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The rate constants for acid-catalyzed hydrolysis of a series of benzaldehyde methyl S-(substituted phenyl) thioacetals have been measured at 30°in 20% dioxane-H20. General acid catalysis could not be detected with any of these compounds. The value of AS* for the hydronium ion catalyzed hydrolysis of the unsubstituted derivative is -4.8 eu. This reaction is much faster in D20 than H20, the ratio kfkn being 1.51. The p value for acid-catalyzed hydrolysis of the series is -1.0. An A1 mechanism is indicated with the carbon-sulfur bond breaking in the rate-determining step. The hydrolysis of the 2,4-dinitrophenyl derivative is pH independent from pH 1.5 to 0.1 M NaOH. This reaction proceeds at approximately the same rate in D20 as in H20 (kD20/kH2o = 0.90). The pH-independent reaction is therefore very likely a unimolecular decomposition to 2,4-dinitrothiophenoxide ion and a resonance-stabilized carbonium ion.

The structural requirements for the hydrolysis of an acetal to be general-acid catalysed are discussed. It is concluded that the hydrolysis of mixed aryl alkyl acetals of benzaldehyde should show general-acid catalysis. This is confirmed by showing that the hydrolysis of benzaldehyde methyl phenyl acetal is general-acid catalysed in acetate, formate, chloroacetate, and phosphate buffers. The a-value of the Brsnsted equation is calculated from the results of the first three of these buffers to be 0.60 at 20'. The solvent isotope effect of the acetic acid-catalysed reaction is k(HOAc)/k(DOAc) = 2.1 1 and of the hydronium ion-catalysed reaction k ( H , O + ) / k ( D , O + ) = 1.02 at 20". The p-value for the acetic acid-catalysed hydrolyses of a series of benzaldehyde methyl aryl acetals is +0.98. A concerted A-SE2 mechanism is preferred to one involving a slow proton transfer followed by a rapid breakdown of the resulting conjugate acid.ALTHOUGH intermolecular general-acid catalysis has never been detected in the hydrolysis of alkyl acetals it is well established for the hydrolysis of the structurally related ortho-esters.2 It is pertinent to enquire what are the structural features in an acetal which would favour general-acid catalysis in its hydrolysis. The freeenergy versus reaction co-ordinate diagram for a specificacid catalysed hydrolysis of an acetal (dimethyl formal) is shown in Figure 1. In order to change from specificto general-acid catalysis it is necessary to change the structure of the acetal so that the free energy of transition state (T.S.) 1 becomes greater than that of T.S.2. This can be achieved by increasing the free energy of T.S.l or decreasing that of T.S.2, or both. If it is assumed that T. s.2 n React ion co-ord i note