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.
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 .
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 ).
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- Peter Sykes: reaction mechanisms - an introduction , 8th edition VCH Weinheim 1982 ISBN 3-527-21090-3 .
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