Benzene is the parent aromatic compound. Its six carbon atoms form a regular hexagon, with each carbon bonded to one hydrogen and participating in a delocalized π-electron system. The delocalization gives benzene its exceptional stability — this is aromaticity.
Benzene: 6 π electrons → n = 1 → (4×1+2) = 6 ✓
| Property | Value |
|---|---|
| State at room temp | Colourless liquid |
| Boiling point | 80.1 °C |
| Melting point | 5.5 °C |
| Density | 0.879 g/cm³ (lighter than water) |
| Solubility in water | Immiscible (non-polar) |
| Smell | Sweet, characteristic aromatic odour |
Benzene undergoes electrophilic aromatic substitution (EAS) — not addition — because substitution preserves aromaticity. The ring acts as an electron donor to the incoming electrophile (E⁺).
Step 2 — Stabilisation: Positive charge delocalised over 3 carbons (resonance)
Step 3 — Loss of H⁺: Base removes H⁺ → aromaticity restored → substituted benzene
Phenol has a hydroxyl (–OH) group directly attached to the benzene ring. The lone pairs on oxygen donate into the ring, making phenol more reactive than benzene toward electrophilic substitution, and directing incoming groups to the ortho and para positions.
- With Na: C₆H₅OH + Na → C₆H₅ONa + ½H₂↑ (sodium phenoxide)
- With NaOH: C₆H₅OH + NaOH → C₆H₅ONa + H₂O (acidic nature of phenol)
- With Br₂ water: C₆H₅OH + 3Br₂ → 2,4,6-tribromophenol↓ (white ppt) + 3HBr — no catalyst needed!
- With HNO₃ (dilute): → ortho-nitrophenol + para-nitrophenol mixture
- With HNO₃ (conc.) + H₂SO₄: → 2,4,6-trinitrophenol (picric acid)
- Kolbe reaction: C₆H₅ONa + CO₂ (high pressure) → sodium salicylate → salicylic acid
- Reimer-Tiemann reaction: C₆H₅OH + CHCl₃ + NaOH → salicylaldehyde (ortho-hydroxybenzaldehyde)
Aniline is aminobenzene — an –NH₂ group directly on the ring. Like phenol, –NH₂ is an ortho/para director and strongly activates the ring. Aniline is a weaker base than aliphatic amines because the lone pair on N is delocalised into the ring.
- With HCl: C₆H₅NH₂ + HCl → C₆H₅NH₃⁺Cl⁻ (aniline hydrochloride salt)
- Acetylation: C₆H₅NH₂ + (CH₃CO)₂O → C₆H₅NHCOCH₃ + CH₃COOH (acetanilide)
- Diazotisation: C₆H₅NH₂ + NaNO₂ + HCl (0–5°C) → C₆H₅N₂⁺Cl⁻ (benzenediazonium chloride)
- Coupling reaction: C₆H₅N₂⁺Cl⁻ + C₆H₅OH → p-hydroxyazobenzene (azo dye, orange-red)
- Bromination: C₆H₅NH₂ + 3Br₂ → 2,4,6-tribromoaniline↓ + 3HBr (no catalyst)
Substituents on the benzene ring control where the next group enters. This is one of the most-tested NEB topics.
| Substituent | Type | Effect on Ring | Directs to | Examples |
|---|---|---|---|---|
| –OH, –NH₂, –OCH₃ | Electron-donating | Activates ring | ortho + para | Phenol, Aniline, Anisole |
| –CH₃, –C₂H₅ | Electron-donating (hyperconj.) | Activates ring | ortho + para | Toluene, Ethylbenzene |
| –NO₂, –SO₃H, –COOH | Electron-withdrawing | Deactivates ring | meta | Nitrobenzene, Benz. sulphonic acid |
| –CHO, –COR, –CN | Electron-withdrawing | Deactivates ring | meta | Benzaldehyde, Acetophenone |
| –Cl, –Br, –F | –I effect but +M effect | Slight deactivation | ortho + para | Chlorobenzene, Bromobenzene |
"MEW" — Meta directors are Electron-Withdrawing groups (–NO₂, –SO₃H, –COOH, –CHO).
Halogens are the exception: they deactivate but still direct ortho/para.
Q2. Describe the electrophilic substitution reaction of benzene with mechanism. [8 marks]
Q3. Why is phenol more acidic than alcohol but less acidic than carboxylic acid? [4 marks]
Q4. What is diazotisation? How is aniline converted to diazonium salt? [5 marks]
Q5. Differentiate between ortho/para and meta directing groups with examples. [4 marks]
→ Write the structural formula of the product clearly
→ For mechanism questions: show the arenium ion intermediate (resonance structures)
→ Always state conditions (temperature, catalyst, concentration) for each reaction
→ For phenol/aniline questions: compare with alcohol/amine to explain acidity/basicity
❌ Saying benzene undergoes addition reactions like alkenes
❌ Forgetting that phenol reacts with Br₂ water without a catalyst
❌ Confusing diazotisation (0–5°C) temperature — higher temp destroys the diazonium salt
❌ Forgetting to balance the combustion equation: 2C₆H₆ + 15O₂ → 12CO₂ + 6H₂O
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