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Relevant academic research and scientific papers

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, CH3OCH2, NCCH2, CF3) with diarylmethyl cations (D+) in acetonitrile/water solutions. In 100% acetonitrile the reactions approach the diffusion limit, 5 × 109 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 pka(RNH3+) not only are curved but also show a change in the sign of their slope on progressing from weakly basic amines (positive βnuc) to strongly basic ones (negative βnuc). 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 β= -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 βnuc values for these are all positive, with a clear trend for βnuc 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.

Electrophilic Reactivity of the Triphenylmethyl Carbocation in Aqueous Solutions

The triphenylmethyl (trityl) carbocation has been generated as a transient intermediate by laser flash photolysis of 1:2 (v/v) acetonitrile:water solutions of trityl acetate and trityl 4-cyanophenyl ether.Identification of the transient as the free carbocation in the ground state was based on its characteristic absorption spectrum and upon conductivity changes.Rate constants have been measured for the reaction of the cation in this solvent with a series of ionic and neutral nucleophiles.The solvent rate constant at 20 deg C is 1.5 x 105 s-1.Azide ions reacts at 4.1 x 109 M-1s-1; the directly measured azide:water ratio is compared to literature values determined by product analysis.Chloride ion reacts at 2 x 106 M-1 s-1; with bromide the equilibrium addition can be observed with k(comb) = 5 x 106 M-1 s-1 and k(ion) for Ph3CBr = 8 x 105 s-1.Rate constants do not adhere to the N+ relationship.This predicts a slope of unity in a plot of log k(Ph3C+) vs.N+, with the better nucleophiles reacting at the 1010 encounter-controlled limit.Azide is the only nucleophile of those studied to approach this.Sulfite and thiolate ions, which are better N+ nucleophiles, react at 2-3 x 108 M-1s-1, while amines react in the 106-107 M-1s-1 range.The plot vs.N+ has a slope of 0.3-0.4.One explanation is that rate constants for the better nucleophiles do level, but this occurs considerably below the 1010 limit.Alternatively, the less than unit slope is real and this more reactive cation, in contrast to more stable analogues, is exhibiting selectivity.