1. Reaction
Reaction
Key Details Reagents: Halogenoalkane (e.g.
) and anhydrous . Conditions: Heat under reflux. Electrophile: Carbocation ( ). Products: alkylbenzene and hydrogen halide. Polyalkylation
- The first alkyl group makes the benzene ring more reactive than it was before.
- The reaction often does not stop at one alkyl group; It keeps going until multiple alkyl groups are attached to the ring.
2. Mechanism:
Alkylation: Electrophilic Substitution
- The carbocation is attracted to the delocalised
bond of benzene. - It eventually breaks the ring of electrons and forms a dative
bond to one of the carbon atoms of the ring. - The remaining four
electrons are spread over the remaining five carbon atoms of the ring, in a five-centre delocalised orbital, forming an intermediate carbocation. - The carbocation loses a proton, the electrons of the former
bond reform the sextet of electrons.
3. Formation of Electrophile
Formation of alkyl carbocation
- The aluminium atom in anhydrous aluminium chloride is electron-deficient, making it a Lewis Acid.
- The aluminium atom accepts a lone pair of electron from one of the chlorine atoms to form a dative bond.
- This highly polarises the
bond, weakening it. - Eventually, the
bond breaks by heterolytic fission. - The products are the strong electrophile
and
4. Regeneration of Catalyst
Regeneration of Catalyst
- The
formed by the mechanism reacts with the formed during the formation of the electrophile. - The halogen carrier catalyst (
) is regenerated.