Nucleophilic Reactivities of Hydrazines and Amines: The Futile Search for the α-Effect in Hydrazine Reactivities

The kinetics of the reactions of amines, hydrazines, hydrazides, and hydroxylamines with benzhydrylium ions and quinone methides were studied in acetonitrile and water by UV-vis spectroscopy, using conventional spectrometers and stopped-flow and laser-flash techniques. From the second-order rate constants k(2), of these reactions, the nucleophilicity parameters N and s(N) were determined according to the linear free energy relationship log k(2) = s(N)(N + E). While methyl groups increase the reactivities of the alpha-position of hydrazines, they decrease the reactivities of the beta-position. Despite the 10(2) times lower reactivities of amines and hydrazines in water than in acetonitrile, the relative reactivities of differently substituted amines and hydrazines are almost identical in the two solvents. In both solvents hydrazine has a reactivity similar to that of methylamine. This observation implies that replacement of one hydrogen in ammonia by Me increases the nucleophilicity more than introduction of an amino group, if one takes into account that hydrazine has two reactive centers. Plots of log k(2) versus the corresponding equilibrium constants (log K) or Bronsted basicities (pK(aH)) do not show enhanced nucleophilicities (alpha-effect) for either hydrazines or hydroxylamine relative to alkylamines.