Some comments concerning the use of static charge distributions for predicting chemical reactivity

Abstract Static charge distributions are frequently used for interpreting and predicting chemical reactivity. As examples, the electrophilic aromatic substitution of nitrobenzene and aniline were specifically examined with the use of a proton as a model electrophile. The electrostatic potentials of these compounds and energies and geometries of the protonated products were calculated. It was found that the minimum in the potential may indeed indicate the site most susceptible to electrophilic attack. Nevertheless, this site may not be the one found substituted under normal laboratory conditions. Some experimental evidence is pointed out that demonstrates the intermediacy of these initially formed protonated species. Consequently, static charge distributions will have limited use in predicting substitution sites of electrophilic reactions, unless prior experience in a series of compounds is available. A generalization is stated: the charge distribution of the reactant most nearly resembles the transition state nearest in energy. Thus, the static charge distribution should indicate sites of kinetic attack by incoming reagents. Similarly, the static charge distribution is most closely related to complexes that perturb the initial distribution the least. Examples are likely to be the formation of encounter complexes and hydrogen-bonded complexes.