Formation of Hemiacetals

Aldehydes and ketones react with alcohols to give hemiacetals. Hemi in Greek means half. Hemiacetals have an –OH and an –OR group attached to the same carbon atom in their molecules.

When an excess of the alcohol is used, a second molecule of the alcohol reacts to give an acetal.

Acetals are stable in basic solutions and are therefore used as protecting groups for aldehydes and ketones. Acetals can be converted back to the carbonyl compounds by treating them with dilute acids because of the reversible nature of the reaction.