Second-order rate constants have been measured for alkaline hydrolysis of O-aryl thionobenzoates (X-C(6)H(4)-CS-OC(6)H(4)-Y) in 80 mol % H(2)O-20 mol % DMSO at 25.0 +/- 0.1 degrees C. The Hammett plot for the reaction of O-4-nitrophenyl X-substituted thionobenzoates (X-C(6)H(4)-CS-OC(6)H(4)-NO(2), 1a-e) exhibits a downward curvature. However, a possible traditional explanation in terms of a change in the rate-determining step (RDS) has been considered but rejected. The proposed explanation involves stabilization of the ground-state (GS) through-resonance interaction between the electron-donating substituent X and the thionocarbonyl functionality on the basis of the linear Yukawa-Tsuno plot obtained for the same reaction. The Brønsted-type plot for the reaction of O-aryl thionobenzoates (C(6)H(5)-CS-OC(6)H(4)-Y, 2a-i) is linear but exhibits many scattered points with a small beta(lg) (-0.35). The Hammett plot for the same reaction shows rather poor correlation with sigma(-) constants but much better correlation with sigma(o) constants. The alkaline hydrolysis of O-aryl thionobenzoates (1a-e and 2a-i) has been proposed to proceed through an addition intermediate in which bond formation is the RDS.

Pseudo-first-order rate constants (kobs) have been measured spectrophotometrically for reactions of O-4-nitrophenyl thionobenzoate (1) with a series of alicyclic secondary amines in MeCN and H2O at 25.0 +/- 0.1 degrees C. The plot of kobs vs amine concentration exhibits an upward curvature in all cases, indicating that the reactions proceed through two tetrahedral intermediates (a zwitterionic T(+/-) and its deprotonated anionic T-) regardless of the amine basicity and the nature of the reaction medium. However, all the amines investigated have been found to be much less reactive in MeCN than in H2O, although the amines are more basic in the former medium by 7-9 pKa units.