Acid‐catalysed hydrolysis of trityl derivatives in strongly acidic aqueous media

Abstract

The kinetics of hydrolysis (deamination or dealcoholation) of tritylamines and 2,2,2‐trifluoroethyl ethers and their corresponding 4‐methoxy, 4,4′‐dimethoxy‐, and 4,4′,4″‐trimethoxy‐substituted analogues in aqueous solutions up to 3.5 mol · dm−3 in strong acid have been investigated at constant ionic strength. In all cases, acid‐catalysed hydrolytic processes have been observed, with finite reactivity at [H3O+] = 0. Strong upward curvature has been observed for kobs versus [HClO4]. Analysis of this dependence in terms of the HR acidity function and the X0 excess acidity scale allow explanation of the observed behaviour in terms of the increasing differences between concentrations and activities of the various species involved in the processes, including water, for which the activity coefficient strongly diverges from its standard state value as the acidity increases. This analysis has shown that, by taking account of the effect of the ionic strength, the same mechanistic models proposed for mildly acidic solutions are valid in more highly concentrated acid media. These comprise (i) protonation of the trityl ether followed by C–O bond heterolysis to give a carbenium ion–alcohol (ion–molecule) pair which can separate, and (ii) C–N bond heterolysis of the protonated tritylamine to give a carbenium ion–amine (ion–molecule) pair followed by separation of the fragments or protonation of the amine and subsequent separation of the ions. Each separated (substituted) trityl carbenium ion, regardless of its provenance, is invariably captured by a solvent molecule (water). Copyright © 2013 John Wiley & Sons, Ltd.