hemiacetal
/ hem-ee-ASS-uh-tal /
Swap the water of a hydrate for an alcohol, and you get a hemiacetal. It is a single carbon that carries both an -OH group and an -OR group (an ether-like oxygen), left over from one molecule of alcohol adding across the C=O of an aldehyde or ketone. The prefix 'hemi-' means half, because it is halfway to the fully protected acetal.
The mechanism is, once more, nucleophilic addition: the oxygen of an alcohol (R'-O-H) attacks the carbonyl carbon, the carbonyl electrons go to oxygen, and a proton transfer leaves you with R-CH(OH)(OR'), a carbon bearing one new ether oxygen and one hydroxyl. Like hydration, this is a reversible equilibrium that for most open-chain aldehydes lies toward the carbonyl, so simple hemiacetals are often hard to isolate. Acid or base catalysis speeds it up without changing where the equilibrium sits.
Hemiacetals are far more than a curiosity because of one beautiful case: sugars. A glucose molecule has both an aldehyde and several hydroxyl groups in the same chain, so it can form a ring by an internal (intramolecular) hemiacetal, where one of its own -OH groups adds to its own C=O. That cyclic hemiacetal is the dominant form of glucose in solution and the reason sugars have an 'anomeric' carbon. So this modest functional group is at the heart of carbohydrate chemistry.
Glucose in water spends most of its time as a six-membered ring, a cyclic hemiacetal formed when its C5 hydroxyl adds to its C1 aldehyde, creating the anomeric carbon at C1.
A hemiacetal: one carbon bearing both -OH and -OR; intramolecular versions make sugar rings.
Strictly, the product from a ketone is a hemiketal, but modern IUPAC usage often calls both hemiacetals. The key point is the diagnostic combination on one carbon: one OH and one OR.