Nucleophilicities of primary and secondary amines in water

The kinetics of the reactions of 26 primary and secondary amines with benzhydrylium ions in water were investigated photometrically. Because the parallel reactions of the benzhydrylium ions with hydroxide and water are much slower, the second-order rate constants for the reactions of amines with benzhydrylium ions could be determined reliably. Reactivities of anilines were also studied in acetonitrile solution. Plots of log k(2,N) for these reactions vs the electrophilicity parameters E of the benzhydrylium ions were linear, which allowed us to derive the nucleophilicity parameters N and s for amines as defined by the equation log k(20 degrees C) = s(E + N). Because the slope parameters for the different amines are closely similar; the relative nucleophilicities are almost independent of the electrophiles and can be expressed by the nucleophilicity parameters N. The correlation between nucleophilicity N and pK(aH) values is poor, and it is found that secondary alkyl amines and anilines are considerably more nucleophilic, while ammonia is much less nucleophilic than expected on the basis of their pK(aH) values.