Hemiacetals decomposition, acid-catalysed

Hemiacetal formation and decomposition are catalysed by acid or base. 

Whereas the base cleavage of 2-arylcyclopropanone hemiacetals proceeds with exclusive C-1—C-2 bond heterolysis, acid-catalysed decomposition occurs by way of C-2—C-3 cleavage to give 1-arylpropanones, a process explicable by cyclopropyl-allyl rearrangement. The rearrangement is also implicated in the acylation of 2-cyclopropylpyridine 1-oxide and in the decomposition of dichlorocarbene adducts of polymethylnaphthalenes. ... 

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 ... 

Hammett plots have been constructed for the acid- and base-catalysed decomposition of methyl hemiacetals of benzaldehydes in aqueous solution. The data are analysed in terms of three-dimensional More O Ferrall-Jencks diagrams and of Cordes interaction effects. 

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