Unit 16: Amines

Introduction to Amines

Amines are organic derivatives of ammonia (NH₃) in which one or more hydrogen atoms are replaced by alkyl or aryl groups. The general formula is RNH₂ (primary), R₂NH (secondary), or R₃N (tertiary), where R represents an alkyl or aryl group.

Classification of Amines

• Primary (1°) amines: One alkyl/aryl group attached to nitrogen (RNH₂).
• Secondary (2°) amines: Two alkyl/aryl groups attached (R₂NH).
• Tertiary (3°) amines: Three alkyl/aryl groups attached (R₃N).

Isomerism in Amines

1. Chain isomerism: Different carbon skeletons (e.g., CH₃CH₂CH₂NH₂ (propylamine) vs. (CH₃)₂CHNH₂ (isopropylamine)).
2. Position isomerism: Variation in the position of the –NH₂ group on the same carbon chain (e.g., 1‑aminopropane vs. 2‑aminopropane).
3. Functional isomerism: Amines can be isomeric with other functional groups containing the same molecular formula, such as imines (RCH=NH) or nitroalkanes (R‑NO₂).

Separation of Amines – Hoffmann's Method

Hoffmann's method exploits the differing solubilities of amine salts in ether and water:

• Primary amines react with Hinsberg's reagent (benzenesulfonyl chloride, C₆H₅SO₂Cl) to form a sulfonamide that is soluble in aqueous alkali.
• Secondary amines give a sulfonamide insoluble in alkali.
• Tertiary amines do not react with Hinsberg's reagent.

After reaction, the mixture is shaken with ether; the ether layer contains unreacted tertiary amines, while the aqueous layer contains the sulfonamide derivatives of primary and secondary amines, which can be further separated by acid‑base extraction.

Preparation of Primary Amines

1. From Haloalkanes (Nucleophilic Substitution)

R‑X + NH₃ → RNH₂ + HX (where X = Cl, Br, I). Excess ammonia minimizes polyalkylation.

2. From Nitriles (Reduction)

RCN + 2[H] → RCH₂NH₂ (catalytic hydrogenation with H₂/Ni or LiAlH₄).

3. From Nitroalkanes (Reduction)

R‑NO₂ + 3[H] → RNH₂ + 2H₂O (using Sn/HCl, Fe/HCl, or catalytic hydrogenation).

4. From Amides – Hofmann Bromamide Reaction

RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O. The amide is treated with bromine in strongly basic solution, leading to loss of the carbonyl carbon as carbonate.

Physical Properties of Amines

• Lower amines (C₁–C₄): Soluble in water due to hydrogen bonding (–NH₂ can act as H‑bond donor and acceptor).
• Boiling points: Lower than corresponding alcohols of similar mass because amines form weaker hydrogen bonds (only one H attached to N).
• Odour: Characteristic fishy smell for volatile amines.

Chemical Properties of Amines

Basicity

Amines act as Brønsted–Lowry bases by accepting a proton on the lone pair of nitrogen:

RNH₂ + H⁺ ⇌ RNH₃⁺

The basicity order in aqueous solution is:

2° > 1° > 3° > NH₃

This trend arises from a balance of electron‑donating inductive effects (increasing basicity) and steric hindrance/solvation effects (decreasing basicity for highly substituted amines).

Reactions of Primary Amines

1. Carbylamine test (with chloroform and alkali): RNH₂ + CHCl₃ + 3KOH → RNC + 3KCl + 3H₂O (produces foul‑smelling isocyanide).
2. Reaction with concentrated HCl: Forms ammonium salt (RNH₃⁺Cl⁻).
3. Alkyl haloalkane (R‑X): Further alkylation to give secondary, tertiary amines, and quaternary ammonium salts.
4. Acylation with acyl chlorides (RCOX) or anhydrides: RNH₂ + R'COCl → RNHCOR' + HCl (amide formation).
5. Reaction with nitrous acid (NaNO₂/HCl):
   1. Primary aliphatic amine → alcohol + N₂ gas + H₂O. RNH₂ + HNO₂ → ROH + N₂ + H₂O
   2. Primary aromatic amine → diazonium salt (ArN₂⁺Cl⁻) at 0‑5 °C.
   3. Secondary amine → N‑nitrosoamine (yellow oil).
   4. Tertiary amine → no reaction (forms a soluble salt).

Aromatic Amines – Aniline

Aniline

1. Reduction of nitrobenzene: C₆H₅NO₂ + 3[H] → C₆H₅NH₂ + 2H₂O (Sn/HCl, Fe/HCl, or catalytic hydrogenation).
2. Ammonolysis of phenol: C₆H₅OH + NH₃ → C₆H₅NH₂ + H₂O (high temperature, pressure, with a catalyst such as ZnO).

Physical Properties of Aniline

• Colourless to pale yellow liquid; darkens on exposure to air and light due to oxidation.
• Sparingly soluble in water (hydrogen bonding limited by the aromatic ring).
• Boiling point: 184 °C.
• Characteristic unpleasant odor.

Chemical Properties of Aniline

Basicity

The lone pair on nitrogen is delocalized into the benzene ring via resonance, reducing its availability for protonation. Consequently, aniline is a weaker base than aliphatic amines and even weaker than ammonia:

Basicity order: aliphatic amines > NH₃ > aniline.

Reactions of Aniline

1. Alkylation: Reaction with alkyl halides gives N‑alkyl anilines (secondary amines) and, with excess, N,N‑dialkyl anilines (tertiary amines).
2. Acylation: With acyl chlorides or anhydrides yields anilides (C₆H₅NHCOR).
3. Diazotization (0‑5 °C): C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O. The diazonium salt is a versatile intermediate for coupling, Sandmeyer, and Gomberg‑Bachmann reactions.
4. Carbylamine test: Aniline (being a primary aromatic amine) gives a positive carbylamine test, producing phenyl isocyanide.
5. Coupling reaction: Diazonium salt couples with phenols or aromatic amines (activated rings) to form azo dyes (Ar‑N=N‑Ar′).
6. Electrophilic substitution (activating, o,p‑directing):
   • Nitration: C₆H₅NH₂ + HNO₃/H₂SO₄ → o‑nitroaniline + p‑nitroaniline (often requires protection of –NH₂ as acetamide to avoid oxidation).
   • Sulphonation: C₆H₅NH₂ + H₂SO₄ → o‑sulfonanilic acid + p‑sulfonanilic acid.
   • Bromination: C₆H₅NH₂ + Br₂ → 2,4,6‑tribromoaniline (white precipitate) in aqueous medium; in non‑polar solvents gives mono‑bromination at ortho/para positions.

Uses of Amines

• Dyes: Azo dyes derived from aniline diazonium salts (e.g., methyl orange, Bismarck brown).
• Drugs: Paracetamol (acetaminophen) is synthesized from p‑aminophenol; many antihistamines, local anesthetics, and alkaloids contain amine moieties.
• Rubber processing: Amines act as accelerators and vulcanization promoters (e.g., diphenylguanidine).
• Corrosion inhibitors, surfactants, and polymer additives.

Summary

Amines constitute a versatile class of organic compounds whose properties are governed by the nitrogen lone pair. Aliphatic amines exhibit stronger basicity and nucleophilicity, while aromatic amines such as aniline show reduced basicity due to resonance delocalization but participate richly in electrophilic substitution and diazonium chemistry. The preparation methods, characteristic reactions (Hoffmann separation, carbylamine test, nitrous acid test, acylation, diazotization), and wide‑ranging applications underscore the importance of amines in synthetic chemistry, pharmaceuticals, and materials science.