Classification of Alcohols, Phenols and Ethers

Alcohols, phenols and ethers are three families of organic compounds built around a single oxygen

atom, and grouping them together in one chapter makes sense because so many of their reactions and

physical trends are best understood by comparing them directly against each other.

An alcohol has the general formula $\text{R--OH}$, where R is a saturated ($sp^3$) alkyl

carbon -- the $-\text{OH}$ (hydroxyl) group is attached to a chain or ring carbon that is not part

of an aromatic system. Alcohols are further classed as monohydric (one $-\text{OH}$ group, e.g.

ethanol), dihydric (two $-\text{OH}$ groups, e.g. ethylene glycol) or polyhydric (three or

more, e.g. glycerol), and, most importantly for how they react, as primary (1°), secondary (2°) or tertiary (3°) according to whether the carbon carrying $-\text{OH}$ is itself attached

to one, two or three other carbon atoms respectively.

A phenol has an $-\text{OH}$ group attached directly to an $sp^2$ carbon of an aromatic

(benzene) ring. This single structural difference from an alcohol -- the $-\text{OH}$ sitting on an

aromatic rather than a saturated carbon -- turns out to change almost everything about the

compound's chemistry: phenols are markedly more acidic than alcohols, and their ring undergoes

substitution reactions that a plain alcohol's carbon chain never does. Phenols too can be

monohydric (phenol itself), dihydric (catechol, resorcinol, hydroquinone -- the three isomeric

dihydroxybenzenes) or trihydric (phloroglucinol, pyrogallol).

An ether has the general formula $\text{R--O--R}'$, where both R and R' are carbon groups

(alkyl or aryl) and neither is a hydrogen -- so, unlike an alcohol or a phenol, an ether's oxygen

has no hydrogen of its own to donate in a hydrogen bond. If R and R' are identical the ether is

called simple or symmetrical (e.g. diethyl ether, $\text{C}_2\text{H}_5\text{--O--} \text{C}_2\text{H}_5$); if they differ it is called mixed or unsymmetrical (e.g.

ethyl methyl ether, $\text{CH}_3\text{--O--C}_2\text{H}_5$). An ether in which one group is aryl

and the other alkyl (e.g. anisole, $\text{C}_6\text{H}_5\text{--O--CH}_3$) is called an alkyl aryl ether, and behaves, as later sections show, differently again from a purely dialkyl ether

when its carbon-oxygen bonds are broken.

This chapter takes the three families through nomenclature, then treats alcohols in depth

(preparation, physical properties, the primary-alcohol reactions, the dehydration mechanism, and

the two commercially vital examples methanol and ethanol), then phenols (preparation, physical

properties, the acidic character that most sharply separates phenols from alcohols, and the

ring's characteristic electrophilic substitution), and finally ethers (nomenclature and the

Williamson synthesis, physical properties, and the cleavage reaction that is the one place an

ether's normally inert $\text{C--O}$ bond does break).