Nitrogen Compounds

 

Formation of Alkyl Amines

Substitution

• R–X + NH₃ → R–NH₂ + HX_(g)
• Type of reaction:
  • nucleophilic substitution
• Reagent:
  • hot conc. NH₃ in ethanol
• Condition:
  • heat under reflux

 

Formation of Aryl Amines

Reduction

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• Type of reaction:
  • reduction of nitrobenzene
• Reagent:
  • conc. HCl with Tin
• Condition:
  • Heat

 

Basicity of Amines

• Bases are proton acceptors (electron donors)
• N-atoms in amines have a lone pair of e^–s
• N donates lone pair and accepts H⁺ forming dative bond

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Relative Basicity

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Reactions of phenyl amine

Bromination

• Type of reaction:
  • electrophilic substitution
• Reagent:
  • aq. Bromine
• Conditions:
  • r.t.p. (no catalyst)
• Forms a white ppt.

 
 

Diazotization

(–N₂⁺ is called diazonium ion)

• Type of reaction:
  • diazotization reaction
• Reagent:
  • nitrous acid (NaNO₂ with excess HCl_(aq))
  • (HNO₂ weak, unstable acid so produced by reaction NaNO₂ with excess HCl)
• Condition:
  • low temp. (5
    ⁰C)
• Note:
  • Diazonium salts of aryl amine stabilized by delocalization of –N₂⁺ ion’s e^–s over benzene ring
  • However, diazonium ion highly unstable and can decompose above 10^oC to

Coupling Reactions of Diazonium Salts

• Type of reaction: electrophilic substitution
• Reagent: aromatic amines or phenols
• Azo compounds are complex compounds involving a minimum of two aromatic rings joined by N=N coupling
• Benzene diazonium ion carries a +ve charge and readily reacts with cold alkaline solutions of aromatic amines and phenols to give brightly coloured azo-compounds

Bright orange dye formed

• By using alterative aryl compounds to phenol, a range of brightly coloured dye can be formed.

   

Formation of Amides

Formation

• Primary Amides:

• Secondary Amides:

• Type of reaction:
  • nucleophilic substitution
• Reagent:
  • conc. NH_3(aq)
• Condition:
  • r.t.p.
• Example:

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ethanoyl chloride + methyl amine  → N-methyl methanamide

 

Neutrality of Amides

• The presence of the electron withdrawing oxygen atom means that the lone pair on the amide’s nitrogen atom is not available to be donated to e.g. H⁺ ions
• Hence, amides are neutral

Hydrolysis of Amides

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Amino Acids

• Optical activity: all amino acids (except glycine) have a chiral carbon therefore they are optically active

 

Acid/Base Properties of Amino Acids

• Basic amino group and acidic carboxyl group interact:
  • Carboxyl group donates a proton to amino group
  • Amino group accepts proton and zwitterion formed

• Zwitterion: ion that contains regions of +ve & –ve charge
• Amino acids solids at r.t.p. due to ionic bonds that exist between zwitterions
• Presence of zwitterions means that amino acids are soluble in water

 

Amino Acids in Acidic/Basic Conditions

• If acid added, the –COO^– part of the zwitterion accepts an H⁺ ion, reforming –COOH group, leaving +ve charge
• If alkali added, the –NH₃⁺ part of the zwitterion donates an H⁺ ion to the OH^–, reforming –NH₂ group and H₂O, leaving a -ve charge

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Peptide Bonds

• Amide link formed by nucleophilic attack of –NH₂ group of one amino acid on –COOH group of another

• Reaction is a condensation reaction as H₂O eliminated
• Reaction can continue to occur as product still has –NH₂ and –COOH group present

 

Dipeptide → Tripeptide  → Polypeptide (protein)

• Proteins are polymers of amino acids; many polypeptide chains held together by intermolecular forces
• Hydrolysis: involves breaking of peptide links by reaction with water catalysed by an acid or alkali catalyst, giving back the amino acids, temp. nearly 90^oC

 

Electrophoresis

• It is used to separate, identify and purify amino acids obtained when protein hydrolysed
• Technique based on separating ions placed in an electric field. When sample placed between two electrodes:
  • +ve charge ions move towards –ve charged electrode
  • –ve charge ions move towards +ve charged electrode

 

 

• Sample placed on absorbent filter paper (or gel)
• Buffer solution carries ions along (back or forth)