Edexcel International A Level Chemistry

Topic 18: Organic Chemistry – Arenes

Benzene is a ring of six carbon atoms sharing six delocalised electrons. That one idea explains why it is unusually stable, why it resists addition, and why it reacts by electrophilic substitution instead.

The one idea

Benzene, C₆H₆, looks like it should have three C=C double bonds. It does not behave as if it has any. The reason is that its π electrons are not held between pairs of carbon atoms. They are spread over the whole ring, and that delocalisation lowers the energy of the molecule.

Everything in this topic follows from that:

1. Evidence. Three kinds of data (energy, bond lengths, infrared) show the ring is not a Kekulé ring of alternating single and double bonds.
2. Model. Overlapping p-orbitals give a ring of delocalised electron density above and below the plane.
3. Behaviour. Low electron density at any one place means benzene resists bromine. Keeping the ring intact means it reacts by substitution, not addition.
4. Phenol. An oxygen lone pair feeds more electron density into the ring, so phenol reacts far more easily.

How to use these notes

• Read pages 1 to 4 in order: they build the model and the evidence for it. Pages 5 to 8 use it to explain the reactions. Page 9 is phenol, page 10 is uses and exam technique.
• Every explanation question in this topic has the same three-part shape: say what the species is, say where the electron density is, say what that does to the reagent. The pages show you the words a mark scheme looks for.
• The mechanisms are worth learning as a pattern. There is one pattern, and only the electrophile changes.
• Each page ends with questions and fully worked answers. Cover the answers, try the questions, then mark yourself.
• Use the topic on a page the night before an exam.

Relative atomic masses are the Edexcel IAL periodic table values to one decimal place: H 1.0, C 12.0, N 14.0, O 16.0, S 32.1, Cl 35.5, Br 79.9. These notes use the circle for benzene in equations and mechanisms, and the Kekulé ring only where the evidence needs it.

Pages

1. Benzene and the energy evidence

   Benzene, C₆H₆, does not behave like three C=C bonds. Its enthalpy of hydrogenation is 152 kJ mol⁻¹ less exothermic than a Kekulé ring predicts, so the ring is unusually stable.

   14 min 18.1

2. Bond lengths and infrared evidence

   X-ray diffraction shows six equal carbon–carbon bonds, all 0.139 nm, and infrared shows no alkene C=C absorption. Together with the energy data, that rules out the Kekulé ring.

   13 min 18.1

3. The delocalised model

   Each carbon in benzene has a p-orbital at right angles to the ring. They overlap sideways into one π system of six delocalised electrons above and below the plane.

   14 min 18.118.2

4. Why benzene resists bromination

   An alkene's π bond is localised, so it has high electron density and polarises bromine at once. Benzene's delocalised π electrons give lower electron density, so bromine is not polarised and nothing happens without a catalyst.

   11 min 18.3

5. Reactions of benzene at a glance

   Benzene burns with a smoky flame, and reacts by substitution with five reagent combinations. Know the reagents, conditions and organic product of each, then learn how they work.

   13 min 18.4

6. Electrophilic substitution and bromination

   Benzene is attacked by a strong electrophile in two steps. The ring donates two π electrons to E⁺, then loses H⁺ and recovers its delocalised ring. Bromination shows the pattern with a catalyst making Br⁺.

   15 min 18.418.5

7. Nitration and sulfonation

   Concentrated nitric and sulfuric acids make the nitronium ion NO₂⁺, which substitutes into benzene to give nitrobenzene. Fuming sulfuric acid gives benzenesulfonic acid; you need its reagents and product, not its mechanism.

   14 min 18.418.5

8. Friedel-Crafts reactions

   A halogenoalkane or an acyl chloride, with an AlCl₃ catalyst, joins a carbon group to the benzene ring. The catalyst makes a carbocation or an acylium ion, which is the electrophile.

   15 min 18.418.5

9. Phenol and bromine water

   Phenol decolourises bromine water at room temperature and gives a white precipitate of 2,4,6-tribromophenol. A lone pair on oxygen is partly delocalised into the ring, so the ring is more electron rich than benzene's.

   13 min 18.6

10. Uses and exam technique

    Know one clear use for each compound in the topic, how to choose a reaction to make a given product, and how to word the two explanations examiners ask for most, benzene against alkenes and phenol against benzene.

    12 min 18.318.418.6

Also in this topic: Topic 18 on a page

Spec checklist

Every point you can be examined on in this topic, and where it is explained. Tick them off as you go.