Further organic chemistry for OCR and Edexcel

6.1 · Free notes

Phenols: acidity, reactions with bromine, bases and acyl chlorides, and the iron(III) chloride test

Key points

• Phenols have an –OH group bonded directly to a benzene ring (e.g. C₆H₅OH). One of the oxygen lone pairs is delocalised into the ring, and this explains most of their properties.
• Phenol is a weak acid (Ka ≈ 1.3 × 10⁻¹⁰ mol dm⁻³, pKa ≈ 9.9). It is more acidic than alcohols (pKa ≈ 16) but far weaker than carboxylic acids (pKa ≈ 4–5).
• Phenol is more acidic than ethanol because the negative charge on the phenoxide ion (C₆H₅O⁻) is delocalised into the ring, which stabilises the ion. In an alkoxide the charge stays on oxygen and is pushed further by the electron-donating alkyl group.
• Phenol reacts with NaOH(aq) in acid–base neutralisation: C₆H₅OH + NaOH → C₆H₅O⁻Na⁺ + H₂O. Ethanol does not do this. Phenol also reacts with sodium: 2C₆H₅OH + 2Na → 2C₆H₅O⁻Na⁺ + H₂.
• Phenol is a weaker acid than carbonic acid, so it does not react with Na₂CO₃ or NaHCO₃ and gives no CO₂ effervescence. Carboxylic acids do effervesce, so this distinguishes phenol from a carboxylic acid.
• Phenol decolourises bromine water at room temperature with no catalyst. It forms a white precipitate of 2,4,6-tribromophenol: C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr. Benzene needs Br₂ with a halogen carrier (FeBr₃) and gives only monosubstitution.
• Phenol is more reactive than benzene because the O lone pair increases the electron density of the ring. This polarises Br₂ more strongly, so the ring is more susceptible to electrophilic substitution. Substitution is directed to the 2, 4 and 6 positions.
• Phenol is nitrated by dilute HNO₃ at room temperature, giving a mixture of 2-nitrophenol and 4-nitrophenol. Benzene needs concentrated HNO₃/H₂SO₄ at about 50 °C.
• Phenol does not form esters directly with carboxylic acids because it is a poor nucleophile. It reacts with acyl chlorides (or acid anhydrides) to give phenyl esters: C₆H₅OH + CH₃COCl → CH₃COOC₆H₅ (phenyl ethanoate) + HCl.
• The iron(III) chloride test: add a few drops of neutral FeCl₃(aq) to the sample. A violet/purple colouration (a complex of Fe³⁺ with phenol) shows a phenol. Alcohols and most other compounds give no such colour.
• Distinguishing tests for exam answers: phenol vs ethanol uses FeCl₃ (violet), Br₂(aq) (white ppt) or NaOH(aq). Phenol vs a carboxylic acid uses Na₂CO₃/NaHCO₃, where only the acid gives CO₂.

In more detail

• Bromination equation: C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr; the product, 2,4,6-tribromophenol, is a white precipitate with an antiseptic smell.
• Acid strength order: carboxylic acids > carbonic acid > phenol > water > ethanol. So phenol reacts with NaOH but not with NaHCO₃, which distinguishes it from carboxylic acids.
• Making phenyl esters: use an acyl chloride or acid anhydride, e.g. phenol + ethanoyl chloride → phenyl ethanoate + HCl.
• Uses of phenols: antiseptics (TCP), resins, and as starting materials for aspirin and some polymers.