Reaction of Halide Ion with 2-(Phenylthio)ethyl Halides in Acetone


2-(Phenylthio)ethyl chloride specifically deuterated α to the sulfur atom (i.e. 2-Cl) was employed to study the mechanism of displacement of chloride ion by iodide ion in dry acetone solution. Experimental evidence including rate studies has been accumulated that allows the conclusion to be drawn that iodide ion displaces chloride ion from 2-Cl by a classical SN2 mechanism to give 2-I. Under the reaction conditions, however, 2-I is reactive toward anchimerically-assisted ionization which leads to product with the deuterium label scrambled. Reaction of 2-Cl with bromide ion follows a similar course. Our displacement reactions of halide ions by halide ions showed the expected nucleophilicity order of Cl- > Br- > I- and the normal leaving group order of I- > Br- > Cl-. An interesting counterion effect was noted with iodides displacing chloride ion. While potassium iodide in acetone converted 2-Cl to the mixture of iodides 2-I and 3-I, tetrabutylammonium iodide caused conversion but the products reverted completely back to the label-scrambled chlorides. The difference in these results is probably because of differences in the thermodynamics of the two systems.

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Journal of Organic Chemistry