Hydroxy Compounds

What an alcohol is

An alcohol is a compound in which a hydroxyl group, $-\text{OH}$, is bonded to a saturated carbon atom. Alcohols with one –OH have the general formula $\text{C}_n\text{H}_{2n+1}\text{OH}$: an alkane, $\text{C}_n\text{H}_{2n+2}$, with one H replaced by OH.

To name an alcohol, find the longest chain that contains the C–OH carbon, number it so the –OH gets the lowest number, and use the ending -ol: $\text{CH}_3\text{CH}_2\text{OH}$ is ethanol; $\text{CH}_3\text{CH(OH)CH}_3$ is propan-2-ol; $(\text{CH}_3)_2\text{CHCH}_2\text{OH}$ is 2-methylpropan-1-ol (the longest chain through the C–OH carbon is three carbons, numbered from the OH end). When there is more than one –OH, keep the e of the alkane and add di- or tri- with a number for each group: $\text{HOCH}_2\text{CH}_2\text{OH}$ is ethane-1,2-diol (used as antifreeze), $\text{HOCH}_2\text{CH(OH)CH}_2\text{OH}$ is propane-1,2,3-triol (glycerol).

The –OH group has two bonds that react, and knowing which one breaks sorts out the whole topic:

• the O–H bond breaks when sodium reacts, when the alcohol is oxidised (together with a C–H bond on the same carbon), when it forms an ester, and when it acts as an acid;
• the C–O bond breaks when –OH is swapped for a halogen and when water is removed to make an alkene.

Primary, secondary and tertiary

Find the carbon that carries the –OH. Count the carbon atoms bonded directly to it:

• primary (1°): one other carbon (or none, in methanol) — ethanol $\text{CH}_3\text{CH}_2\text{OH}$, butan-1-ol $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$
• secondary (2°): two — propan-2-ol $\text{CH}_3\text{CH(OH)CH}_3$, butan-2-ol
• tertiary (3°): three — 2-methylpropan-2-ol $(\text{CH}_3)_3\text{COH}$

Another way to see it: the C–OH carbon of a primary alcohol carries two H atoms, a secondary carries one H, and a tertiary carries none.

When a molecule has more than one –OH, classify each –OH separately. In ethane-1,2-diol both –OH groups are on end $\text{CH}_2$ carbons, so both are primary. Glycerol, $\text{HOCH}_2\text{CH(OH)CH}_2\text{OH}$, has two primary –OH groups and one secondary. Citric acid has one tertiary –OH as well as three $-\text{CO}_2\text{H}$ groups.

This is the same count you used for halogenoalkanes. Here it decides what happens on oxidation: primary alcohols can go to an aldehyde and then a carboxylic acid, secondary alcohols to a ketone, and tertiary alcohols are not oxidised. The “Oxidation: products and colour changes” section explains why.

Hydrogen bonding: boiling points and solubility

The O–H bond is very polar, and the oxygen has two lone pairs. So the H of one alcohol molecule is attracted to a lone pair on the O of another: a hydrogen bond between molecules (AS Chemical Bonding). This explains two physical properties.

• Boiling points are high. Ethanol ($M_\text{r}$ = 46) boils at 78 °C, but propane ($M_\text{r}$ = 44) boils at −42 °C. Both have similar van der Waals' forces, but only ethanol also has hydrogen bonds to break when it boils. Aldehydes and ketones cannot form hydrogen bonds with each other (they have no O–H), so they boil lower than the alcohols they come from. That is why an aldehyde can be distilled off as soon as it forms (see “Oxidation: products and colour changes”).
• Small alcohols dissolve in water. Alcohol molecules form hydrogen bonds with water molecules, so methanol, ethanol and propanol mix with water in any proportion. As the hydrocarbon chain gets longer, more of the molecule cannot hydrogen bond, and solubility falls.

More –OH groups mean more hydrogen bonds: ethane-1,2-diol boils at 197 °C and glycerol at 290 °C.