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Acetal Formation

If there is an excess of alcohol, the hemiacetal can react with a second molecule of alcohol to give a "full" acetal.

Ketals▶ Watch the video on YouTube
Mechanism of acetal formation from the hemiacetal: protonation of the OH, loss of water, attack of a second alcohol molecule on the carbocation and loss of a proton
Dean-Stark apparatus: the azeotrope vapor rises, condenses and drips; the water separates at the bottom and the dry solvent returns to the flask

The reaction is still an equilibrium, which has to be shifted by removing the water formed with a desiccant or a Dean-Stark apparatus.

Acetals can be isolated as perfectly stable compounds.

In an acetal, the former carbonyl carbon has lost the π cloud of the C=O bond and is now sp3. Hence, the acetal is much less reactive than the starting aldehyde or ketone: it withstands the attack of many nucleophiles that would easily react with a C=O double bond.

Acetal formation is therefore used to "protect" the C=O group of aldehydes and ketones (not of carboxylic acid derivatives). Look at the "Problems and exercises" to understand this protection with a very simple but enlightening example.