Nitrogen Containing Organic Compounds - Preparation of Anilines

• Introduction

  • Aniline is a nitrogen-containing organic compound with the chemical formula C₆H₅N.
  • It is also known as aminobenzene, phenylamine, or benzenamine.
  • Aniline is an important precursor for a variety of chemical compounds and dyes.
  • In this lecture, we will discuss the preparation of anilines.

• Preparation of Anilines

  • Reduction of Nitrobenzene
    • Nitrobenzene can be reduced to aniline by using reducing agents such as iron and hydrochloric acid (Fe/HCl).
    • The reaction can be represented as:

      C₆H₅NO₂ + 6H₂ → C₆H₅NH₂ + 2H₂O.

    • Example: Reaction of nitrobenzene with Fe/HCl to form aniline.

• Preparation of Anilines (contd.)

  • Reduction of Nitrobenzene (contd.)
    • Another method involves the use of tin and hydrochloric acid (Sn/HCl) as reducing agents.
    • The reaction can be represented as:

      C₆H₅NO₂ + 3Sn + 6HCl → C₆H₅NH₂ + 3SnCl₂ + 2H₂O.

    • Example: Reaction of nitrobenzene with Sn/HCl to form aniline.

• Preparation of Anilines (contd.)

  • Reduction of Nitrobenzene (contd.)
    • Sodium sulfide (Na₂S) can also be used as a reducing agent to convert nitrobenzene into aniline.
    • The reaction can be represented as:

      C₆H₅NO₂ + 6Na₂S + 4H₂O → C₆H₅NH₂ + 6NaHSO₃.

    • Example: Reaction of nitrobenzene with Na₂S to form aniline.

• Preparation of Anilines (contd.)

  • Reduction of Nitrobenzene (contd.)
    • Reduction can also be achieved using a catalyst such as platinum, palladium, or nickel.
    • The reaction takes place in the presence of hydrogen gas (H₂).
    • Example: Reaction of nitrobenzene with catalytic hydrogenation to form aniline.

• Preparation of Anilines (contd.)

  • Reduction of Nitrobenzene (contd.)
    • Alternatively, reduction of nitro compounds can be carried out using molecular hydrogen (H₂) and a metal catalyst such as Raney nickel or palladium on carbon.
    • Example: Reaction of nitrobenzene with hydrogen (H₂) and Raney nickel catalyst to form aniline.

• Preparation of Anilines (contd.)

  • Reduction of Nitrobenzene (contd.)
    • Reduction using ammonium sulfide (NH₄)₂S can also be employed to convert nitrobenzene into aniline.
    • Example: Reaction of nitrobenzene with (NH₄)₂S to form aniline.

• Preparation of Anilines (contd.)

  • Reduction of Nitrobenzene (contd.)
    • Aniline can also be prepared by the reduction of aryldiazonium salts with hypophosphorus acid.
    • The reaction can be represented as:

      C₆H₅N₂⁺X^- + H₃PO₂ → C₆H₅NH₂ + H₃PO₃ + X^-.

    • Example: Reaction of aryldiazonium salt with hypophosphorus acid to form aniline.

• Preparation of Anilines (contd.)

  • Reduction of Nitrobenzene (contd.)
    • Aniline derivatives can also be obtained by the reduction of nitro compounds using various reducing agents.
    • Example: Reduction of nitrobenzene derivatives with appropriate reducing agents to form respective aniline derivatves.

11. Preparation of Anilines (contd.)

• Reduction of Aromatic Nitro Compounds with Iron and Sodium Hydroxide
  • Aromatic nitro compounds can be reduced to anilines by using a combination of iron and sodium hydroxide (Fe/NaOH).
  • The reaction can be represented as:

    ArNO₂ + 6Fe + 12NaOH → ArNH₂ + 6Fe(OH)₃ + 6NaNO₂ + 6H₂O.

  • Example: Reaction of aromatic nitro compound with Fe/NaOH to form aniline.

