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Organic conversion

Benzene to fluorobenzene

4 steps: substitution, then reduction, then diazotisation, then Balz-Schiemann reaction

Class 11, Chapter 9 Hydrocarbons; Class 12, Chapter 9 Amines

  1. BenzeneC₆H₆Arene
  2. Step 1

    conc. HNO₃ + conc. H₂SO₄

    323 to 333 K

    i

    Nitration: the nitronium ion is the electrophile; keep below 333 K to avoid dinitration.

    O−N+O
    NitrobenzeneC₆H₅NO₂Nitro compound
  3. Step 2

    Sn + conc. HCl

    i

    Metal in acid reduces the nitro group; the amine comes out as its salt, so add NaOH at the end.

    Also written: Fe + HCl; H₂, Pd (in ethanol)

    NH2
    AnilineC₆H₅NH₂Amine
  4. Step 3

    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
  5. Step 4

    (i) HBF₄ (ii) heat

    Balz-Schiemann reaction
    i

    Balz-Schiemann reaction: the tetrafluoroborate salt precipitates and decomposes on heating.

    F
    FluorobenzeneC₆H₅FHaloarene

Exam answer

  1. Step 1: Benzene to nitrobenzene: conc. HNO₃ + conc. H₂SO₄, 323 to 333 K.
  2. Step 2: Nitrobenzene to aniline: Sn + conc. HCl.
  3. Step 3: Aniline to benzene diazonium chloride: NaNO₂ + HCl, 273 to 278 K.
  4. Step 4: Benzene diazonium chloride to fluorobenzene: (i) HBF₄ (ii) heat.
Why these reagents

Reagent and reason, step by step

  • Step 1: Benzene to nitrobenzene: conc. HNO₃ + conc. H₂SO₄, 323 to 333 K Nitration: the nitronium ion is the electrophile; keep below 333 K to avoid dinitration.
  • Step 2: Nitrobenzene to aniline: Sn + conc. HCl Metal in acid reduces the nitro group; the amine comes out as its salt, so add NaOH at the end.
  • Step 3: 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 4: Benzene diazonium chloride to fluorobenzene: (i) HBF₄ (ii) heat Balz-Schiemann reaction: the tetrafluoroborate salt precipitates and decomposes on heating.
Questions

Benzene to fluorobenzene: quick answers

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

4 steps: Benzene, then nitrobenzene, then aniline, then benzene diazonium chloride, then fluorobenzene.

Which named reaction is used to convert benzene to fluorobenzene?

Diazotisation and Balz-Schiemann reaction. 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 benzene to fluorobenzene?

Benzene to nitrobenzene: conc. HNO₃ + conc. H₂SO₄, 323 to 333 K. Nitration: the nitronium ion is the electrophile; keep below 333 K to avoid dinitration.

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