32.2 Phenol

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Recall how phenol is produced from phenylamine via a diazonium salt intermediate.
  2. Recall the key reactions of phenol: with NaOH(aq), with Na(s), with diazonium salts in NaOH(aq), nitration with dilute HNO₃, and bromination with Br₂(aq).
  3. Explain why phenol is acidic.
  4. Describe and explain the relative acidities of water, phenol, and ethanol.
  5. Explain why the conditions for nitrating and brominating phenol differ from those used for benzene.
  6. Recall that the –OH group directs incoming groups to the 2-, 4-, and 6-positions on the ring.
  7. Apply your knowledge of phenol's reactions to other phenolic compounds such as naphthol.

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What is Phenol?

Phenol is an organic compound made up of a benzene ring (a ring of six carbon atoms with alternating double bonds) with a hydroxyl group (–OH) attached directly to it.

Its molecular formula is C₆H₅OH.

The fact that the –OH group is joined directly to the benzene ring is what makes phenol special and very different in behaviour from alcohols like ethanol.

Section 1: How Phenol is Produced — From Phenylamine via a Diazonium Salt

Phenol is made in the lab starting from phenylamine (C₆H₅NH₂), which is an amine with an –NH₂ group attached to a benzene ring.

This is a two-step process:

Step 1 — Making the Diazonium Salt

Reagents: Phenylamine + HNO₂ (nitrous acid) — OR equivalently, NaNO₂ (sodium nitrite) + a dilute acid (e.g. HCl or H₂SO₄)

Note: HNO₂ is not stable and is generated in situ (meaning: made on the spot in the reaction mixture) by mixing NaNO₂ with dilute HCl or dilute H₂SO₄.

Condition: Temperature must be kept below 10°C (usually in an ice bath).

Why below 10°C? The diazonium salt formed is unstable — if the temperature rises above 10°C, it decomposes immediately before you can use it. Keeping it cold keeps it intact.

What happens: The –NH₂ group on phenylamine reacts with HNO₂ to form a diazonium salt, specifically benzenediazonium chloride (if HCl is used).

The diazonium salt contains a –N≡N⁺ group (called the diazonium group) attached to the benzene ring.

Equation:

C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N≡N⁺ Cl⁻ + 2H₂O

(Phenylamine + nitrous acid + hydrochloric acid → benzenediazonium chloride + water)

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