Amines

🎯 Key Points

• Aqueous basicity (aliphatic): 2° > 1° > 3° > NH₃ (solvation effects dominate); gas phase: 3° > 2° > 1° > NH₃ (pure +I effect)
• Aniline is a much weaker base than aliphatic amines — its N lone pair delocalises into the benzene ring
• HNO₂ test: 1° aliphatic amine → N₂ gas; 1° aromatic amine → stable diazonium salt (0-5°C); 2° amine → N-nitroso oil; 3° amine → no reaction
• Gabriel synthesis gives pure 1° amines only (no aryl amines possible); Hofmann bromamide degradation shortens the chain by one carbon
• Diazonium salt is a versatile synthetic intermediate: → phenol, → benzene (with H₃PO₂), → azo dyes (coupling with phenol/aniline)

Classification of Amines

• Primary (1°): One alkyl or aryl group on N (R-NH₂); e.g., methylamine CH₃NH₂
• Secondary (2°): Two groups on N (R₂NH); e.g., dimethylamine
• Tertiary (3°): Three groups on N (R₃N); e.g., trimethylamine

Basicity of Amines

• Amines are Lewis and Brønsted bases (lone pair on N)
• Aliphatic amines: 2° > 1° > 3° > NH₃ (in aqueous solution due to solvation effects)
• Aromatic amines (aniline) are much weaker bases than aliphatic amines (lone pair delocalised into ring)
• Electron-donating groups on ring increase basicity of arylamines; -NO₂ decreases it

Important Reactions

• Reaction with nitrous acid (HNO₂):
  • 1° aliphatic amine: unstable diazonium salt → N₂ gas
  • 1° aromatic amine: stable diazonium salt (ArN₂⁺Cl⁻) at 0-5°C
  • 2° amine: N-nitroso compound (yellow oil)
  • 3° amine: no visible reaction
• Coupling reaction: Diazonium salt + phenol or aniline → azo dye (orange or red colour)
• Hofmann's bromamide degradation: RCONH₂ + Br₂/KOH → RNH₂ (one carbon less)

Preparation

• Reduction of nitro compounds: ArNO₂ + Fe/HCl → ArNH₂
• Gabriel phthalimide synthesis: gives only 1° amines
• Reductive amination of carbonyl with NH₃ and H₂

More Preparation Methods

• Gabriel phthalimide synthesis: Phthalimide + KOH gives potassium salt, which is alkylated with RX, then hydrolysed to give a pure 1° amine (avoids over-alkylation); cannot be used to prepare aromatic amines since aryl halides do not undergo SN2 easily
• Ammonolysis of alkyl halides: RX + excess NH₃ → RNH₂ (tends to give a mixture of 1°, 2°, 3° amines and quaternary salt)
• Reduction of nitriles: RCN + H₂/Ni or LiAlH₄ → RCH₂NH₂
• Hoffmann bromamide degradation mechanism: Amide reacts with Br₂/KOH to form an isocyanate intermediate via nitrene, which hydrolyses to the amine, the product has one carbon less than the starting amide

Basicity Order Explained

• In the gas phase (no solvation), basicity order is 3° > 2° > 1° > NH₃, purely due to the +I effect of alkyl groups increasing electron density on N
• In aqueous solution, the order changes to 2° > 1° > 3° > NH₃ because 3° amines have fewer N-H bonds for hydrogen bonding with water, reducing solvation stabilisation of the resulting cation
• Ethylamine (pKb about 3.3) is a stronger base than aniline (pKb about 9.4) because the lone pair on aniline's N is delocalised into the benzene ring, making it less available for protonation

Diazonium Salt Reactions

• Replacement by -OH: ArN₂⁺ + H₂O (warm) → ArOH + N₂
• Replacement by -Cl, -Br: with corresponding Cu(I) halide (Sandmeyer reaction)
• Replacement by -CN: ArN₂⁺ + CuCN → ArCN + N₂ (Sandmeyer-type, used to extend carbon chain)
• Replacement by -F: via Balz-Schiemann reaction (using HBF₄)
• Reduction to ArH: ArN₂⁺ + H₃PO₂/H₂O → ArH + N₂ (removes the amino group entirely)
• Coupling with phenol/aniline: gives brightly coloured azo dyes used in the dye and textile industry

