All subjects · Chemistry · WCH15 (Unit 5)

Friedel-Crafts reactions and phenol

Friedel-Crafts reactions attach carbon chains to a benzene ring, which is a key step in making many useful compounds. Phenol shows how a group on the ring changes its reactivity.

Friedel-Crafts alkylation (spec 18.4)

A halogenoalkane with an aluminium chloride catalyst, under anhydrous conditions:

C6H6 + CH3CH2Cl -> C6H5CH2CH3 + HCl (ethylbenzene)

Electrophile: CH3CH2Cl + AlCl3 -> CH3CH2+ + AlCl4-. Regeneration: H+ + AlCl4- -> AlCl3 + HCl.

Friedel-Crafts acylation (spec 18.4)

An acyl chloride with AlCl3, warmed under reflux in dry conditions, gives an aromatic ketone:

C6H6 + CH3COCl -> C6H5COCH3 + HCl (phenylethanone)

Electrophile: CH3COCl + AlCl3 -> CH3CO+ + AlCl4-. The mechanism is the same electrophilic substitution as nitration. AlCl3 reacts with water, so everything must be dry.

Phenol (spec 18.6)

Phenol, C6H5OH, reacts with bromine water at room temperature, without a catalyst. The orange bromine water is decolourised and a white precipitate of 2,4,6-tribromophenol forms:

C6H5OH + 3Br2 -> C6H2Br3OH + 3HBr

A lone pair on the oxygen overlaps with the ring and is partly delocalised into it. This increases the electron density of the ring, so it polarises bromine molecules and attracts electrophiles much more strongly than benzene does. The ring is activated.

Worked example

Give the reagents, conditions and electrophile for making phenylethanone from benzene.

Reagents: ethanoyl chloride with an aluminium chloride catalyst
Conditions: anhydrous, warm under reflux
CH3COCl + AlCl3 -> CH3CO+ + AlCl4-

Answer: Ethanoyl chloride and AlCl3, dry, warm; electrophile CH3CO+

Key idea: Alkylation: RCl + AlCl3 gives R+; acylation: RCOCl + AlCl3 gives RCO+ (aromatic ketone). Dry conditions. Phenol decolourises bromine water at room temperature, giving white 2,4,6-tribromophenol, because the O lone pair activates the ring.

Check yourself

Which reagents convert benzene into ethylbenzene?
- Ethene with nickel
- Ethanol with concentrated sulfuric acid
- Chloroethane with aluminium chloride
- Ethanoyl chloride with aluminium chloride
Show answer
Chloroethane with aluminium chloride. Friedel-Crafts alkylation uses a halogenoalkane and AlCl3.
What is the electrophile when benzene reacts with ethanoyl chloride and AlCl3?
- CH3CO+
- CH3+
- Cl+
- AlCl4-
Show answer
CH3CO+. AlCl3 removes Cl- from the acyl chloride.
What is the organic product of benzene with propanoyl chloride and AlCl3?
- Propylbenzene
- Phenyl propanoate
- Phenylpropanone, C6H5COCH2CH3
- Benzoic acid
Show answer
Phenylpropanone, C6H5COCH2CH3. Acylation attaches the C=O group to the ring, giving a ketone.
Why must Friedel-Crafts reactions be carried out in dry conditions?
- Benzene reacts with water
- Aluminium chloride reacts with water
- Water is the electrophile
- Water would form a precipitate of the product
Show answer
Aluminium chloride reacts with water. Water destroys the catalyst.
What is seen when bromine water is added to phenol?
- No change
- A silver mirror forms
- Purple fumes are given off
- The bromine water is decolourised and a white precipitate forms
Show answer
The bromine water is decolourised and a white precipitate forms. 2,4,6-tribromophenol is a white solid.
Why does phenol react with bromine much more readily than benzene?
- Phenol has no delocalised electrons
- A lone pair on oxygen is delocalised into the ring, increasing its electron density
- The O-H bond breaks easily
- Phenol is an alkene
Show answer
A lone pair on oxygen is delocalised into the ring, increasing its electron density. The activated ring attracts electrophiles strongly.
How many moles of Br2 react with one mole of phenol in bromine water?
Show answer
3. C6H5OH + 3Br2 -> C6H2Br3OH + 3HBr: three H atoms on the ring are replaced.

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Last updated 7 November 2026. Spotted a mistake? Tell us.