Nitrogen compounds

🔷 Primary and Secondary Amines

🔹 Preparation of Amines

(a) Reaction of halogenoalkanes with NH₃ (ethanol, heat under pressure)

• Mechanism: Nucleophilic substitution.
• Reagents: Excess concentrated NH₃ in ethanol.
• Conditions: Heated under pressure in a sealed tube.
• Reaction:
  R–X + NH₃ → R–NH₂ + HX
  (R = alkyl group, X = halogen)

(b) Reaction of halogenoalkanes with primary amines (ethanol, sealed tube/pressure)

• Further substitution forms secondary amines.
• Reaction:
  R–X + R'–NH₂ → R'–NH–R + HX
• Note: May lead to tertiary amines if continued.

(c) Reduction of amides with LiAlH₄

• Converts amides (R–CONH₂) to amines (R–CH₂NH₂).
• Reagents: LiAlH₄ in dry ether.
• Conditions: Room temperature under anhydrous conditions.

(d) Reduction of nitriles

• Nitrile → Primary amine.
• Two methods:
  • LiAlH₄ in dry ether:
    RCN + 4[H] → RCH₂NH₂
  • Hydrogenation with H₂/Ni catalyst under heat and pressure.

🔹 Condensation with Acyl Chloride

• Ammonia or amine + acyl chloride → amide + HCl
• Reagents: Ammonia or primary amine.
• Conditions: Room temperature.
• Example:
  CH₃COCl + NH₃ → CH₃CONH₂ + HCl

🔹 Basicity of Amines

• Amines act as Brønsted–Lowry bases by accepting protons.
• The availability of the lone pair on nitrogen affects basicity.
• Factors:
  • Alkyl groups increase electron density → stronger base.
  • Aromatic rings (e.g., phenylamine) delocalize lone pair → weaker base.

🔷 Phenylamine and Azo Compounds

🔹 Preparation of Phenylamine

1. Nitration of benzene:
   C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O
   (using H₂SO₄ as catalyst, < 55 °C).
2. Reduction of nitrobenzene:
   C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O
   • Reagents: Sn/concentrated HCl under heat.
   • Followed by NaOH (aq) to liberate phenylamine.

🔹 Reactions of Phenylamine

(a) With Br₂ (aq)

• Forms 2,4,6-tribromophenylamine, a white precipitate.
• Rapid due to activated benzene ring by –NH₂ group.

(b) With NaNO₂ and HCl (<10 °C)

• Formation of benzenediazonium chloride:
  • C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O
  • NaNO₂ + HCl → HNO₂ (in situ)
• Further warming:
  • C₆H₅N₂⁺Cl⁻ + H₂O → C₆H₅OH + N₂ + HCl

🔹 Relative Basicity

• Aqueous ammonia < phenylamine < ethylamine
• Explanation:
  • Ethylamine: Alkyl group pushes electrons → strongest base.
  • Phenylamine: Lone pair delocalized into ring → weakest.
  • NH₃: Intermediate basicity.

🔹 Azo Compounds

(a) Coupling Reaction

• Diazonium salt + phenol (in alkaline medium) → azo dye.
  • Example:
    C₆H₅N₂⁺Cl⁻ + C₆H₅OH → C₆H₅–N=N–C₆H₄OH (in NaOH)

(b) Identification

• Azo group: –N=N–

(c) Use

• Azo compounds are colorful and used as dyes in textiles, indicators, etc.

(d) Other Azo Dyes

• Can be synthesized with different activated aromatic compounds like amines and phenols.

🔷 Amides

🔹 Formation of Amides

(a) From ammonia + acyl chloride

• Room temperature:
  • CH₃COCl + NH₃ → CH₃CONH₂ + HCl

(b) From primary amine + acyl chloride

• CH₃COCl + CH₃NH₂ → CH₃CONHCH₃ + HCl

🔹 Reactions of Amides

(a) Hydrolysis

• Acidic:
  RCONH₂ + HCl + H₂O → RCOOH + NH₄Cl
• Alkaline:
  RCONH₂ + NaOH → RCOONa + NH₃

(b) Reduction

• Reagent: LiAlH₄ in dry ether
• Converts CONH₂ → CH₂NH₂

🔹 Basicity of Amides

• Very weak bases.
• Due to resonance between lone pair on N and carbonyl group (C=O), reducing its availability.

🔷 Amino Acids

🔹 Acid-Base Properties and Zwitterions

• Amino acids contain both:
  • –NH₂ (basic)
  • –COOH (acidic)
• In aqueous solution:
  • Zwitterion: Internal salt (NH₃⁺ and COO⁻ on same molecule).
• Isoelectric point (pI): pH where net charge is zero.

🔹 Peptide Bond Formation

• Condensation reaction between –NH₂ and –COOH groups.
• Forms amide bond (peptide bond):
  • H₂N–CHR–COOH + H₂N–CHR'–COOH → H₂N–CHR–CONH–CHR'–COOH + H₂O
• Dipeptides and tripeptides are formed via successive linkages.

🔹 Electrophoresis

• Amino acids move in electric field based on their net charge at a specific pH:
  • Below pI → positive (move to cathode).
  • Above pI → negative (move to anode).
  • At pI → no movement.
• Used to separate amino acids in a mixture.

✅ Summary

• Amines: Prepared by substitution/reduction; act as bases; undergo condensation to form amides.
• Phenylamine: Made from benzene; reacts to form azo dyes.
• Amides: Less basic; formed from amines/ammonia and acyl chlorides; hydrolyzed or reduced.
• Amino acids: Amphoteric; form peptides and are separated by electrophoresis.

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