Friedel-Crafts alkylation
| Friedel-Crafts reaction | |
|---|---|
| Named after |
Charles Friedel James Crafts |
| Reaction type | Coupling reaction |
| Identifiers | |
| RSC ontology ID | RXNO:0000369 |
| Friedel-Crafts acylation | |
|---|---|
| Named after |
Charles Friedel James Crafts |
| Reaction type | Coupling reaction |
| Identifiers | |
| Organic Chemistry Portal | friedel-crafts-acylation |
| RSC ontology ID | RXNO:0000045 |
The Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877 to attach substituents to an aromatic ring. There are two main types of Friedel–Crafts reactions: alkylation reactions and acylation reactions. Both proceed by electrophilic aromatic substitution. The general reaction scheme is shown below.
Several reviews have been written.
Template:Reaction-box Friedel–Crafts alkylation involves the alkylation of an aromatic ring with an alkyl halide using a strong Lewis acid catalyst. With anhydrous ferric chloride as a catalyst, the alkyl group attaches at the former site of the chloride ion. The general mechanism is shown below.
This reaction has one big disadvantage, namely that the product is more nucleophilic than the reactant due to the electron donating alkyl-chain. Therefore, another hydrogen atom is substituted with an alkyl-chain, which leads to overalkylation of the molecule. Also, if the chloride ion is not attached to a tertiary carbon atom or secondary carbon atom, then the carbocation formed (R+) will undergo a carbocation rearrangement reaction. This reactivity is due to the relative stability of the tertiary and secondary carbocation over the primary carbocations.
Steric hindrance can be exploited to limit the number of alkylations, as in the t-butylation of 1,4-dimethoxybenzene.
Alkylations are not limited to alkyl halides: Friedel–Crafts reactions are possible with any carbocationic intermediate such as those derived from alkenes and a protic acid, Lewis acid, enones, and epoxides. An example is the synthesis of neophyl chloride from benzene and methallyl chloride:
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