During the Pschorr cyclisation of 2-aroylphenyl radicals, a rearrangement occurs reversibly by 1,5-hydrogen transfer to give 2-benzoylaryl radicals. Rate constants of (1.2 ± 0.2) × 10 6 s Ϫ1 at 293 K are estimated for both the forward and back reactions in the equilibrium between 2-(4-methylbenzoyl)phenyl and 2-benzoyl-5-methylphenyl radicals. Assuming an empirical estimate of 1.6 × 10 Ϫ2 dm 3 mol Ϫ1 s Ϫ1 for the hypothetical rate of abstraction of hydrogen from benzene by phenyl radicals, the radical translocation is calculated to occur with a statistically corrected effective molarity of 2.2 × 10 8 mol dm Ϫ3 . By contrast, the competing cyclisation, though occurring at a rate of (8.0 ± 0.9) × 10 5 s Ϫ1 , exhibits an effective molarity of only 5.3 mol dm Ϫ3 . The causes of these differences are analysed in terms of reaction mechanism.
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