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Acid-base equilibria of aromatic ketyl radicals and of their dimers (pinacols) were studied by flash photolysis of ketones and by electrochemical methods. pK values of the ketyls were found : fluorenone, 9.5 ; xanthone, 9.8 ; benzophenone, 9.15 ; 4-chlorobenzophenone, 9.2. The pK of the corresponding pinacols are 13.7, 13.5, 12.6 and 13-6, respectively. The difference in acidity between free radicals and their dimers is due to different hybridization of the C atom bonded to the OH group. Some new data are reported on the normal red-ox potentials of the ketone-ketyl anion systems, on the absorption spectra and dimerization rate constants of the ketyls. 918

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Solvent effects on the Hammett ρ value for the cathodic reduction of substituted benzophenones were determined. The electrochemistry of a series of 11 compounds was studied in acetonitrile, acetone, dimethyl sulphoxide, propylene carbonate, N,N‐dimethylformamide, N,N‐dimethylacetamide, N,N‐diethylformamide and hexamethylphosphoric triamide. The ρ values for the reversible one‐electron transfer are described by the Lewis acid‐base model ρ = − 0.006AN + 0.003DN + 0.391, where AN = solvent acceptor number and DN = solvent donor number.