CBSE Class 12 Chemistry Amines Notes Set 06

Revision Notes

Amines: Amines are considered as amino derivatives of hydrocarbons or alkyl derivatives of ammonia. In these compounds, one, two or three hydrogen atoms are replaced by alkyl or aryl groups. Aliphatic amino compounds are called amino alkanes and aromatic amines are called amino arenes. e.g., \( CH_3NH_2 \), \( C_2H_5NH_2 \), \( C_6H_5NH_2 \), etc.
 

Physical Properties

• Lower members are combustible gases, members from \( C_3 \) to \( C_{11} \) are volatile liquids and \( C_{12} \) onwards are solids. Lower aromatic amines are liquids.

• Pure amines are colourless but develop colour on keeping in air for long time.
• Boiling point increases with an increase in molecular weight. The order of boiling points of isomeric amines: Primary > Secondary > Tertiary.
• Lower members are readily soluble in water which decreases in water and increases in organic solvents with an increase in molecular weight.

Reactions:

• Alkylation: Amines react with alkyl halides to form higher amines.
• Acylation: \( R—NH_2 + R'—COCl \rightarrow R—NH—COR' + HCl \) (N-substituted amide)
• Benzoylation: \( CH_3NH_2 + C_6H_5COCl \rightarrow CH_3NHCOC_6H_5 + HCl \) (N-Methylbenzamide)
• Carbylamine reaction: Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide form isocyanides or carbylamines (unpleasant odour). Secondary and tertiary amines do not give this reaction.

Identification of primary, secondary and tertiary amines

• Reaction with nitrous acid:
  Primary: Gives alcohol with effervescence of \( N_2 \) gas.
  Secondary: Gives oily nitrosoamine which gives Liebermann's nitrosoamine test.
  Tertiary: Forms nitrite in cold soluble in water and on heating gives nitrosoamine.

• Hinsberg Test:
  Primary: Gives N-alkyl benzene sulphonamide which is soluble in alkali.
  Secondary: Gives N, N-dialkyl benzene sulphonamide which is insoluble in alkali.
  Tertiary: No reaction.
   
• Hoffmann's mustard oil reaction:
  Primary: Forms N-substituted isothiocyanate with characteristic unpleasant smell of mustard oil.

Electrophilic Substitution Reactions for Aniline

• Bromination: Aniline reacts with bromine water to give 2, 4, 6-tribromoaniline (white ppt). To get monobromo product, the amino group is first protected by acetylation.

• Nitration: Direct nitration gives a mixture of ortho, meta and para nitroanilines. (Para 51%, Meta 47%, Ortho 2%).
• Sulphonation: Aniline reacts with conc. \( H_2SO_4 \) to form anilinium hydrogensulphate which on heating forms sulphanilic acid (Zwitter ion).

Mnemonics

Concept: Amines
Mnemonic: Passive Avoidance Conditioned Reflex
Interpretation: Primary amines give Carbylamine Reaction.

Know the Terms

• Gabriel phthalimide synthesis: Phthalimide gives potassium salt with ethanolic KOH, which on heating gives alkyl halide which in turn on alkaline hydrolysis gives primary amine. It is used for \( 1^\circ \) amine synthesis.

• Hoffmann Bromamide degradation: Treating amide with bromine in aqueous or ethanolic NaOH solution, results in degradation of amide leading to formation of primary amine. \( 1^\circ \) amine formed contains one carbon less than the parent amide.
• Sandmeyer’s reaction: Introduction of nucleophiles \( Cl^- \), \( Br^- \), \( CN^- \) to benzene ring in presence of \( Cu(I) \) ion.
• Schotten Baumann reaction: Benzoylation of amines with benzoyl chloride is known as Schotten Baumann reaction.