This chemistry grades 11-12 lesson on benzene notes gives the key ideas first, then worked examples and diagrams. After that, practise with the worksheet on benzene and check yourself with the quiz.
Quick answer: Benzene, C₆H₆, is a planar ring with six delocalised π electrons and six equal C–C bonds of 0.140 nm. Delocalisation makes it 152 kJ/mol more stable than a Kekulé triene, so benzene prefers electrophilic substitution, such as nitration, to addition reactions.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.3.10
Benzene notes: the key ideas
What is the structure of benzene?
Benzene, C₆H₆, is a planar ring of six carbon atoms, each bonded to one hydrogen atom. Each carbon uses one electron in a p orbital, and these six electrons form a delocalised π system above and below the ring. The ring has no separate double bonds: every C–C bond has the same length, 0.140 nm, between a single bond (0.154 nm) and a double bond (0.134 nm).
An arene is a hydrocarbon containing a benzene ring. Delocalisation is the spreading of π electrons over several atoms, which lowers the energy of the molecule. Electrophilic substitution is a reaction in which an electrophile (an electron-pair acceptor) replaces a hydrogen atom of the ring.
What evidence shows that benzene is more stable than a triene?
Hydrogenating one C=C bond in cyclohexene releases 120 kJ/mol. If benzene had three isolated C=C bonds, hydrogenation would release 3 × 120 = 360 kJ/mol. The measured value is only 208 kJ/mol. The molecule is therefore 360 − 208 = 152 kJ/mol more stable than the Kekulé structure suggests; this is the delocalisation energy.
The same conclusion comes from chemistry. Benzene does not decolourise bromine water, as an alkene does, and it gives only one product 1,2-dibromobenzene, whereas the Kekulé ring predicts two isomers.
How does electrophilic substitution happen?
The delocalised ring is a region of high electron density, so it attracts electrophiles. Attack on the ring temporarily breaks the delocalisation, which costs energy, so benzene reacts more slowly than an alkene and always needs a strong electrophile. The reaction ends by losing H⁺, which restores the stable ring: the result is substitution, not addition.
| Reaction | Reagents and conditions | Product | Electrophile |
|---|---|---|---|
| nitration | conc. HNO₃ + conc. H₂SO₄, below 55 °C | nitrobenzene | NO₂⁺ |
| bromination | Br₂ + FeBr₃ (halogen carrier), room temperature | bromobenzene | Br⁺ (polarised Br₂) |
| alkylation | CH₃Cl + AlCl₃, dry | methylbenzene | CH₃⁺ |
| acylation | CH₃COCl + AlCl₃, dry | phenylethanone | CH₃CO⁺ |
Why are these reactions useful?
Each substitution attaches a functional group to the stable ring, so benzene becomes a starting material for dyes, medicines and plastics. For example, nitrobenzene is reduced by tin and concentrated hydrochloric acid to phenylamine, which is used to make azo dyes. Friedel–Crafts reactions build new C–C bonds and need dry conditions, because water destroys the aluminium chloride catalyst.
HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻
C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (catalyst H₂SO₄)
C₆H₆ + Br₂ → C₆H₅Br + HBr (catalyst FeBr₃)
Use the data: ΔH(hydrogenation) is −120 kJ/mol for cyclohexene and −208 kJ/mol for benzene.
Predicted for three localised C=C bonds: 3 × (−120) = −360 kJ/mol.
Difference: −208 − (−360) = +152 kJ/mol, so benzene is 152 kJ/mol more stable.
Aarav nitrates 7.80 g of benzene (M = 78.0 g/mol) and collects 9.84 g of nitrobenzene (M = 123.0 g/mol).
Moles of benzene = 7.80 ÷ 78.0 = 0.100 mol, so the theoretical mass is 0.100 × 123.0 = 12.3 g.
Percentage yield = 9.84 ÷ 12.3 × 100 = 80.0 %.
Do not say benzene undergoes addition with bromine like an alkene. The ring keeps its delocalised electrons and substitutes one hydrogen with bromine, producing HBr as the second product.
Remember the mechanism as three beats: make the electrophile, let the ring attack it, then lose H⁺ to rebuild the ring. In mark schemes the curly arrow must start from the ring’s π cloud.
- Benzene: planar, C–C bonds all 0.140 nm, bond angles 120°.
- Stabilisation energy: −360 − (−208) = 152 kJ/mol.
- Nitration: conc. HNO₃ + conc. H₂SO₄, below 55 °C, electrophile NO₂⁺.
- Bromination: Br₂ with FeBr₃; Friedel–Crafts: RCl or RCOCl with AlCl₃.
- Mechanism: electrophile attacks the ring, then H⁺ is lost to restore it.
Benzene structure and electrophilic substitution: frequently asked questions
Why does benzene not react like an alkene?
Its six π electrons are delocalised over the whole ring, which makes it 152 kJ/mol more stable. Addition would destroy this stability, so substitution happens instead.
What is the role of concentrated sulfuric acid in nitration?
It protonates nitric acid so that water can leave, forming the nitronium ion NO₂⁺, which is the electrophile. The acid is regenerated at the end of the reaction, so it acts as a catalyst.
Why is the temperature kept below 55 °C in nitration?
A higher temperature lets a second nitro group substitute, giving dinitrobenzene. Keeping the temperature low gives mostly mononitrobenzene.
What is a halogen carrier?
A halogen carrier such as FeBr₃ or AlCl₃ is a Lewis acid that polarises a halogen molecule to produce a stronger electrophile. It is regenerated at the end, so it is a catalyst.
How do you know benzene has delocalised bonds?
Its C–C bonds are all equal at 0.140 nm, its hydrogenation enthalpy is 152 kJ/mol less exothermic than predicted, it gives only one 1,2-disubstituted isomer and it does not decolourise bromine water.
