Electrophilicity as a Possible Descriptor of the Kinetic Behaviour

An understanding of the precise nature of a chemical reactivity descriptor is of utmost interest to quantum chemists. In the present investigation an attempt has been made to analyze whether the electrophilicity index is a reliable descriptor of the kinetic behaviour. 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 are studied as representative examples of electrophilic addition and substitution reactions respectively. The correlation is not very good to assess that which, however, 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 is turned 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.