hemiacetal
Summary
A hemiacetal is a carbon bonded to one −OH and one −OR group (plus two other substituents), formed when an alcohol adds to an aldehyde. Open-chain hemiacetals are usually unfavorable, but cyclic five- or six-membered ones, such as the ring forms of glucose, are favored.
Detail
Meaning: A hemiacetal carbon carries an −OH, an −OR, and two other groups (H or carbon). It forms when an alcohol oxygen, acting as a nucleophile, attacks an aldehyde carbonyl carbon: RCHO + R′OH ⇌ RCH(OH)OR′. The same addition to a ketone gives a hemiketal. Open-chain hemiacetals usually favor the carbonyl side, but when the −OH and the carbonyl are in one molecule, a five- or six-membered ring forms readily. Glucose cyclizes this way: the C5 hydroxyl attacks the C1 aldehyde, creating the anomeric carbon and the α and β anomers.
On the MCAT: Expect a drawn ring where you must find the anomeric carbon (one −OH, one ring −O−). Adding a second alcohol under acid catalysis converts a hemiacetal to an acetal, as in a glycosidic bond. Acetals resist base and are hydrolyzed by aqueous acid, so they serve as protecting groups for carbonyls.
Common confusion: A hemiacetal has one −OR and one −OH; an acetal has two −OR groups. Only the hemiacetal can reopen to the free carbonyl. That is why a sugar with a free anomeric −OH can mutarotate and act as a reducing sugar, while a glycoside (an acetal) cannot.
Sources
- Organic Chemistry (Wade)
- Organic Chemistry: Structure and Function (Vollhardt and Schore)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.