Under acid conditions, alcohols attack the carbonyl carbon of aldehydes and ketones in two successive nucleophilic additions forming first a hemiacetal, which then loses water to form a carbocation intermediate, to then form an acetal through the second nucleophilic addition of alcohol.

Although we have mentioned hemiacetal and acetal formation here in 'Reactions of Alcohols', it is better to treat the reaction in full depth later in the chapter on 'Reactions of Aldehydes and Ketones'.

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