Theoretical scales of reactivity and selectivity are important tools to explain and to predict reactivity patterns, including reaction mechanisms. The main achievement of these efforts has been the incorporation of such concepts in advanced texts of organic chemistry. In this way, the modern organic chemistry language has become more quantitative, making the classification of organic reactions an easier task. The reactivity scales are also useful to set up a number of empirical rules that help in rationalizing and in some cases anticipating the possible reaction mechanisms that can be operative in a given organic reaction. In this review, we intend to give a brief but complete account on this matter, introducing the conceptual basis that leads to the definition of reactivity indices amenable to build up quantitative models of reactivity in organic reactions. The emphasis is put on two basic concepts describing electron-rich and electron-deficient systems, namely, nucleophile and electrophiles. We then show that the regional nucleophilicity and electrophilicity become the natural descriptors of electrofugality and nucleofugality, respectively. In this way, we obtain a closed body of concepts that suffices to describe electron releasing and electron accepting molecules together with the description of permanent and leaving groups in addition, nucleophilic substitution and elimination reactions. 1. Introduction The development of reactivity indices to describe organic reactivity has been an active area of research from the dawn of theoretical physical organic chemistry [1–4]. From the earlier semiempirical models proposed in Hückel molecular orbital (HMO) theory, reactivity was described with the aid of static first order reactivity descriptors like atomic charges, free valence index, and bond orders [5, 6]. These reactivity indices were formerly developed around the ground state of reactants. The next generation of reactivity indices began with the elegant theory proposed by Coulson and Longuet-Higgins, in a series of papers describing the response functions, including second order quantities like atomic and bond polarizabilities [7–9]. It is worth emphasizing that, at that time, HMO and Coulson-Longuet-Higgins theories were conceived not as methods to approximately solve the Schr？edinger equation but as models of chemical bond. The third generation of reactivity indices started after the pioneering work of Gilles Klopman, who introduced the concept of charge and frontier controlled reactions, including solvation effects [4]. Nowadays, the treatment of
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