Lecture on Amines - AQA Chemistry

Introduction to Amines

• Amines are organic compounds derived from ammonia.
• Basic reaction example:
  • CH₃NH₂ + H₂O → CH₃NH₃⁺ + OH⁻
  • NH₃ (aq) + H₂O (l) → NH₄⁺ (aq) + OH⁻ (aq)

Base Properties

• Primary Aliphatic Amines
  • Act as Bronsted-Lowry bases.
  • Lone pair of electrons on nitrogen easily forms dative covalent bonds with H⁺.
  • Weaker bases due to low concentration of hydroxide ions.
  • Stronger than ammonia due to electron-releasing alkyl groups.
• Aromatic Amines
  • Example: Phenylamine
  • Lone pair of electrons delocalizes with benzene ring, reducing basicity.

Reactions as Bases

• Amines react with acids to form ammonium salts.
  • Example: CH₃NH₂ (aq) + HCl (aq) → CH₃NH₃⁺Cl⁻ (aq)
  • Addition of NaOH to ammonium salt regenerates the amine.
• Solubility
  • Ionic salts from reactions are generally soluble.
  • Phenylamine is not very soluble in water but its salts are.

Comparative Base Strength

• Order of Base Strength: Aromatic amines < Ammonia < Primary amines < Tertiary amines < Secondary amines
• Secondary Amines
  • Stronger than primary due to increased alkyl groups.
• Tertiary Amines
  • Less soluble; weaker than secondary amines.

Nucleophilic Properties

• Primary Amines Formation
  • Formed via nucleophilic substitution reaction with halogenoalkanes and ammonia.
  • Lone pair on nitrogen allows further reactions to secondary, tertiary amines, and quaternary salts.
  • Primary amine formation is not efficient due to side reactions.

Reaction Mechanisms

• Primary Amine Formation
  • Ammonia attacks halogenoalkane forming intermediate.
  • Second ammonia molecule removes proton, forming amine.
  • Reaction example: CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br
• Secondary and Tertiary Amines
  • Further reactions of primary amine with halogenoalkanes.
• Quaternary Ammonium Salt Formation
  • Uses excess halogenoalkane.
  • Used as cationic surfactants in various products.

Preparation of Amines

• From Nitriles
  • Two-step process:
    1. Convert halogenoalkane to nitrile using KCN in aqueous ethanol.
    2. Reduce nitrile to primary amine using LiAlH₄ or H₂ with a catalyst.
• Reducing Nitroarenes
  • Reduction to amines using Sn/Fe and HCl or catalytic hydrogenation.

Other Reactions

• Reaction with acyl chlorides and acid anhydrides to form amides.
  • Nucleophilic addition-elimination reaction.

Conclusion

• Amines exhibit various chemical properties and reactivity patterns.
• Understanding their reaction mechanisms is crucial for synthesis and practical applications.