What are Friedel Crafts Reactions?
Friedel Crafts reaction is essentially an example of an electrophilic substitution reaction, in which an alkyl or acyl group replaces the hydrogen atom of an aromatic compound to form a hydrocarbon or a ketone. The alkylation or acylation of the aromatic compound takes place in the presence of an acid catalyst, such as AlCl3, BF3, ZnCl2, FeCl3 etc. The catalyst has the function of generating the attacking particle, an alkyl or an acyl cation. This reaction was discovered by C. Friedel and J. Crafts in 1877-78 who developed this reaction to attach substituents to an aromatic ring.
It should be observed that in both the processes of alkylation and acylation, the hydrogen atom which is initially attached to the aromatic ring gets replaced with an electrophile. The most commonly used catalyst is aluminium trichloride because it performs the function of a Lewis acid by coordinating with the halogen to produce an effective electrophile.
A representation of Friedel Crafts alkylation and acylation of benzene is shown below.
Friedel Crafts Alkylation
In Friedel Crafts alkylation, an alkyl halide is treated with a Lewis acid in the presence of an aromatic ring. The alkyl group attaches itself to the ring forming, a C-C bond, while the C-H Bond gets cleaved. Only an alkyl halide (chloride, bromide, iodide), and not alkenyl or alkynyl and aryl halides, should be used, as the reaction will otherwise fail. The carbocations of these species are highly unstable, and also difficult to produce.
This process is mainly used in industry to synthesize high octane fuels, surfactants, perfumes, antioxidants, etc., and other significant products such as cumene and thymol.
Generic Example:
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Specific Example:
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Mechanism of Friedel Crafts Alkylation
Step 1
The alkyl halide will react with the Lewis acid, to form a carbocation, which is a more electrophilic C.
Step 2
The resultant carbocation tries to attack the aromatic ring, to form cyclohexadienyl cation, which is intermediate. At this stage, the aromatic property of the ring is lost, due to the cleavage of the carbon-carbon double bond.
Step 3
The intermediate cation undergoes deprotonation, after which the carbon-carbon double bond is restored, and the compound again becomes aromatic. The released proton forms HCl and regenerates the AlCl3 catalyst.
Friedel Crafts Acylation
This electrophilic aromatic substitution reaction takes place between arenes and acyl halide or anhydrides leading to the synthesis of monoacylated products, in the presence of a Lewis acid catalyst such as AlCl3. The halogen present in the acyl halide forms a complex with the Lewis acid to produce a highly electrophilic acylium ion (RCO+), which gets stable due to resonance. This reaction can be used only to produce ketones.
The Friedel Crafts acylation is an important process for industry. It is used for the preparation of chemical feedstock, synthetic intermediates, and fine chemicals.
Generic Example:
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Specific Example:
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Mechanism of Friedel Crafts Acylation
This reaction follows a four-step mechanism.
Step 1
A reaction takes place between the acyl halide and the Lewis acid catalyst, leading to the formation of a complex, and loss of halide iron by the acyl halide. An acylium ion is formed, which gets stable due to resonance.
Step 2
The acylium ion launches an electrophilic attack on the ring. As a result of this, the aromatic property of the ring is lost during the formation of a complex.
Step 3
At this stage, deprotonation of the intermediate complex takes place, and the ring regains its aromaticity. The released proton combines with a chloride ion to form HCl, and the catalyst AlCl3 is formed in the process.
Step 4
The regenerated catalyst tries to attack the oxygen of the carbonyl group. As a result, a ketone product is released by adding water to the products of step 3.
As we know that the acylium ion is stabilized by resonance, no rearrangement occurs. Also, because the product is deactivated, it is no longer susceptible to electrophilic attack, it, therefore, does not undergo any further reaction.
Hence, the Friedel Crafts acylation reaction yields the required acyl benzene.
FAQs on Friedel Crafts Alkylation and Acylation
Question 1: What is the Role of Lewis Acid in Friedel Crafts Reaction?
Solution: For an electrophilic aromatic substitution reaction to take place, the Lewis acid, which acts as the catalyst, activates the electrophile. The Lewis acid coordinates with the electrophile to make it a better electrophile.
For example, in the case of isopropyl chloride, AlCl3 coordinates with the chlorine atom. This weakens the carbon-chlorine bond so that the chlorine can easily depart to form a secondary carbocation, which is a better electrophile than isopropyl chloride.
After the completion of the reaction, the Lewis acid is regenerated again. Hence, it only increases the rate of the reaction but is not consumed by it.
This reaction does not take place with alkenyl or alkynyl halides because the carbocations of these species are highly unstable, and also very difficult to generate.
Question 2: What are the Limitations or Drawbacks of the Friedel Crafts Reaction?
Solution: Some drawbacks of Friedel Crafts alkylation are:
If a deactivating group is present on the aromatic ring, the catalyst may get deactivated because of the formation of complexes.
The carbocations formed by aryl and vinyl halides cannot be used in this reaction as they are highly unstable.
The aromatic compound to be used must be taken in excess so that polyalkylation (crowding of the aromatic compound by multiple alkyl groups) does not take place.
Aromatic compounds which are less reactive as compared to mono-halobenzenes are not fit for participation in the Friedel Crafts alkylation.
Some drawbacks of Friedel Crafts acylation are as follows:
The Friedel Crafts acylation only leads to the formation of ketones. This happens because the formyl chloride decomposes to form CO and HCl.
Aromatic compounds which are less reactive as compared to mono-halobenzenes are not fit for participation in the Friedel Crafts acylation.
Because of the formation of unreactive complexes with the Lewis acid, arylamines cannot be used in this reaction.
