Nitrogen Compounds

Primary Amine

Secondary Amine

Tertiary Amine

 

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

Reduction

R–C ‎≡‎ N + 4[H] ‎→‎ R–CH2NH2

R–CO–NH2 + 4[H] → R–CH2NH2 + H2O

Type of reaction: reduction of a nitrile

Type of reaction: reduction of an amide

Reagent: hydrogen gas

Reagent: hydrogen gas

Reducing Agents (catalyst):

LiAlH4 in dry ether

Nickel

Reducing Agent (catalyst):

LiAlH4 in dry ether

Conditions: high temp. and pressure

Conditions: r.t.p.

 

Formation of Aryl Amines

Reduction

  • 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 es
  • N donates lone pair and accepts H+ forming dative bond

 

Relative Basicity

Phenyl amine weaker than ammonia

Ammonia

Ethylamine stronger than ammonia

 

Lone pair of es on N gets partially delocalized by interaction with benzene e cloud

Ethyl e donating group, increases e density on N

Lone pair less available for coordination to proton

Enhanced ability to donate lone pair of es to proton

 

Alkyl ammonium cation formed more stable than NH4+ cation from ammonia

 

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 es over benzene ring
    • However, diazonium ion highly unstable and can decompose above 10oC 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. NH3(aq)
  • Condition:
    • r.t.p.
  • Example:

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

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

 

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 90oC

 

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)

 

Factors that Determine the

Direction of Travel

Speed of Movement

pH of buffer solution

Charge on amino acid

Voltage applied

Temperature

Size (Mr) of amino acid

Magnitude of charge

Larger ions with longer side chain move slower

More highly charge ions move faster

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