Base-Catalyzed Halogenation of Acetone

Article abstract of DOI:10.1021/ja00385a034

The kinetics of iodination and bromination of acetone were measured in basic aqueous solutions, buffered with trifluoroethanol, under conditions in which halogenation of the enolate ion is the rate-determining step.With the assumption of diffusion-controlled halogenation of the enolate ion by I2 and HOBr with k = 5 * 10⁹ M^-1 s^-1, rate constants for other halogenating agents (M^-1 s^-1) are BrO^-, 8 * 10⁷; IO^-, 10⁸; IOH, 2.5 * 10⁸; and I3^-, 3.6 * 10⁸; the pK_a of acetone is 18.9 +/- 0.3 at 25 deg C, ionic strength 1.0 M (KCl) or 1.0 M (KNO3).Iodoform was identified spectrofotometrically as the product of iodination.Values of pK_a^SH = 10.8 and 10.6 for the enol give pK_E = 8.1 and 8.3, respectively, for formation of the enol from acetone.It is suggested that the reactions of IO^- and BrO^- may proceed through the formation of a complex with the enolate, followed by protonation and halogenation, and that I3^- may serve as an iodine donor through a preassociation mechanism.The observed dependence of the absorbance of I3^- on buffer concentration and pH indicates the formation of a significant amount of a species with the composition ROIOH^- at high concentrations of trifluoroethoxide and hydroxide ions.