A comparison of the relative energies of isomeric intermediates formed in electrophilic, radical, and nucleophilic aromatic substitution reactions

Abstract

The relative energetics of isomeric σ-complex intermediates formed in electrophilic, free radical, and nucleophilic attack of Cl, H, and Me on PhCl and PhMe are compared for all positions: o, m, p, and ipso. The results are presented as ΔH, computed by the MINDO/3 method, for the appropriate isodesmic reaction with benzene. For the chlorobenzene intermediates, there is a marked increase in the relative preference for the ipso-position as the reactions change from electrophilic to free radical to nucleophilic. For toluene intermediates, the order of stability- p > o > m > ipso-is seen in all reactions except for one case, free radical methylation. In general, the free radical intermediates show the smallest range of energy differences. Comparison of predictions from these calculations with experimental results (largely partial rate factors and product ratios) shows some qualitative agreement.