A2-Level Benzene, Aromatic Chemistry
The bromination of benzene involves the electrophilic substitution of a bromine onto a benzene ring:
A halogen carrier is needed (usually AlBr ) to create an electrophile, as the delocalised electrons in the benzene are unable to polarize the bromine molecule sufficiently (unlike with bromine and alkenes).
Bromination of Arenes
When bromine is reacted with an alkene, electrophilic addition occurs. The high electron density in the double carbon bond of the alkene polarises the bromine molecule, enabling an electrophile (Br ) to be formed that can react with the double bond.
When bromine is reacted with a benzene ring, no such substitution occurs without ‘extra help’. This is because benzene is unable to polarise the bromine molecule sufficiently to produce the Br electrophile.
As a result, a halogen carrier is needed. A halogen carrier (usually AlBr ) polarises the bond in the bromine molecule sufficiently to allow an electrophilic substitution reaction to occur.
Why is a halogen carrier needed and how is this different to the halogenation of an alkene?
In an alkene, the electrons within the double carbon bond are located close together – between the two carbon atoms. This means there is a high electron density between the two carbon atoms, this density can polarise the halogen molecule enough to create a halogen electrophile.
In benzene however, the electrons that make up the pi-system are not as close together – each carbon atom in benzene has a ‘half share’ of a pi-bond.
Although there are more pi-bonded electrons in benzene compared to an alkene, they are more spread out. This means when a halogen molecule comes near benzene, it is not polarised as much as if it comes near an alkene. The polarisation is required to create the electrophile.
How does a halogen carrier work?
A halogen carrier is usually made by the reaction of iron or aluminium with the same halogen that is to be added to the benzene ring.
The reaction produces a metal tri-halide (iron tri-bromide (FeBr ) or aluminium tri-chloride (AlCl )).
When these species come near to a halogen molecule, the positive charge of the metal ion pulls electrons from the halogen molecule towards it. This causes a large polarisation of the bond within the halogen molecule.
The halogen molecule is now already polarised, so the benzene ring can provide just enough extra polarisation to force the molecule to break and form an electrophile.