  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.
