In organic chemistry, the –OH functional group that characterizes alcohols is called the hydroxyl group. It consists of one oxygen atom covalently bonded to one hydrogen atom. When this group is attached to a saturated carbon atom, the resulting compound is generally classified as an alcohol.
The oxygen in hydroxyl gives alcohols important chemical properties. Its electronegativity makes the O–H bond polar, allowing alcohol molecules to form hydrogen bonds with one another and with substances such as water. This helps explain why small alcohols, including methanol and ethanol, mix readily with water, while larger hydrocarbon chains make alcohols progressively less water-soluble.
A frequent mix-up is between hydroxyl and hydroxide. Hydroxyl refers to the –OH group within a molecule or radical, while hydroxide is the OH⁻ ion with an overall negative charge. The group is also different from amino (–NH₂), carbonyl (C=O), and carboxyl (–COOH), although a carboxyl group contains both a carbonyl and a hydroxyl component.
Alcohol names often end in “-ol”: ethanol contains one hydroxyl group, while ethylene glycol contains two and is therefore a diol.