Reaction with electrophiles
The π electrons in benzene are distributed around the ring, making the ring electron-rich. This allows it to attract electrophiles: electron-deficient species that may carry a positive charge or contain a positively polarised atom.
Electrophilic attack on benzene normally results in substitution. One hydrogen attached to the ring is replaced by the incoming atom or group, so the reactions considered here are monosubstitutions.
The reactive electrophile is often produced within the reaction mixture from suitable reagents rather than being supplied directly in its reactive form.
Substitution rather than addition
Delocalisation makes the benzene ring particularly stable. The important difference between substitution and addition is therefore what happens to this stabilised π system by the end of the reaction.
| Substitution | Addition |
|---|---|
| A ring hydrogen is replaced by another atom or group. | New atoms or groups become added to the ring. |
| Delocalisation is interrupted during the mechanism. | The delocalised π system is broken. |
| The delocalised ring is re-formed in the product. | The final product no longer has the same aromatic stabilisation. |
For example, hydrogenation of benzene to cyclohexane is an addition reaction: the delocalised π system is lost in the product. Electrophilic substitution is favoured because the aromatic, delocalised ring is restored.
Exam Tip: When asked why benzene undergoes substitution in preference to addition, compare the final products. State that addition would remove the stabilised delocalised π system, whereas substitution allows that system to be re-formed.