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pKa values in aqueous solution have been determined for seven p-substituted acetophenones. The determinations are based on the diffusion-controlled reaction of an enolate with hypochlorous acid. The values determined for the acetophenones are as follows: substituent, value; H, 18.4; p-MeO, 19.0; p-F, 18.5; p-Cl, 18.1; p-Br, 18.0; p-NO2, 16.7; p-Me3N+, 17.1. ρ for these pKa values is −1.95. Using literature values for the enol content we show that most of this ρ value reflects substituent effects upon enolization. With relatively high hydroxide and low hypochlorite concentrations the major products from chlorination of acetophenones are the corresponding mandelic acids. Under similar conditions bromination of acetophenone gives benzoic acid.

The pAa of a simple ketone can be determined by analysis of the kinetics of halogenation in alkaline solution (J. Am. Chem. Soc. 1982,104, 895). Details of the determination of the pAa of acetone are reported. The stoichiometry of chlorination of acetone is 2.15 hypohalites per acetone; bromination consumes 2.83 hypobromites. The major product from chlorination is lactate and not acetate; lactate is a minor product from bromination of acetone. The kinetic significance of the possible side reactions is discussed. The determination of the pAa requires knowledge of the rate constant for a diffusion-controlled reaction of nonspherically symmetrical molecules in water; the state of our knowledge of this rate constant is reviewed critically. It is shown that for reactions involving diffusion together of reactants which then undergo direct heavy-atom bond making it is very unusual for the diffusion limit to be reached, although the observed rate constant may become insensitive to the thermodynamic reactivity of the reactants.

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