diols

Diols are organic compounds that contain two hydroxyl (-OH) groups within a single molecule. They can be acyclic or cyclic, and their two hydroxyl groups may occupy different positions on the carbon framework. When the two -OH groups are on adjacent carbons, the compound is a vicinal diol (for example, ethylene glycol). If both hydroxyls sit on the same carbon, the compound is a geminal diol (also called a hydrate in many cases, though many geminal diols are unstable and not isolated). Diols where the hydroxyls are separated by one or more carbons are termed 1,3-diols, 1,4-diols, etc.

Nomenclature for diols follows the parent hydrocarbon with the position numbers of the hydroxyl groups, for example ethanediol for two carbons with -OH on C1 and C2, or more commonly ethylene glycol. Common names are widely used, such as ethylene glycol (ethane-1,2-diol) and propylene glycol (propan-1,2-diol).

Diols occur widely in nature and industry. They can be prepared by reduction of carbonyl compounds, by dihydroxylation of alkenes, or by opening epoxides. They are versatile building blocks in organic synthesis and serve as solvents, antifreezes, and humectants. In polymers, diols are important monomers or chain-extending agents; for example, 1,4-butanediol is used to make polyesters and polyurethanes, while ethylene glycol is a common antifreeze and a precursor to polyethylene terephthalate.

Property-wise, diols typically engage in strong hydrogen bonding, giving higher boiling and melting points than many nonpolar organics. They can act as bidentate ligands in coordination chemistry, form acetals with carbonyl compounds, and be protected as cyclic acetals in synthetic sequences. Safety and handling depend on the specific diol; for example, ethylene glycol is toxic if ingested.