Cambridge International AS and A Level Chemistry 32: Hydroxy compounds

Hydroxy Compounds is Cambridge International Chemistry 9701 Topic 32. The A Level boundary adds alcohol acylation and develops phenol through diazonium-salt preparation, acid-base reactions, azo coupling, nitration, bromination, acidity, ring activation and application to other phenolic compounds.

1. Alcohols with acyl chlorides

An alcohol reacts with an acyl chloride at room temperature to form an ester and hydrogen chloride. Ethanol plus ethanoyl chloride gives ethyl ethanoate.

The alcohol oxygen attacks the electrophilic carbonyl carbon. Addition is followed by elimination of chloride and proton transfer, so the carbonyl is restored.

This route is rapid and does not require the concentrated sulfuric acid and reflux associated with reversible carboxylic-acid esterification.

2. Recognising phenol

Phenol has hydroxyl bonded directly to an aromatic carbon. An aromatic molecule with hydroxyl on a side-chain carbon is an alcohol instead.

Direct ring attachment allows an oxygen lone pair to interact with the delocalised pi system. That interaction affects both O-H acidity and ring reactivity.

Connectivity therefore explains why phenol cannot be treated as merely an aromatic-looking ethanol.

3. Phenylamine to diazonium salt

Phenylamine reacts with nitrous acid below 10 degrees Celsius to form a benzenediazonium salt. Nitrous acid is commonly generated in situ from sodium nitrite and dilute acid.

The low temperature keeps the diazonium salt sufficiently stable for use. Warming too early causes decomposition before the intended intermediate is established.

State both the nitrite-acid reagents and the below-10-degree condition.

4. Diazonium salt to phenol

Further warming the aqueous diazonium salt replaces the diazonium group with hydroxyl, producing phenol and nitrogen gas.

This is a two-stage preparation: cold diazotisation first, then warming with water. Combining the stages without their different temperatures loses the control logic.

Effervescence of nitrogen is evidence for decomposition of the diazonium intermediate.

5. Phenol with aqueous alkali

Phenol reacts with aqueous sodium hydroxide to form sodium phenoxide and water. This shows phenol is acidic enough to be deprotonated by hydroxide.

Ethanol does not react appreciably with aqueous sodium hydroxide under the same conditions because ethoxide is less stabilised relative to ethanol.

The product charge is delocalised through resonance involving the aromatic ring.

6. Phenol with sodium

Phenol reacts with sodium metal to form sodium phenoxide and hydrogen gas. Two phenol molecules produce one hydrogen molecule.