Amines

Amines are organic derivatives of ammonia (NH3) in which one, two, or three hydrogen atoms are replaced by alkyl or aryl groups. They are fundamental in dyes, drugs, alkaloids, and vitamins.

Classification

Primary (1°): RNH2 — one carbon on N (e.g., CH3NH2, methylamine).
Secondary (2°): R2NH — two carbons on N (e.g., (CH3)2NH).
Tertiary (3°): R3N — three carbons on N (e.g., (CH3)3N).
Quaternary ammonium salt: R4N^+ — no lone pair, not a base.
Aryl amines (e.g., aniline, C6H5NH2): lone pair on N delocalised into ring → weaker bases than alkylamines.

Note: amine class = number of carbon substituents on N (unlike alcohols where it is on the alpha-C).

Physical Properties

Lower members are gases (fishy smell). Primary and secondary amines form N–H...N H-bonds (weaker than O–H...O); BPs higher than alkanes but lower than alcohols. Soluble in water via N–H...OH2 H-bonding.

Preparation

1.Reduction of nitro compounds: C6H5NO2 + 6[H] -(Fe/HCl or H2/Ni)→ C6H5NH2 + 2H2O.
2.Reduction of nitriles: RCN + 4[H] -(LiAlH4)→ RCH2NH2 (primary amine).
3.Gabriel phthalimide synthesis: Potassium phthalimide + RX → N-alkylphthalimide → alkaline hydrolysis → RNH2. Only primary amines produced.
4.Hoffmann bromamide degradation: RCONH2 + Br2 + 4NaOH → RNH2 + Na2CO3 + 2NaBr + 2H2O. Product has one fewer carbon than amide.

Chemical Properties

Basicity: Amines accept H^+ via the N lone pair.
Aliphatic (aqueous): (CH3)2NH > CH3NH2 > (CH3)3N > NH3. (CH3)3N is weaker than (CH3)2NH because the bulky trimethylammonium ion is poorly solvated by water.
Aryl amines weaker than aliphatic: lone pair in conjugation with ring = less available.

With nitrous acid (NaNO2/HCl, 0–5°C):
1° aliphatic: unstable diazonium → N2 evolved.
1° aromatic: stable diazonium salt ArN2^+Cl^- (used in dye synthesis).
2°: N-nitrosamine (yellow oil).
3°: no reaction.

Electrophilic substitution on aniline: –NH2 is strongly activating (+M) → EAS at ortho/para. Bromination with Br2(aq) gives 2,4,6-tribromoaniline instantly. To get monobromo product, protect –NH2 by acetylation (aniline → acetanilide), then brominate, then hydrolyse.

Carbylamine reaction: RNH2 + CHCl3 + alc. KOH → isocyanide (foul smell). Only primary amines.

Azo coupling: ArN2^+Cl^- + activated ArH (phenol/amine, alkaline) → Ar–N=N–Ar' (azo dye, coloured).

Diazonium Salts — synthetic uses

- N2^+ replaced by –OH (warm water), –CN (CuCN, Sandmeyer), –Cl (CuCl), –Br (CuBr), –I (KI), –F (HBF4, Balz-Schiemann).

Worked Examples

Example 1

Increasing basicity: C6H5NH2, (C2H5)2NH, C2H5NH2.
Aryl amine weakest (lone pair in ring). Among aliphatic, 2° > 1° in solution.
Answer: C6H5NH2 < C2H5NH2 < (C2H5)2NH.

Example 2

Preparation of aniline from nitrobenzene.
C6H5NO2 + 6[H] -(Fe/HCl)→ C6H5NH2 + 2H2O.
Answer: Aniline.

Example 3

Aniline + NaNO2/HCl at 0–5°C.
Diazotisation: C6H5NH2 + NaNO2 + HCl → C6H5N2^+Cl^- + NaCl + H2O.
Answer: Benzene diazonium chloride (stable below 5°C).

Example 4

Why does Hoffmann degradation give a shorter-chain amine?
RCONH2 → RNH2 — the carbonyl carbon is lost as CO2 in the reaction.
Answer: Product (RNH2) has one fewer carbon than starting amide (RCONH2).

Example 5

Why is (CH3)3N weaker base than (CH3)2NH in water?
The trimethylammonium ion [(CH3)3NH^+] is bulky and poorly solvated — less stable cation — so equilibrium lies less to the right.
Answer: Poor solvation of (CH3)3NH^+ reduces effective basicity of (CH3)3N in water.

Example 6

Carbylamine test on methylamine.
CH3NH2 + CHCl3 + KOH(alc.) → CH3NC (methyl isocyanide, extremely foul smell).
Answer: Positive carbylamine test — confirms primary amine.

Example 7

Azo dye from benzene diazonium chloride + phenol.
C6H5N2^+Cl^- + C6H5OH -(NaOH)→ p-HO–C6H4–N=N–C6H5 (orange azo dye).
Answer: Para-hydroxyazobenzene; contains –N=N– azo linkage.

Common mistakes

> Watch out: Gabriel synthesis gives only primary amines (no 2° or 3°). Carbylamine test is only for primary amines. Despite three +I methyl groups, (CH3)3N is weaker base than (CH3)2NH in water (solvation effect). Tertiary amines give no reaction with nitrous acid.

Summary

Amines: 1°/2°/3° classified by N-substitution. Basicity: aliphatic > NH3 > aryl amines. Key preparations: reduction of nitro/nitrile compounds, Gabriel synthesis, Hoffmann degradation. Diazotisation gives diazonium salts — versatile intermediates for many aromatic substitutions and azo dyes. Carbylamine test confirms 1° amines.