The standard definition of alcohols is: compounds whose molecules contain a hydroxyl group bonded to a carbon atom on a hydrocarbon group or a side chain of a benzene ring, with the functional group being a hydroxyl group (-OH).
It is important to note that compounds formed by the direct substitution of a hydrogen atom on an aromatic hydrocarbon ring by a hydroxyl group are not alcohols but are classified as phenols. This is the core distinguishing point between alcohols and phenols.
The general structural formula of an alcohol can be represented as R-OH, where R represents an aliphatic hydrocarbon group, an alicyclic hydrocarbon group, or an aromatic side chain, and the hydroxyl group must be attached to a saturated carbon atom (a non-aromatic ring carbon).
From a classification perspective, alcohols can be divided according to different dimensions:
By the number of hydroxyl groups: classified as monohydric alcohols, dihydric alcohols, and trihydric alcohols. Those containing two or more hydroxyl groups are collectively called polyhydric alcohols.
By the type of carbon atom to which the hydroxyl group is attached: classified as primary alcohols, secondary alcohols, and tertiary alcohols.
By the type of hydrocarbon group: classified as aliphatic alcohols, alicyclic alcohols, and aromatic alcohols.