Amines

Nitro Compounds – Introduction

Nitro compounds contain the –NO₂ functional group.

Preparation of Nitro Compounds

• Direct nitration of benzene: Benzene + HNO₃/H₂SO₄ → Nitrobenzene
• Vapour phase nitration of alkanes: 475–675 K, free radical mechanism
• From halo compounds:
  • R–X + AgNO₂ → R–NO₂ (major)
  • R–X + KNO₂ → R–ONO (major)
• From amines: Oxidation using KMnO₄

Physical Properties

• Nitroalkanes: colourless, pleasant-smelling liquids
• Nitrobenzene: yellow oily liquid, heavier than water
• High boiling points compared to alkyl nitrites

Chemical Properties

• Reduction: Nitro compounds give different products based on reducing agent
• Acidic α-hydrogen: Nitroalkanes undergo condensation reactions
• With chlorine: Nitromethane forms chloropicrin (insecticide)
• Nitrobenzene: Meta-directing, deactivating in EAS

Cyanides & Isocyanides

Alkyl cyanides (R–CN) and alkyl isocyanides (R–NC) are functional isomers.

Preparation of Cyanides

• R–X + alc. KCN → R–CN (major)
• From amides: dehydration using P₄O₁₀
• Using Grignard reagent + cyanogen chloride
• From oximes (dehydration)

Preparation of Isocyanides

• R–X + AgCN → R–NC (major)
• Carbylamine reaction: Only 1° amines

Chemical Properties

• Hydrolysis: R–CN → amide → acid
• Reduction:
  • Complete → 1° amines
  • Partial (Stephen reaction) → aldehydes
• R–NC on heating converts to R–CN

Amines – Introduction

Amines are derivatives of ammonia where one or more H atoms are replaced by alkyl/aryl groups.

• 1° amine: R–NH₂
• 2° amine: R₂NH
• 3° amine: R₃N

Structure

• Aliphatic amines: pyramidal shape
• Chiral nitrogen possible, but optical activity not observed due to rapid inversion
• Quaternary ammonium salts may be optically active

Preparation of Amines

Reduction Methods

• Nitro compounds → amines (Sn/HCl, Fe/HCl)
• Amides → amines (LiAlH₄)
• Nitriles → 1° amines
• Oximes → amines

Other Methods

• Hofmann bromamide reaction: One carbon less
• Curtius rearrangement
• Schmidt reaction
• Gabriel phthalimide synthesis: Only 1° amines
• Reductive amination of aldehydes/ketones

Physical Properties of Amines

• Boiling point: 3° < 2° < 1°
• Lower amines are water soluble (H-bonding)

Basicity of Amines

• Due to lone pair on nitrogen
• Aliphatic amines are stronger bases than NH₃
• Observed order (lower aliphatic amines): 2° > 1° > 3°
• Aniline is weaker base due to resonance
• EDG increases basicity; EWG decreases basicity

Chemical Reactions of Amines

With Acids

• RNH₂ + HCl → RNH₃⁺Cl⁻

With Nitrous Acid (NaNO₂ + HCl)

• 1° aliphatic amines → alcohol + N₂
• 1° aromatic amines → diazonium salts (0–5°C)
• 2° amines → nitroso compounds
• 3° aliphatic amines → salts

Carbylamine Test

• Only 1° amines give isocyanide (foul smell)

Hofmann Exhaustive Alkylation

• Gives least substituted alkene (Hofmann elimination)

Aniline

• Strongly activating, ortho–para directing
• Undergoes bromination to give 2,4,6-tribromoaniline
• Nitration gives meta product due to anilinium ion formation

Diazonium Salts

Preparation

• Aniline + NaNO₂ + HCl (0–5°C) → Benzene diazonium chloride

Reactions

• Sandmeyer reaction (Cl, Br, CN)
• Gattermann reaction
• Balz–Schiemann reaction (F)
• Coupling reactions (azo dyes)

Tests for Amines

Hinsberg’s Test

• 1° amine: soluble product in alkali
• 2° amine: insoluble product
• 3° amine: no reaction