Class 12 Chemistry Amines Notes

Class 12 Chemistry Amines Notes

9.1 Structure of Amines

  • Amines are derivatives of ammonia (NH₃) in which one or more hydrogen atoms are replaced by alkyl or aryl groups.
  • General formula: R–NH₂, R₂–NH, R₃–N

9.2 Classification

  1. Primary amines (1°): R–NH₂
  2. Secondary amines (2°): R₂–NH
  3. Tertiary amines (3°): R₃–N
  4. Aromatic amines: Amines attached to an aromatic ring (e.g., aniline)

9.3 Nomenclature

IUPAC Rules

  1. Identify the longest chain containing –NH₂
  2. Replace –e in alkane with –amine
  3. Number the chain so –NH₂ gets lowest number
  4. For aromatic amines, use aniline as parent

Examples:

  • CH₃CH₂NH₂ → Ethylamine
  • C₆H₅NH₂ → Aniline

9.4 Preparation of Amines

Aliphatic Amines

  1. From halides: RX + NH₃ → R–NH₂
  2. Reduction of nitro compounds: R–NO₂ + H₂ → R–NH₂
  3. Reductive amination of aldehydes/ketones

Aromatic Amines

  1. Reduction of nitroarenes: C₆H₅NO₂ + Sn/HCl → C₆H₅NH₂
  2. Gabriel phthalimide synthesis

9.5 Physical Properties

  • Primary and secondary amines are polar → hydrogen bonding → higher boiling points
  • Soluble in water (smaller molecules)
  • Aromatic amines less soluble due to resonance stabilization

9.6 Chemical Reactions

  1. Reaction with acids: R–NH₂ + HCl → R–NH₃⁺Cl⁻
  2. Alkylation: R–NH₂ + RX → R₂–NH + HX
  3. Acylation: R–NH₂ + R’COCl → R–NH–COR’
  4. Diazotization (for aromatic amines):
    C₆H₅NH₂ + NaNO₂ + HCl → C₆H₅N₂⁺Cl⁻

9.7 Preparation of Diazonium Salts

  • Aromatic amines react with nitrous acid at 0–5°C:

C6H5NH2+HNO2+HCl→C6H5N2+Cl−+2H2OC₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂OC6​H5​NH2​+HNO2​+HCl→C6​H5​N2+​Cl−+2H2​O

  • Important for azo coupling reactions

9.8 Physical Properties of Diazonium Salts

  • Soluble in water
  • Generally unstable at room temperature
  • Explosive in dry form

9.9 Chemical Reactions of Diazonium Salts

  1. Replacement reactions:
    • Sandmeyer reaction: C₆H₅N₂⁺Cl⁻ + CuCl → C₆H₅Cl
    • Gattermann reaction: C₆H₅N₂⁺Cl⁻ + CuCN → C₆H₅CN
  2. Coupling reactions: Formation of azo dyes

9.10 Importance of Diazonium Salts in Synthesis of Aromatic Compounds

  • Used to introduce halogens, CN, OH into benzene ring
  • Formation of azo dyes: C₆H₅N₂⁺Cl⁻ + ArOH → Ar–N=N–Ar

📌 Important Exam Points

  • Amines are basic due to lone pair on nitrogen
  • Diazonium salts are key intermediates for aromatic substitution
  • Aromatic amines less basic than aliphatic amines
  • Sandmeyer reaction is important for halogenation