Hydroxy compounds

📚The chemistry

Making alcohols

• Hydration of an alkene — steam, H₂O(g), with an H₃PO₄ catalyst (electrophilic addition).
• Cold, dilute, acidified KMnO₄ with an alkene — gives a diol.
• Hydrolysis of a halogenoalkane — NaOH(aq), heat (nucleophilic substitution).
• Reduction of an aldehyde or ketone — NaBH₄ or LiAlH₄. Aldehydes give primary alcohols; ketones give secondary alcohols.
• Reduction of a carboxylic acid — LiAlH₄ only (in dry ether); NaBH₄ is not strong enough. Gives a primary alcohol.
• Hydrolysis of an ester — dilute acid or dilute alkali, heat. Gives the alcohol and the carboxylic acid (or its salt).

Primary, secondary and tertiary

As for halogenoalkanes, count the alkyl groups on the carbon carrying the OH: primary (one) — propan-1-ol; secondary (two) — propan-2-ol; tertiary (three) — 2-methylpropan-2-ol. Each OH group in a molecule is classified separately: in propane-1,2-diol, HOCH₂CH(OH)CH₃, one OH is primary and the other secondary.

Reactions of alcohols

Combustion

Alcohols burn with a clean, almost colourless flame, which is why ethanol is used as a fuel.

Substitution to halogenoalkanes

The OH group is replaced by a halogen, using HX(g), KCl with concentrated H₂SO₄ or H₃PO₄, PCl₃ and heat, PCl₅, or SOCl₂ — the equations are on the topic 15 page.

With sodium

Sodium fizzes steadily and dissolves, releasing hydrogen and forming sodium ethoxide. The reaction is noticeably slower than sodium with water.

Acidity: alcohols against water

Both water and alcohols can lose H⁺ from an O–H bond, but alcohols do so less readily: ethanol is a weaker acid than water. The reason is the alkyl group. It is electron-donating (positive inductive effect), so it pushes electron density onto the oxygen of the alkoxide ion, RO⁻. That intensifies the negative charge on oxygen and makes RO⁻ less stable than OH⁻, which has no alkyl group. The less stable the anion, the less the O–H bond ionises. It is the same idea that makes sodium react more gently with ethanol.

Oxidation

Heat the alcohol with acidified potassium dichromate(VI), K₂Cr₂O₇ with dilute H₂SO₄ (orange → green as Cr³⁺ forms), or acidified KMnO₄ (purple → colourless). What you get depends on the class of alcohol and the apparatus:

• Primary alcohol, distil as it forms → aldehyde. The aldehyde has a lower boiling point than the alcohol (it cannot hydrogen bond with itself), so it boils off before it can be oxidised further. \[ \mathrm{CH_3CH_2OH + [O] \rightarrow CH_3CHO + H_2O} \]
• Primary alcohol, heat under reflux → carboxylic acid. Reflux returns the aldehyde to the flask, so it is oxidised again. Use excess oxidising agent. \[ \mathrm{CH_3CH_2OH + 2[O] \rightarrow CH_3COOH + H_2O} \]
• Secondary alcohol → ketone (distillation or reflux). Ketones are not oxidised further. \[ \mathrm{CH_3CH(OH)CH_3 + [O] \rightarrow CH_3COCH_3 + H_2O} \]
• Tertiary alcohol → no reaction. The carbon carrying the OH has no H atom to lose, and oxidation would mean breaking a C–C bond. The dichromate stays orange.

That last contrast is the distinguishing test: warm with acidified K₂Cr₂O₇. Primary and secondary alcohols turn it green; tertiary alcohols leave it orange. Primary and secondary are then told apart by testing the product: an aldehyde (from a primary alcohol) reacts with Tollens’ or Fehling’s reagent, a ketone does not (topic 17).

Dehydration

Pass the vapour over heated Al₂O₃, or heat with concentrated H₂SO₄ or H₃PO₄: water is eliminated and an alkene forms. C₂H₅OH → CH₂=CH₂ + H₂O. As with halogenoalkanes, an unsymmetrical alcohol can give more than one alkene.

Esterification

Heat an alcohol with a carboxylic acid and a few drops of concentrated H₂SO₄ as catalyst. It is a reversible condensation reaction:

The product is ethyl ethanoate, with a sweet, fruity smell. Esters are used as solvents and flavourings.

The tri-iodomethane (iodoform) test

Warm the compound with alkaline aqueous iodine (I₂ with NaOH). A pale yellow precipitate of tri-iodomethane, CHI₃, with an antiseptic smell, shows the presence of a CH₃CH(OH)– group. The iodine first oxidises it to CH₃CO–, which is then split into CHI₃ and a carboxylate ion with one carbon fewer:

Ethanol (R = H) gives a positive test — the only primary alcohol that does. Propan-2-ol and butan-2-ol give it; propan-1-ol and butan-1-ol do not.