. Can. J. Chem. 63, 3 155 (1985).The hydrolysis of methyl dimethylphosphinothionate in D2S04-D20 solution gives as products the phosphinic acid and methyl dimethylphosphinate. The bell-shaped pH-rate profiles for both reactions as well as the values of Bunnett's w and w* (3.3 1 and -0.21 respectively), Bunnett-Olsen's c$ (0.82), and Yates-McClelland's r (6.56) support an A-2 pathway. The results are interpreted in terms of the presence of a common pentacoordinate intermediate along the reaction pathway. The pK, of the above ester as well as that of methyl dimethylphosphinodithioate are reported (-5.53 and -5.38 Introduction From our previous studies on the acid-catalyzed hydrolysis of phosphinates we reported that methyl dialkylphosphinates hydrolyze by an A-2 process and, based on relative rate data for the esters l a -d of 11 : 2.5: 1.2: 1, we suggested that water attacks the carbon of the protonated ester with subsequent C-0 bond cleavage (2). On the other hand, McClelland has shown by 0 -1 8 studies that compounds 2a-c undergo acidcatalyzed hydrolysis with approximately 90% P-0 bond cleavage (3). In the alkaline hydrolysis of phosphinates, P-0 bond cleavage is the norm (4) except in the case when phosphorus is substituted with large alkyl groups like iC,H7 where Haake and Rahil (5) have shown that there is mixed cleavage (approximately 25% C-0 cleavage). In other phosphorus esters P-0 cleavage predominates in base and C-0 cleavage in acid (6).

Hydrolysis of the title compound in strong acid leads not only to the phosphinic acid but also to methyl dimethylphosphinate which could only be formed by the intermediacy of a pentaco-ordinate species.The hydrolysis of the title compound, (l), has been studied over the concentration range 25-85% D2SO4 at 75 "C. The reactions were followed by n.m.r. spectroscopy in a manner similar to that which has already been reported.1 The most striking feature of this reaction is that (1) undergoes both loss of methanol and exchange of sulphur (equation 1).Similar observations have been reported for the hydrolysis of thiobenzoates2.3 and thiobenzoic acids .4 Over a wide range of acid concentrations, the thiobenzoates hydrolyse by an A,,2