OHGruppe

OHGruppe, or the hydroxyl group, is a functional group consisting of an oxygen atom bonded to a hydrogen, usually written as -OH. It is common in alcohols, phenols, and many biomolecules. The neutral hydroxyl group is distinct from the hydroxide ion (OH−), which forms in aqueous solutions when the group acts as a base.

Properties and behavior: The -OH group is polar and capable of hydrogen bonding, which raises boiling points and enhances solubility in water. Its acidity varies: alcohols typically have pKa around 16–18, whereas phenols are more acidic with pKa around 9–10. This difference affects reactivity and deprotonation in chemical reactions.

Reactivity: As a functional group, -OH participates in esterification with carboxylic acids to form esters, and in oxidation of primary and secondary alcohols to aldehydes, carboxylic acids, or ketones. Dehydration of alcohols can yield alkenes. Methanol, ethanol, and glycerol illustrate neutral alcohols with one or more hydroxyls, while phenols show the distinctive chemistry of an OH group on an aromatic ring.

Nomenclature and occurrence: The hydroxyl group is named hydroxy- as a substituent, and compounds with it as the principal functional group are called alcohols. OH groups occur widely in carbohydrates, nucleotides, and many lipids, contributing to solubility and to hydrogen-bonding networks in biological systems.

Spectroscopy: In infrared spectroscopy, the OH stretch appears around 3200–3550 cm−1, often broad due to hydrogen bonding. In NMR, exchangeable OH protons show signals that depend on solvent and temperature.