Amines

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.

In water: 2° > 1° > NH₃ > aniline (for direct comparison).

Dimethylamine (2°) > methylamine (1°) > NH₃ > aniline (C₆H₅NH₂)

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?

Soluble in NaOH → the sulfonamide product has an acidic N-H → reacts with NaOH to form sodium salt → soluble.

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.

Carbylamine reaction (only for 1° amines):

CH₃NH₂ + CHCl₃ + 3KOH → CH₃-N≡C (methyl isocyanide) + 3KCl + 3H₂O

Methyl isocyanide has a very foul, nauseating odour.

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.

Step 1: Aniline + NaNO₂ + HCl (0–5°C) → C₆H₅N₂⁺Cl⁻ (benzene diazonium chloride)

Step 2: C₆H₅N₂⁺Cl⁻ + Cu₂Cl₂ → C₆H₅Cl + N₂ (Sandmeyer reaction)

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.

The trap: Only 1° aliphatic amines give N₂ gas (very unstable diazonium → immediate decomposition).

1° AROMATIC amines give diazonium SALT (stable at 0–5°C — used in Sandmeyer reactions). No N₂ gas at 0°C.

2° amines: give N-nitroso compound (R₂N-NO) — yellow oily liquid or solid.

3° amines: essentially no reaction with HNO₂ in the same sense (N has no H to replace).

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

Carbylamine test (1° amines)

Diazonium salt reactions (Sandmeyer)

Hinsberg test for amine type

Gabriel and Hofmann preparation

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.

Concept Core

Formula Vault

Worked Examples

Mistake DNA

Chapter Intelligence

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