The 4,4 '-dimethoxytrityl carbenium ion by ionization of 4,4 '-dimethoxytrityl alcohol in acetonitrile - aqueous perchloric and nitric acids containing electrolytes: kinetics, equilibria, and ion-pair formation

We have studied the equilibration shown in eq. [3] of 4,4'-dimethoxytrityl alcohol in aqueous perchloric and nitric acids containing low proportions o f acetonitrile using stopped-flow kinetics techniques. The rate constants f or the overall progress to equilibrium, k(obs), have been resolved into for ward and reverse components using the equilibrium UV absorbance and a value for the molar absorptivity of the 4,4'-dimethoxytrityl carbenium ion deter mined in concentrated aqueous perchloric acid. The forward reaction (rate c onstant k(f)) is first order in both the alcohol and the acid concentration s; the reverse reaction (rate constant k(r)) is pseudo first order with res pect to the carbocation. At constant hydronium ion concentration, the forwa rd rate constant increases linearly with the concentration of electrolyte, whereas the reverse rate constant decreases. These effects depend upon the nature of the anion, but not the cation, and are not ionic strength effects . At constant anion concentrations, k(f) in both acids, and k(r) in perchlo ric acid, are independent of hydronium ion concentration; however, k(r) dec reases with increasing hydronium ion concentration at constant nitrate conc entration. At nonconstant ionic strength, changes in k(f) and k(r)observed in increasing concentrations of perchloric acid are attributable wholly to changes in perchlorate concentration. A mechanism is proposed which involve s pre-equilibrium protonation of the alcohol, heterolysis of the protonated alcohol to give a 4,4'-dimethoxytrityl carbenium ion - water ion-molecule pair, then conversion of this into a dissociated carbenium ion in equilibri um with ion pairs. To account for the strong effects of perchlorate and nit rate upon the forward rate constants, it is proposed that these anions prov ide additional reaction channels from the ion-molecule pair. However, we fi nd no evidence of acid catalysis in the reaction of the ion-molecule pair ( in contrast to our finding for the reaction of the corresponding ion-molecu le pair formed from dimethoxytritylamine in acidic media). Some of the elem entary rate and equilibrium constants of the proposed mechanism have been e valuated.