The mechanism of the vapor‐phase chlorination of benzene derivatives

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Abstract
The title reaction, displaying peculiar characteristics as to relative rates and isomer distributions, has been studied in detail. Prior to this study, different mechanisms had been advanced by several groups. Kinetic features (isomer patterns, relative and absolute rates, reaction orders, influences of additives, H/D isotope effects) strongly point to a free‐radical (chain) process, in which (1) is a crucial step. This abstraction reaction, endothermal by about 6 kcal/mol, apparently proceeds via a transition state closely resembling the free aryl radical. Relative rates and isomer distributions therefore reflect differences in stabilization energies, or in DH°(Ar – H). With high arene–Cl2 intake ratios or, more pronounced, with CCl4 as the reagent, aryl radicals also lead to biaryl, where arene successfully competes with the halogenating agent. This interpretation is quantitatively supported by our observation that “added,˝” recognizable aryl radicals yield the same chlorination–arylation product ratio, and by the results of competitive chlorination of benzene and chloroform over a temperature range of 200°C, where the latter study substantiates the value DH0(C6H5 – H) ≈ 109 kcal/mol.

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