What you'll learn

• How to make primary and secondary alcohols from halogenoalkanes and carbonyl compounds.
• How alcohols react with hydrogen halides, ethanoyl chloride and carboxylic acids.
• Why phenol is acidic and unusually reactive towards bromine.
• How to use bromine water and FeCl₃(aq) to identify phenols.

Starting points: alcohols, phenols and functional groups

A functional group is the atom or group of atoms responsible for the characteristic reactions of an organic compound.

Alcohols are classified by the carbon atom bonded to –OH:

• Primary alcohol: the –OH carbon is bonded to one other carbon atom, e.g. ethanol, CH₃CH₂OH. Methanol is usually treated with this group.
• Secondary alcohol: the –OH carbon is bonded to two other carbon atoms, e.g. propan-2-ol, CH₃CH(OH)CH₃.

Making primary and secondary alcohols

There are two key routes in this section:

1. Hydrolysis of halogenoalkanes using aqueous hydroxide ions.
2. Reduction of carbonyl compounds using a hydride reducing agent.

From halogenoalkanes: nucleophilic substitution

A halogenoalkane contains a carbon–halogen bond, such as C–Br or C–Cl. The halogen is more electronegative than carbon, so the carbon is δ⁺ and can be attacked by a nucleophile.

To make an alcohol, heat the halogenoalkane under reflux with aqueous sodium hydroxide or potassium hydroxide:

CH₃CH₂Br(l) + OH⁻(aq) → CH₃CH₂OH(aq) + Br⁻(aq)

The curly-arrow mechanism is nucleophilic substitution: OH⁻ attacks the δ⁺ carbon, while the C–Br bond breaks to Br⁻.

From carbonyl compounds: reduction

A carbonyl compound contains the C=O group. Aldehydes and ketones are reduced to alcohols.

• Aldehyde → primary alcohol
• Ketone → secondary alcohol

A common reducing agent is sodium tetrahydridoborate, NaBH₄, in water or ethanol, followed by protonation.

For example:

CH₃CHO + 2[H] → CH₃CH₂OH

CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃

In the mechanism, a hydride ion, H⁻, attacks the δ⁺ carbonyl carbon. The C=O π bond breaks onto oxygen, forming an alkoxide ion, which is then protonated to form the alcohol.

Reactions of primary and secondary alcohols

Alcohols can be converted into halogenoalkanes and esters. The important idea is that the oxygen lone pair often starts the reaction, but the –OH group itself is a poor leaving group unless it is activated.

With hydrogen halides: forming halogenoalkanes

Alcohols react with hydrogen halides, HX, to form halogenoalkanes:

R–OH + HX → R–X + H₂O

For example:

CH₃CH₂OH(l) + HBr(aq) → CH₃CH₂Br(l) + H₂O(l)

Typical conditions include warming with HBr, or generating HBr in situ from KBr or NaBr and concentrated H₂SO₄. Chloroalkanes can be made using concentrated HCl, often with ZnCl₂.

Mechanism summary:

1. The alcohol oxygen is protonated, forming R–OH₂⁺.
2. Water becomes the leaving group.
3. The halide ion attacks the carbon, forming R–X.

With ethanoyl chloride: forming esters

An ester has the functional group –COO–. Alcohols react rapidly with ethanoyl chloride, CH₃COCl, at room temperature:

R–OH + CH₃COCl → CH₃COOR + HCl

For example:

CH₃CH₂OH(l) + CH₃COCl(l) → CH₃COOCH₂CH₃(l) + HCl(g)

The product is ethyl ethanoate. You may observe steamy white fumes of HCl.

The mechanism is nucleophilic addition-elimination: the alcohol oxygen attacks the acyl carbon, a tetrahedral intermediate forms, Cl⁻ leaves, then deprotonation gives the ester.

With carboxylic acids: acid-catalysed esterification

Alcohols react with carboxylic acids to form esters, but this reaction is slower and reversible:

R–OH + R′COOH ⇌ R′COOR + H₂O

Conditions:

• concentrated H₂SO₄ catalyst
• heat under reflux
• often use excess reactant or remove water to improve yield

For example:

CH₃CH₂OH(l) + CH₃COOH(l) ⇌ CH₃COOCH₂CH₃(l) + H₂O(l)

Phenol: acidity and ring reactions

Phenol, C₆H₅OH, behaves differently from aliphatic alcohols because the oxygen lone pair interacts with the delocalised π system of the benzene ring.

Acidity of phenol

Phenol is a weak acid:

C₆H₅OH(aq) ⇌ C₆H₅O⁻(aq) + H⁺(aq)

It reacts with sodium hydroxide to form sodium phenoxide:

C₆H₅OH(aq) + NaOH(aq) → C₆H₅O⁻Na⁺(aq) + H₂O(l)

Phenol is more acidic than ethanol because the phenoxide ion is stabilised by delocalisation of the negative charge into the benzene ring. Ethoxide ions do not have this aromatic delocalisation.

However, phenol is much weaker than carboxylic acids, so it usually does not fizz with sodium carbonate.

Reaction with bromine water

Phenol reacts readily with bromine water at room temperature. The orange/brown bromine water is decolourised and a white precipitate of 2,4,6-tribromophenol forms:

C₆H₅OH(aq) + 3Br₂(aq) → C₆H₂Br₃OH(s) + 3HBr(aq)

The –OH group donates electron density into the ring, activating the 2, 4 and 6 positions. Unlike benzene, phenol does not need a halogen carrier such as FeBr₃.

Reaction with ethanoyl chloride

Phenol reacts with ethanoyl chloride to form phenyl ethanoate:

C₆H₅OH + CH₃COCl → CH₃COOC₆H₅ + HCl

The mechanism follows the same nucleophilic addition-elimination pattern as alcohols reacting with ethanoyl chloride: oxygen attacks the acyl carbon, chloride leaves, and deprotonation forms the ester.

FeCl₃(aq) test for phenols

Neutral iron(III) chloride solution is used to test for phenols. Add a few drops of FeCl₃(aq) to the sample, usually dissolved in water or ethanol.

A positive test gives a purple, violet, blue or green colour due to formation of an iron(III) complex with phenoxide ions.