Hemiacetals, acid/base catalysed formation

You can see why hemiacetals are unstable they are essentially tetrahedral intermediates containing a leaving group and, just as acid or base catalyses the formation of hemiacetals, acid or base also catalyses their decomposition back to starting aldehyde or ketone and alcohol. That s why the title of this section indicated that acid or base catalysts increase the rate of equilibration of hemiacetals with their aldehyde and alcohol components—the catalysts do not change the position of that equilibrium ... 

In glucosides, hemiacetal hydroxyl activation/substitution can be achieved using a sulfonic anhydride and a nucleophile, plus a base as acid scavenger.12 The reaction is catalysed by dibutyl sulfoxide (Bu2S=0), and shows evidence of sulfur-covalent catalysis. Using benzenesulfonic anhydride [(PhS0)20], it is proposed to involve initial formation of a sulfonium sulfonate (6), the S(IV) centre of which then reacts with... 

Hemiacetal formation and decomposition are catalysed by acid or base. 

Hemiacetal formation is catalysed by acid or base, but acetal formation is possible only with an acid catalyst because an OH group must be made into a good leaving group. 

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