A hemiacetal or a hemiketal have the general formula R1R2C(OH)OR, where R1 or R2 is hydrogen or an organic substituent. They generally results from the addition of an alcohol to an aldehyde or a ketone, although the latter are sometimes called hemiketals. Most sugars are hemiacetals.
According to the IUPAC definition, in R1R2C(OH)OR R1 and R2 may or may not be a hydrogen. In a hemiketal, neither R-group can be a hydrogen. Hemiketals are regarded as hemiacetals where none of the R-groups are H, and are therefore a subclass of the hemiacetals. The Greek prefix hèmi means half, refers to the fact that a single alcohol has been added to the carbonyl group, in contrast to acetals or ketals, which are formed when a second alkoxy group has been added to the structure.
Cyclic hemiacetals and hemiketals are sometimes called lactols. They often form readily, especially when they are 5- and 6-membered rings. In this case an intramolecular OH group reacts with the carbonyl group. Glucose and many other aldoses exist as cyclic hemiacetals whereas fructose and similar ketoses exist as cyclic hemiketals.
Solutions of aldehydes in alcohols mainly consist of the hemiacetal. The reaction is sensitive to steric effects.
|Carbonyl compound||alcohol solvent||%hemiacetal|
|Formation of hemiacetals|
|Formation of hemiketals|
|Left, glucose, a cyclic hemiacetal.|
Right a lactol of fructose, a cyclic hemiketal.
Hemiacetals and hemiketals may be thought of as intermediates in the reaction between alcohols and aldehydes or ketones, with the final product being an acetal or a ketal:
- R2C=O + 2 R'OH ⇌ R2C(OH)(OR)
- R2C(OH)(OR) + R'OH ⇌ R2C(OR')2 + H2O
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "hemiketals". doi:10.1351/goldbook.H02776
- Fox, Marye Anne; Whitesell, James K. (2004). Organic Chemistry. Jones & Bartlett Learning. p. 590. ISBN 9780763721978.
- IUPAC Gold Book lactols
- Schmitz, Ernst; Eichhorn, Inge (1967). "Acetals and hemiacetals". The Ether Linkage (PATAI'S Chemistry of Functional Groups). pp. 309–351. doi:10.1002/9780470771075.ch7. ISBN 9780470771075.