Alcohols — Mono, Di, Tri or Polyhydric Alcohols

By the number of hydroxyl groups

Alcohols (and, in the same way, phenols) are grouped as monohydric, dihydric, trihydric, or, more generally, polyhydric, according to whether the molecule carries one, two, three, or many $-\text{OH}$ groups.

• Monohydric — one $-\text{OH}$ group, e.g. ethanol, $\text{C}_2\text{H}_5\text{OH}$.
• Dihydric — two $-\text{OH}$ groups on different carbons, e.g. ethylene glycol, $\underset{\underset{\displaystyle \text{CH}_2\text{OH}}{|}}{\text{CH}_2\text{OH}}$.
• Trihydric — three $-\text{OH}$ groups on different carbons, e.g. glycerol, $\underset{\underset{\displaystyle \underset{\underset{\displaystyle \text{CH}_2\text{OH}}{|}}{\text{CHOH}}}{|}}{\text{CH}_2\text{OH}}$.

Further classification of monohydric alcohols

A monohydric alcohol can be classified further by the hybridisation of the carbon atom that carries the $-\text{OH}$ group.

(i) Compounds with a $\text{C}_{sp^3}-\text{OH}$ bond

Here the $-\text{OH}$ is attached to an $sp^3$-hybridised carbon of an alkyl group. This class splits into the following types.

Primary, secondary and tertiary alcohols — named by whether the $-\text{OH}$-bearing carbon is itself attached to one, two, or three other carbon atoms:

Allylic alcohols — the $-\text{OH}$ is on an $sp^3$-hybridised carbon that sits right next to a carbon–carbon double bond (an allylic carbon). Depending on how many other carbons are also attached to that same hydroxyl-bearing carbon, an allylic alcohol can itself be primary, secondary, or tertiary — for instance, $\text{CH}_2=\text{CH}-\text{CH}_2-\text{OH}$ is a primary allylic alcohol, while $\text{CH}_2=\text{CH}-\overset{\overset{\displaystyle \text{H}}{|}}{\underset{\underset{\displaystyle -\text{C}-}{|}}{\text{C}}}-\text{OH}$ is a secondary allylic alcohol and $\text{CH}_2=\text{CH}-\overset{\overset{\displaystyle -\text{C}-}{|}}{\underset{\underset{\displaystyle -\text{C}-}{|}}{\text{C}}}-\text{OH}$ is a tertiary allylic alcohol.

Benzylic alcohols — the $-\text{OH}$ is on an $sp^3$-hybridised carbon directly attached to an aromatic (benzene) ring. As with allylic alcohols, a benzylic alcohol can be primary, secondary, or tertiary depending on the number of carbon substituents on the hydroxyl-bearing carbon.

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