The Reactivity—Selectivity Principle and its Mechanistic Applications

Abstract

This chapter focuses on the status of reactivity–selectivity relationships by exploring the basis and variety of such relationships, and outlines the limitations of this principle. A qualitative statement of reactivity–selectivity principle is that highly reactive species are unselective in their choice of reactants compared to stable and, therefore, unreactive species. In a particular reaction series, an increase in the reactivity of one of the reactants results in a corresponding decrease in the selectivity of that species. In certain mechanistic areas, for example electrophilic substitution and carbene chemistry, mechanistic information can be derived by the reactivity–selectivity principle, which serves as a key probe into the stability and the structure of highly active species. The observation of a reactivity–selectivity relationship for a given reaction series suggests a certain uniformity in mechanism; a sudden break or the total failure to obtain such a relationship suggests the opposite. In view of the basic nature of the assumptions on which the reactivity–selectivity principle is based, reactivity–selectivity relationships serve as a probe into some of the fundamental tenets of theoretical chemistry. Some limitations of this principle are that it only applies to processes which obey a rate- equilibrium relationship; it only operates for relatively simple processes; reactions in which solvation factors contribute substantially to the overall energy change may also bring about a breakdown in the principle. The utility of the reactivity–selectivity principle is illustrated for a number of diverse areas of mechanistic interest. © 1977, Academic Press Inc. (London) Ltd