reactivities toward variation of the counter ion strongly depends on the structure of the carbanions. The reactivity parameters N and sN of the free carbanions were derived from the linear plots of log k2 against the electrophilicity parameters E of the reference electrophiles, according to the linear‐free energy relationship log k2(20 °C)=sN(N+E). These reactivity parameters can be used to predict absolute rate constants for the reactions of these carbanions with other electrophiles of known E parameters.