Electrophile affinity: a reactivity measure for aromatic substitution.

The reactivity and regioselectivity of the electrophilic chlorination, nitration, and alkylation of benzene derivatives were rationalized by comparing literature data for the partial rate factors (ln f) for these S(E)Ar processes with theoretical reactivity parameters. The Electrophile Affinity (Ealpha), a new quantity, is introduced to characterize reactivity and positional selectivity. Ealpha is evaluated theoretically by the energy change associated with formation of an arenium ion by attachment of a model electrophile to the aromatic ring. The dependence between Ealpha and ln f values for chlorination for 11 substitutions of benzene and methyl benzenes had a high correlation coefficient (r = 0.992). Quite satisfactory correlations between Ealpha values and partial rate factors also were obtained for the nitration of substituted benzenes (r = 0.971 for 12 processes) and benzylation of benzene and halobenzenes (r = 0.973 for 13 processes). These results provide clear evidence for the usefulness of the electrophile affinity in quantifying reactivity and regiochemistry. Satisfactory relationships (r >0.97) also were found between EPN (electrostatic potential at nuclei) values, which reflect the variations of electron density at the different arene ring positions, and the experimental partial rate factors (ln f) for the chlorination and nitration reactions, but not for the benzylation. This disaccord is attributed to strong steric influences on the reaction rates for substitutions involving the bulky benzyl moiety.