5.4.2Unit 5. Transition Metals and Organic Nitrogen Chemistry
Benzene Reactions — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 28 of 43 Unit quiz
Part 1 of 5Benzene Versus Alkenes
Bromine water
- Alkenes decolourise bromine water at room temperature, whereas benzene does not react.
Electron density
- In an alkene the pi electrons are localised between two carbons,
- but in benzene they are spread over six,
- so the density at any one place is lower.
Attack by electrophiles
- Benzene is less able to polarise an approaching Br₂ molecule, so stronger electrophiles are needed.
Reaction type
- Alkenes mainly undergo addition, while benzene undergoes substitution.
Part 2 of 5Why Substitution
Why Substitution
Addition
- Adding to the ring would destroy the delocalised system and lose the extra stability of about 152 kJ mol⁻¹.
Substitution
- Replacing H by another group keeps the delocalised ring intact, so the product is as stable as the starting material.
Energy profile
- Substitution has a manageable activation energy once a strong electrophile is supplied, so it is the favoured route.
Part 3 of 5Summary of Reactions
Summary of Reactions
Nitration
- Concentrated HNO₃ with concentrated H₂SO₄ below about 55 °C gives nitrobenzene.
Halogenation
- Br₂ with anhydrous FeBr₃ (or Fe) gives bromobenzene and HBr.
Friedel-Crafts
- A chloroalkane or acyl chloride with AlCl₃ gives an alkylbenzene or a ketone.
Hydrogenation
- Heating with hydrogen and a nickel catalyst gives cyclohexane, requiring more severe conditions than for an alkene.
Combustion
- Benzene burns with a smoky, sooty flame because of its high proportion of carbon.
| Reagent | Alkene | Benzene |
|---|---|---|
| Bromine water | Decolourised | No reaction |
| Type of reaction | Addition | Substitution |
| Conditions | Mild | Catalyst or strong reagent needed |
Part 4 of 5Worked Example
Question Write the equation for the complete hydrogenation of benzene and state the type of product.
- Benzene is C₆H₆; each of the three delocalised pairs of electrons can take up H₂.
- Three moles of H₂ are requiredC₆H₆ + 3H₂ → C₆H₁₂.
- The product is cyclohexane, a saturated cyclic alkane.
Answer C₆H₆ + 3H₂ → C₆H₁₂ (nickel catalyst, heat)
Part 5 of 5Exam Checklist & Tips
5.4.2 – Key Points for the Exam
- Electrophilic substitution Replacement of H on the ring by an electrophile.
- Electrophile A species that accepts a pair of electrons.
- Halogen carrier A catalyst, such as FeBr₃, that helps generate the electrophile.
- Addition reaction A reaction in which atoms add across a double bond.
Common Mistakes
- Saying benzene readily undergoes addition at room temperature.
- Using bromine water instead of bromine with a catalyst for substituting benzene.
- Forgetting the temperature limit in nitration.
- Describing benzene as unreactive; it reacts under the right conditions.
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