Nomenclature, Structure and Reactivity of Alcohols: MDCAT Chemistry notes
Nomenclature, Structure and Reactivity of Alcohols for MDCAT: 1°, 2°, 3° alcohols, Lucas test, dehydration, oxidation, iodoform test, industrial ethanol.
Classification and names
Alcohols contain an $-\mathrm{OH}$ group on a saturated (sp$^3$) carbon, the carbinol carbon. IUPAC names end in -ol, with the chain numbered to give the OH the lowest locant.
| Type | Carbinol carbon carries | Examples |
|---|---|---|
| Primary (1$^\circ$) | one alkyl group | ethanol; 1-propanol; 2-methyl-1-propanol |
| Secondary (2$^\circ$) | two alkyl groups | 2-propanol (isopropyl alcohol); 2-butanol |
| Tertiary (3$^\circ$) | three alkyl groups | 2-methyl-2-propanol (tert-butyl alcohol); 2-methyl-2-butanol |
Polyhydric alcohols: ethylene glycol (ethane-1,2-diol), used as automobile antifreeze; glycerol (propane-1,2,3-triol). Note: $\mathrm{CH_3OCH_3}$ is an ether, $\mathrm{CH_3COOH}$ an acid, $\mathrm{CH_3CH_2Br}$ an alkyl halide; $\mathrm{CH_3CH_2OH}$ is the alcohol.
Structure and physical properties
The oxygen is sp$^3$ with two lone pairs, and the C–O–H angle is bent. The O–H bond allows hydrogen bonding, so alcohols boil much higher than alkanes of similar mass. Boiling point rises with chain length and falls with branching, so n-propyl alcohol boils above isopropyl alcohol and ethanol.
Industrial ethanol
- Hydration of ethene: ethene + steam over $\mathrm{H_3PO_4}$ at high temperature and pressure — the main industrial route.
- Ethene absorbed in cold conc. $\mathrm{H_2SO_4}$ gives ethyl hydrogen sulphate, which on boiling with water hydrolyses to ethanol.
- Fermentation gives rectified spirit (95.6% ethanol, an azeotrope). The last water is removed with quicklime (CaO) to get absolute alcohol.
- Denatured spirit is ethanol made unfit to drink by adding poisonous or nauseating substances such as methanol and pyridine. Lactic acid is not a denaturant.
Reactions
O–H bond breaks
With sodium: $\mathrm{2ROH + 2Na \rightarrow 2RONa + H_2}$, giving a sodium alkoxide. Reactions in which the alcohol acts through its O–H (sodium, esterification) are fastest for 1$^\circ$ > 2$^\circ$ > 3$^\circ$, because the oxygen is least crowded and least electron-rich in primary alcohols.
C–O bond breaks
Here a carbocation character develops, so the order is 3$^\circ$ > 2$^\circ$ > 1$^\circ$. With HCl (or HBr, $\mathrm{PCl_5}$, $\mathrm{SOCl_2}$) an alcohol gives an alkyl halide.
Lucas test (conc. HCl + anhydrous $\mathrm{ZnCl_2}$): the insoluble alkyl chloride appears as a cloudy oily layer.
- Tertiary: immediately at room temperature
- Secondary: within about 5–10 minutes
- Primary: no visible reaction at room temperature (only on heating)
Dehydration with conc. $\mathrm{H_2SO_4}$
- Ethanol at 140$^\circ$C (excess alcohol) → diethyl ether.
- Ethanol at about 170–180$^\circ$C (excess acid) → ethene.
- Ease of dehydration: 3$^\circ$ > 2$^\circ$ > 1$^\circ$ (via carbocation).
- Methanol has one carbon, so it cannot give an alkene; it gives dimethyl ether, not $\mathrm{CH_3CHO}$, $\mathrm{C_2H_4}$ or HCHO.
Oxidation (acidified $\mathrm{K_2Cr_2O_7}$)
- 1$^\circ$ alcohol → aldehyde → carboxylic acid (complete oxidation). To stop at the aldehyde it is distilled off as it forms, since it boils lower than the alcohol and would otherwise be oxidized further to the acid.
- 2$^\circ$ alcohol → ketone.
- 3$^\circ$ alcohol resists oxidation under these conditions.
Iodoform test
Alcohols containing $\mathrm{CH_3CH(OH)-}$ (ethanol, 2-propanol, 2-butanol) give yellow iodoform with $\mathrm{I_2}$ and NaOH. Methanol, 1-propanol and 1-butanol do not, so the test distinguishes ethanol from methanol.
Common MDCAT traps
- 140$^\circ$C gives ether; higher temperature gives alkene.
- Only ethanol, among methanol, ethanol and 1-alcohols, gives iodoform.
- Tertiary alcohols form the oily layer immediately in the Lucas test, without heating.
- Complete oxidation of a primary alcohol gives an acid, not an aldehyde.
- C–O cleavage: 3$^\circ$ fastest; O–H cleavage: 1$^\circ$ fastest.
Quick revision
- Glycerol = propane-1,2,3-triol; glycol = antifreeze.
- Alcohol + Na → alkoxide + $\mathrm{H_2}$.
- 2$^\circ$ alcohol oxidizes to ketone.
- Absolute alcohol: dry rectified spirit with CaO.