Hydroxyl group

Hydroxy group ( marked in blue ) as a functional group of an alcohol. R is then an alkyl group .

The hydroxyl group (older, still used term hydroxyl group ) −OH is the functional group of alcohols and phenols and also occurs in carbohydrates or in hydroxycarboxylic acids as a structural element of an alcohol. The hydroxyl group is also a sub-element of the carboxy group in carboxylic acids , but there it is bound to the C atom of a carbonyl group , which has a higher oxidation state. The hydroxyl group is therefore not a structural element of an alcohol in carboxylic acids, but has the acidic properties of a carboxylic acid.

The hydroxyl group is neither identical with the negatively charged OH ion ( ^- OH, see hydroxide ion ) nor with the neutral OH radical (· OH, called hydroxyl for short, see hydroxyl radical ).

Alcohols

An alcoholic hydroxyl group gives the molecule polar properties . For example, hydrogen bonds can form between ethanol molecules . This results in an attractive and cross-linking of neighboring alcohol molecules, for example, the liquid state of aggregation due to the ethanol. However, a hydroxyl group can also contribute to lowering the boiling and melting point if the hydroxyl group can form a hydrogen bond with other polar groups within a molecule (intramolecularly) . This weakens the polarity of the molecule towards the outside (intermolecular). For example, the boiling point of ricinoleic acid is significantly below the boiling point of oleic acid .

Phenols

Comparison between a hydroxyl group in phenol and in an enol : Phenol (top left) is in equilibrium with vanishingly tiny amounts of the ortho- quinoid ketone (top right). The ketone cyclohexanone (bottom left) is in equilibrium with small amounts of the enol (bottom right). The migrating proton is marked in green .

Compared to alcoholic hydroxyl groups, phenolic hydroxyl groups have more acidic properties, since the phenolate anion formed during deprotonation is mesomeric stabilized. The + M effect outweighs the −I effect . This also applies to aliphatic enols , but in keto-enol tautomerism, in contrast to the phenols, the ketone is almost exclusively present because it is energetically favored. In the case of phenols, however, it is exactly the opposite because of the formation of the aromatic system (see Hückel rule ).

literature

• Phillip Fresenius, Klaus Görlitzer: Organisch-Chemische Nomenklatur , 3rd edition Wissenschaftliche Verlagsgesellschaft Stuttgart 1991, ISBN 3-8047-1167-7 .
• Peter Sykes: reaction mechanisms - an introduction , 8th edition VCH Weinheim 1982 ISBN 3-527-21090-3 .
• Organikum , 16th edition, VEB Deutscher Verlag der Wissenschaften Berlin 1985, ISBN 3-326-00076-6 .

Web links

Wiktionary: Hydroxy group  - explanations of meanings, word origins, synonyms, translations

Individual evidence

1. ↑ Entry on hydroxy group. In: Römpp Online . Georg Thieme Verlag, accessed on August 28, 2019.