Friedel–Crafts reaction

What is the Friedel–Crafts reaction, and what are the two types?

Friedel–Crafts reactions are electrophilic substitution reactions of benzene using a haloalkane or acyl chloride in the presence of a halogen carrier catalyst (usually AlCl₃). They are important because they form new carbon–carbon bonds to the aromatic ring, making them very useful in organic synthesis.

-Alkylation

-Acylation

What is Alkylation

benzene reacts with a haloalkane (e.g. CH₃Cl) and AlCl₃ to introduce an alkyl group onto the ring. The haloalkane/AlCl₃ generates a carbocation (R⁺) as the electrophile.

Example: C₆H₆ + CH₃Cl → C₆H₅CH₃ + HCl (methylbenzene formed).

What is Acylation

benzene reacts with an acyl chloride (e.g. CH₃COCl, ethanoyl chloride) and AlCl₃ to introduce an acyl group (RCO–) onto the ring, forming an aryl ketone.

Example: C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl (phenylethanone formed).

Why is Friedel–Crafts acylation more useful in synthesis than alkylation?

In alkylation, the alkyl group introduced onto the ring activates it, making it even more reactive than the starting benzene — multiple alkylations can occur, giving a mixture of products. In acylation, the acyl group actually deactivates the ring slightly, so only monosubstitution occurs, giving a pure product in better yield. Acylation is therefore a more controlled and synthetically useful reaction.