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 α-position of hydrazines, they decrease the reactivities of the β-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 Brønsted basicities (pK(aH)) do not show enhanced nucleophilicities (α-effect) for either hydrazines or hydroxylamine relative to alkylamines.

Rate constants for solvolyses of benzhydryl chlorides, which take place on the 10 ms to minute time scale, have been determined in aqueous acetone and acetonitrile by conductometry, using conventional conductometers as well as stopped-flow techniques. Secondary and tertiary amines were used to suppress ion recombination (common-ion return) thus giving access to the ionization rate constants k(1). The observed common-ion rate depressions can be rationalized by the correlation equation for electrophile-nucleophile combinations, log k(20 degrees C) = s(E + N), where electrophiles (here: carbocations) are characterized by the parameter E and nucleophiles (here: chloride anions and solvents) are characterized by N and s.

The rates of the hydride abstractions from the 2-aryl-1,3-dimethyl-benzimidazolines 1a-f by the benzhydrylium tetrafluoroborates 3a-e were determined photometrically by the stopped-flow method in acetonitrile at 20 degrees C. The reactions follow second-order kinetics, and the corresponding rate constants k2 obey the linear free energy relationship log k2(20 degrees C) = s(N+E), from which the nucleophile-specific parameters N and s of the 2-arylbenzimidazolines 1a-c have been derived. With nucleophilicity parameters N around 10, they are among the most reactive neutral C--H hydride donors which have so far been parameterized. The poor correlation between the rates of the hydride transfer reactions and the corresponding hydricities (DeltaH0) indicates variable intrinsic barriers.

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