Classification

An alcohol has an –OH group on an aliphatic carbon; a phenol has –OH directly on a carbon of an aromatic ring. Replacing the H of –OH by an alkyl or aryl group gives an ether, R–O–R′.

Counting –OH groups sorts alcohols and phenols into monohydric (one), dihydric (two), trihydric (three) and polyhydric (many). Ethane-1,2-diol is dihydric and propane-1,2,3-triol is trihydric.

Monohydric alcohols with the –OH on an sp³ carbon are primary (1°), secondary (2°) or tertiary (3°), matching the kind of carbon that holds the –OH.

Allylic alcohols carry –OH on the sp³ carbon beside a C=C; benzylic alcohols carry it on the sp³ carbon joined to an aromatic ring. Each of these can itself be 1°, 2° or 3°.

When –OH sits on an sp² carbon of a C=C the compound is a vinylic alcohol, such as CH₂=CH–OH. Phenols are the aromatic case of –OH on an sp² carbon.

Ethers are symmetrical when both groups on oxygen are the same (C₂H₅OC₂H₅) and unsymmetrical, or mixed, when they differ (C₂H₅OCH₃, C₂H₅OC₆H₅).

These classes are the starting points for detergents (alcohols), antiseptics (phenols) and fragrances (ethers). Furniture-polishing spirit is mostly ethanol, and sugar, cotton and paper are all built from –OH compounds.