Friedel-Crafts Reaction
Definition and meaning of Friedel-Crafts Reaction in chemistry.
The Friedel-Crafts reaction attaches a carbon group to an aromatic ring using a Lewis acid catalyst such as aluminum chloride (AlCl3). It comes in two forms: alkylation, which adds an alkyl group, and acylation, which adds an acyl group to make a ketone.
In more detail
Both versions are examples of electrophilic aromatic substitution, in which an electron-rich benzene ring attacks a strong electrophile and swaps out one of its hydrogen atoms. The Lewis acid catalyst is essential because it helps create that electrophile. In alkylation, AlCl3 pulls a halide off an alkyl halide to make a carbocation.
In acylation, it does the same to an acyl halide to make an acylium ion. In Friedel-Crafts alkylation, the ring attacks the carbocation and, after losing a proton, ends up with a new alkyl group. This reaction has two well-known drawbacks.
First, the carbocation can rearrange to a more stable form, so the group that ends up on the ring may not match the starting halide. Second, adding an alkyl group makes the ring more reactive, so the product can react again and give multiple substitutions.
Friedel-Crafts acylation avoids both problems. The acylium ion is stabilized by resonance and does not rearrange, so the product is clean and predictable. Because the new acyl group is electron-withdrawing, it deactivates the ring toward further reaction, which stops multiple additions.
The immediate product is an aryl ketone. Both reactions have limits. They do not work well on rings that already carry strong electron-withdrawing groups, such as nitro groups, because those rings are too electron-poor to attack the electrophile.
Chemists often use acylation followed by a reduction of the ketone as a two-step way to place a straight-chain alkyl group without the rearrangement that plagues direct alkylation.
Key facts
| Field | Organic Chemistry |
|---|---|
| Reaction class | electrophilic aromatic substitution |
| Two types | alkylation and acylation |
| Catalyst | Lewis acid (often AlCl3) |
| Alkylation electrophile | carbocation |
| Acylation electrophile | acylium ion |
| Alkylation drawback | rearrangement and over-reaction |
| Named for | Charles Friedel and James Crafts |
Reacting benzene with acetyl chloride (CH3COCl) and aluminum chloride produces acetophenone, an aryl ketone. The AlCl3 generates the acylium ion CH3CO<sup>+</sup>, which the benzene ring attacks before losing a proton.
Frequently asked questions
Why does acylation give cleaner products than alkylation?
The acylium ion does not rearrange, and the acyl group it adds deactivates the ring. This prevents both carbocation rearrangement and multiple substitutions, which are common problems in alkylation.
Why is a Lewis acid catalyst needed?
The Lewis acid removes a halide from the alkyl or acyl halide to generate the strong electrophile (a carbocation or acylium ion) that the aromatic ring can attack.
Can Friedel-Crafts reactions be done on any benzene ring?
No. Rings carrying strong electron-withdrawing groups, such as nitrobenzene, are too electron-poor to react. The ring generally must be at least as reactive as benzene itself.
How can you add a straight-chain alkyl group without rearrangement?
Use Friedel-Crafts acylation to install a ketone, then reduce the C=O group to a CH2. This two-step route avoids the carbocation rearrangement of direct alkylation.