ChemistryModerate Weightage ★★★Class 12
Amines
Classification, basicity comparison, preparation (Gabriel, Hofmann degradation), reactions (Hinsberg, diazonium) — expect 2–3 EAPCET questions.
2–3Questions in EAPCET
~3%Paper Weightage
7Key Reactions
3Mistake Traps
Concept Core
Amines — classification, basicity, preparation methods, and key reactions.
Classification and Basicity Order
Based on number of H replaced on NH₃: 1° (RNH₂), 2° (R₂NH), 3° (R₃N), 4° (R₄N⁺ — not a base).
In aqueous solution (water as solvent):
Basicity order: 2° > 1° > 3° > NH₃ (for aliphatic amines in water)
Reason: Two effects compete: (1) +I effect of alkyl groups increases electron density on N → makes it more basic; (2) Steric effect and solvation of conjugate acid (R₂NH₂⁺) by water.
Aniline vs aliphatic amines: Aniline (PhNH₂) is much weaker base than methylamine (CH₃NH₂). In aniline, the lone pair on N is delocalised into the ring (−NH₂ is +M) → less available for protonation → weaker base.
Preparation Methods
1. Reduction of nitro compounds: Ar-NO₂ + 6[H] → Ar-NH₂ (Fe/HCl or Sn/HCl)
2. Gabriel phthalimide synthesis: phthalimide + KOH + R-X → 1° amine only (pure, no 2° or 3°)
3. Hofmann degradation: RCONH₂ + Br₂/KOH → RNH₂ + CO₂ + KBr (chain shortened by 1 C)
4. Reductive amination: RCHO + NH₃ + [H] → RCH₂NH₂
5. From nitriles: RCN + 4[H] → RCH₂NH₂ (chain extended by 1 C)
Hinsberg Test (Distinguish 1°, 2°, 3°)
Hinsberg reagent: benzenesulfonyl chloride (C₆H₅SO₂Cl)
| Amine | Reaction | Solubility in NaOH |
|---|
| 1° R-NH₂ | Reacts → sulfonamide | Soluble (acidic N-H) |
| 2° R₂NH | Reacts → sulfonamide | Insoluble (no acidic N-H) |
| 3° R₃N | Does NOT react | — |
Diazonium Salts and Reactions
Preparation: ArNH₂ + NaNO₂ + HCl (0–5°C) → ArN₂⁺Cl⁻ (diazonium salt)
Sandmeyer reaction: ArN₂⁺ + CuCN/Cu₂Cl₂/Cu₂Br₂ → ArCN/ArCl/ArBr
Balz-Schiemann: ArN₂⁺BF₄⁻ → ArF + N₂ + BF₃ (fluoride introduction)
Coupling: ArN₂⁺ + phenol/aniline → azo dye (−N=N− bond; coloured)
Diazonium salts are very reactive and must be kept at 0–5°C (decompose on warming).
Reactions of Amines
With HCl: R-NH₂ + HCl → R-NH₃⁺Cl⁻ (salt formation; tests basicity)
Acetylation: R-NH₂ + CH₃COCl → R-NHCOCH₃ + HCl (acetamide)
Carbylamine reaction (1° amines only): R-NH₂ + CHCl₃/KOH → R-NC (isocyanide; foul smell — diagnostic)
With HNO₂ (NaNO₂/HCl at 0–5°C): 1° aliphatic → N₂ gas; 1° aromatic → diazonium salt; 2° → N-nitroso; 3° → no reaction
Aromatic Amines — Special Behaviour
Aniline is weaker base but stronger nucleophile in some reactions due to lone pair on N. It undergoes:
• EAS (directing: −NH₂ is ortho/para activator, most powerful)
• Bromination without Lewis acid catalyst (3 Br atoms added — 2,4,6-tribromoaniline white ppt)
Acetylation protects −NH₂ (converts to −NHCOCH₃) for controlled monosubstitution reactions.
Formula Vault
Amine reactions and basicity facts for EAPCET.
