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Short Notes: Amines
4.1 Classification and Nomenclature
| Type | Structure | Example |
|---|---|---|
| Primary (1°) | R-NH₂ | CH₃NH₂ (Methylamine) |
| Secondary (2°) | R₂NH | (CH₃)₂NH (Dimethylamine) |
| Tertiary (3°) | R₃N | (CH₃)₃N (Trimethylamine) |
| Aliphatic | Alkyl group attached to N | C₂H₅NH₂ |
| Aromatic | Aryl group attached to N | C₆H₅NH₂ (Aniline) |
4.2 Preparation of Amines
| Method | Reaction | Notes |
|---|---|---|
| Reduction of Nitro Compounds | R-NO₂ + 6[H] → R-NH₂ + 2H₂O (Sn/HCl or Fe/HCl or H₂/Ni) | Best for aromatic amines |
| Reduction of Nitriles | R-CN + 4[H] → R-CH₂-NH₂ (LiAlH₄ or H₂/Ni) | Gives 1° amine |
| Reduction of Amides | R-CONH₂ + 4[H] → R-CH₂-NH₂ + H₂O (LiAlH₄) | Gives 1° amine |
| Gabriel Phthalimide Synthesis | Phthalimide + KOH + R-X → R-NH₂ (followed by hydrolysis) | Pure 1° aliphatic amine No 2° or 3° amine |
| Hoffmann Bromamide Degradation | R-CONH₂ + Br₂ + 4KOH → R-NH₂ + K₂CO₃ + 2KBr + 2H₂O | Loss of one carbon Produces 1° amine |
| Ammonolysis of Alkyl Halides | R-X + NH₃ → R-NH₂ (1°) → R₂NH (2°) → R₃N (3°) | Mixture of products |
4.3 Physical Properties
- Hydrogen bonding: 1° and 2° amines form H-bonds; 3° amines cannot (no H on N)
- Boiling points: 1° > 2° > 3° amines (due to H-bonding)
- Solubility: Lower amines soluble in water (H-bonding with water)
- Smell: Lower aliphatic amines have fishy odor
- Basicity: Lone pair on nitrogen can accept protons
4.4 Basicity of Amines
Basicity Order in Gaseous Phase:
Tertiary > Secondary > Primary > NH₃
(Due to +I effect: more alkyl groups → more electron density on N)
Basicity Order in Aqueous Solution:
Secondary > Primary > Tertiary > NH₃
(Due to combination of inductive effect and solvation/steric factors)
Aromatic Amines:
Less basic than aliphatic amines (lone pair delocalized into benzene ring)
Aliphatic amines > NH₃ > Aromatic amines
Tertiary > Secondary > Primary > NH₃
(Due to +I effect: more alkyl groups → more electron density on N)
Basicity Order in Aqueous Solution:
Secondary > Primary > Tertiary > NH₃
(Due to combination of inductive effect and solvation/steric factors)
Aromatic Amines:
Less basic than aliphatic amines (lone pair delocalized into benzene ring)
Aliphatic amines > NH₃ > Aromatic amines
| Compound | pKb Value | Relative Basicity |
|---|---|---|
| Aniline (C₆H₅NH₂) | 9.4 | Weak base |
| Methylamine (CH₃NH₂) | 3.4 | Strong base |
| p-Nitroaniline | 13.0 | Very weak (NO₂ is EWG) |
| p-Toluidine (p-CH₃C₆H₄NH₂) | 8.9 | More basic than aniline (CH₃ is EDG) |
4.5 Chemical Reactions of Amines
| Reaction Type | Reaction | Notes |
|---|---|---|
| Basic Character | R-NH₂ + HCl → R-NH₃⁺Cl⁻ | Forms salt |
| Alkylation | R-NH₂ + R'-X → R-NH-R' → R-NR'₂ → R₃N | Successive alkylation |
| Acylation | R-NH₂ + R'COCl → R-NH-COR' + HCl (in presence of base) | N-substituted amide Stops after one acyl group |
| Benzoylation (Schotten-Baumann) | R-NH₂ + C₆H₅COCl → R-NH-COC₆H₅ (in NaOH) | Benzamide derivative |
| Carbylamine Reaction | R-NH₂ + CHCl₃ + 3KOH → R-NC (isocyanide - foul smell) | Test for 1° amines only |
| Reaction with HNO₂ | See separate table below | Different for 1°, 2°, 3° amines |
| Electrophilic Substitution (Aniline) | Activates benzene ring (ortho-para directing) | NH₂ is strongly activating |
4.6 Reaction with Nitrous Acid (HNO₂)
| Amine Type | Reaction | Product |
|---|---|---|
