Nucleophilicity Parameters for Carbanions in Methanol

Abstract

AbstractThe 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)