Aliphatic Basicity
2° > 1° > 3° > NH₃
In water; steric + solvation effects
Aniline vs Methylamine
CH₃NH₂ >> C₆H₅NH₂ (basicity)
Aniline: lone pair delocalised into ring
Gabriel Synthesis
Phthalimide → 1° amine only
Pure 1° amine; no 2° or 3° byproducts
Hofmann Degradation
RCONH₂ + Br₂/KOH → RNH₂
Loss of CO; chain shortened by 1 C
Carbylamine (1° only)
RNH₂ + CHCl₃/KOH → RNC
Foul-smelling isocyanide; diagnostic for 1° amine
Diazonium Formation
ArNH₂ + NaNO₂/HCl (0–5°C) → ArN₂⁺Cl⁻
Kept cold; decomposes above 5°C
Sandmeyer Reaction
ArN₂⁺ + Cu₂Cl₂ → ArCl + N₂
Also: CuBr (→ArBr), CuCN (→ArCN)
Azo Coupling
ArN₂⁺ + PhOH → Ar-N=N-C₆H₄-OH
Azo dye; coloured
Worked Examples
5 problems — basicity ordering, Hinsberg test, carbylamine, Sandmeyer, and diazonium.
EasyArrange: CH₃NH₂, (CH₃)₂NH, C₆H₅NH₂, NH₃ in decreasing basicity▾
Arrange methylamine, dimethylamine, aniline, and ammonia in decreasing order of basicity in aqueous solution.
1
In water: 2° > 1° > NH₃ > aniline (for direct comparison).
2
Dimethylamine (2°) > methylamine (1°) > NH₃ > aniline (C₆H₅NH₂)
3
Aniline is least basic: lone pair on N delocalised into benzene ring → less available for H⁺.
✓ Decreasing: (CH₃)₂NH > CH₃NH₂ > NH₃ > C₆H₅NH₂
EasyIdentify the amine type: Hinsberg test → product soluble in NaOH▾
An amine gives a product with Hinsberg reagent that is SOLUBLE in NaOH. What type is it?
1
Soluble in NaOH → the sulfonamide product has an acidic N-H → reacts with NaOH to form sodium salt → soluble.
2
Only primary amines (R-NH₂) give a sulfonamide with an N-H that is acidic.
✓ Primary amine (1°)
MediumWrite the product of carbylamine test with CH₃NH₂▾
Methylamine (CH₃NH₂) is treated with CHCl₃ and KOH. Identify the product and its significance.
1
Carbylamine reaction (only for 1° amines):
2
CH₃NH₂ + CHCl₃ + 3KOH → CH₃-N≡C (methyl isocyanide) + 3KCl + 3H₂O
3
Methyl isocyanide has a very foul, nauseating odour.
4
This test is diagnostic for PRIMARY amines only. 2° and 3° amines do not give this reaction.
✓ Product: CH₃NC (methyl isocyanide — foul smell); diagnostic for primary amines
EAPCET LevelConvert aniline to chlorobenzene using diazonium salt (Sandmeyer)▾
Starting from aniline, convert it to chlorobenzene. Write the reaction steps.
1
Step 1: Aniline + NaNO₂ + HCl (0–5°C) → C₆H₅N₂⁺Cl⁻ (benzene diazonium chloride)
2
Step 2: C₆H₅N₂⁺Cl⁻ + Cu₂Cl₂ → C₆H₅Cl + N₂ (Sandmeyer reaction)
3
The Cu₂Cl₂ catalyst facilitates the replacement of −N₂⁺ with −Cl.
✓ Aniline → [NaNO₂/HCl, 0°C] → Diazonium → [Cu₂Cl₂] → Chlorobenzene
Trap QuestionAll amines give N₂ gas with HNO₂ (NaNO₂ + HCl)▾
A student claims all amines react with HNO₂ to give nitrogen gas. Evaluate.
1
The trap: Only 1° aliphatic amines give N₂ gas (very unstable diazonium → immediate decomposition).
