Sandmeyer reactions. Part 5. Estimation of the rates of 1,5-aryl/aryl radical translocation and cyclisation during Pschorr fluorenone synthesis with a comparative analysis of reaction energetics

During the Pschorr cyclisation of 2-aroylphenyl radicals, a rearrangement o ccurs reversibly by 1,5-hydrogen transfer to give 2-benzoylaryl radicals. R ate constants of (1.2 +/-0.2)x10(6) s(-1) at 293 K are estimated for both t he forward and back reactions in the equilibrium between 2-(4-methylbenzoyl )phenyl and 2-benzoyl-5-methylphenyl radicals. Assuming an empirical estima te of 1.6x10(-2) dm(3) mol(-1) s(-1) for the hypothetical rate of abstracti on of hydrogen from benzene by phenyl radicals, the radical translocation i s calculated to occur with a statistically corrected effective molarity of 2.2x10(8) mol dm(-3). By contrast, the competing cyclisation, though occurr ing at a rate of (8.0 +/-0.9)x10(5) s(-1), exhibits an effective molarity o f only 5.3 mol dm(-3). The causes of these differences are analysed in term s of reaction mechanism.