Friedel Crafts Reaction and its Types
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
1. What is the Friedel-Crafts reaction and what are its main types as per the CBSE syllabus for 2025-26?
The Friedel-Crafts reaction is a fundamental method in organic chemistry for attaching substituents to an aromatic ring. It is a type of electrophilic aromatic substitution where a hydrogen atom on the aromatic ring is replaced by an electrophile. The reaction is typically catalysed by a strong Lewis acid, such as anhydrous aluminium chloride (AlCl₃). There are two primary types of this reaction:
- Friedel-Crafts Alkylation: This involves the substitution of an aromatic proton with an alkyl group.
- Friedel-Crafts Acylation: This involves the substitution of an aromatic proton with an acyl group (R-C=O).
2. What is the specific role of a Lewis acid, like anhydrous AlCl₃, in a Friedel-Crafts reaction?
The role of a Lewis acid like anhydrous aluminium chloride (AlCl₃) is to act as a catalyst by generating the necessary electrophile. In alkylation, it reacts with an alkyl halide to form a carbocation (R⁺). In acylation, it reacts with an acyl halide to form a resonance-stabilised acylium ion ([R-C=O]⁺). This newly formed, highly reactive electrophile is then able to attack the electron-rich benzene ring, which would not happen without the catalyst's involvement.
3. Can you provide a simple example of the Friedel-Crafts alkylation of benzene?
A classic example of Friedel-Crafts alkylation is the reaction of benzene with chloromethane (CH₃Cl) in the presence of anhydrous aluminium chloride as a catalyst. The methyl group from chloromethane substitutes a hydrogen atom on the benzene ring to form toluene (methylbenzene). The overall reaction is: C₆H₆ + CH₃Cl --(Anhydrous AlCl₃)--> C₆H₅CH₃ + HCl.
4. What is the difference between Friedel-Crafts Alkylation and Acylation?
The primary difference lies in the type of group attached to the aromatic ring and the resulting product. Key distinctions include:
- Group Attached: Alkylation attaches an alkyl group (-R), while acylation attaches an acyl group (-COR).
- