Nucleophilicity Parameters for Carbanions in Methanol

The kinetics of the reactions between eleven carbanions 2a – k (nitronates and malonic acid derivatives) and benzhydrylium ions 1a – g were investigated photometrically in methanol/acetonitrile (91:9, v/v) at 20 °C. Since the competing reactions of the benzhydrylium ions with methanol and methoxide ions are generally slower, the second‐order rate constants of the reactions between the benzhydrylium ions 1a – g and the carbanions 2a – k can be determined with high precision. The second‐order rate constants (log  k ) correlate linearly with the electrophilicity parameters E of the benzhydrylium ions, allowing us to determine the nucleophilicity parameters N and s of the carbanions in methanol/acetonitrile (91:9, v/v). The nucleophilic reactivities do not correlate with the acidity constants of the conjugate CH acids, and from the poor correlation of the reactivities of the substituted α‐nitrobenzyl anions 2d – h with Hammett’s σ – constants it can be inferred that the nucleophilic reactivities are strongly controlled by solvation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)