REACTIVITIES OF DIARYLMETHYL AND TRIARYLMETHYL CATIONS WITH PRIMARY AMINES IN AQUEOUS ACETONITRILE SOLUTIONS - THE IMPORTANCE OF AMINE HYDRATION

By use of the technique of laser flash photolysis, rate constants k(RNH2) have been directly measured for the reactions of primary amines RCH2NH2 (R = CH3CH2, CH30CH2, NCCH2, CF3) with diarylmethyl cations (D+) in acetonitrile/water solutions. In 100% acetonitrile the reactions approach the diffusion limit, 5 x 10(9) M-1 s-1, although they are slower, k(RNH2) for a given cation increasing with increasing amine basicity and for a given amine increasing with decreased electron donation from substituents in D+. In the mixed solvents the rate constants decrease in a regular fashion with increasing water content. The changes can be large, being on the order of 10-100 proceeding from 100% acetonitrile to 100% water. Moreover, the rate-retarding effect of water is more pronounced with more basic amines, with the consequence that in water-rich solutions the reactivity order no longer parallels amine basicity. Plots of log k(RNH2) versus pK(a)(RNH3+) not only are curved but also show a change in the sign of their slope on progressing from weakly basic amines (positive-beta(nuc)) to strongly basic ones (negative-beta(nuc)). This behavior is explained by a mechanism in which a hydrated amine RNH2...HOH is unreactive and an equilibrium desolvation to form the unhydrated amine precedes reaction with the cation. Quantitative treatment is carried out, using the rate constants in 100% acetonitrile to model the reaction of the free amine. This approach reproduces the experimental data within an average of +/- 0.04 log unit and results in equilibrium constants for the desolvation with the expected beta = -0.2 dependency on amine basicity. Rate constants have also been measured in 33% acetonitrile/water for a series of triarylmethyl cations ranging from 4,4'-(Me2N)2T+ to 4,4'-(CF3)2T+. The beta(nuc) values for these are all positive, with a clear trend for beta(nuc) to decrease with increasing cation reactivity, this being true even for the relatively stable cations 4,4'-(Me2N)2T+, 4-Me2NT+, and 4,4',4"-(MeO)3T+. Thus, amine nucleophiles do not adhere to the N+ constant selectivity relation, even for stable cations.