The effect of medium on rate and mechanism: aminolysis of O-4-nitrophenyl thionobenzoate in MeCN and H2O.
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.