Electrophilic Substitution
Chemistry: IB Diploma Higher Level, topic 6.5.1
Electrophilic Substitution of Benzene
Electrophilic substitution reactions can be used to introduce halogens or nitro (NO2) groups onto a benzene ring.
Nitration of benzene
- In nitration reactions of benzene, benzene reacts with a nitronium ion, NO2+, which is produced from the reaction of nitric acid with sulphuric acid.
- Sulphuric acid acts as an acid catalyst and the reaction requires heating to 50°C.
Chlorination of benzene
- The chlorination reaction requires a Lewis acid catalyst such as AlCl3 and a dry solvent, which is normally ether.
General mechanism of electrophilic substitution
- Electrophiles are attracted to the high electron density of the benzene ring.
- The attack of an electrophile on benzene breaks up the continuous cycle of delocalised electrons to form a positively charged intermediate.
- This intermediate is unstable as it loses aromaticity.
- A hydrogen ion (H+) is then lost from the non-aromatic intermediate to regain the complete delocalisation of electrons over the six carbons.
Nitration (mechanism)
- The electrophile is the nitronium ion (NO2+).
- The nitronium ion is formed by an acid-base reaction in which sulphuric acid acts as an acid and nitric acid, the weaker of the two acids, as a base:
- HNO3(aq) + 2H2SO4(aq) → NO2+(aq) + 2HSO4-(aq) + H3O+(aq)
Nitration (mechanism) cont.
- On the formation of nitronium (NO2+), which is the electrophile, the reaction mechanism is analogous to the general mechanism.
- The sulphuric acid is a catalyst as it is regenerated by protonation of HSO4- in the final step.
Test yourself on Electrophilic Substitution
Which of these is the electrophile in the nitration of benzene?