Topic 18 · page 4 of 10

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 read18.3

Read first: The delocalised model

Alkenes and bromine: a reminder 18.3

Cyclohexene, like every alkene, decolourises bromine water at room temperature in a few seconds, with no catalyst:

CX6HX10+BrX2→CX6HX10BrX2 \ce{C6H10 + Br2 -> C6H10Br2}

This works because the π bond in C=C is localised between two carbon atoms. Two electrons in a small region means high electron density. When a bromine molecule approaches, the electron density repels the electrons in the Br–Br bond and induces a dipole. The nearer bromine atom becomes δ+, and the alkene attacks it. This is electrophilic addition.

Benzene and bromine 18.3

Benzene does not decolourise bromine water. The mixture stays orange and nothing happens, even though benzene also has π electrons.

The difference is the delocalisation. In benzene the six π electrons are spread over six carbon atoms. The electron density between any two carbons is lower than in the localised π bond of an alkene. A bromine molecule approaching a benzene ring is not polarised enough to react.

There is also an energy reason. Adding Br₂ across the ring would destroy the delocalised π system and lose the extra 152 kJ mol⁻¹ of stability. A reaction that keeps the ring intact, substitution, is far more favourable. The next pages cover it.

Cyclohexene Benzene
π electrons localised between 2 carbons delocalised over 6 carbons
electron density high lower
effect on Br₂ polarised easily not polarised enough
with bromine water decolourised quickly no change
type of reaction addition needs a catalyst, then substitution

Check yourself

  1. State what you would see when bromine water is added to (a) cyclohexene and (b) benzene.
  2. Explain why cyclohexene reacts with bromine water but benzene does not.
  3. Give a second, energy-based reason why bromine does not add across the ring of benzene.
  4. Write the equation for the reaction of cyclohexene with bromine.
  5. A student writes "benzene is unreactive". Make the statement more accurate.

Answers cover these until you have tried the questions

  1. (a) The orange bromine water is decolourised quickly. (b) No change: it stays orange.
  2. In cyclohexene the π electrons are localised between two carbon atoms, giving high electron density, which polarises the bromine molecule (induces a dipole), so it is attacked. In benzene the π electrons are delocalised over six carbon atoms, so the electron density between any two carbons is lower and cannot polarise the bromine enough to react.
  3. Addition would destroy the delocalised π system and so lose the 152 kJ mol⁻¹ of extra stability. Substitution keeps the ring.
  4. CX6HX10+BrX2→CX6HX10BrX2\ce{C6H10 + Br2 -> C6H10Br2} (1,2-dibromocyclohexane)
  5. Benzene is resistant to addition (for example with bromine water) because of its delocalised ring, but it does react by electrophilic substitution when a strong electrophile and a catalyst are used.