Friedel–Crafts alkylation: practice and mechanism
AlCl₃ turns CH₃Cl into an electrophile and the methyl group goes onto the ring. Activating groups direct ortho/para; on deactivated rings (e.g. nitrobenzene) Friedel–Crafts fails.
Example
AlCl₃ turns the haloalkane into a carbocation (or a strongly polarised complex) that attacks the aromatic ring. The alkyl group activates the ring, so over-alkylation is a risk; strongly deactivated rings (e.g. nitrobenzene) do not react at all.
Mechanism step by step
- 1
Forming the electrophile: AlCl₃ pulls the Cl off CH₃Cl, giving a strongly polarised CH₃–Cl–AlCl₃ complex or a carbocation.
- 2
The ring π electrons attack the electrophile. A carbocation forms (σ-complex, arenium ion) with its charge spread over three carbons; aromaticity is lost for a moment.
- 3
A base removes the H from the attacked carbon. The ring becomes aromatic again and the electrophile has replaced an H.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- The existing substituent is ortho/para-directing, so the methyl group goes ortho or para, not meta.
Often confused with: Friedel–Crafts acylation
Frequently asked questions
What are the drawbacks of alkylation?
The product is more reactive than the starting material (alkyl activates), so multiple alkylation happens easily. Longer alkyl groups rearrange as carbocations.
How is this avoided?
With Friedel–Crafts acylation followed by reduction (Clemmensen or Wolff–Kishner). The acyl group deactivates and does not rearrange.