What is Friedel Crafts Reaction Definition Mechanism Types Catalysts Examples and Differences
The Friedel-Crafts reaction is an aromatic chemical reaction that undergoes an electrophilic aromatic substitution.
Two scientists, French Charles Friedel and American James Crafts invented this well-known reaction. The aromatic compound undergoes an electrophilic substitution in the Friedel-Crafts reaction.
In the presence of a Lewis acid, such as anhydrous aluminium chloride, the hydrogen atom in benzene is swapped with an electrophile.
The two forms of Friedel-Crafts Reactions are-
Friedel-Crafts Alkylation
Friedel-Crafts Acylation
Both types of Acetyl Chloride Benzene ions, Friedel craft alkylation and Friedel craft acylation involve electrophilic aromatic substitution.
Steps and Limitations
Friedel-Crafts Alkylation
Friedel-Crafts Alkylation is a chemical reaction in which an aromatic compound's proton is substituted with an alkyl group. In the presence of anhydrous aluminium chloride, this reaction takes place. Other Lewis acids, such as Ferric chloride, can be used in place of anhydrous aluminium chloride.
Friedel craft alkylation reaction can be represented in short form as follows-
Aromatic ring + Alkyl halide Lewis acid→ Alkyl aromatic compound
(Alkylbenzene)
Friedel crafts alkylation of Benzene - On treating benzene with an alkyl halide, in presence of Lewis acid such as anhydrous aluminium chloride, it forms alkylbenzene. This reaction is known as the Friedel craft alkylation reaction.
Steps:
The steps below illustrate the mechanism of the Friedel-Crafts Alkylation process.
The formation of an electrophilic carbocation is the first step.
The alkyl halide reacts with the Lewis acid used for the process, which is either anhydrous aluminium chloride or ferric chloride. As a result, an electrophilic carbocation is produced.
Intermediate cation formation
The aromatic ring is attacked by the electrophilic carbocation generated by the interaction of Lewis acid and alkyl halide. When it hits the aromatic ring, it forms a cyclohexadienyl cation intermediate. Because the carbon-carbon double bond breaks, the aromatic ring loses its scent.
Alkyl halide formation
Deprotonation occurs when the cyclohexadienyl cation loses one proton. The aromatic ring's carbon-carbon double bond is reformed, restoring the ring's aromaticity as well. The aluminium chloride catalyst is regenerated by the proton released during deprotonation.
Limitations
Aryl and vinyl halides can’t be used in this reaction as their carbocations are very reactive and highly unstable.
Deactivating groups in aromatic rings may not be suitable for Friedel-Crafts alkylation. Because the deactivating group can create a compound with the Lewis acid, inactivating it. For example, aniline's amine group deactivates anhydrous aluminium chloride. It's a half-reaction
Polyalkylation occurs commonly in alkyl halide and aromatic chemical reactions. To avoid this, take the aromatic sample in huge quantities.
Since mono halobenzenes are least reactive, they do not respond or participate in Friedel-Crafts alkylation.
Reaction doesn’t take place if benzene has a substituent group that is more deactivating than halogens.
As alkyl benzene is more reactive than benzene, so polyalkylation takes place.
Friedel-Crafts Acylation
The acylation reaction of Friedel-Crafts is analogous to the alkylation reaction. The only difference is that, unlike the alkylation reaction, the Friedel-Crafts acylation reaction produces a ketone.
Friedel craft acylation reaction can be represented in short form as follows-
Aromatic ring + RCOX Lewis acid → Acyl aromatic compound
Various aromatic compounds can be acylated using the Friedel-Crafts procedure. When the reactant is an alcohol or an amine, the oxygen and nitrogen atoms are acylated.
Friedel crafts acylation of Benzene - On treating benzene with an acyl halide, in presence of Lewis acid, it forms acyl benzene. This is known as Friedel craft’s acylation reaction.
Steps:
The steps below illustrate the mechanism of the Friedel-Crafts Acylation process:
The formation of the acylium ion is the first step.
An acylium ion is formed when anhydrous aluminium chloride combines with an acyl halide. Resonance stabilises the acylium ion that results.
Intermediate cation formation
The aromatic ring is attacked by the acylium ion generated by the interaction of Lewis acid and acyl halide. An intermediate is generated when it attacks the aromatic ring. Because the carbon-carbon double bond breaks, the aromatic ring loses its scent.
Intermediate complex deprotonation
Deprotonation occurs in the intermediate complex, which means it loses one proton. The aromatic ring's carbon-carbon double bond is reformed, restoring the ring's aromaticity as well. The aluminium chloride catalyst is regenerated by the proton released during deprotonation.
The ketone molecule is released.
The carbonyl oxygen is attacked by anhydrous aluminium chloride that has been regenerated by proton addition. The ketone product is produced and released in the presence of excess water.
Limitations
The Friedel-Crafts process produces exclusively ketone compounds. Formyl chloride breaks into HCl and CO2 under certain conditions (CO).
Since mono halobenzenes are the least reactive, they do not respond or participate in Friedel-Crafts acylation reaction.
Aryl amines form highly unreactive complexes with lewis acid catalysts so we can’t use them in this reaction.
The aromatic compound, which is less reactive than mono halobenzene, cannot be used in this reaction.
