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Organic conversion

Aniline to benzene

2 steps: diazotisation, then reduction

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

    H₃PO₂ + H₂O

    i

    Hypophosphorous acid replaces N₂⁺ by H (ethanol also does this).

    Also written: CH₃CH₂OH, heat

    BenzeneC₆H₆Arene

Exam answer

  1. Step 1: Aniline to benzene diazonium chloride: NaNO₂ + HCl, 273 to 278 K.
  2. Step 2: Benzene diazonium chloride to benzene: H₃PO₂ + H₂O.
Alternative routes (3)
  • Aniline → Benzene diazonium chloride → Phenol → Benzene
    1. Aniline to benzene diazonium chloride: NaNO₂ + HCl, 273 to 278 K.
    2. Benzene diazonium chloride to phenol: H₂O, warm.
    3. Phenol to benzene: Zn dust, heat.
  • Aniline → Benzene diazonium chloride → Chlorobenzene → Phenol → Benzene
    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⁺.
    4. Phenol to benzene: Zn dust, heat.
  • Aniline → Benzene diazonium chloride → Bromobenzene → Phenylmagnesium bromide → Benzene
    1. Aniline to benzene diazonium chloride: NaNO₂ + HCl, 273 to 278 K.
    2. Benzene diazonium chloride to bromobenzene: Cu₂Br₂, HBr.
    3. Bromobenzene to phenylmagnesium bromide: Mg, dry ether.
    4. Phenylmagnesium bromide to benzene: H₂O.
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 benzene: H₃PO₂ + H₂O Hypophosphorous acid replaces N₂⁺ by H (ethanol also does this).
Questions

Aniline to benzene: quick answers

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

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

Which named reaction is used to convert aniline to benzene?

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 benzene?

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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