QUANTITATIVE COMPARISON OF NUCLEOPHILIC SUBSTITUTION AT SULFONYL VS SULFINYL SULFUR . HYDROLYSIS OF ARYL ALPHA-DISULFONES IN AQUEOUS DIOXANE

The spontaneous (uncatalyzed) hydrolysis of aryl a-disulfones (eq 1) has been studied kinetically in acidic aqueous dioxane. Because this reaction differs from the previously investigated2 spontaneous hydrolysis of aryl sulfinyl sulfones (eq 2) only in that eq 1 involves nucleophilic substitution at sulfonyl (>SO2) sulfur while eq 2 involves substitution at sulfinyl (>SO) sulfur, the data for the two reactions (Table II) provide a quantitative comparison of the influence of various important reaction variables on nucleophilic substitution at sulfonyl vs. sulfinyl sulfur. This comparison reveals that, despite the fact that the substitution at sulfinyl sulfur is 104 times faster than the one at sulfonyl sulfur, the two reactions show a remarkable similarity in (1) dependence of rate on aryl group structure, (2) increase in rate with water content of the solvent, (3) solvent isotope effect, and (4) ∆S℠. The large difference in rate arises solely as a result of a 6.0-kcal/mole difference in activation energy. The conclusion is that analogous nucleophilic substitutions at sulfonyl and sulfinyl sulfur do not differ significantly in their detailed mechanism: nucleophilic substitution at sulfonyl sulfur is much slower merely because of a less favorable activation energy. The large solvent isotope effect ([formula-ommited] = 2.3) for α-disulfone hydrolysis indicates that a proton transfer is part of the rate-determining step of that reaction. Various mechanisms including this feature are discussed, and it is concluded that a mechanism (eq 8) involving a concerted proton transfer from the attacking water molecule to the departing ArSO2 group is the most satisfactory one on the basis of presently available evidence. Finally, comparison of the spontaneous hydrolysis of α-disulfones with that of aryl sulfonic anhydrides (eq 3) shows that in reactions of the type under consideration a change in the character of the leaving group is apparently much more likely to lead to a significant change in mechanism than is a change in the site where substitution occurs from sulfonyl to sulfinyl sulfur. © 1969, American Chemical Society. All rights reserved.