Alcohols
Alcohols contain an –OH group on a saturated carbon. They are classified by the number of alkyl groups on the carbon carrying the –OH: primary (one: CH₃CH₂OH), secondary (two: CH₃CH(OH)CH₃) and tertiary (three: (CH₃)₃COH). Hydrogen bonding gives them higher boiling points than alkanes of similar mass, and the small alcohols dissolve in water.
| Preparation | Method |
|---|---|
| Hydration of alkenes | Alkene + steam, phosphoric acid catalyst, 300 °C |
| Fermentation of sugars | Yeast, 25–35 °C, no air |
| From halogenoalkanes | Warm with aqueous NaOH |
| Reduction of aldehydes and ketones | NaBH₄ or LiAlH₄ |
| Hydrolysis of esters | Heat with aqueous alkali |
| Reaction | Result |
|---|---|
| With sodium | Hydrogen and sodium alkoxide |
| With a carboxylic acid (acid catalyst) | Ester + water |
| Oxidation of a primary alcohol with acidified K₂Cr₂O₇ | Aldehyde (distil off immediately), or carboxylic acid (reflux with excess oxidant) |
| Oxidation of a secondary alcohol | Ketone |
| Tertiary alcohol | Not oxidised (no hydrogen on the carbon with –OH) |
| Dehydration with concentrated H₂SO₄ or Al₂O₃, 170 °C | Alkene |
| With PCl₅ or SOCl₂ | Chloroalkane (a test for –OH: steamy fumes of HCl) |
Choose the number of alkyl groups on the carbon bearing the –OH group (1 = primary, 2 = secondary, 3 = tertiary). Primary and secondary alcohols are oxidised; tertiary are not.
Phenol
Phenol, C₆H₅OH, has the –OH group attached directly to a benzene ring. It is weakly acidic: it reacts with sodium hydroxide (but not with sodium carbonate) to form sodium phenoxide, because the phenoxide ion is stabilised by delocalisation of its charge into the ring. Alcohols are not acidic enough to react with NaOH. The –OH group makes the ring very reactive: phenol is brominated by bromine water without a catalyst, giving a white precipitate of 2,4,6-tribromophenol. Phenol gives a purple colour with neutral iron(III) chloride. It is used in disinfectants, plastics and dyes.
Key terms
- Primary alcohol
- An alcohol in which the C–OH carbon is bonded to one alkyl group.
- Dehydration
- Removal of water from a molecule.
- Phenol
- A compound with an –OH group on a benzene ring.
- Esterification
- The reaction of an alcohol with a carboxylic acid to make an ester.
Summary
- Primary, secondary and tertiary alcohols differ in how they are oxidised.
- Alcohols react with sodium, acids (esters), oxidising agents and dehydrating agents.
- Phenol is weakly acidic and its ring is very reactive.
- Phenol turns purple with iron(III) chloride.