Acylation reactions generally form only ketones.
FAQs on Friedel Crafts Reaction Mechanism of Alkylation and Acylation
1. What is Friedel–Crafts alkylation?
Friedel–Crafts alkylation is an electrophilic aromatic substitution reaction that introduces an alkyl group onto a benzene ring using an alkyl halide and a Lewis acid catalyst. In this reaction, benzene reacts with an alkyl halide (R–Cl) in the presence of AlCl3 to form an alkylbenzene.
General reaction:
C6H6 + R–Cl → C6H5R + HCl (in presence of AlCl3)
It proceeds via formation of a carbocation electrophile and is widely used in organic chemistry for carbon–carbon bond formation.
2. What is Friedel–Crafts acylation?
Friedel–Crafts acylation is an electrophilic aromatic substitution reaction that introduces an acyl group (–COR) onto an aromatic ring using an acyl chloride and a Lewis acid catalyst. Benzene reacts with an acyl chloride (RCOCl) in the presence of AlCl3 to form a ketone.
General reaction:
C6H6 + RCOCl → C6H5COR + HCl (in presence of AlCl3)
The reaction proceeds via formation of an acylium ion (RCO+) and typically gives only one substitution product.
3. What is the difference between Friedel–Crafts alkylation and acylation?
The main difference between Friedel–Crafts alkylation and acylation is that alkylation adds an alkyl group (–R), while acylation adds an acyl group (–COR) to an aromatic ring.
- Reagent used: Alkyl halide (R–Cl) in alkylation; acyl chloride (RCOCl) in acylation.
- Electrophile formed: Carbocation (R+) in alkylation; acylium ion (RCO+) in acylation.
- Rearrangement: Possible in alkylation; not observed in acylation.
- Polyalkylation: Common in alkylation; usually absent in acylation.
- Product: Alkylbenzene vs aromatic ketone.
4. What is the mechanism of Friedel–Crafts alkylation?
The mechanism of Friedel–Crafts alkylation involves formation of a carbocation followed by electrophilic substitution on the benzene ring.
Steps:
- 1. Electrophile formation: R–Cl + AlCl3 → R+ + AlCl4-
- 2. Sigma complex formation: Benzene attacks R+ to form a resonance-stabilized carbocation intermediate.
- 3. Deprotonation: Loss of H+ restores aromaticity, forming alkylbenzene and regenerating AlCl3.
5. What is the mechanism of Friedel–Crafts acylation?
The mechanism of Friedel–Crafts acylation involves formation of an acylium ion followed by electrophilic substitution on the aromatic ring.
Steps:
- 1. Acylium ion formation: RCOCl + AlCl3 → RCO+ + AlCl4-
- 2. Attack on benzene: Benzene forms a sigma complex with RCO+.
- 3. Deprotonation: Loss of H+ restores aromaticity, yielding an aromatic ketone.
6. Why does Friedel–Crafts alkylation lead to polyalkylation?
Friedel–Crafts alkylation leads to polyalkylation because the introduced alkyl group activates the benzene ring toward further substitution. Alkyl groups are electron-donating and increase electron density on the ring through the +I effect.
- The first alkyl group makes the ring more reactive than benzene.
- The activated ring reacts again with more carbocation electrophile.
- This produces di- or tri-alkylated products.
7. Why is carbocation rearrangement common in Friedel–Crafts alkylation?
Carbocation rearrangement is common in Friedel–Crafts alkylation because unstable carbocations rearrange to form more stable ones before reacting with benzene. Primary carbocations can undergo hydride or alkyl shifts to form secondary or tertiary carbocations.
- Example: A primary carbocation may rearrange to a more stable tertiary carbocation.
- This leads to unexpected alkylbenzene products.
8. What are the limitations of Friedel–Crafts reactions?
The main limitations of Friedel–Crafts reactions are that they do not work well with strongly deactivated aromatic rings or certain functional groups.
- Rings with strong electron-withdrawing groups (e.g., –NO2, –SO3H) do not undergo the reaction.
- Amines (–NH2) form complexes with AlCl3, preventing reaction.
- Alkylation may cause polyalkylation and rearrangement.
- Vinyl and aryl halides do not undergo Friedel–Crafts alkylation.
9. What catalyst is used in Friedel–Crafts alkylation and acylation?
The most common catalyst used in Friedel–Crafts alkylation and acylation is the Lewis acid AlCl3. Aluminum chloride accepts an electron pair from the halide to generate a strong electrophile.
- Other Lewis acids: FeCl3, BF3, and AlBr3.
- The catalyst helps form R+ (alkylation) or RCO+ (acylation).
- It is regenerated at the end of the reaction.
10. Can you give an example of Friedel–Crafts alkylation and acylation reactions?
An example of Friedel–Crafts alkylation is the reaction of benzene with methyl chloride, while an example of acylation is the reaction of benzene with acetyl chloride.
Alkylation example:
C6H6 + CH3Cl → C6H5CH3 + HCl (AlCl3)
Product: Toluene
Acylation example:
C6H6 + CH3COCl → C6H5COCH3 + HCl (AlCl3)
Product: Acetophenone
Both reactions are classic examples of Friedel–Crafts reactions in aromatic chemistry.
