Quick Revision Notes

Class 12 Chemistry: Amines (अमीन)

### Introduction & Structure

• Amines: Derivatives of ammonia (NH_3) where one or more hydrogen atoms are replaced by alkyl or aryl groups.
• General Formula: R-NH_2 (Aliphatic) or Ar-NH_2 (Aromatic).
• Classification: Based on the number of alkyl/aryl groups attached to the nitrogen atom:
  • Primary (1°): R-NH_2
  • Secondary (2°): R_2NH
  • Tertiary (3°): R_3N

### Nomenclature (नामकरण)

• Common System: Alkylamine (e.g., Methylamine, CH_3NH_2).
• IUPAC System: Alkanamine (e.g., Methanamine, CH_3NH_2).
• For secondary and tertiary amines, the prefix 'N' is used for substituent groups (e.g., N-Methylmethanamine, CH_3NHCH_3).

### Methods of Preparation (बनाने की विधियाँ)

1. Reduction of Nitro Compounds (नाइट्रो यौगिकों का अपचयन):
   • R-NO_2 + 6[H] \xrightarrow{Fe/HCl \text{ or } H_2/Ni} R-NH_2 + 2H_2O
2. Ammonolysis of Alkyl Halides (हैलोजन व्युत्पन्न का अमोनिया अपघटन):
   • R-X + NH_3 \rightarrow R-NH_2 \xrightarrow{R-X} R_2NH \xrightarrow{R-X} R_3N \xrightarrow{R-X} R_4N^+X^- (Ammonium salt)
3. Reduction of Nitriles (Cyanides) (साइनाइड का अपचयन):
   • R-CN + 4[H] \xrightarrow{Na/EtOH \text{ or } H_2/Ni} R-CH_2NH_2
4. Reduction of Amides (ऐमाइड का अपचयन):
   • R-CONH_2 + 4[H] \xrightarrow{LiAlH_4} R-CH_2NH_2 + H_2O
5. Gabriel Phthalimide Synthesis (गेब्रियल थैलिमाइड संश्लेषण):
   • Used exclusively for Primary Aliphatic Amines. Phthalimide reacts with ethanolic KOH followed by alkyl halide and alkaline hydrolysis to give 1^\circ amine.
6. Hofmann Bromamide Degradation Reaction (हॉफमैन ब्रोमामाइड निम्नीकरण अभिक्रिया):
   • An amide is treated with Br_2 and aqueous or alcoholic NaOH to form a primary amine with one carbon atom less than the starting amide.
   • R-CONH_2 + Br_2 + 4NaOH \rightarrow R-NH_2 + Na_2CO_3 + 2NaBr + 2H_2O

### Physical Properties (भौतिक गुण)

• Boiling Points: 1^\circ > 2^\circ > 3^\circ (due to intermolecular hydrogen bonding in 1° and 2° amines). Tertiary amines do not have intermolecular H-bonding.
• Solubility: Lower aliphatic amines are soluble in water due to hydrogen bonding with water molecules. Solubility decreases with an increase in molar mass of the hydrophobic alkyl group.

### Chemical Properties (रासायनिक गुण)

1. Basic Character (क्षारीय प्रकृति):
   • Amines are basic due to the lone pair of electrons on the nitrogen atom.
   • Order of Basicity in Aqueous Solution:
     • For methyl group: (CH_3)_2NH > (CH_3)_3N > CH_3NH_2 > NH_3 (2° > 3° > 1°)
     • For ethyl group: (C_2H_5)_2NH > (C_2H_5)_3N \approx C_2H_5NH_2 (2° > 3° $\approx$ 1°)
   • Effect of Substituents: Electron-donating groups (+I effect) increase basicity; Electron-withdrawing groups (-I effect) decrease basicity. Aromatic amines are weaker bases than ammonia.
2. Alkylation (ऐल्किलन):
   • Amines react with alkyl halides to form secondary, tertiary amines, and finally quaternary ammonium salts.
3. Acylation (ऐसिडylation):
   • Primary and secondary amines react with acid chlorides, anhydrides, or esters to form amides (acylation). This reaction is also known as Schotten-Baumann reaction.
4. Carbylamine Reaction (कार्बिलऐमीन अभिक्रिया / आइसोसाइनाइड परीक्षण):
   • Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide form foul-smelling alkyl/aryl isocyanides (carbylamines).
   • R-NH_2 + CHCl_3 + 3KOH \xrightarrow{\Delta} R-NC + 3KCl + 3H_2O
   • (Note: Secondary and tertiary amines do not give this test; used to distinguish 1° amines).
5. Reaction with Nitrous Acid (HNO_2):
   • Aliphatic 1° amines: Form unstable aliphatic diazonium salts which rapidly decompose to give alcohols and N_2 gas.
   • Aromatic 1° amines (Aniline): React with HNO_2 at 0-5^\circ C (273-278 K) to form Benzene Diazonium Chloride (Diazotisation reaction).
6. Electrophilic Substitution Reactions of Aniline (ऐनिलीन की इलेक्ट्रोफिलिक प्रतिस्थापन अभिक्रियाएँ):
   • Bromination: Aniline reacts with bromine water at room temperature to give a white precipitate of 2,4,6-tribromoaniline. (To get mono-bromo derivative, protection of -NH_2 group is done via acetylation).
   • Nitration: Direct nitration gives a mixture containing p-nitroaniline, m-nitroaniline, and o-nitroaniline. (m-isomer is formed due to protonation in acidic medium).
   • Sulphonation: Aniline reacts with conc. H_2SO_4 to form sulphanilic acid, which exists as a zwitter ion.

### Diazonium Salts (डाइएजोनियम लवण)

• General Formula: Ar-N_2^+ X^- where X^- = Cl^-, Br^-, HSO_4^-, BF_4^-
• Preparation (Diazotisation):
  • Ar-NH_2 + NaNO_2 + 2HX \xrightarrow{273-278 K} Ar-N_2^+ X^- + NaX + 2H_2O
• Important Name Reactions of Diazonium Salts:
  1. Sandmeyer Reaction: Replacement of diazonium group using Cu_2Cl_2/HCl, Cu_2Br_2/HBr, or CuCN/KCN to give aryl halides or cyanides.
  2. Gattermann Reaction: Replacement using copper powder and corresponding halogen acid (Cu/HCl or Cu/HBr).
  3. Balz-Schiemann Reaction: Ar-N_2^+Cl^- + HBF_4 \rightarrow Ar-N_2^+BF_4^- \xrightarrow{\Delta} Ar-F + BF_3 + N_2
  4. Coupling Reactions: Reaction with phenol or aniline in alkaline/mild acidic medium to form brightly colored azo dyes (Electrophilic substitution).
     • `Ar-N_2^+Cl^- + C_6H_5OH \text{ (Phenol)} \rightarrow p\text{-Hydroxyazobenzene (Orange dye)}$$

### Distinction Tests (विभेद करने के परीक्षण)