hemiacetal

thumb|The general structure of a hemiacetal (left) and hemiketal (right). In organic chemistry, a hemiacetal is a functional group with the general formula {{chem2|R^{1}R^{2}C(OH)OR}}, where {{chem2|R^{1}, R^{2}|}} is a hydrogen atom or an organic substituent. They generally result from the nucleophilic addition of an alcohol (a compound with at least one hydroxy group) to an aldehyde () or a ketone () under acidic conditions. The addition of an alcohol to a ketone is more commonly referred to as a hemiketal. Common examples of hemiacetals include cyclic monosaccharides. Hemiacetals have use as a protecting group and in synthesizing oxygenated heterocycles like tetrahydrofurans.

==Nomenclature== According to the IUPAC definition of a hemiacetal, the R1 and R2 groups may or may not be hydrogen. In a hemiketal, both of these R-groups must not be hydrogen. Thus, hemiketals are regarded as a subclass of hemiacetals. The prefix hemi, meaning half, refers to the one alcohol added to the carbonyl group. This is half of the required alcohols to form acetals or ketals. Cyclic hemiacetals can sometimes be referred to as lactols. ==Formation ==