Friedel–Crafts alkylation is a chemical reaction in which an alkyl group (a chain of carbon and hydrogen atoms) is added to an aromatic ring like benzene. It is a type of electrophilic aromatic substitution, meaning the aromatic ring donates electrons to form a bond with an electron-loving species called an electrophile.
The reaction requires a Lewis acid catalyst, usually aluminum chloride (AlCl₃), which helps generate a highly reactive species called a carbocation from the alkyl halide. This carbocation is the actual electrophile that attacks the aromatic ring.
Once the carbocation attaches to the ring, the aromatic system temporarily loses its stability. Then, a proton is removed to restore the ring’s aromaticity, giving the alkylated aromatic compound as the final product.
Key points to remember:
- The reaction introduces an alkyl group onto an aromatic ring.
- A Lewis acid catalyst is necessary to generate the reactive species.
- Carbocations can rearrange, so the final product might differ from the original alkyl halide.
- Multiple alkylations can occur because the product is often more reactive than the starting aromatic compound.
- Deactivated aromatic rings (with electron-withdrawing groups) do not undergo this reaction easily.