Journal of the American Chemical Society p. 5758 - 5765 (1982)
Update date:2022-08-28
Topics:
Tapuhi, Eliana
Jencks, William P.
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 pKa 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 pKaSH = 10.8 and 10.6 for the enol give pKE = 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.
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