What Are Hemiacetals and Acetals

Curved arrows in each solution show the formation of the new carbon-carbon bond and the alkoxide ion, and labels on the final product show which set of reagents forms each bond  

Show how these three compounds can be synthesized from the same Grignard reagent  

The addition of a molecule of alcohol to the carbonyl group of an aldehyde or a ketone forms a hemiacetal (a half-acetal). This reaction is catalyzed by both acid and base Oxygen adds to the carbonyl carbon and hydrogen adds to the carbonyl oxygen  

Hemiacetal A molecule containing an —OH and an —OR or —OAr group bonded to the same carbon. 

The functional group of a hemiacetal is a carbon bonded to an —OH group and an —OR or —OAr group  

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Because monosaccharides contain alcohol functional groups and aldehyde (or ketone) functional groups, the reactions of monosaccharides are an extension of what you have already learned about the reactions of alcohols, aldehydes, and ketones. For example, an aldehyde group in a monosaccharide can be oxidized or reduced and can react with nucleophiles to formimines, hemiacetals, and acetals. When you read the sections that deal with the reactions of monosaccharides, you will find cross-references to the sections in which the same reactivity for simple organic compounds is discussed. As you study, refer back to these sections they will make learning about carbohydrates a lot easier and will give you a good review of some chemistry that you have already learned about. 

When one follows the reaction arrows in Figure 9.12 from the bottom upward, the following important information can be noted In an acidic water-containing solution 0,0-acetals are hydrolyzed to give carbonyl compounds and alcohols. Such a hydrolysis consists of seven elementary reactions. First, the hemiacetal A (Nu = OR3) and one equivalent of alcohol are produced from the 0,0-acetal and water in an exact reversal of the latter s formation reaction, i.e., through a proton-catalyzed SN1 substitution (in four steps). What follows is a three-step decomposition of this hemiacetal to the carbonyl compound and a second equivalent of the alcohol. 

First you must decide what products are formed by hydrolysis of the acetal. In dealing with acetals and hemiacetals, any carbon atom with two bonds to oxygen is derived from a carbonyl group. 

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