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Organic conversion

Aniline to phenol

2 steps: diazotisation, then hydrolysis

Class 12, Chapter 9 Amines

  1. NH2
    AnilineC₆H₅NH₂Amine
  2. Step 1

    NaNO₂ + HCl

    273 to 278 K

    Diazotisation
    i

    Diazotisation: keep at 273 to 278 K or the salt decomposes. The acid generates HNO₂ in situ.

    N+NCl−
    Benzene diazonium chlorideC₆H₅N₂⁺Cl⁻Diazonium salt
  3. Step 2

    H₂O, warm

    i

    Warming with water replaces N₂⁺ by OH; keep the diazonium salt cold until then.

    HO
    PhenolC₆H₅OHPhenol

Exam answer

  1. Step 1: Aniline to benzene diazonium chloride: NaNO₂ + HCl, 273 to 278 K.
  2. Step 2: Benzene diazonium chloride to phenol: H₂O, warm.
Alternative routes (4)
  • Aniline → Benzene diazonium chloride → Chlorobenzene → Phenol
    1. Aniline to benzene diazonium chloride: NaNO₂ + HCl, 273 to 278 K.
    2. Benzene diazonium chloride to chlorobenzene: Cu₂Cl₂, HCl.
    3. Chlorobenzene to phenol: (i) NaOH, 623 K, 300 atm (ii) H⁺.
  • Aniline → Benzene diazonium chloride → Benzene → Chlorobenzene → Phenol
    1. Aniline to benzene diazonium chloride: NaNO₂ + HCl, 273 to 278 K.
    2. Benzene diazonium chloride to benzene: H₃PO₂ + H₂O.
    3. Benzene to chlorobenzene: Cl₂, anhydrous FeCl₃.
    4. Chlorobenzene to phenol: (i) NaOH, 623 K, 300 atm (ii) H⁺.
  • Aniline → Benzene diazonium chloride → Benzene → Cumene → Phenol
    1. Aniline to benzene diazonium chloride: NaNO₂ + HCl, 273 to 278 K.
    2. Benzene diazonium chloride to benzene: H₃PO₂ + H₂O.
    3. Benzene to cumene: CH₃CH₂CH₂Cl, anhydrous AlCl₃.
    4. Cumene to phenol: (i) O₂ (ii) H₃O⁺.
  • Aniline → Benzene diazonium chloride → Benzene → Benzene sulphonic acid → Phenol
    1. Aniline to benzene diazonium chloride: NaNO₂ + HCl, 273 to 278 K.
    2. Benzene diazonium chloride to benzene: H₃PO₂ + H₂O.
    3. Benzene to benzene sulphonic acid: fuming H₂SO₄ (oleum).
    4. Benzene sulphonic acid to phenol: (i) NaOH (molten), heat (ii) H⁺.
Why these reagents

Reagent and reason, step by step

  • Step 1: Aniline to benzene diazonium chloride: NaNO₂ + HCl, 273 to 278 K Diazotisation: keep at 273 to 278 K or the salt decomposes. The acid generates HNO₂ in situ.
  • Step 2: Benzene diazonium chloride to phenol: H₂O, warm Warming with water replaces N₂⁺ by OH; keep the diazonium salt cold until then.
Questions

Aniline to phenol: quick answers

How many steps does it take to convert aniline to phenol?

2 steps: Aniline, then benzene diazonium chloride, then phenol.

Which named reaction is used to convert aniline to phenol?

Diazotisation. A primary aromatic amine reacts with nitrous acid (NaNO2 and HCl) at 273 to 278 K to give a diazonium salt.

What is the reagent for step 1 of aniline to phenol?

Aniline to benzene diazonium chloride: NaNO₂ + HCl, 273 to 278 K. Diazotisation: keep at 273 to 278 K or the salt decomposes. The acid generates HNO₂ in situ.

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