Amines & Diazonium Salts

Amines are derivatives of ammonia where one or more hydrogen atoms are replaced by alkyl or aryl groups. They are the most important nitrogen-containing organic compounds, serving as building blocks for pharmaceuticals, dyes, and polymers.

Classification

By substitution: (1) Primary (1°) — R-NH₂ (one H replaced). (2) Secondary (2°) — R₂NH (two H replaced). (3) Tertiary (3°) — R₃N (all three H replaced). Quaternary ammonium salt — R₄N+X- (four groups on N, positively charged).

By carbon type: (1) Aliphatic amines — alkyl groups on N (methylamine CN, diethylamine CCNCC). (2) Aromatic amines — aryl group directly on N (aniline ).

Key amine structures:

Methylamine (simplest 1° aliphatic amine)

Dimethylamine (2° aliphatic amine)

Aniline (1° aromatic amine)

N,N-Dimethylaniline (3° aromatic amine)

Important: Do NOT confuse primary amine (1 group on N) with primary alcohol (OH on 1° C). A "primary amine" classification refers to the nitrogen, not the carbon.

Structure and Bonding

Nitrogen in amines is sp3 hybridised with a lone pair occupying one tetrahedral position. This gives a pyramidal shape (like NH₃). The lone pair is responsible for basicity and nucleophilicity. In aniline, the lone pair is partially delocalised into the benzene ring (+M effect), reducing basicity and nucleophilicity compared to aliphatic amines.

Preparation of Amines

1. Alkylation of ammonia (Hofmann method): NH₃ + RX → RNH₂ + R₂NH + R₃N + R₄N+X- (mixture of all classes). Not selective — over-alkylation is a major problem.

2. Gabriel phthalimide synthesis: Phthalimide + KOH → potassium phthalimide; then + RX (SN₂) → N-alkylphthalimide; then + NaOH/H₂O or N₂H₄ → RNH₂ (pure 1° amine). Only for primary amines. Does NOT work for aryl amines (ArX doesn't undergo SN₂).

3. Reduction methods: (a) Nitro compounds: ArNO₂ + Sn/HCl or H₂/Pd or Fe/HCl → ArNH₂. This is the standard route to aniline. (b) Nitriles: RCN + LiAlH₄ or H₂/Ni → RCH₂NH₂. (c) Amides: RCONH₂ + LiAlH₄ → RCH₂NH₂.

4. Hofmann bromamide degradation (rearrangement): RCONH₂ + Br₂ + NaOH → RNH₂ + Na₂CO₃. Converts an amide to a primary amine with one fewer carbon. Key feature: the R group migrates from C to N (1,2-shift). Only gives 1° amines.

5. Curtius and Schmidt reactions: RCON₃ (heat) → RNCO → RNH₂ (Curtius). RCOOH + HN₃ → RNH₂ + CO₂ + N₂ (Schmidt). Both give 1° amines.

Physical Properties

Boiling points: Amines have higher b.p. than alkanes but lower than alcohols (N-H...N weaker than O-H...O). 1° > 2° > 3° for H-bonding ability (3° has no N-H).

Solubility: Lower amines (up to ~5C) are water-soluble (H-bonding with water). Higher amines are insoluble.

Odour: Lower aliphatic amines have fishy, unpleasant smell. Aniline has a characteristic sweet smell.

Basicity of Amines

In gas phase: 3° > 2° > 1° > NH₃ (more alkyl groups = more +I effect = more electron density on N).

In aqueous solution: 2° > 1° > 3° > NH₃ (for aliphatic amines). The order is irregular because basicity in solution depends on: (1) Inductive effect (+I of alkyl groups increases basicity), (2) Solvation of the conjugate acid (smaller, less substituted cations are better solvated), (3) Steric effects (bulky groups hinder protonation and solvation).

Aromatic vs Aliphatic: Aniline (pKb ~9.4) is much weaker base than methylamine (pKb ~3.3) because the lone pair on N in aniline is delocalised into the benzene ring (+M effect reduces availability for protonation).

Substituent effects on aniline: EWGs (NO₂, CN) decrease basicity further. EDGs (CH₃, OCH₃) increase basicity. p-Nitroaniline (pKb ~13) is extremely weak base. p-Methoxyaniline is stronger base than aniline. The basicity order of substituted anilines parallels the electron-donating/withdrawing ability of substituents.

