CoachA hemiacetal is a compound in which one carbon carries both an OH and an OR group, formed by the addition of an alcohol to a carbonyl group. An acetal is a compound in which one carbon carries two OR groups, formed from a hemiacetal and a second alcohol molecule, with loss of water.
Examples
Example 23.8 (A ketone in the way)
To add a Grignard reagent to the aldehyde group of a molecule that also carries a ketone, no simple acetal choice exists (aldehydes form acetals more readily than ketones, so the wrong group would be protected); the order of the steps or the reagents must change. Protection is a strategy, not a reflex.