An aldehyde or a ketone reacts with alcohol in the presence of an acid catalyst to form a geminal diether product called an acetal. Acetal formation begins with protonation of the ketone or aldehyde carbonyl oxygen by an acid catalyst, increasing the attractiveness of the carbonyl group to the approaching alcohol nucleophile. A tetrahedral intermediate forms, which is typical of nucleophilic reactions with carbonyl compounds. After a subsequent protonation, the hydroxyl group leaves as water, forming a cation intermediate. This intermediate is approached by another alcohol nucleophile.
One interesting use of acetal formation is to protect carbonyl groups from hostile reaction conditions. Ethylene glycol is often used in this way to form a cyclic acetal, which can be converted back into the aldehyde or ketone at a later stage.
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