Organic Chemistry: Nitrogen Compounds
1. Introduction
Nitrogen-containing organic compounds include amines, diazonium salts, cyanides, and isocyanides. They are fundamental in biochemistry, pharmaceuticals, and dyes.
2. Amines
2.1 Classification
Primary (1°): R-NH₂. Secondary (2°): R₂NH. Tertiary (3°): R₃N. Quaternary: R₄N⁺X⁻.
2.2 Preparation
- Reduction of nitro compounds (R-NO₂ → R-NH₂ with Sn/HCl or H₂/catalyst).
- Hoffmann degradation of amides (R-CONH₂ + Br₂ + KOH → R-NH₂).
- Gabriel phthalimide synthesis (primary amines only — no 2° or 3° side products).
- Reduction of nitriles (R-CN → R-CH₂NH₂ with LiAlH₄ or H₂/Ni).
- Reduction of amides (R-CONH₂ → R-CH₂NH₂ with LiAlH₄).
2.3 Basicity
Amines are basic due to the lone pair on nitrogen.
Basicity order in gas phase: 3° > 2° > 1° > NH₃. Basicity order in aqueous solution: 2° > 1° > 3° > NH₃ (steric hindrance to solvation).
Electron-donating groups increase basicity; electron-withdrawing groups decrease it.
2.4 Reactions
- Acylation: R-NH₂ + R'COCl → R'CONHR + HCl. Primary and secondary amines react; tertiary amines do not.
- Carbylamine reaction: R-NH₂ + CHCl₃ + 3KOH → R-NC + 3KCl + 3H₂O (test for 1° amines).
- Hinsberg test: C₆H₅SO₂Cl with 1° (forms soluble salt), 2° (insoluble), 3° (no reaction).
- Diazotisation: Ar-NH₂ + NaNO₂ + 2HCl → Ar-N₂⁺Cl⁻ (at 0-5°C).
3. Diazonium Salts
3.1 Preparation
Diazotisation requires:
- Temperature 0-5°C (below 5°C is critical).
- Excess acid to prevent coupling.
- Freshly prepared NaNO₂.
3.2 Reactions (Synthetic Utility)
Replacement reactions (N₂ leaves):
- ArN₂⁺Cl⁻ + H₂O → ArOH (phenol).
- ArN₂⁺Cl⁻ + CuCl → ArCl (Sandmeyer reaction).
- ArN₂⁺Cl⁻ + KI → ArI.
- ArN₂⁺Cl⁻ + H₃PO₂ → ArH (replacement by H).
- ArN₂⁺BF₄⁻ → ArF (Balz-Schiemann reaction).
Coupling reactions (N₂ retained):
- With phenol (alkaline): Azo dye (orange dye).
- With aniline (acidic): Azo dye.
4. Basicity of Amines — Detailed Explanation
4.1 Factors Affecting Basicity
- Inductive effect: Alkyl groups are electron-donating (+I effect), which increases electron density on nitrogen, making it more basic.
- Solvation effect: In aqueous solution, protonated amines (R-NH₃⁺) are stabilised by hydrogen bonding with water. More H atoms on N means better solvation.
- Steric hindrance: In tertiary amines, three alkyl groups hinder the approach of a proton and reduce solvation.
4.2 Order of Basicity in Aqueous Solution
Secondary > Primary > Tertiary > NH₃ (for aliphatic amines)
This is because secondary amines balance the inductive effect (two alkyl groups) with good solvation (two H atoms).
4.3 Aromatic Amines (Aniline)
Aniline is much weaker than aliphatic amines because the lone pair on N is delocalised into the benzene ring (resonance). The delocalisation reduces electron density on N, making it less available for protonation.
Basicity: Aliphatic amine > NH₃ > Aromatic amine
5. Cyanides and Isocyanides
Cyanides (R-CN): Prepared from R-X + KCN. Used in chain elongation. Isocyanides (R-NC): Prepared from R-X + AgCN. Foul smell.
5. Worked Problems
Problem 1: Distinguish between aniline and N-methylaniline using Hinsberg test. Solution: Aniline (1°) + Hinsberg reagent → soluble in KOH. N-methylaniline (2°) → insoluble precipitate.
Problem 2: Write the product of reaction of aniline with Br₂ water. Solution: 2,4,6-Tribromoaniline (white precipitate). The -NH₂ group strongly activates the ring.
6. Common Mistakes
'Students often forget that diazonium salts are unstable above 5°C and decompose to give phenol. All diazotisation reactions must be carried out at 0-5°C.'
7. ISC Exam Focus
| Topic | Theory Marks | Practical Marks |
|---|---|---|
| Amine preparation | 3 | 2 |
| Basicity of amines | 3 | 1 |
| Diazonium salts | 4 | 2 |
| Distinction tests | 2 | 2 |
8. Self-Test Questions
- Describe the preparation of aniline from nitrobenzene.
- Arrange the following in increasing order of basicity: C₆H₅NH₂, NH₃, CH₃NH₂, (CH₃)₂NH, (CH₃)₃N. Explain.
- What is the carbylamine reaction? Which class of amines gives this test?
- Explain Sandmeyer's reaction with an example.
- Complete: C₆H₅N₂⁺Cl⁻ + C₆H₅OH (alkaline medium) → ?
9. Quick Reference — Key Reactions of Amines
| Reaction | Reagent | Product | Test For |
|---|---|---|---|
| Carbylamine | CHCl₃ + KOH | Isocyanide (foul smell) | 1° amines only |
| Hinsberg | C₆H₅SO₂Cl | Sulphonamide | 1°, 2°, 3° distinction |
| Diazotisation | NaNO₂ + HCl (0-5°C) | Diazonium salt | Aromatic 1° amines |
| Acylation | RCOCl / (RCO)₂O | Amide | 1° and 2° amines |
| Hoffmann elimination | CH₃I then AgOH | Alkene | Quaternary ammonium salts |
10. Synthetic Importance of Diazonium Salts
Diazonium salts are versatile intermediates for introducing various functional groups into the benzene ring:
C₆H₅N₂⁺ → C₆H₅OH (with H₂O, hydrolysis) C₆H₅N₂⁺ → C₆H₅Cl (with CuCl, Sandmeyer) C₆H₅N₂⁺ → C₆H₅Br (with CuBr, Sandmeyer) C₆H₅N₂⁺ → C₆H₅I (with KI) C₆H₅N₂⁺ → C₆H₅F (with HBF₄, Balz-Schiemann) C₆H₅N₂⁺ → C₆H₅CN (with CuCN) C₆H₅N₂⁺ → C₆H₅H (with H₃PO₂, reduction) C₆H₅N₂⁺ → C₆H₅NO₂ (with NaNO₂ + Cu)
This ability to replace the diazonium group makes these compounds extremely valuable in organic synthesis.
