Friedel-Crafts alkylation
Benzene reacts with an alkyl halide in the presence of anhydrous AlCl3 to give an alkylbenzene.
BenzeneC₆H₆Arene - Step 1
CH₃Cl, anhydrous AlCl₃
TolueneC₆H₅CH₃Arene
How it works
- AlCl₃ polarises the alkyl halide to give a carbocation (R⁺).
- The carbocation attacks the benzene ring (electrophilic substitution).
- Loss of H⁺ restores aromaticity and regenerates AlCl₃.
What it makes
Toluene, ethylbenzene and cumene.
Where it fails
Carbocations rearrange (n-propyl chloride gives cumene). Polyalkylation occurs. It fails on rings carrying strongly deactivating groups.
Class 11, Chapter 9 Hydrocarbons
Friedel-Crafts alkylation: quick answers
Why does n-propyl chloride give cumene?
The primary carbocation rearranges to the more stable secondary isopropyl cation before attacking the ring.
Why is polyalkylation a problem?
The alkyl group activates the ring, so the product reacts faster than benzene.
Which catalyst is used?
Anhydrous aluminium chloride.