Hydroxyl

The hydroxy group (deprecated also hydroxyl )-OH is the functional group of alcohols and phenols. They also come in carbohydrates or hydroxycarboxylic acids, for example, in front of as structural elements. The hydroxy group is a sub-element of the carboxy group of the carboxylic acids. The functional group of sulfonic acids can be considered as a combination of a sulfonyl group with a hydroxy group.

The hydroxy group may not be with the negatively charged OH ion ( -OH, see hydroxide ion ), or with the neutral OH radical ( · OH, named hydroxyl, see hydroxyl radical ) are mixed.

The alcoholic hydroxyl group gives the molecule polar properties. So can form, for example, between ethanol molecules hydrogen bonds. This leads to an attraction and networking neighboring alcohol molecules, due for instance to the liquid state of the ethanol. However, a hydroxy group also contribute to lower boiling and melting point, when the hydroxyl group can form a hydrogen bond with other polar groups within a molecule (intramolecular ). So that the polarity of the molecule to the outside ( intermolecular) is weakened. Thus, for example, the boiling point of ricinoleic acid well below the boiling point of oleic acid.

Phenolic hydroxyl groups have to alcoholic as compared stronger acidic properties, as formed during the deprotonation of the phenolate anion is resonance-stabilized. The M effect outweighs the - I effect. This is true even for aliphatic enols, but these is the keto -enol tautomerism, in contrast to almost all phenols ketone before, because it is energetically favorable. For the phenols but it is due to the formation of the aromatic system exactly reversed (see Hückel rule).