Board exam focus — Amines (CBSE & HBSE)

Classification and nomenclature of amines, preparation (nitro reduction, Hoffmann's degradation), basicity (alkyl vs aryl), diazotization, Sandmeyer and coupling reactions.

Classification and Preparation

Amines

Classification

Amines: derivatives of NH3 with alkyl/aryl groups replacing H.

Based on degree of N-substitution:

• Primary (1 degrees): RNH2 - one substituent
• Secondary (2 degrees): R2NH - two substituents
• Tertiary (3 degrees): R3N - three substituents

Aliphatic vs Aromatic: Alkylamines: R = alkyl (CH3NH2, (C2H5)2NH) Arylamines: R = aryl (C6H5NH2 aniline, C6H5NHCH3 N-methylaniline)

Naming: Alkylamines: prefix alkyl + amine (methylamine, dimethylamine) Complex: N-substituted (N-methylaniline, N,N-dimethylaniline)

Preparation

1. Reduction of Nitro Compounds: ArNO2 + 6[H] → ArNH2 + 2H2O Laboratory: C6H5NO2 + Sn + 6HCl → C6H5NH2 + SnCl4 + 2H2O (then NaOH) Industrial: C6H5NO2 + 3H2 (Fe2O3/Cr2O3) → C6H5NH2 + 2H2O

2. Reduction of Nitrile: RCN + 4[H] (LiAlH4 or H2/Ni) → RCH2NH2 (one extra C than starting material)

3. Reduction of Amide: RCONH2 + 4[H] (LiAlH4) → RCH2NH2

4. Hoffmann's Bromamide Degradation: RCONH2 + Br2 + 4NaOH → RNH2 + Na2CO3 + 2NaBr + H2O Key point: Product amine has ONE FEWER carbon than starting amide. Mechanism: Amide → N-bromamide → isocyanate (rearrangement) → amine

5. Gabriel Phthalimide Synthesis: Phthalimide + KOH → K-phthalimide → K-phthalimide + RX → N-alkylphthalimide → hydrazine → primary amine Advantage: Gives PURE primary amine (no secondary or tertiary products).

Industrial Preparation of Aniline

2C6H5NO2 + 3Fe + 6HCl → 2C6H5NH2 + 3FeCl2 + 2H2O (Bechamp reduction) Aniline: colorless oil, characteristic smell, bp 184 C, slightly water-soluble, turns brown in air.

Basicity of Amines

Basicity of Amines

Amines as Bases

Amines have lone pair on N; accept H+ (Lewis/Bronsted base). RNH2 + H2O ⇌ RNH3+ + OH- Kb = [RNH3+][OH-]/[RNH2]; pKb = -log Kb Lower pKb = stronger base.

Basicity of Aliphatic Amines

Alkyl groups donate electrons (inductive +I effect) to N → more electron density → better H+ acceptor.

Gas phase order: 3 degrees > 2 degrees > 1 degrees > NH3

In aqueous solution - solvation also matters:

• RNH3+ (primary ammonium) has 3 N-H bonds for H-bonding with water (better solvated)
• R2NH2+ (secondary) has 2 N-H
• R3NH+ (tertiary) has only 1 N-H (least solvated)

Net in water: secondary (2 degrees) > primary (1 degrees) approximately equal tertiary (3 degrees) > NH3 Typically: (C2H5)2NH > C2H5NH2 > (C2H5)3N > NH3

pKb values: methylamine 3.38, dimethylamine 3.27, trimethylamine 4.22, NH3 4.74

Basicity of Aromatic Amines

Aniline (C6H5NH2) pKb = 9.38 - MUCH WEAKER than aliphatic amines.

Reason: Resonance delocalization of N lone pair into benzene ring. C6H5-NH2 has multiple resonance structures with N+ and negative charge on ring. Lone pair on N partially delocalized → less available for protonation → weaker base.

Order: Aliphatic amines >> NH3 >> Aryl amines Methylamine (pKb 3.38) >> NH3 (pKb 4.74) >> Aniline (pKb 9.38)

Effect of Substituents on Aniline Basicity

• Electron-donating groups (CH3, OCH3 at o/p): increase basicity of aniline
• Electron-withdrawing groups (NO2, CN, Cl at o/p): decrease basicity

p-Nitroaniline (pKb 13.0): much weaker than aniline p-Methylaniline (pKb 8.9): slightly stronger than aniline (9.38)

Chemical Reactions of Amines

Chemical Reactions of Amines

Alkylation

RNH2 + R'X → R-NH-R' (secondary amine) Continued alkylation → tertiary, then quaternary ammonium salt: R4N+X-

Acylation and Benzoylation

With acyl chloride or anhydride: RNH2 + CH3COCl → RNHCOCH3 (acetamide) + HCl RNH2 + (CH3CO)2O → RNHCOCH3 + CH3COOH Schotten-Baumann: ArNH2 + C6H5COCl + NaOH → ArNHCOC6H5 + NaCl (benzamide) Acylation used to protect -NH2 in synthesis (amide less reactive than amine).

Reaction with HNO2 (Nitrous Acid)

Prepared: NaNO2 + HCl (0-5 C) → HNO2 + NaCl

Primary aliphatic amine: unstable diazonium, decomposes → N2 + alcohol + alkene (mixture) Primary aromatic amine (0-5 C): stable arenediazonium salt (ArN2+Cl-) Secondary amine: N-nitrosamine (R2N-N=O, yellow oily liquid) Tertiary aliphatic amine: no reaction (no N-H)

Diazotization

ArNH2 + NaNO2 + 2HCl (0-5 C) → ArN2+Cl- + NaCl + H2O

Stable at 0-5 C due to resonance delocalization. Decomposes above 5 C.

Sandmeyer Reactions (Replacement)

ArN2+Cl- + CuCl → ArCl + N2 + CuCl (Sandmeyer) ArN2+Cl- + CuBr → ArBr + N2 ArN2+Cl- + CuCN → ArCN + N2 ArN2+Cl- + HBF4 → ArF + N2 (Balz-Schiemann) ArN2+Cl- + H3PO2 → ArH + N2 (replacement by H) ArN2+Cl- + H2O (boil) → ArOH + N2 (replacement by OH) ArN2+Cl- + KI → ArI + N2 + KCl (without Cu)

Coupling Reaction

ArN2+Cl- + C6H5OH (alk.) → Ar-N=N-C6H4-OH (para, orange-red azo dye) ArN2+Cl- + ArNH2 → Ar-N=N-Ar-NH2 (aminoazo compound)

Electrophilic aromatic substitution on activated ring (phenol or amine ring). Para position preferred. Azo dyes: intensely colored, used in textile dyeing.

Hinsberg Test

Hinsberg reagent = benzenesulfonyl chloride (C6H5SO2Cl)

• 1 degrees amine: forms sulfonamide + dissolves in NaOH (N-H acidic)
• 2 degrees amine: forms sulfonamide + insoluble in NaOH (no N-H)
• 3 degrees amine: no reaction (no N-H)

Carbylamine Reaction (only 1 degrees amines)

RNH2 + CHCl3 + 3KOH → RNC (isocyanide, carylamine) + 3KCl + 3H2O Offensive smell of isocyanide; test for primary amines only.

Frequently asked questions

Are these Amines notes free?

Yes — the Amines notes for chemistry (Class 12) on Siksha Sarovar are completely free to read, with no account required.

Do these notes follow CBSE and HBSE?

Yes. The Amines notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.

What does the Amines chapter cover?

Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Amines.