In organic chemistry, a hemiacetal is a functional group the general formula R1R2C(OH)OR, where R1, R2 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 (R−CH=O) or a ketone (R2C=O) 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.