ACID-CATALYZED HYDROLYSIS OF 2-(SUBSTITUTED PHENYL)-1,3-OXATHIOLANES

The rates of acid-catalyzed hydrolysis of a series of 2-(substituted phenyl)-1,3-oxathiolanes have been measured in H2O and 50% dioxane-H2O. A plot of the logarithms of the rate constants vs. ó, the Hammett substituent constant, is curved, the point for the p-methoxy-substituted compound showing considerable positive deviation from the line established by the use of meta-substituted compounds and compounds having electron-withdrawing substituents in the para position (p =–2.8). The p-methoxy group produces negative deviation when ó+ constants are employed and H2O is the solvent. Reasonable linearity is obtained, however, with ó+ and the logarithms of rate constants measured in 50% dioxane-H2O. A plot of log kobsd for hydrolysis of 2-(p-nitro phenyl)-1,3-oxathiolane in various aqueous HC1 solutions vs.–H0 is linear with a slope of 1.23. Thus, the transition state in these hydrolysis reactions must resemble a carbonium ion ith the most likely mechanism involving a unimolecular decomposition of a protonated intermediate. The value of kD/kH for hydrolysis of 2-(p-meth oxyphenyl)-1,3-oxathiolane (1.93) is considerably less than normally observed for hydrolysis of analogous acetals and may indicate that the protonated intermediate has sulfur protonated. The value of ∆S* for hydrolysis of 2-phenyl-1,3-oxathiolane is–13.2 eu. © 1969, American Chemical Society. All rights reserved.