Benzenediazonium chloride (C₆H₅N₂⁺ Cl⁻) forms when aniline is treated with nitrous acid (NaNO₂ + HCl) at 0–5°C. Its −N₂⁺ group is an excellent leaving group, making these salts key intermediates for making phenols, haloarenes and azo dyes. Image: Benjah-bmm27, Public Domain, via Wikimedia Commons.

Structure and Nomenclature of Amines

• Nitrogen in amines is sp³ hybridised with a lone pair, giving a pyramidal shape (like ammonia); the bond angles are close to 108°
• Common names: alkyl groups named alphabetically followed by "amine" (e.g. ethylmethylamine); C₆H₅NH₂ is aniline
• IUPAC names: alkanamine — the terminal -e of the alkane is replaced by -amine (methanamine for CH₃NH₂, ethanamine for C₂H₅NH₂)
• In secondary and tertiary amines the other groups on nitrogen take the locant "N-" (e.g. N-methylethanamine, N,N-dimethylmethanamine)

Distinguishing Tests: Hinsberg and Carbylamine

• Carbylamine (isocyanide) test: only 1° amines, on warming with CHCl₃ and alcoholic KOH, give foul-smelling isocyanides — RNH₂ + CHCl₃ + 3KOH → RNC + 3KCl + 3H₂O; 2° and 3° amines give no reaction, so it is a test for 1° amines
• Hinsberg's test: amines are shaken with benzenesulphonyl chloride (C₆H₅SO₂Cl). A 1° amine gives a product soluble in KOH (its remaining N-H is acidic); a 2° amine gives a product insoluble in KOH; a 3° amine does not react — distinguishing all three classes

Other Chemical Reactions of Amines

• Alkylation: amines react with alkyl halides to give successively 2° and 3° amines and finally a quaternary ammonium salt
• Acylation: 1° and 2° amines react with acid chlorides or anhydrides to give substituted amides (e.g. aniline + acetyl chloride → acetanilide); acetylation lowers the reactivity of the amino group
• Electrophilic substitution in aromatic amines: -NH₂ is a strong activating ortho/para director, so aniline + bromine water instantly gives 2,4,6-tribromoaniline (white precipitate)
• To limit substitution to one position (mono-bromination or nitration), the amine is first acetylated to acetanilide, which also protects it from oxidation by the acidic reaction mixture and favours the para product

🚀 JEE Advanced Edge

Why diazonium salts are unstable above 5°C: The N≡N⁺ group is a good leaving group, and the salt readily decomposes to release N₂ gas and form a highly reactive aryl cation/free radical, especially as thermal energy increases — this instability is exactly why diazonium chemistry must be carried out cold, and why it's so useful synthetically (the N₂ leaving group can be replaced by almost anything: -OH, -CN, -X, -H).

Why aniline doesn't undergo Friedel-Crafts reactions: AlCl₃ (the Lewis acid catalyst) coordinates with aniline's lone pair on nitrogen, forming a salt that makes the ring strongly electron-withdrawing (deactivated) instead of activated — this self-poisoning of the catalyst is why aniline must first be protected (e.g., by acetylation to acetanilide) before any Friedel-Crafts-type reaction can be attempted.

Worked problem: Rank the basicity of ammonia, methylamine, and aniline. Approach: Aniline < ammonia < methylamine. Methylamine's alkyl group donates electron density (+I), increasing N's availability to accept a proton, making it MORE basic than plain ammonia. Aniline's lone pair is delocalised into the ring (resonance), making it LESS available than even ammonia's lone pair — resonance delocalisation has a much bigger basicity-reducing effect than the inductive effect's basicity-increasing effect.