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High Yield Topic

Amines & Diazonium Salts

Chemistry Unit 13
25 min read
IAT Advanced
Hot Topic

1. Core Concept

Amines are derivatives of ammonia (NH3) where one or more H atoms are replaced by alkyl or aryl groups. They are Lewis bases due to the lone pair on Nitrogen. Diazonium salts are highly reactive intermediates that serve as "synthetic bridges" to introduce various functional groups onto an aromatic ring.

2. Amines: Structure & Properties

Structure: N is sp3 hybridized. The lone pair creates a pyramidal geometry (bond angle ≈ 107°).

Basicity Order (Kb):

  • Gas Phase: 3° > 2° > 1° > NH3 (Due to +I effect of alkyl groups).
  • Aqueous Solution (CH3-group): 2° > 1° > 3° > NH3 (Balance of +I effect, solvation/H-bonding, and steric hindrance).
  • Aromatic: Aniline is much less basic than aliphatic amines because the lone pair is involved in resonance with the benzene ring.

3. Preparation of Amines

  • Reduction: R-NO2 → R-NH2 (using Fe/HCl or Sn/HCl).
  • Ammonolysis: RX + NH3 → Mixture of 1°, 2°, 3° amines and quaternary ammonium salt.
  • Gabriel Phthalimide Synthesis: Specifically for pure 1° aliphatic amines. (Cannot prepare aromatic amines).
  • Hofmann Bromamide Degradation: R-CONH2 + Br2 + 4NaOH → R-NH2 + Na2CO3 + 2NaBr + 2H2O. (Decreases carbon chain by 1).

4. Chemical Reactions

Carbylamine Test:

R-NH2 + CHCl3 + KOH → R-NC (Foul smell). Only for 1° amines.

Hinsberg Test (Distinguishing amines):

  • 1° amine + PhSO2Cl: Product is soluble in NaOH.
  • 2° amine + PhSO2Cl: Product is insoluble in NaOH.
  • 3° amine + PhSO2Cl: No reaction.

Nitrous Acid (HNO2):

  • 1° Aliphatic amine → Alcohol (N2 gas evolved).
  • 1° Aromatic amine → Diazonium salt (stable at 0-5°C).

5. Diazonium Salts (Ar-N2+X-)

Ar-NH2 + NaNO2 + 2HX → Ar-N2+X-
Diazotisation (0-5°C).

Synthetic Bridge Reactions:

  • Sandmeyer: CuCl/HCl or CuBr/HBr → Ar-Cl / Ar-Br.
  • Gattermann: Cu/HCl → Ar-Cl.
  • Iodide: KI → Ar-I.
  • Fluoride (Schiemann): HBF4 → Δ → Ar-F.
  • Deamination: H3PO2 + H2O → Benzene.

Coupling Reactions: Ar-N2+X- + Phenol/Aniline → Azo dyes (Yellow/Orange/Red). Reaction occurs at the para position.

6. Conceptual Insights

Resonance in Aniline

The lone pair on the nitrogen is delocalized into the benzene ring. This makes the nitrogen less available to donate its lone pair to protons, thus explaining why aniline is a weaker base than aliphatic amines.

Azo Dye Utility

The extended conjugation in azo compounds (Ar-N=N-Ar') makes them excellent chromophores, which is why they are widely used as dyes.

7. Common Mistakes

  • Gabriel Phthalimide: Forgetting it only makes 1° aliphatic amines. Aromatic amines cannot be made this way.
  • Hofmann Reaction: Forgetting the product has one carbon atom less than the starting amide.
  • Sandmeyer vs. Gattermann: Remember Sandmeyer uses Cu(I) salts; Gattermann uses Cu powder.

8. IAT Exam Focus Points

Key Exam Focus:

  • Hinsberg Test: High yield. Know how 1°, 2°, and 3° amines react with Benzenesulphonyl chloride.
  • Basic Strength: Rank p-nitroaniline (weakest) < aniline < p-toluidine (strongest due to +I of CH3).
  • Azo Coupling: Recognize the formation of colored dyes in basic or slightly acidic conditions.
  • Carbylamine: Specific test to identify 1° amines.

9. Practice Mock Test

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End of Chapter

Nitrogen-Containing Organic Compounds

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