KINETICS OF THE SILVER ION-PROMOTED HYDROLYSIS OF 2-METHYL-2-(SUBSTITUTED PHENYL)-1,3-DITHIANES IN 10-PERCENT DIOXANE-WATER MIXTURES - IMPLICATIONS FOR CYCLIC ACETAL HYDROLYSIS CATALYZED BY HYDROGEN-IONS

The hydrolysis of 1,3-dithianes derived from para-substituted acetophe nones is promoted by sliver ions. Kinetic study using a 10% (v/v) diox ane-water solvent shows that, when [Ag+] equal to or less than 0.2 mol dm(-3), the p-NO2, -Cl, -H and -Me derivatives hydrolyse via rapidly- formed 1 Ag+ : 1 dithiane-complexes, but that the p-MeO derivative for ms an unreactive 1 : 1-complex and hydrolyses via a 2 Ag+ : 1 dithiane -complex. Comparison of the kinetic parameters with those available fo r analogous open-chain S,S-acetals reveals that cyclisation leads to a substantial (>10(4)-fold) overall loss of reactivity and that this lo ss arises both from a lowering of acetal basicity towards Ag+ and from a slower rate of hydrolysis of the 1 : 1-complex. The implications fo r hydrogen ion-catalysed hydrolyses of cyclic acetals are discussed, a s are the reasons for the lower reactivity of the cyclic acetals.