Is electrophilicity a kinetic or a thermodynamic concept?

Abstract

An understanding of the precise nature of a chemical reactivity descriptor is of utmost interest to quantum chemists. An attempt has been made here to analyze whether the electrophilicity index is a reliable descriptor of the kinetic behaviour or not. Relative experimental rates of Friedel-Crafts benzylation, acetylation and benzoylation reactions correlate well with the corresponding calculated electrophilicity values. Chlorination of various substituted ethylenes and nitration of toluene and chlorobenzene have been studied as representative examples of electrophilic addition and substitution reactions, respectively. The correlation is not very good although it improves drastically by removing a few data points to show that the electrophilicity is a kinetic quantity with inherent thermodynamic information. The correlation between the experimental and the calculated activation energies is studied for some Markovnikov and anti-Markovnikov addition reactions and it turns out to be reasonably well. Reaction electrophilicity, local electrophilicity and activation hardness are used together to provide a transparent picture of reaction rates as well as the orientation of aromatic electrophilic substitution reactions. Ambiguity in the definition of the electrophilicity is highlighted.