Friedel-Crafts Reaction: Alkylation & Acylation
The Friedel-Crafts Reaction is a set of electrophilic aromatic substitution reactions used to attach alkyl or acyl groups to benzene rings. It utilizes a Lewis acid catalyst (like anhydrous $AlCl_3$) to generate a strong electrophile.
1. Friedel-Crafts Alkylation
This reaction introduces an alkyl group ($-R$) to the benzene ring. Benzene reacts with an alkyl halide in the presence of anhydrous Aluminium Chloride.
Mechanism
- Generation of Electrophile: The Lewis acid ($AlCl_3$) abstracts the halogen to form a carbocation ($R^+$). $$ R-Cl + AlCl_3 \rightarrow R^+ + [AlCl_4]^- $$
- Attack: The benzene ring attacks the carbocation.
- Proton Loss: Aromaticity is restored by removing a proton.
Crucial Point: Rearrangement
Since the intermediate is a Carbocation, it can undergo rearrangement to a more stable form (e.g., $1^\circ \to 2^\circ$ or $3^\circ$).
Example: Reaction of Benzene with n-Propyl Chloride yields Isopropylbenzene (Cumene) as the major product, not n-propylbenzene.
2. Friedel-Crafts Acylation
This reaction introduces an acyl group ($-COR$) to the ring, forming a ketone. It uses an acyl halide ($RCOCl$) or acid anhydride.
Mechanism & Advantages
The electrophile here is the resonance-stabilized Acylium Ion ($R-C^+=O \leftrightarrow R-C \equiv O^+$).
- No Rearrangement: The acylium ion is stable and does not rearrange. The n-propyl group remains n-propyl.
- No Polyacylation: The product (Ketone) contains a deactivating carbonyl group, which prevents further reaction.
3. Limitations of Friedel-Crafts
These reactions fail under specific conditions, which is a favorite topic for examiners.
- Deactivated Rings: Reaction fails with strong electron-withdrawing groups (e.g., $-NO_2, -CN, -COOH$). The ring is too electron-deficient to attack the electrophile.
- Aniline ($C_6H_5NH_2$): Fails because the Lewis acid ($AlCl_3$) reacts with the lone pair on nitrogen to form a salt, which strongly deactivates the ring.
- Polyalkylation: In alkylation, the product (Alkylbenzene) is more reactive than benzene, leading to multiple alkyl groups attaching.
4. Comparison
| Feature | Alkylation | Acylation |
|---|---|---|
| Reagent | $R-X$ (Alkyl Halide) | $R-COX$ (Acyl Halide) |
| Electrophile | Carbocation ($R^+$) | Acylium Ion ($RCO^+$) |
| Rearrangement | Possible | Not Possible |
| Poly-substitution | Common problem | Does not occur |
Friedel-Crafts Quiz
Test your concepts on Alkylation & Acylation. 10 MCQs with explanations.
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