Vapour-phase chemistry of arenes. Part 11. Autoxidation of chlorobenzene between 500–1 100 K. Mechanisms of formation of chlorophenols and phenol

Abstract

Slow combustion of chlorobenzene (1) has been studied at low degrees of conversion (< 5%); mostly at atmospheric pressure, but at ca. 1 080 K under FVT-like conditions. Emphasis was on formation of the hydroxy derivatives o/m/p-ClC₆H₄OH (2) and the ipso substitution product phenol (3).

A variety of ingredients, such as Bu^tOOH, cyclohexane, nitroethane, and hydrogen peroxide, forming ˙OH in situ by thermolysis or autoxidation, has been used to induce the reaction of (1).

The change in product composition (2, 3) with temperature revealed a major change in mechanism of hydroxylation between 600–700 K. Above ca. 600 K, H-abstraction (1)+˙OH→o/m/p-ClC₆H₄˙[accompanied by (1)+˙OH→Cl˙+(3)] is followed by rapid addition of O₂(step 4) to give C₆H₄OO˙ radicals (v).

At moderate temperatures (v) reacts bimolecularly, with itself or with HO₂˙, leading to ClC₆H₄O˙(iv) and therefrom to (2). Thermokinetic analysis emphasises that at elevated temperatures (v) decomposes into (iv) and O(³P). The latter species adds to (1) and forms (2) rather efficiently, with an isomer distribution different from that associated with the pathway Ar˙→ArO₂˙→ArOH.

Data on model runs, including kinetic isotope effect measurements on p-DC₆H₄Cl and C₆H₆/C₆D₆, support our mechanistic interpretation.