Reactions of benzene

Benzene is electron rich.

Electrophiles are electron pair acceptors.

So electrophiles might be expected to add to benzene, saturating it.

Reaction with acidified potassium manganate (vii)

Alkenes reduce purple Manganese +7…

To colourless +2

But benzene is unable to react.

Reaction with bromine

Alkenes decolourise bromine in both the dark and the light.

But benzene will not react.

So benzene does not undergo electron addition reactions like alkenes.

This is because the ring of delocalised electrons is an extremely stable structure and so is conserved when benzene reacts.

This means that electrophiles react by substituting for one or more hydrogen atoms attatched to the ring.

Important electrophilic subsitution reactions include A) nitration, B) halogenation, C) alkylation and D) acylation.

Electrophilic Substitution

• Arenes are electron rich.

• So are attractive to electrophiles (electron pair acceptors).

• But the ring is very stable and must be conserved.

• So electrophiles substitute for one or more of the hydrogens, rather than adding to the molecule.

Eg; Halogenation

• Halogenation involves substituting a halogen for one of the hydrogens;

• C6H6 +[X] → C6H5 X + [H]• In order for a reaction to take place

there must also be a halogen carrier.

• Eg; Aluminium halides, Iron (iii) halides and iron filings.

Step 1; Formation of the electrophile

• The halogen carrier polarises the halogen molecule;

• Eg; δ+ Cl – Clδ- ------AlCl3

• The positive dipole formed on one halogen atom enables it to act as an electrophile.

Step 2; Attack on the ring.

• A chlorine atom accepts a pair of electrons from benzene.

• To do this the ring has to be broken.

• C6H6 + δ+Cl–Clδ---- AlCl3

↓

• (C6H6Cl)+ + AlCl4-

Step 3; Restoration of the ring

• The ring is restored by the loss of a proton.

• (C6H6Cl)+ → C6H5Cl + H+

Step 4; Regeneration of the halogen carrier.

• The proton reacts with the aluminium tetra chloride ion

• H+ + AlCl4- → HCl + AlCl3

• As the halogen carrier has been regenerated it has aced as a catalyst, which explains why they are referred to as Friedel-Crafts catalysts.

• Giving an overall reaction;

• C6H6 + Cl2 → C6H5Cl + HCl

Combustion

They give smoky flames, as the high C:H ratii means that unburnt carbon is present.

Arenes are flammable;C6H6 +7.5O2 → 6CO2 + 3H2O