Organic Compounds Containing Nitrogen

Amines are derivatives of NH₃ with one, two, or three alkyl/aryl groups replacing H.

• Primary (1°): R-NH₂ (e.g., methylamine CH₃NH₂)
• Secondary (2°): R₂NH (e.g., dimethylamine (CH₃)₂NH)
• Tertiary (3°): R₃N (e.g., trimethylamine (CH₃)₃N)

Note: 1°, 2°, 3° here refers to substitution on nitrogen, not on adjacent carbon (different from alcohols).

Basicity comes from N's lone pair: amine + H⁺ → ammonium ion.

In gas phase / non-aqueous solvents (no solvation effects):
3° > 2° > 1° > NH₃ — more alkyl groups → more inductive electron donation → more available lone pair → more basic.

In aqueous solution (solvation of the conjugate acid matters):
2° > 1° > 3° > NH₃ (for aliphatic amines).

Why the order changes: smaller cations (R-NH₃⁺ with fewer alkyl groups) are better solvated by water — more H-bonds. The trade-off between inductive donation (favoring tertiary) and solvation (favoring fewer substituents) flips the order.

For aromatic amines (aniline, C₆H₅NH₂):
Aniline is much less basic than ammonia. Reason: nitrogen lone pair is delocalized into the benzene ring → less available for protonation.

General trend: alkyl amines > NH₃ > aryl amines.

Substituent effects on aniline basicity:

• Electron-donating groups (-NH₂, -OCH₃) increase basicity.
• Electron-withdrawing groups (-NO₂, -CN) decrease basicity.
• p-nitroaniline pKa ~1 (very weak base); aniline ~9; p-methoxyaniline ~10.

Hinsberg test (distinguish 1°, 2°, 3° amines):

Treat amine with benzenesulfonyl chloride (PhSO₂Cl) in alkaline (NaOH) solution.

1° amine: forms a sulfonamide R-NH-SO₂-Ph. The H on N is acidic (due to electron-withdrawing SO₂) → soluble in NaOH (forms sodium salt). On acidification → ppt re-forms.

2° amine: forms R₂N-SO₂-Ph. No acidic H on N → insoluble in NaOH, but the solid is formed (different from 3°).

3° amine: no N-H to substitute → no reaction. Stays as the free amine.

Summary:

• Soluble in NaOH → 1°.
• Insoluble in NaOH, but solid formed → 2°.
• No reaction → 3°.

Key preparations:

• Reduction of nitro compounds: Ar-NO₂ + H₂/Ni (or Sn/HCl) → Ar-NH₂. Classic synthesis of aniline from nitrobenzene.
• Hoffmann bromamide: R-CONH₂ + Br₂/NaOH → R-NH₂ + CO₂ (loses one C). Useful for ladder-down syntheses.
• Gabriel synthesis: phthalimide route to pure 1° amines (no 2°/3° contamination).

Key reactions:

• Diazotization (1° aromatic amines only, 0-5°C): Ar-NH₂ + HNO₂ + HCl → Ar-N₂⁺Cl⁻ (diazonium salt).
• Diazonium salts → countless products: phenols (with water), aryl halides (with CuX), aryl cyanides (with CuCN), azo dyes (with phenols).

Sandmeyer reaction: Ar-N₂⁺Cl⁻ + CuCl/CuBr → Ar-Cl/Ar-Br.