acetal
Summary
An acetal is a carbon bonded to two alkoxy (OR) groups, made from an aldehyde or ketone plus two alcohols with acid catalysis. Most testable: acetals are stable to base, nucleophiles and hydrides but hydrolyze in aqueous acid, so they serve as carbonyl protecting groups.
Detail
Meaning: An acetal is a carbon bonded to two alkoxy (OR) groups, with its other two bonds to H or carbon groups: R₂C(OR')₂. It forms when an aldehyde or ketone reacts with two alcohols (or one diol, giving a cyclic acetal) under acid catalysis. The carbonyl is protonated, an alcohol adds to give a hemiacetal, loss of water gives an oxocarbenium ion, and a second alcohol adds. Removing water pushes the equilibrium toward the acetal. Older texts call ketone-derived products ketals; many current texts call them acetals too.
On the MCAT: Acetals are the standard protecting group for carbonyls. To reduce an ester with LiAlH₄ in a molecule that also has a ketone, convert the ketone to a cyclic acetal with ethylene glycol and acid, reduce the ester, then remove the acetal with aqueous acid. Glycosidic linkages in carbohydrates are acetals formed at the anomeric carbon.
Common confusion: A hemiacetal has one OR and one OH on the same carbon and is usually unstable, while an acetal has two OR groups and can be isolated. Acetals are cleaved by aqueous acid, not by base, which is why they protect a carbonyl during base or hydride reactions.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.