The Journal of Organic Chemistry
1022 SMITHAND FELDT
Evidence for the Formation of Two Tetrasubstituted Intermediates in the Alkaline Hydrolysis of Ethyl Thionbenzoate'" STANLEY G. SMITH'^ AND RAYMOND J. FELDT The Noyes Chemical Laboratory, University of Illinois, Urbana, Illinois 61801 Received September 16, 1967 The effect of pH on the rate and the yield of ethyl benzoate and thiobenzoic acid in the hydrolysis of ethyl thionbenzoate in 50% aqueous dioxane has been studied. Over the pH range of 4-12 the product of hydrolysis is very largely ethyl benzoate arising from sulfur-oxygen exchange. Under more alkaline conditions the fraction of the reaction giving ethyl benzoate drops to 0.4 with thiobenzoic acid becoming the major product. The change in the course of the reaction with acidity may be accounted for by a pH-dependent equilibrium between a t least two intermediates.
Tetrasubstituted2 intermediates in the reaction of nucleophiles with carboxylic esters have generally been invoked to explain I 8 0 e ~ c h a n g e~, ~p e c t r a l and , ~ kinetic data.3p5 Thion esters,* I, constitute a particularly instructive system for the study of such intermediates since in aqueous medium the process which formally corresponds to oxygen exchange with carboxylic esters gives rise to sulfur-oxygen exchange and the formation of the corresponding oxo ester (eq 1).
R-C-OR' I
0
+
II
SH
I R-C-OR I OH
H20
f
+
R-C-OR'
Ha
0
(1)
II R-C-SH
+
R'OH
Previously6 it was found that ethyl thionbenzoate (11) in 40% aqueous acetone a t 25.0" with 0.05 M sodium hydroxide gave 22% ethyl benzoate (111)resulting from sulfur-oxygen exchange and 78% hydrolysis to thionbenzoic acid (IV) and ethanol. Using hydrochloric acid as an hydrolysis catalysis a t 125.0°, 85% of the reaction was found to proceed through sulfur-oxygen exchange. These studies have now been extended to provide more information on the effect of pH on the fmction of the reaction proceeding by each route. S
II
CcH,-C-OC2H5 nlk*
k
-b HL'
C6H5C
