Amines

Definition of Amines

Amines are organic compounds derived from ammonia (NH₃) by substituting one or more hydrogen atoms with alkyl or aryl groups.

They are classified as primary, secondary, or tertiary based on the number of hydrogen atoms replaced.

Acidity

Amines are very weak bases.

Formation of Primary Aliphatic Amines

  1. From Halogenoalkanes:

    • Reaction: Halogenoalkane + Excess Ammonia → Primary Amine + Ammonium Halide

    • Mechanism: Nucleophilic substitution

    • Conditions: Heat in a sealed tube with ethanol as solvent

    • Example: CH₃CH₂Br + NH₃ → CH₃CH₂NH₂ + HBr

  2. From Nitriles:

    • Reaction: Nitrile + Hydrogen → Primary Amine

    • Mechanism: Reduction

    • Conditions: LiAlH₄ in diethyl solution + dilute acid

    • Example: CH₃CN + 4[H] → CH₃CH₂NH₂

Formation of Aromatic Amines from Nitrobenzenes

  • Reaction: Nitrobenzene + 6[H] → Phenylamine + Water

  • Mechanism: Reduction

  • Conditions: Heat under reflux with tin + concentrated hydrochloric acid, neutralize with sodium hydroxide

  • Example: C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O

Basicity of Amines

  • Aliphatic Amines: Stronger bases than ammonia due to electron-donating alkyl groups, increasing nitrogen's electron density.

    • Example: CH₃NH₂ > NH₃ in basicity.

  • Aromatic Amines: Weaker bases than ammonia as the nitrogen lone pair delocalizes into the benzene ring, reducing proton acceptance.

    • Example: C₆H₅NH₂ < NH₃ in basicity.

  • Order of Basicity: Aliphatic amines > Ammonia > Aromatic amines.

Ethanoylation of Primary Amines Using Ethanoyl Chloride

  • Reaction: Primary Amine + Ethanoyl Chloride → N-substituted Amide + HCl

  • Mechanism: Nucleophilic acyl substitution

  • Example: CH₃NH₂ + CH₃COCl → CH₃CONHCH₃ + HCl

Reaction of Primary Amines with Cold Nitric(III) Acid

  1. For Primary Aliphatic Amine:

    • Reaction: Primary Amine + HNO₂ → Alcohol + Nitrogen Gas + Water

    • Example: CH₃CH₂NH₂ + HNO₂ → CH₃CH₂OH + N₂ + H₂O

  2. For Primary Aromatic Amine:

    • Reaction: Primary Aromatic Amine + HNO₂ → Benzene-Diazonium Salt + Water (at 0-5°C)

    • Example: C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O

Coupling of Benzene-Diazonium Salts

  1. With Phenols:

    • Reaction: Benzene-Diazonium Salt + Phenol → Azo Compound + HCl

    • Conditions: Alkaline medium (NaOH), 0-5°C

    • Example: C₆H₅N₂⁺Cl⁻ + C₆H₅OH → C₆H₅-N=N-C₆H₄OH + HCl

  2. With Aromatic Amines:

    • Reaction: Benzene-Diazonium Salt + Aromatic Amine → Azo Compound + HCl

    • Conditions: Alkaline medium, 0-5°C

    • Example: C₆H₅N₂⁺Cl⁻ + C₆H₅NH₂ → C₆H₅-N=N-C₆H₄NH₂ + HCl

Summary of Key Reactions

Formation of aliphatic amines via nucleophilic substitution/reduction; aromatic amines from nitrobenzenes.

Basicity depends on nitrogen lone pair availability. Ethanoylation forms N-substituted amides.

Primary amines react with HNO₂ to yield varied products based on aliphatic or aromatic structure. Benzene-diazonium salts couple with phenols or aromatic amines to create azo compounds.