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(+) <Na(+) <Li(+) , 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.