Hemiacetals base-catalysed

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 ... 

Chapter 9). Hydration of (52) gives hemiacetal (53) which can be used for (54) in base-catalysed reactions. All these compounds are therefore readily available. Other surprising sequences are given in Table 40.3. 

Likewise, the aldehydic and not the hemiacetal form of pyri-doxal undergoes polarographic reduction. Because the ringopening reaction of the hemiacetal group is acid-base catalysed, an increase of the limiting current at low and high pH-values was observed (Fig. 10). The acid-base catalysis can probably be explained by the difference in reactivity of different protonized forms of pyridoxal. In agreement with the explanation of the... 

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. 

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. 

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. ... 

Alkali metal t-butoxides, hydrides and bis(TMS)amides efficiently catalyse Claisen-Tishchenko disproportionation of aldehydes to the corresponding carboxylic esters. Potassium bases were more effective than sodium, and 18-crown-6 further accelerates the reaction. Kinetic studies suggest that the rate-determining step is a second-order concerted hydride transfer from a potassium hemiacetal to another molecule of aldehyde. 

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