Quick Summary
Organic nitrogen compounds include amines, nitro compounds, and diazonium salts. This chapter carries 8–10 marks in TN HSE. Reactions and properties of amines, methods of preparation, and the chemistry of diazonium salts are exam favourites.
Key Concepts
- Amines: Derivatives of NH₃ where H atoms are replaced by alkyl/aryl groups
- Diazonium salts: ArN₂⁺X⁻ — highly reactive, key in organic synthesis
- Nitro compounds: R–NO₂; reduced to amines
Classification of Amines
| Type | Structure | Example |
|---|---|---|
| Primary (1°) | RNH₂ | CH₃NH₂ (methylamine) |
| Secondary (2°) | R₂NH | (CH₃)₂NH (dimethylamine) |
| Tertiary (3°) | R₃N | (CH₃)₃N (trimethylamine) |
| Quaternary ammonium salt | R₄N⁺X⁻ | (CH₃)₄N⁺Cl⁻ |
Preparation of Amines
1. Reduction of nitro compounds: RNO₂ + 6[H] → RNH₂ + 2H₂O (Fe/HCl or H₂/Ni) 2. Ammonolysis of alkyl halides: RX + NH₃ → RNH₂ → R₂NH → R₃N (mixture) 3. Reduction of nitriles: RCN + 2H₂ → RCH₂NH₂ (primary amine) 4. Reduction of amides: RCONH₂ + LiAlH₄ → RCH₂NH₂ 5. Gabriel synthesis: Phthalimide + RX → N-alkylphthalimide → primary amine (no 2° or 3° formed) 6. Hofmann bromamide reaction: RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O (product has one less C)
Chemical Reactions of Amines
Basicity
- Aliphatic: R₃N > R₂NH > RNH₂ > NH₃ > C₆H₅NH₂
- In water: 2° > 1° > 3° > NH₃ (due to solvation effects)
Kb expression: RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻
Acylation and Sulphonylation
- RNH₂ + CH₃COCl → RNHCOCH₃ + HCl (acetylation)
- RNH₂ + C₆H₅SO₂Cl → RNHSO₂C₆H₅ + HCl (Hinsberg's reaction)
Carbylamine Reaction (Test for 1° amines)
- RNH₂ + CHCl₃ + 3KOH → RNC (isocyanide, foul smell) + 3KCl + 3H₂O
Reaction with HNO₂
- 1° aliphatic: RNH₂ + HNO₂ → ROH + N₂ + H₂O
- 1° aromatic: ArNH₂ + NaNO₂ + HCl (0–5°C) → ArN₂⁺Cl⁻ (diazonium salt)
- 2° amine: forms N-nitroso compound (yellow oil)
- 3° amine: forms salt (no N–H to replace)
Diazonium Salts
Preparation (Diazotisation): ArNH₂ + NaNO₂ + HCl → ArN₂⁺Cl⁻ + NaCl + H₂O (at 0–5°C)
Reactions (Sandmeyer's reactions):
| Reagent | Product |
|---|---|
| CuCl / HCl | ArCl (chlorobenzene) |
| CuBr / HBr | ArBr |
| CuCN / KCN | ArCN |
| H₃PO₂ / H₂O | ArH (deamination) |
| NaOH, warm | ArOH |
| HBF₄ | ArF (Balz-Schiemann reaction) |
| Coupling with phenol | Azo dye (ArN=NAr') |
Nitro Compounds
- Preparation: ArH + HNO₃/H₂SO₄ → ArNO₂ (nitration)
- Reduction: ArNO₂ → ArNH₂ (acidic: Fe/HCl; neutral: NH₄Cl/Zn; alkaline: Na/alcohol → azobenzene pathway)
- Nitro group is electron-withdrawing — deactivates ring for electrophilic substitution
Important Questions
One Mark
1. Name the test used to distinguish primary, secondary, and tertiary amines. 2. What is Gabriel's synthesis used for? 3. Name the product when aniline reacts with Br₂ water. 4. State Hofmann bromamide reaction. 5. What is the Sandmeyer reaction?
Short Answer (3 marks)
1. Explain the basicity order of aliphatic amines in water. 2. How are diazonium salts prepared? Give two reactions of diazonium salts. 3. Explain the Gabriel synthesis for preparing primary amines.
Long Answer (5 marks)
1. Describe the preparation and chemical reactions of diazonium salts, including Sandmeyer's reactions and azo coupling. 2. Discuss the methods of preparation and distinguishing tests for primary, secondary, and tertiary amines.
Board Exam Tips
- Carbylamine test is specific for primary amines — write the foul smell detail
- Coupling reaction gives azo dyes — used in industry (mention this)
- Gabriel synthesis gives only primary amines — no contamination with 2° or 3°
- Hofmann bromamide: product has one carbon less than starting amide — mark this
Revision Checklist
- [ ] Classification and nomenclature of amines
- [ ] All 6 preparation methods
- [ ] Basicity order with explanation
- [ ] Hinsberg's test procedure and results
- [ ] Carbylamine test
- [ ] Diazotisation conditions
- [ ] All Sandmeyer reactions
- [ ] Azo coupling reaction and azo dyes
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