Chapter: Amines

• Introduction & Structure

  • Definition: Derivatives of ammonia ($NH_3$)
  • General Formula: $R-NH_2$, $R_2NH$, $R_3N$
  • Classification: Primary ($1^\circ$), Secondary ($2^\circ$), Tertiary ($3^\circ$) amines
  • Hybridization: $sp^3$ with pyramidal shape

• Nomenclature

  • Common System: Alkylamine (Aliphatic) / Aniline (Aromatic)
  • IUPAC System: Alkanamine (Prefix amino-)
  • Examples:
    • $CH_3NH_2$: Methanamine
    • $C_6H_5NH_2$: Aniline / Benzenamine

• Preparation of Amines

  • Reduction of Nitro compounds ($NO_2 \rightarrow NH_2$ using $H_2/Ni$ or $Fe/HCl$)
  • Ammonolysis of alkyl halides (Hofmann's method)
  • Reduction of nitriles ($R-CN$) and amides ($R-CONH_2$)
  • Gabriel Phthalimide Synthesis (Exclusive for $1^\circ$ aliphatic amines)
  • Hofmann Bromamide Degradation (Step-down reaction, removes one carbonyl carbon)

• Physical Properties

  • State: Lower amines are gases; higher are liquids/solids
  • Solubility: Lower aliphatic amines are soluble in water due to hydrogen bonding
  • Boiling Points: $1^\circ > 2^\circ > 3^\circ$ (due to extent of intermolecular H-bonding)

• Chemical Properties

  • Basic Character:
    • Due to lone pair on nitrogen
    • Gas phase basicity: $3^\circ > 2^\circ > 1^\circ > NH_3$ (due to +I effect)
    • Aqueous phase basicity: Affected by +I effect, steric hindrance, and solvation effect
    • Comparison: Aliphatic amines are stronger bases than ammonia and aniline
  • Alkylation & Acylation: Formation of secondary/tertiary amines and amides
  • Carbylamine Test: Identification test for $1^\circ$ amines (forms foul-smelling isocyanides)
  • Reaction with Nitrous Acid ($HNO_2$):
    • Aliphatic: Forms unstable diazonium salts $\rightarrow$ alcohols
    • Aromatic: Forms stable benzenediazonium chloride at 0-5°C
  • Electrophilic Substitution in Aniline:
    • Resonance effect directs to ortho/para positions
    • Bromination: Gives 2,4,6-tribromoaniline (requires protection of amino group via acetylation)
    • Nitration: Yields meta-isomer due to acidic medium protonation (anilinium ion)
    • Sulphonation: Forms sulfanilic acid (zwitter ion)

• Diazonium Salts

  • General Formula: $Ar-N_2^+ X^-$
  • Preparation: Diazotization of aniline using $NaNO_2 + HCl$ at $0-5^\circ C$
  • Chemical Reactions:
    • Replacement reactions (Sandmeyer/Gattermann reactions using $Cu_2Cl_2/HCl$, $Cu/HCl$)
    • Replacement by iodide, fluoride, cyanide, and hydrogen
    • Coupling reactions: Formation of azo dyes (e.g., orange/yellow dyes with phenol/aniline)
  • Synthetic Importance: Introduction of groups into aromatic rings that cannot be done directly