2
1° AROMATIC amines give diazonium SALT (stable at 0–5°C — used in Sandmeyer reactions). No N₂ gas at 0°C.
3
2° amines: give N-nitroso compound (R₂N-NO) — yellow oily liquid or solid.
4
3° amines: essentially no reaction with HNO₂ in the same sense (N has no H to replace).
5
Summary: 1° aliphatic → N₂. 1° aromatic → diazonium salt. 2° → N-nitroso compound. 3° → no reaction.
✓ False — only 1° aliphatic → N₂; 1° aromatic → diazonium salt; 2° → N-nitroso; 3° → no reaction
Mistake DNA
3 amine errors from EAPCET distractor analysis.
🔄
3° Amine is Most Basic (More Alkyl Groups = More Basic)
In aqueous solution, 3° amines are LESS basic than 2° amines due to steric effects preventing effective hydration of the protonated form.
❌ Wrong
3° amine most basic
(3 alkyl groups, max +I) ✗
(ignores steric/solvation)
✓ Correct
In water: 2° > 1° > 3° > NH₃ ✓
2° strikes best balance of
+I effect and solvation ✓
The +I inductive effect increases N basicity: 3° > 2° > 1° > NH₃. But in water, solvation of R-NH₃⁺ matters — 3° (R₃NH⁺) is bulky, poorly solvated → less stable cation → 3° is less basic than 2°.
🧪
Carbylamine Test is Positive for All Amines
Carbylamine reaction (isocyanide formation with CHCl₃/KOH) is specific to PRIMARY amines only.
❌ Wrong
2° or 3° amine +
CHCl₃/KOH → foul smell ✗
(only 1° amines react)
✓ Correct
Carbylamine: 1° only ✓
R-NH₂ + CHCl₃/KOH → R-NC ✓
2° and 3° amines: no reaction ✓
The carbylamine reaction requires two N-H bonds for the mechanism. 2° amines have one N-H (insufficient). 3° amines have none. Only 1° amines (two N-H bonds) give isocyanide.
❄️
Diazonium Salt Stable at Room Temperature
Diazonium salts are thermally unstable and must be kept at 0–5°C. They decompose on warming (which is why they're used immediately).
❌ Wrong
Benzene diazonium Cl:
stable at room temp ✗
(decomposes above 5°C!)
✓ Correct
Kept at 0–5°C only ✓
Warm → C₆H₅OH + N₂ ✓
Use immediately for
Sandmeyer or coupling ✓
Diazonium salts are highly reactive intermediates. The N₂ group is a superb leaving group. Above 5°C, they lose N₂ spontaneously. This is why all diazonium chemistry is done at ice-cold temperature.
Chapter Intelligence
Amines connects to GOC (basicity theory), haloalkanes (Gabriel, nitrile reduction), and dyes (azo coupling).
EAPCET Weightage (2019–2024)
Basicity order comparison~8 Carbylamine test (1° amines)~7 Diazonium salt reactions (Sandmeyer)~6 Hinsberg test for amine type~5 Gabriel and Hofmann preparation~4 High-Yield PYQ Patterns
Basicity: 2° > 1° > 3° > NH₃Aniline less basic than CH₃NH₂Carbylamine test → only 1° amineSandmeyer: diazonium → aryl halideHNO₂ reactions: 1°/2°/3° aminesGabriel synthesis: pure 1° amineCoupling reaction → azo dye formation
Exam Strategy
- Basicity: in aqueous solution, 2° > 1° > 3° > NH₃ for aliphatic. Aniline is much weaker than all aliphatic amines (ring resonance withdraws lone pair).
- Carbylamine reaction: foul-smelling isocyanide (RNC) forms only with 1° amines + CHCl₃/KOH. Diagnostic test — appears as identification question regularly.
- Diazonium chemistry (0–5°C only): Sandmeyer (Cu₂X₂ → ArX), Balz-Schiemann (→ ArF), coupling (→ azo dyes). Know which reagent gives which product.
- Hofmann degradation: RCONH₂ → RNH₂ (loses one carbon). Useful for descending the homologous series of amines.