A comprehensive view on stabilities and reactivities of triarylmethyl cations (tritylium ions)

Abstract

By combining conventional photometric and conductimetric methods with stopped‐flow techniques, we were able to investigate ionization rate constants of trityl bromides, chlorides, and carboxylates in aqueous acetonitrile in a reactivity range from 10 −5 to 10 3 s −1 . In this way, it became possible to compare ionization processes yielding tritylium ions which differ by 21 units in p K R+ , corresponding to 120 kJ mol −1 in free energy. Analysis of such far‐reaching correlations provided new insights into several aspects of organic reactivity. We observed (i) the change of solvolysis mechanisms from S N 1 reactions without and with common ion return, (ii) reactions where formation and consumption of the carbocations could be observed visually, (iii) solvolyses where the ionization step is almost complete before the reaction with water becomes significant, and (iv) ionizations that yield solutions of persistent carbocations. The continuous change from reactions with carbocation‐like transition states to reactions with transition states of little carbocation character caused large variations of Winstein–Grunwald m values. Values of m = 0.2 for ionization processes show that low values of m do not necessarily imply S N 2 reactions. Although the 4,4′,4″‐trimethoxytritylium ion reacts more than 1000 times faster with water than the 4‐dimethylaminotritylium ion and has a p K R+ value that is more negative by three units, both carbocations are formed with almost equal rates by ionization of the corresponding acetates in aqueous acetonitrile. This comparison again illustrates that electrofugality is not simply the inverse of electrophilicity and presents a challenge for further study of the importance of intrinsic barriers. Copyright © 2012 John Wiley & Sons, Ltd.