Hemiacetals definition

A similar mechanism is operative in the reduction of carboxylic esters with DIBAL (Figure 17.61). The tetrahedral intermediate A is formed by addition of an A1—H bond of the reducing agent to the ester C=0 bond. This tetrahedral intermediate A does not necessarily decompose immediately to an aldehyde and ROAl(iBu)2. In nonpolar media, A definitely decomposes quite slowly. In fact, at very low temperatures A remains unchanged until it is protonated to a hemiacetal during aqueous workup. The latter eliminates water to give the aldehyde. 

Establish the orientation of the asymmetric carbon created by hemiacetal or hemiketal formation. By definition, an a-anomeric OF is on the same side of the Fischer projection as the... 

For (1 —> 4)-linked polysaccharides, intramolecular-hemiacetal formation may occur, to give five- or six-membered rings. Authors generally draw all their formulas of this type with one or other ring, although there is as yet no definite evidence as to which is correct, except possibly for oxycellu-loses. Some authors favor the five-membered, hemiacetal ring, because of... 

To answer this problem, one must know the structures of the molecules in question and a couple of definitions. By definition, epimers are a pair of molecules that differ from each other only in their configuration at a single asymmetric center. Anomers are special epimers that differ only in their configuration at a carbonyl carbon hence, they are usually acetals or hemiacetals. An aldose-ketose pair is obvious. Inspection of Fischer representations of the molecular pairs leads to the conclusion that (a), (c), and (e) are aldose-ketose pairs (b) and (f) are epimers and (e) are anomers. 

Alcohols. In the case of the simpler aliphatic alcohols, the formation of formaldehyde hemiacetals takes place vith the evolution of considerable heat e.g, 15 kcale am evolved when one gram molecule of formaldehyde gas is absorbed in methanol (page 37). The fact that a concentrated solution of anhydrous formaldehyde in methanol has a higher boiling point than either pure formaldehyde or methanol itself is definite proof of compound formation. It is probable that hemiacetals are present in alcoholic formaldehyde, just as polj -oxymethylene glycols are present in aqueous solutions. 

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