Role of structure, medium and pathway in electrophilic aromatic substitutions. Part 2. Incidence of π-complexes and influence of medium on stability of incipient moieties in molecular chlorination of aromatic hydrocarbons

Abstract

The kinetics of molecular chlorination of twelve benzenoid compounds has been studied at 25 °C in pure acetic acid as solvent, and over a wide range of initial substrate concentrations. For each kinetic run the rate coefficient remains constant to within experimental error. However, in the case of eleven of these compounds a significant sustained decrease in the rate coefficient of the substrate is observed to follow a regular increase in initial substrate concentration. For example, the rate constant of p-xylene decreases steadily to effect a 55% rate drop (k₂= 3.42 – 1.55 × 10^–3 dm³ mol^–1 s^–1) over a tenfold increase in initial xylene concentration (0.191–1.98 mol dm^–3). The results obtained in the present kinetic study are amenable to: (i) qualitative interpretation in terms of a differential effect of solvent on incipient charged moieties associated with the transition state of the reaction; and (ii) quantitative rationalization when a pathway is accepted in which a pre-equilibrium π-complex precedes the transition state, and with the kinetic rate equation incorporating both the equilibrium constant of the complex and its specific rate of further reaction.