6.4.11Additional lessons
Electrophilic Substitution in Benzene — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 157 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Why Substitution
Benzene undergoes electrophilic substitution, replacing a hydrogen with another group.
Delocalisation
- Delocalisation Benzene has six pi electrons delocalised over the ring, which gives it extra stability.
Addition would destroy it
- Addition would destroy it An addition reaction would break the delocalised system, which costs a large amount of energy.
Substitution keeps it
- Substitution keeps it Substituting a hydrogen atom keeps the ring intact, so the delocalisation is restored.
Bromine water
- Bromine water Benzene does not decolourise bromine water, unlike alkenes, because its pi electrons are less available.
Part 2 of 4Nitration of Benzene
1
Benzene is heated with a mixture of concentrated nitric acid and concentrated sulfuric acid at about 50 °C.
2
The acids react to form the nitronium ion, NO₂⁺: HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O.
3
The nitronium ion is an electrophile, and a pair of pi electrons from the ring attacks it.
4
A positively charged intermediate forms with the delocalisation partially broken, and the carbon bonded to NO₂ also has its hydrogen.
5
The hydrogen is lost as H⁺, and the electrons return to the ring, restoring the delocalised system and forming nitrobenzene.
Part 3 of 4Details of the Reaction
Details of the Reaction
Overall equation
- Overall equation C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O, with concentrated sulfuric acid as a catalyst.
Catalyst regenerated
- Catalyst regenerated The H⁺ lost from the ring reacts with HSO₄⁻ to re-form H₂SO₄, so the acid is not used up.
Temperature control
- Temperature control Keeping the temperature below about 55 °C prevents a second nitro group from being added.
Uses
- Uses Nitrobenzene can be reduced to phenylamine, which is used to make dyes and pharmaceuticals.
| Feature | Alkene | Benzene |
|---|---|---|
| Reaction type | Electrophilic addition | Electrophilic substitution |
| Bromine water | Decolourised | No reaction |
| Reason | Localised pi bond | Delocalised ring |
Part 4 of 4Exam Checklist & Tips
6.4.11 – Key Points for the Exam
- Electrophilic substitution A reaction in which an electrophile replaces a hydrogen atom on an aromatic ring.
- Nitronium ion The NO₂⁺ electrophile formed from nitric and sulfuric acids.
- Delocalisation Sharing of pi electrons over several atoms, giving extra stability.
- Nitrobenzene The aromatic compound C₆H₅NO₂ formed by nitration of benzene.
Common Mistakes
- Saying benzene undergoes addition with bromine, when it needs a catalyst and gives substitution.
- Forgetting the concentrated sulfuric acid, which makes the electrophile.
- Leaving the positive charge off the intermediate in the mechanism.
- Not showing that H⁺ is lost in the final step to restore the ring.
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