The Reactions of Aldehydes and Ketones with Alcohols

The product formed when one equivalent of an alcohol adds to an aldehyde or a ketone is called a hemiacetal. The product formed when a second equivalent of alcohol is added is called an acetal (ass-ett-AL). Like water, an alcohol is a poor nucleophile, so an acid catalyst is required for the reaction to take place at a reasonable rate. (Occasionally you see the terms hemiketal and ketal used instead of hemiacetal and acetal for the products of the reaction of an alcohol with a ketone.) 

Notice that the pattern of three tetrahedral intermediates occurs again in this mechanism —namely, 

Hemi is the Greek word for half. When one equivalent of alcohol has added to an aldehyde or a ketone, the compound is halfway to the final acetal, which contains groups 

Although the sp carbon of an acetal is bonded to two oxygens, which suggests that it is not stable, the acetal can be isolated if the water that is eliminated is removed from the reaction mixture. If water is not available, the only compound the acetal can form is the O-aUcylated intermediate, which is even less stable than the acetal. 

The acetal can be hydrolyzed back to the aldehyde or ketone in an acidic aqueous solution. 

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