Addition of alcohols hemiacetal and acetal formation

Alcohols are relatively weak nucleophiles (like water) but add rapidly to aldehydes and ketones on acid catalysis. The initial product is a hydroxyether or hemiacetal. 

Hemiacetal formation is fundamental to the chemistry of carbohydrates (see Section 11.1). Glucose, for example, contains an aldehyde and several alcohol groups. The reaction of the aldehyde with one of the alcohols leads to the formation of a cyclic hemiacetal (even without acid catalysis) in an intramolecular reaction. 

Hemiacetals can undergo further reaction, with a second equivalent of alcohol, to yield an acetal. 

Acetals are extremely useful compounds in synthesis because they can act as protecting groups for aldehydes and ketones. 

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