Abstract

This chapter discusses the uses of reactivity–selectivity descriptor in organic chemistry. The reactivity–selectivity descriptor that characterizes the reactive behavior of the different topological regions within a molecule is used to study the reactivity and selectivity of various classical organic reactions. It is shown that the intrinsic electrophilic/nucleophilic character of the different molecular regions determined through reactivity–selectivity descriptor predicts correctly the regio-and stereo-selectivity of these reactions. In contrast to the frontier molecular orbital (FMO) theory where the couple nucleophile–electrophile needs to be identified to perform the molecular orbital analysis, the use of the reactivity–selectivity descriptor does not need an a priori classification of the reacting molecules because all molecules present topological regions, in which both nucleophilic and electrophilic characters are locally allowed. In the rationalization of the mechanism of a chemical reaction characterized by soft–soft interactions, the only thing that matters is the matching of the electrophilic region of one molecule with nucleophilic region of the other; this complies with the principle of maximum hardness (PMH) and with the hard and soft acids and bases (HSAB) postulate. While in the case of hard–hard interactions extra chemical considerations should be added for a correct identification of the reactive sites.