Concepts: classification, naming and structure of the three families

2.1 The three families

An alcohol has an OH group on an sp3 carbon of an aliphatic chain. A phenol has the OH group on an sp2 carbon of an aromatic ring. An ether has two carbon groups joined through an oxygen, written R-O-R. You can think of an ether as an alcohol whose hydrogen has been replaced by a second organic group.

Alcohols are sorted by the number of OH groups. Monohydric alcohols carry one, dihydric carry two such as ethylene glycol, trihydric carry three such as glycerol, and polyhydric carry many. Monohydric alcohols are sorted again by the carbon holding the OH group: primary, secondary or tertiary. Two more classes appear where the OH group sits next to special carbons. Allylic alcohols have the OH on an sp3 carbon beside a carbon carbon double bond, and benzylic alcohols have it on an sp3 carbon beside an aromatic ring. A vinylic alcohol has the OH on an sp2 carbon of a double bond, and these are unstable compared with the other classes.

Phenols follow the same counting logic: monohydric, dihydric and trihydric. The dihydric benzene derivatives have common names worth knowing, catechol for the 1,2 compound, resorcinol for 1,3 and hydroquinone for 1,4. Ethers are either symmetrical when both groups are the same, or unsymmetrical when the two groups differ.

2.2 Naming

For alcohols, take the alkane name and replace the final e with ol, then number the longest chain from the end that gives the OH group the lower locant. So CH3CH2CH2OH is propan-1-ol and the branched isomer CH3CH(OH)CH3 is propan-2-ol. When more than one OH is present, keep the e and add diol or triol with locants, so ethylene glycol becomes ethane-1,2-diol and glycerol becomes propane-1,2,3-triol. Cyclic alcohols take the cyclo prefix with the OH on carbon 1.

Phenol is both the common and the accepted IUPAC name, and substituted phenols use ortho, meta and para in common names and 2, 3, 4 in IUPAC names. Cresols are methylphenols, and 2-methylphenol is also called o-cresol.

Ethers are named as alkanes with an alkoxy substituent, with the larger group as the parent. Methoxyethane, ethoxypropane and methoxybenzene are IUPAC names, while the common names read the two groups in alphabetical order and add ether, as in ethyl methyl ether. Anisole is the common name of methoxybenzene.

2.3 The carbon oxygen bond

In methanol the oxygen uses an sp3 orbital, the angle at oxygen is a little under the tetrahedral value because two lone pairs repel the bonded pairs, and the C-O bond measures about 142 pm. In phenol the oxygen is joined to an sp2 carbon, the lone pair on oxygen overlaps with the ring pi system, and the C-O bond shortens to 136 pm. That partial double bond character is the first hint that the OH group of phenol will not behave exactly like the OH group of an alcohol.

Ethers keep a C-O length of about 141 pm, close to the alcohol value, but the angle at oxygen is wider than tetrahedral because the two organic groups are bulky and push against each other.

2.4 Physical properties explained by hydrogen bonding

Alcohols and phenols can both donate a hydrogen bond and accept one. Along a chain, each OH can attach to the OH of a neighbour, so the molecules stick together and the boiling points come out high: higher than ethers, haloalkanes and hydrocarbons of similar mass. Ethanol and propane have almost the same molecular mass, yet ethanol boils far above propane because propane cannot hydrogen bond at all. Methoxymethane has no OH group, cannot donate a hydrogen bond, and so boils between the two.

Among alcohols, boiling point rises as the carbon chain grows, because the van der Waals contact area grows. Branching lowers the boiling point because the molecule becomes rounder and touches its neighbours less. Phenol boils high for its size because of strong hydrogen bonding and a fairly rigid ring.

Solubility in water comes from the same property. The OH group makes hydrogen bonds with water, so the small alcohols, methanol, ethanol and propan-1-ol, mix with water in any proportion. As the hydrocarbon part grows, the hydrophobic tail starts to dominate and the solubility falls. Ethers can accept hydrogen bonds from water through their oxygen, so ethoxyethane dissolves to about 7.5 g per 100 mL, close to the 9 g per 100 mL of butan-1-ol. Pentane, with no oxygen, does not dissolve at all.

2.5 Acidity

Alcohols and phenols both lose the OH hydrogen to a strong base, so both are Bronsted acids. Water beats an alcohol: an alkoxide reacts with water to give the alcohol back plus hydroxide, which shows water is the stronger acid. Phenol is a much stronger acid than either, and it reacts with aqueous sodium hydroxide, which alcohols cannot do. The reason is the phenoxide ion. Its negative charge spreads over the ring and the oxygen through resonance, which is far more stable than an alkoxide that keeps the charge on one oxygen atom. Electron withdrawing groups such as nitro stabilise the phenoxide further and raise acidity, which is why p-nitrophenol is a stronger acid than phenol. Alkyl groups push electron density towards the oxygen and make cresols slightly weaker acids than phenol.