Reactions of Amines

1. Acylation: RNH₂ + CH₃COCl → RNHCOCH₃ + HCl (acetylation). Used to protect -NH₂ group. 3° amines cannot be acylated (no N-H).

2. Carbylamine reaction (isocyanide test): RNH₂ + CHCl₃ + KOH → RNC (isocyanide, foul smell). Only 1° amines give this test. Key distinction test.

3. Reaction with nitrous acid (HNO₂): This is the most important reaction for distinguishing amine classes:

• 1° aliphatic: RNH₂ + HNO₂ → ROH + N₂ + H₂O (unstable diazonium, decomposes immediately).
• 1° aromatic: ArNH₂ + HNO₂ (0-5°C) → ArN₂+Cl- (stable diazonium salt — very useful).
• 2° (both): R₂NH + HNO₂ → R₂N-N=O (N-nitrosoamine, yellow oily liquid).
• 3° aliphatic: R₃N + HNO₂ → R₃N+H NO₂- (salt formation only).
• 3° aromatic: Ar-NR₂ + HNO₂ → p-nitroso-Ar-NR₂ (C-nitrosation on ring).

4. Hinsberg test: RNH₂ + C₆H₅SO₂Cl (benzenesulphonyl chloride) → C₆H₅SO₂NHR (soluble in NaOH). R₂NH + C₆H₅SO₂Cl → C₆H₅SO₂NR₂ (insoluble in NaOH). R₃N + C₆H₅SO₂Cl → no reaction. Distinguishes 1°, 2°, 3° amines.

5. Electrophilic substitution on aniline: -NH₂ is a powerful activating group (+M, o/p director). Bromination: C₆H₅NH₂ + Br₂(aq) → 2,4,6-tribromoaniline (no catalyst needed). For monosubstitution: protect -NH₂ by acetylation first, then brominate, then deprotect. Nitration: direct nitration with conc. HNO₃ gives oxidation. Use dilute HNO₃ or protect as acetamide first.

Acetanilide (protected aniline):

p-Nitroaniline (EWG-substituted):

Diazonium Salts

Preparation: ArNH₂ + NaNO₂ + HCl (0-5°C) → ArN₂+Cl- + NaCl + 2H₂O. This is diazotisation. Temperature must be 0-5°C to prevent decomposition. Aliphatic diazonium salts are too unstable to isolate.

Benzenediazonium ion:

Reactions — replacement of N₂+:

1. Sandmeyer reaction: ArN₂+Cl- + CuCl → ArCl + N₂ (also: CuBr → ArBr; CuCN → ArCN). Uses Cu(I) halide/cyanide catalyst.

2. Gattermann reaction: ArN₂+Cl- + Cu/HCl → ArCl + N₂. Uses Cu powder (not Cu salt). Similar products to Sandmeyer but different catalyst.

3. Replacement by OH: ArN₂+ + H₂O (warm) → ArOH + N₂ + H+ (phenol synthesis).

4. Replacement by H: ArN₂+ + H₃PO₂ (hypophosphorous acid) or C₂H₅OH → ArH + N₂ (removes the diazo group).

5. Replacement by I: ArN₂+ + KI → ArI + N₂ + KCl. No catalyst needed (unlike Cl, Br).

6. Replacement by F: ArN₂+BF₄- (heat) → ArF + N₂ + BF₃ (Balz-Schiemann reaction).

7. Replacement by NO₂: ArN₂+ + NaNO₂/Cu → ArNO₂ + N₂.

Coupling reactions (retention of N₂+):

ArN₂+ + C₆H₅OH → p-HO-C₆H₄-N=N-C₆H₅ (p-hydroxyazobenzene, orange dye). ArN₂+ + C₆H₅NH₂ → p-H₂N-C₆H₄-N=N-C₆H₅ (p-aminoazobenzene, yellow dye). ArN₂+ + C₆H₅N(CH₃)₂ → p-(CH₃)₂N-C₆H₄-N=N-C₆H₅.

Coupling occurs at the para position of the activated ring (phenol, aniline, N,N-dimethylaniline ). The diazonium ion is a weak electrophile — it can only attack highly activated rings. These azo compounds (-N=N-) are coloured and form the basis of azo dyes.

p-Hydroxyazobenzene (azo dye from phenol coupling):

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