Alcohols and Phenols

An alcohol carries a hydroxyl group, $OH$, on a saturated carbon. The monohydric family is written $ROH$, where $R$ can be a simple alkyl group such as $CH_3$ or $CH_3CH_2$, or a group carrying a ring such as $C_6H_5CH_2$. The classification runs on two axes at once. Count the hydroxyl groups first: one gives a monohydric alcohol, two a dihydric, three a trihydric, and more a polyhydric alcohol. Then look at the carbon that carries the hydroxyl, the carbinol carbon, and count the alkyl groups on it. One alkyl group makes the alcohol primary, two secondary, and three tertiary. Methanol is the special case, since its carbinol carbon has no alkyl group at all but is still classed as primary.

The oxygen of the hydroxyl is $sp^3$ hybridised, and so is the carbon it bonds to, so the group is bent rather than linear. The C-O-H angle is $108.9^\circ$, slightly wider than the $104.5^\circ$ of water, and the two lone pairs sit in the remaining hybrid orbitals. That geometry explains the physical behaviour of the family, because the oxygen can both donate a hydrogen bond through its O-H and accept one with its lone pairs.

Naming runs on two systems. The common names add the word alcohol to the alkyl group, giving methyl, ethyl and benzyl alcohol, and several exam favourites live here: 1,2-ethanediol is glycol, 1,2,3-propanetriol is glycerol, 1,3-benzenediol is resorcinol, 1,2-benzenediol is catechol, and the trihydric trihydroxybenzene is pyrogallol. IUPAC replaces the final e of the parent alkane with ol and numbers the chain from the end nearer the hydroxyl, so $CH_3CH_2CH_2OH$ is propan-1-ol and $CH_3CH(OH)CH_3$ is propan-2-ol. An alcohol whose hydroxyl sits on a carbon attached to a ring, such as benzyl alcohol, $C_6H_5CH_2OH$, is an aromatic alcohol, which is not a phenol: a phenol has the hydroxyl bonded directly to the ring.