A hydroxyl or hydroxy group is a chemical functional group containing one oxygen atom connected by a covalent bonding to one hydrogen atom (−OH). It is sometimes called the alcohol functional group because when bonded to carbon in a molecule that otherwise contains only hydrogen and carbon the hydroxy (not hydroxyl) group defines the molecule as an alcohol, resulting in a name ending in -ol. A hydroxyl group bonded covalently to the carbon of a carbonyl group (C=O) produces a carboxyl group (C(O)OH) that is the defining group of a carboxylic acid. When the −OH group participates in an ionic bond, the [OH⁻] anion is called the hydroxide ion. As a free radical, it is the hydroxyl radical.

Hydroxy group

According to IUPAC rules the term hydroxyl refers to the radical OH only, while the functional group −OH is called hydroxy group.

The oxygen atom in the hydroxy group is much more electronegative than the hydrogen, so that the electrons are closer to the oxygen atom. Similar to water, compounds containing the hydroxy group are capable of forming hydrogen bonds. Primary alcohols are miscible with water. In complex compounds, the portion of the molecule containing the hydroxy group is hydrophilic.