NCERT Solutions for Class 12 Chemistry Chapter 9 Amines
NCERT Solutions for Class 12 Chemistry Chapter 9 Amines covers organic nitrogen compounds, classification, IUPAC nomenclature, basic character order of aliphatic and aromatic amines, preparation methods (Hoffmann bromamide & Gabriel phthalimide synthesis), distinction tests (Hinsberg & Carbylamine test), and Diazonium salts chemistry.
Overview of Topics & Subtopics (Click to Jump)
1. Classification, Structure & IUPAC Nomenclature
Short Note:
• Structure: Nitrogen in amines is sp3 hybridized with a lone pair, possessing a pyramidal geometry. The C-N-H or C-N-C bond angle is less than 109.5° (approx 108° in trimethylamine) due to lone pair-bond pair repulsion.
• Classification: Based on the number of hydrogen atoms replaced in ammonia (NH3): Primary (1°, R-NH2), Secondary (2°, R2NH), and Tertiary (3°, R3N).
• Nomenclature: IUPAC names are derived as alkanamines. For example, CH3NH2 is Methanamine and C6H5NH2 is Aniline (Benzenamine).
2. Methods of Preparation & Basicity Order of Amines
Short Note:
• Preparation Methods:
- Reduction: Reduction of nitro compounds (NO2 → NH2 using Fe/HCl or H2/Pd), nitriles (RCN → RCH2NH2 with LiAlH4), and amides.
- Gabriel Phthalimide Synthesis: Prepares pure 1° aliphatic amines only. Aromatic 1° amines cannot be prepared because aryl halides do not undergo nucleophilic substitution with phthalimide anion.
- Hoffmann Bromamide Degradation: RCONH2 + Br2 + 4NaOH → R-NH2 + Na2CO3 + 2NaBr + 2H2O. (Yields primary amine with one carbon atom less than starting amide).
• Basic Character of Amines: Amines are basic due to presence of unshared electron pair on nitrogen atom.
- In Gas Phase: Basicity order is 3° > 2° > 1° > NH3 (+I effect of alkyl groups).
- In Aqueous Solution: Combined effect of +I effect, hydration energy, and steric hindrance:
* Methyl substituted: 2° > 1° > 3° > NH3 [(CH3)2NH > CH3NH2 > (CH3)3N > NH3]
* Ethyl substituted: 2° > 3° > 1° > NH3 [(C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3]
- Aromatic vs Aliphatic Amines: Aniline is less basic than aliphatic amines because the lone pair on N is delocalised into the benzene ring via resonance.
3. Chemical Reactions & Diazonium Salts
Short Note:
• Chemical Distinction Tests:
- Carbylamine Reaction (Isocyanide Test): Only 1° amines (aliphatic & aromatic) on heating with CHCl3 + alcoholic KOH give foul-smelling isocyanides (R-NC).
- Hinsberg Test: Reaction with Benzenesulphonyl chloride (C6H5SO2Cl):
* 1° amine forms sulfonamide soluble in alkali.
* 2° amine forms sulfonamide insoluble in alkali.
* 3° amine does not react.
• Diazonium Salts (ArN2+X-): Prepared by Diazotisation (Aniline + NaNO2 + 2HCl at 273-278 K → C6H5N2+Cl-).
- Sandmeyer Reaction: ArN2+Cl- + Cu2Cl2/HCl → Ar-Cl + N2.
- Azo Coupling Reaction: C6H5N2+Cl- + C6H5OH → Orange dye (p-hydroxyazobenzene).Intext & Conceptual Questions
(i) CH3CH2NH2
(ii) (CH3)3C-NH2
(iii) C6H5-NH-CH3
1. CH3CH2NH2: Ethanamine
2. (CH3)3C-NH2: 2-Methylpropan-2-amine
3. C6H5-NH-CH3: N-Methylbenzenamine (N-Methylaniline)
Hinsberg Reagent is Benzenesulphonyl chloride (C6H5SO2Cl).
| Amine Type | Reaction with C6H5SO2Cl | Solubility in Aqueous NaOH |
|---|---|---|
| Primary Amine (1°) | Forms N-alkylbenzenesulphonamide (contains acidic N-H hydrogen). | Soluble in NaOH solution. |
| Secondary Amine (2°) | Forms N,N-dialkylbenzenesulphonamide (no N-H hydrogen). | Insoluble in NaOH solution. |
| Tertiary Amine (3°) | Does not react (no hydrogen attached to nitrogen). | Remains unreacted and insoluble. |
Textbook Chapter End Exercise Questions
Aniline (C6H5NH2) is significantly weaker base than Methylamine (CH3NH2) due to two reasons:
• Resonance Delocalisation: In aniline, the unshared lone pair of electrons on nitrogen atom is in conjugation with the benzene ring and gets delocalised over five resonance structures. Thus, the lone pair is less available for protonation.
• Stability of Conjugate Acid: Anilinium ion formed after protonation has only 2 resonance structures, whereas unprotonated aniline has 5 resonance structures. In contrast, in methylamine, the methyl group (+I effect) increases electron density on nitrogen, making it a stronger base.
Hoffmann Bromamide Degradation Reaction: When a primary acid amide is treated with bromine (Br2) in an aqueous or ethanolic solution of sodium hydroxide (NaOH), it yields a primary amine containing one carbon atom less than the starting amide.
General Chemical Equation:
R-CONH2 + Br2 + 4NaOH → R-NH2 + Na2CO3 + 2NaBr + 2H2O
Example:
CH3CONH2 (Acetamide) + Br2 + 4NaOH → CH3NH2 (Methanamine) + Na2CO3 + 2NaBr + 2H2O
Read More & Chapter Resources
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