Nucleophilic Reactivities of Primary and Secondary Amines in Acetonitrile

The kinetics of the reactions of primary and secondary amines with benzhydrylium ions and quinone methides in acetonitrile have been studied under pseudo‐first‐order conditions (high excess of amines) by UV/Vis spectrophotometry. Generally, the plots of k obs versus amine concentration were linear, in line with a second‐order rate law. However, for some reactions of secondary amines with quinone methides, the plots of k obs versus amine concentration showed an upward curvature, which indicates rate‐limiting deprotonation of the initially formed adduct by a second molecule of amine. From the second‐order rate constants k 2 for the attack on the electrophiles by the amines, the nucleophilicity parameters N and s for the amines were determined from the linear free energy relationship log k 2 (20 °C) = s ( N + E ). The rates of the reactions of the amines with benzhydrylium ions are strongly affected by solvent polarity, in sharp contrast to the analogous reactions of other neutral nucleophiles.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)