Hemiacetals formation, acid-catalysed

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. 

Purists might criticize the avoidance of equilibrium arrows in the mechanisms shown. Some reactions, e.g. hemiacetal formation or acid-catalysed ester hydrolysis, are undoubtedly reversible, yet we have shown them as proceeding only in the forward direction. We believe it is more important to develop the skills for predicting a rational mechanism rather than remembering whether the reaction is reversible or not. Unless there is any specific comment regarding reversible reactions, we should concentrate on the reaction in the sense given in the question. 

Hemiacetal formation and decomposition are catalysed by acid or base. 

Loss of a proton to give the acetal acid-catalysed acetal formation from hemiacetal... 

Why is elimination preferred to hemiacetal formation in the acid-catalysed cyclization of this ketone ... 

Acetic acid catalyses die formation of a cyclic acetal by reaction of die remaining aldehyde with the enol and the hemiacetal hydroxy groups. 

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... 

Alkynylbenzaldehydes yield isochromenes in a Pd-catalysed reaction with alcohols. The Pd plays a dual role, behaving as a Lewis acid to effect the formation of a hemiacetal and coordinating to the alkyne moiety to promote cyclic ether formation <02JA764>. 

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 ... 

The process for the industrial production of permethric acid by Roussel-Uclaf starts from racemic trons-chrysanthemic acid, which is in turn accessed by the Martel synthesis (cf. 1,3-cycloelimination). After separation of the enantiomers with an amino-alcohol, the (IR)-enantiomer is subjected to ozonolysis. Basic epimerisation gives (IR)-cis-caronaldehyde hemiacetal. Water is added to the (IS)-enantiomer in the presence of a catalytic amount of sulfuric acid then the carboxylic add function is epimerised with the formation of a lactone. Magnesium bromide-catalysed ring-opening leads to (lR)-c s-chrysanthemic acid, which is converted into (IR)-ds-caronaldehyde hemiacetal by an analogous route. [105]... 

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