Amines & Diazonium Salts
Program: COMEDK – Chemistry
Audience: 12th‑grade students preparing for undergraduate entrance exams
Why study this?
Amines are the backbone of many biologically important molecules (proteins, drugs, dyes).
Diazonium salts are key intermediates that allow us to transform an aniline into a wide variety of functional groups (halides, nitriles, phenols, etc.) by a one‑step substitution.
1. Simple Explanation
| Term | What it is | Key Feature |
|---|---|---|
| Primary, Secondary, Tertiary Amines | R–NH₂, R₂NH, R₃N | Number of R groups attached to N. |
| Aromatic amines | Aniline & derivatives | N attached to an aromatic ring. |
| Diazonium salt | Ar–N₂⁺ X⁻ (X = Cl, Br, NO₃, etc.) | Very reactive, readily undergoes substitution. |
| Sandmeyer reaction | Ar–N₂⁺ Cl⁻ → Ar–X (X = Cl, Br, CN, etc.) | Uses Cu(I) salts + reducing agent. |
Aniline (C₆H₅NH₂) → Diazonium salt (C₆H₅N₂⁺ Cl⁻) → Substituted benzene (C₆H₅X)
2. Key Points
2.1. Amines
| Property | Details |
|---|---|
| Basicity | Amines are stronger bases than water; pKₐ (conjugate acids) ≈ 9–10. |
| Acidity | N–H bonds are slightly acidic (pKₐ ≈ 30–35). |
| Nucleophilicity | Primary > secondary > tertiary (steric hindrance). |
| Reactivity | Electrophilic acylation, alkylation, nitrosation, oxidation. |
Common reactions
| Reaction | Example |
|---|---|
| Acylation | RCOCl + R₂NH → RCO–NR₂ + HCl |
| Alkylation | R₂NH + R′Cl → R₂N–R′ + HCl |
| Oxidation | R₂NH → R₂NO (nitrosamine) with NaNO₂/H⁺ |
| Nitrosation → Diazonium | Aniline + NaNO₂ + HCl → Ar–N₂⁺Cl⁻ |
2.2. Diazonium Salts
| Feature | Details |
|---|---|
| Formation | 1‑step nitrosation of aromatic amines (NaNO₂/HCl). |
| Stability | Stable at < 5 °C; decompose above 10–15 °C. |
| Reactivity | Acts as an electrophile; undergo substitution (Sandmeyer). |
| Safety | Can explode if concentrated or heated; handle with care. |
2.3. Sandmeyer & Related Reactions
| Transformation | Reagents | Products |
|---|---|---|
| Halogenation | Ar–N₂⁺Cl⁻ + CuCl | Ar–Cl + N₂ + CuCl₂ |
| Bromination | Ar–N₂⁺Cl⁻ + CuBr | Ar–Br + N₂ + CuBr₂ |
| Amination | Ar–N₂⁺Cl⁻ + NH₄Cl | Ar–NH₂ + N₂ + CuCl₂ |
| Nitration | Ar–N₂⁺Cl⁻ + CuCN | Ar–CN + N₂ + CuCl₂ |
| Reduction | Ar–N₂⁺Cl⁻ + SnCl₂/HCl | Ar–H (deamination) |
3. Worked Example: Preparation of 4‑Chlorobenzonitrile from Aniline
Step‑by‑step synthesis
| Step | Reaction | Details |
|---|---|---|
| 1 | Diazotization | 4‑Aniline (C₆H₄‑NH₂) + NaNO₂ + 2 HCl (ice) → 4‑Diazonium‑chloride |
| 2 | Sandmeyer (cyanation) | 4‑Diazonium‑chloride + CuCN + HCl |
| 3 | Work‑up | Extract product with ether; wash, dry, evaporate. |
Balanced equations
Diazotization
[
\text{C}_6\text{H}_4\text{–NH}_2 + \text{NaNO}_2 + 2\text{HCl} \xrightarrow{0–5^{\circ}\text{C}} \text{C}_6\text{H}_4\text{–N}_2^+\text{Cl}^- + \text{NaCl} + \text{H}_2\text{O}
]
Sandmeyer (cyanation)
[
\text{C}_6\text{H}_4\text{–N}_2^+\text{Cl}^- + \text{CuCN} + \text{HCl} \rightarrow \text{C}_6\text{H}_4\text{–CN} + \text{CuCl}_2 + \text{N}_2\uparrow
]
Overall
[
\text{C}_6\text{H}_4\text{