Equilibria/equilibrium thioacetal formation

Formation of a stable hydro complex with thioacetic acid is particularly interesting, since the reaction with formic or acetic acids, which have similar acidities, leads to an equilibrium in which the hydro complex reductively eliminates in the absence of excess acid (279). 

Cyclic Acetals Formation of an acetal using a diol as the alcohol gives a cyclic acetal Cyclic acetals often have more favorable equilibrium constants, since there is a smaller entropy loss when two molecules (a ketone and a diol) condense than when three molecules (a ketone and two molecules of an alcohol) condense. Ethylene glycol is often used to make cyclic acetals its acetals are called ethylene acetals (or ethylene ketals). Dithiane (Section 18-8) and its alkylated derivatives are examples of cyclic thioacetals (sulfur acetals). 

Condensation of 2-mercaptoethanol or 3-mercaptopropanoI with ketones is usually achieved with the aid of an acid catalyst. Hydrogen chloride has been used but more common agents are boron trifluoride , freshly fused zinc chloride or p-toluenesulphonic acid . An exchange method between 2,2-dimethyl-1,3-oxathiolane or 2,2-dimethyl-1,3-oxathiane and a non-volatile ketone leads to formation of the new mono-thioacetal and acetone . The equilibrium is displaced by continuous distillation of the acetone formed (equation 71). With saturated ketones, mostly steroids, the yields of the above methods are comparable and are usually in the 60-90% range. With a,j3-unsaturated ketones, the yields were significantly lower . 

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