| Aliphatic 1° | R-NH₂ + HNO₂ → R-OH + N₂ + H₂O | Alcohol + Nitrogen gas |
| Aromatic 1° (Aniline) | C₆H₅NH₂ + HNO₂ + HCl (273-278K) → C₆H₅N₂⁺Cl⁻ | Benzenediazonium chloride (stable at 273-278K) |
| Secondary (2°) | R₂NH + HNO₂ → R₂N-N=O | N-Nitrosamine (yellow oily liquid) |
| Tertiary (3°) | R₃N + HNO₂ → R₃N⁺-NO₂⁻ | Salt formation (soluble) |
4.7 Diazonium Salts (from Aniline)
| Reaction | Reagent | Product |
|---|---|---|
| Sandmeyer Reaction | CuCl/HCl | C₆H₅Cl (Chlorobenzene) |
| CuBr/HBr | C₆H₅Br (Bromobenzene) | |
| CuCN/KCN | C₆H₅CN (Benzonitrile) | |
| Gattermann Reaction | Cu/HCl | C₆H₅Cl |
| Cu/HBr | C₆H₅Br | |
| Replacement by -I | KI | C₆H₅I (Iodobenzene) |
| Replacement by -F | HBF₄ | C₆H₅F (Fluorobenzene) |
| Replacement by -OH | H₂O (warm) | C₆H₅OH (Phenol) |
| Replacement by -H | H₃PO₂ | C₆H₆ (Benzene) |
| Coupling Reaction | C₆H₅OH (phenol) at 273-278K | p-Hydroxyazobenzene (orange dye) |
| C₆H₅NH₂ (aniline) | p-Aminoazobenzene (yellow dye) |
4.8 Electrophilic Substitution in Aniline
- -NH₂ group: Strongly activating, ortho-para directing
- Bromination: C₆H₅NH₂ + 3Br₂ → 2,4,6-Tribromoaniline (white ppt)
- Nitration: Protect -NH₂ by acetylation first, then nitrate, then deacetylate
- Sulphonation: Forms sulphanilic acid (C₆H₄(NH₂)SO₃H) - zwitterion
4.9 Distinction Tests
| Test | 1° Amine | 2° Amine | 3° Amine |
|---|---|---|---|
| Carbylamine Test | Foul smell (isocyanide) | No reaction | No reaction |
| Hinsberg's Test | Soluble in alkali (sulphonamide with acidic H) | Insoluble in alkali (no acidic H) | No reaction (remains unreacted) |
| With HNO₂ | N₂ gas evolved | Yellow oily N-nitrosamine | Clear solution (salt) |
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FAQs on Short Notes: Amines
| 1. What are amines and how are they classified? |  |
Ans. Amines are organic compounds derived from ammonia (NH₃) by replacing one or more hydrogen atoms with alkyl or aryl groups. They are classified into three categories: primary amines (RNH₂), where one hydrogen atom is replaced; secondary amines (R₂NH), where two hydrogen atoms are replaced; and tertiary amines (R₃N), where all three hydrogen atoms are replaced.
| 2. What is the general formula for amines? | |
Ans. The general formula for amines can be expressed as RNH₂ for primary amines, R₂NH for secondary amines, and R₃N for tertiary amines, where R represents an alkyl or aryl group.
| 3. How do amines participate in hydrogen bonding? | |
Ans. Amines can form hydrogen bonds due to the presence of a nitrogen atom with a lone pair of electrons. This allows amines to interact with water and other polar solvents, leading to increased solubility in polar environments compared to hydrocarbons.
| 4. What are the common methods for the preparation of amines? | |
Ans. Common methods for preparing amines include the reduction of nitro compounds (such as nitrobenzene), alkylation of ammonia, and reductive amination of carbonyl compounds. Each method involves different starting materials and conditions to yield various types of amines.
| 5. What are the uses of amines in everyday applications? | |
Ans. Amines are widely used in various applications, including as intermediates in the synthesis of pharmaceuticals, agrochemicals, dyes, and surfactants. They also play a role in the production of polymers and as additives in food and cosmetics.
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