Friedel-Crafts alkylation
The Friedel-Crafts alkylation is a name reaction in organic chemistry. It is used to add alkyl groups to aromatic rings using a Lewis acid like aluminium chloride as a catalyst.
Materials
The Friedel-Crafts alkylation starts with an aromatic compound like benzene (C
6H
6) and an alkyl halide like ethyl chloride (CH
3CH
2Cl). A small amount of a Lewis acid, usually aluminium chloride, is added to be a catalyst.
Alkenes can be used in place of alkyl halides by using a Brønsted acid as the catalyst. This version of the reaction is common in the chemical industry.
Mechanism
The Friedel-Crafts alkylation is a type of electrophilic aromatic substitution with two carbocation intermediates.[1] First, the alkyl halide reacts with the catalyst, making a carbocation and an anionic Lewis adduct:
- CH
3CH
2Cl + AlCl
3 → CH
3CH+
2 + AlCl−
4
If starting from an alkene and Brønsted acid, the carbocation is instead made from protonation of the alkene:
- H
2C=CH
2 + H
2SO
4 → CH
3CH+
2 + HSO−
4
The carbocation is a strong electrophile, so it is attracted to the many electrons in the aromatic ring. The carbocation reacts with the ring, making a new covalent bond between carbon atoms to form a larger arenium intermediate,
- CH
3CH+
2 + C
6H
6 → C
6H
6(CH
2CH
3)+
Finally, the adduct abstracts a hydrogen ion from the larger carbocation, which regenerates the catalyst and releases the hydrogen halide byproduct:
- C
6H
6(CH
2CH
3)+
+ AlCl−
4 → C
6H
5(CH
2CH
3) + AlCl
3 + HCl
The net reaction is
- C
6H
6 + CH
3CH
2Cl → C
6H
5(CH
2CH
3) + HCl
Friedel-Crafts Alkylation Media
Friedel-Crafts Equation Overview
ethylation of benzene
Alkylation of benzene with propylene in cumene process
t-butylation of 1,4-dimethoxybenzene
Neophyl chloride synthesis
Structure of the Friedel-Crafts-Alkylation
1,3-Diisopropylbenzene is produced via transalkylation, a special form of Friedel–Crafts alkylation.
Friedel–Crafts acylation overview
mechanism of the Friedel-Crafts acylation reaction
Friedel–Crafts hydroxyalkylation
References
- ↑ John C. McMurray. "16.3 Alkylation and Acylation of Aromatic Rings: The Friedel–Crafts Reaction". Organic Chemistry, a Tenth Edition. Houston, TX: OpenStax.