16.2 Aromatic Amines

BySubarnam
16.2 Aromatic Amines (Aniline)

🎯 Learning Outcomes — By the end of the chapter, students should be able to:

  • Show preparation of aniline from nitrobenzene and phenol.
  • State the physical properties of aromatic amines.
  • Explain the chemical properties of aromatic amines, i.e.
    • basicity of aniline, comparison of the basic nature of aniline with aliphatic amines and ammonia,
    • alkylation,
    • acylation,
    • diazotisation,
    • carbylamines,
    • coupling reaction, and
    • electrophilic substitution (nitration, sulphonation, and bromination).
  • State important uses of aniline.
  1. From nitrobenzene (studied in Nitrobenzene)
  2. From Phenol (studied in properties of Phenols)
  1. Colourless liquid with an unpleasant odour and very sensitive to oxidation, so it rapidly becomes brown when exposed to air.
  2. B.pt 184°C and M.pt 6.2°C.
  3. Sparingly soluble in water but readily soluble in organic solvents.
  1. Alkylation
    NH₂ NHCH₃ N(CH₃)₂ N(CH₃)₃⁺ I⁻ CH₃–I −HI CH₃–I −HI CH₃–I Aniline (1° amine) N-methylaniline (2° amine) N,N-Dimethylaniline (3° amine) Trimethylanilinium iodide (Quaternary ammonium salt)
  2. Acylation
    NH₂ NH–CO–CH₃ + CH₃COCl Pyridine −HCl Aniline N-phenylethanamide (Acetanilide)
  3. Carbylamine reaction (test of 1° amine and chloroform)
    NH₂ N≡C + CHCl₃ + 3KOH (alc.) Δ + 3KCl + 3H₂O Aniline Phenyl carbylamine (Phenyl isocyanide)
  4. Diazotisation
    NaNO₂ + HCl HNO₂ + NaCl NH₂ N₂Cl + HNO₂ + HCl 0–10°C + 2H₂O Aniline Benzene diazonium chloride
  5. Coupling reaction
    N=N–Cl NH₂ + pH = 4–5 40°C NH₂ N=N + HCl Diazonium salt Aniline p-Aminoazobenzene (orange-yellow solid) M.Pt = 126°C

Aromatic Electrophilic Substitution

  1. Nitration

    Aniline cannot be nitrated directly with conc. HNO₃ and conc. H₂SO₄ because the –NH₂ group in such cases gets oxidised into a –NO₂ group.

    NH₂ NH–CO–CH₃ Aniline Acetanilide Protection (of –NH₂) (CH₃CO)₂O, Acylation NH–CO–CH₃ NO₂ NH–CO–CH₃ NO₂ HNO₃, H₂SO₄, 15°C + p-Nitroacetanilide (90%) o-Nitroacetanilide (1–2%) Can be separated by chromatography NH₂ NO₂ p-Nitroaniline H₂O/OH⁻ Deprotection (of –NH₂)
  2. Sulphonation
    NH₂ ⁺NH₃HSO₄⁻ NH₂ SO₃H ⁺NH₃ SO₃⁻ + H₂SO₄ 5hr, 180–200°C ‘baking’ ≡ + H₂O Aniline Anilinium hydrogen sulfate p-Aminobenzene sulfonic acid (Sulfanilic acid) — Zwitter ion
  3. Bromination
    NH₂ NH₂ Br Br Br + 3 Br₂(aq) + 3 HBr Aniline 2,4,6-Tribromoaniline (White ppt)

(Study in detail from the book)

All amines are basic due to the lone pair of electrons on the nitrogen atom. They are stronger bases than water and are therefore protonated by water in their aqueous solutions.

RNH₂ + H₂O RNH₃⁺ + OH⁻ Aminoalkane Alkyl ammonium ion

The basicity constant can be expressed as:

Kb = Keq × [H₂O] = [RNH₃⁺][OH⁻] / [RNH₂]

The following order of basicity is observed from the basicity constant of the different amines:

N-methylaminomethane > Aminomethane > N,N-dimethylaminomethane > Ammonia
Aminomethane > Aminoethane > Aminopropane
  1. Aliphatic amines are more basic than ammonia.
  2. Aromatic amines are less basic than ammonia.
  3. Basic strength of 1°, 2°, and 3° amines
    1. Hydration effect
    2. Inductive effect
    3. Steric effect
  4. Effect of the substituent on the basicity of aniline.
1. Which of the following has the most basic character? Explain.
a) NH₃   b) (CH₃)₂NH   c) CH₃NH₂   d) C₆H₅NH₂

Ans: (CH₃)₂NH > CH₃NH₂ > NH₃ > C₆H₅NH₂ (Study in detail from the book)

1 mark2. How is Nitrobenzene converted to Phenol?

NO₂ NH₂ N₂Cl + [H] Zn/HCl −H₂O HNO₂/HCl ice cold Nitrobenzene Aniline Benzene diazonium chloride OH H₂O/boil −N₂, −HCl Phenol

3 marks3. How is nitrobenzene converted to p-hydroxyazobenzene?

Hint — identify intermediates A and B:

Nitrobenzene Sn/HCl, Δ→ A NaNO₂/HCl, 0°C→ B Phenol→ p-hydroxyazobenzene

4 marks4. Starting with nitrobenzene, how would you prepare TNT?

NO₂ NH₂ +[H] Sn/HCl, −H₂O Nitrobenzene Aniline NH₂ OH +HNO₂ NaNO₂/HCl, warm (−N₂,−HCl) Aniline Phenol OH Zn-dust, Δ Phenol Benzene CH₃ +CH₃Cl, AlCl₃ −HCl Benzene Toluene CH₃ CH₃ NO₂ NO₂ NO₂ Conc. HNO₃ conc. H₂SO₄ Toluene 2,4,6-trinitrotoluene (TNT)

2 marks5. Starting from Nitrobenzene, how would you obtain Picric acid?

NO₂ NH₂ OH OH NO₂ NO₂ NO₂ Sn/HCl HNO₂ NaNO₂/HCl, warm Conc. HNO₃ conc. H₂SO₄ Nitrobenzene Aniline Phenol 2,4,6-trinitrophenol (Picric acid)

2.5 marks6. Convert Benzene to m-bromophenol.

NO₂ HNO₃/H₂SO₄ −H₂O Benzene Nitrobenzene NO₂ NO₂ Br Br₂/FeBr₃ Nitrobenzene 3-Bromonitrobenzene NO₂ Br NH₂ Br Sn/HCl 3-Bromonitrobenzene 3-Bromoaniline NH₂ Br OH Br HNO₂ warm 3-Bromoaniline m-Bromophenol

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