12. Preparation of Anilines (contd.)

• Reduction of Aromatic Nitro Compounds with Hydrogen Sulphide
  • Aromatic nitro compounds can also be reduced to anilines by using hydrogen sulfide (H₂S).
  • The reaction can be represented as:

    ArNO₂ + 3H₂S + 2H₂O → ArNH₂ + 3H₂SO₄.

  • Example: Reaction of aromatic nitro compound with H₂S to form aniline.

13. Preparation of Anilines (contd.)

• Reduction of Aromatic Nitro Compounds with Zinc and Sodium Hydroxide
  • Aromatic nitro compounds can also be reduced to anilines by using zinc and sodium hydroxide (Zn/NaOH).
  • The reaction can be represented as:

    ArNO₂ + Zn + 2NaOH → ArNH₂ + Na₂ZnO₂ + H₂O.

  • Example: Reaction of aromatic nitro compound with Zn/NaOH to form aniline.

14. Preparation of Anilines (contd.)

• Reduction of Aromatic Nitro Compounds with Hydrogen and a Catalyst
  • Aromatic nitro compounds can be reduced to anilines by using hydrogen gas and a metal catalyst such as platinum, palladium, or nickel.
  • The reaction takes place at high pressure and temperature.
  • Example: Reaction of aromatic nitro compound with hydrogen (H₂) and platinum catalyst to form aniline.

15. Preparation of Anilines (contd.)

• Reduction of Aromatic Nitro Compounds with Sodium Hydroxide
  • Aromatic nitro compounds can also be reduced to anilines by using sodium hydroxide (NaOH).
  • The reaction can be represented as:

    ArNO₂ + 6NaOH + 4H₂O → ArNH₂ + 6NaNO₃ + 9H₂O.

  • Example: Reaction of aromatic nitro compound with NaOH to form aniline.

16. Preparation of Anilines (contd.)

• From Aryl Halides
  • Anilines can be prepared by reducing aryl halides (halobenzenes) with a strong reducing agent like zinc and ammonium chloride (Zn/NH₄Cl).
  • The reaction can be represented as:

    ArX + Zn + 2NH₄Cl → ArNH₂ + ZnCl₂ + 2NH₄X.

  • Example: Reaction of aryl halide with Zn/NH₄Cl to form aniline.

17. Preparation of Anilines (contd.)

• From Diazonium Salts and Cuprous Chloride
  • Anilines can be prepared by treating aryl diazonium salts with cuprous chloride (CuCl) in the presence of acids or acid salts.
  • The reaction can be represented as:

    ArN₂⁺X^- + CuCl → ArNH₂ + CuX + N₂.

  • Example: Reaction of aryldiazonium salt with CuCl to form aniline.

18. Preparation of Anilines (contd.)

• From Aryl Halides and Ammonia
  • Anilines can also be prepared by heating aryl halides (halobenzenes) with ammonia (NH₃) in the presence of a catalyst such as copper powder (Cu).
  • The reaction can be represented as:

    ArX + NH₃ + Cu → ArNH₂ + HX.

  • Example: Reaction of aryl halide with NH₃ and Cu to form aniline.

19. Preparation of Anilines (contd.)

• From Phenols
  • Anilines can be prepared by treating phenols with concentrated hydrochloric acid (HCl) or bromine (Br₂) in the presence of a reducing agent like tin (Sn).
  • The reaction can be represented as:

    ArOH + HCl + Sn → ArNH₂ + SnCl₂ + H₂O.

  • Example: Reaction of phenol with HCl and Sn to form aniline.

20. Preparation of Anilines (contd.)

• From Nitritic Acid and Sulfinic Acids
  • Anilines can be prepared by subjecting aromatic compounds to the action of nitritic acid (HNO₂) in the presence of sulfinic acids.
  • The reaction can be represented as:

    ArH + HNO₂ + H₂SO₃ → ArNH₂ + H₂SO₄.

  • Example: Reaction of aromatic compound with HNO₂ and H₂SO₃ to form aniline.

