hydroxyl group
/ hy-DROK-sil /
Add an O-H to an organic molecule and you have given it the hydroxyl group, the small piece of structure that makes a molecule an alcohol. It is the OH in the ethanol of drinks, in the glycerol of soap-making, in the sugar on your table. Tiny as it is, the hydroxyl transforms how a molecule dissolves, boils, and reacts.
A hydroxyl group is an oxygen bonded to a hydrogen, with its other bond going to a carbon: C-O-H. Both bonds in it are polar because oxygen is strongly electronegative, and the oxygen also carries two lone pairs. Those features give hydroxyl its three signature behaviours. First, the O-H can both donate and accept hydrogen bonds, which is why alcohols boil far higher than comparable hydrocarbons and why small alcohols mix with water. Second, the O-H is weakly acidic — a strong base can pull off that hydrogen to leave an alkoxide. Third, the oxygen's lone pairs let it act as a nucleophile, and the whole OH can, under the right conditions, be converted into a leaving group.
The hydroxyl group is everywhere in chemistry and life: it defines alcohols, and a hydroxyl attached directly to a benzene ring makes a phenol, which behaves rather differently. Sugars are studded with hydroxyls, which is why they dissolve so readily in water and link together through them. One precise caveat: a hydroxyl group (covalently bonded OH on carbon) is not the same as a free hydroxide ion (OH-), the strong base — same letters, very different species.
Ethanol, CH3CH2OH, owes its water-miscibility and its 78 °C boiling point (versus -89 °C for ethane, C2H6) entirely to its single hydroxyl group, which lets molecules hydrogen-bond to one another.
One small OH lifts the boiling point by over 150 degrees through hydrogen bonding.
A hydroxyl group (covalent C-OH) is not a hydroxide ion (OH-, a strong base). And OH on a benzene ring is a phenol, which is far more acidic than an ordinary alcohol.