21. Preparation of Anilines (contd.)

• From Aryl Halides and Toluene
  • Anilines can be prepared by reacting aryl halides with toluene in the presence of a reducing agent like sodium hydroxide (NaOH).
  • The reaction can be represented as:

    ArX + C₆H₅CH₃ + NaOH → ArNH₂ + C₆H₅CH₂ONa + HX.

  • Example: Reaction of aryl halide with toluene and NaOH to form aniline.

22. Preparation of Anilines (contd.)

• From Acetanilide
  • Anilines can be prepared by reducing acetanilide with a strong reducing agent such as tin and hydrochloric acid (Sn/HCl).
  • The reaction can be represented as:

    CH₃C₆H₄NHCOR + Sn + 4HCl → CH₃C₆H₄NH₂ + SnCl₂ + CO₂ + H₂O + R.

  • Example: Reaction of acetanilide with Sn/HCl to form aniline.

23. Preparation of Anilines (contd.)

• From Cyanobenzene
  • Anilines can be prepared by reducing cyanobenzene with a strong reducing agent like hydrogen gas and a metal catalyst such as platinum or palladium (H₂/Pt or Pd).
  • The reaction can be represented as:

    C₆H₅CN + 2H₂ + Pt or Pd → C₆H₅NH₂ + H₂O.

  • Example: Reaction of cyanobenzene with H₂/Pt or Pd to form aniline.

24. Preparation of Anilines (contd.)

• From Benzene via Gabriel Synthesis
  • Anilines can be prepared by using the Gabriel synthesis method.
  • The reaction involves the use of potassium phthalimide (C₈H₅NO₂K) followed by hydrolysis and decarboxylation steps.
  • Example: Gabriel synthesis to form aniline.

25. Preparation of Anilines (contd.)

• From Diazonium Salts and Phenols
  • Anilines can be prepared by reacting aryldiazonium salts with phenols.
  • The reaction can be represented as:

    ArN₂⁺X^- + ArOH → ArNH₂ + X^- + ArO^-.

  • Example: Reaction of aryldiazonium salt with phenol to form aniline.

26. Preparation of Anilines (contd.)

• From Peroxides
  • Anilines can be prepared by reacting aromatic compounds with hydrogen peroxide (H₂O₂) in the presence of a catalyst like palladium on carbon.
  • The reaction can be represented as:

    ArH + H₂O₂ → ArNH₂ + H₂O.

  • Example: Reaction of aromatic compound with H₂O₂ and palladium on carbon to form aniline.

27. Preparation of Anilines (contd.)

• From Nitroso Compounds
  • Anilines can be prepared by reducing nitroso compounds with a reducing agent like sodium bisulfate (NaHSO₄) or sodium hydrosulfite (Na₂S₂O₄).
  • The reaction can be represented as:

    ArN=O + NaHSO₄ + 2H₂O → ArNH₂ + NaHSO₃ + H₂SO₄.

  • Example: Reaction of nitroso compound with NaHSO₄ to form aniline.

28. Preparation of Anilines (contd.)

• From Nitroso Compounds (contd.)
  • Another method involves the use of ferrous sulfate (FeSO₄) as a reducing agent.
  • Example: Reaction of nitroso compound with FeSO₄ to form aniline.

29. Preparation of Anilines (contd.)

• From Nitroso Compounds (contd.)
  • Anilines can also be prepared by reducing nitroso compounds using sodium sulfide (Na₂S).
  • Example: Reaction of nitroso compound with Na₂S to form aniline.

30. Summary

• Anilines can be derived from various starting materials including nitrobenzenes, aryl diazonium salts, phenols, acetanilide, and other aromatic compounds.
• Various reducing agents and catalysts can be employed to carry out the reduction reactions.
• The choice of the reducing agent and reaction conditions depends on the specific starting material and desired aniline derivative.
• Anilines are important compounds in organic synthesis and find extensive applications in the production of dyes, pharmaceuticals, and other chemical compounds.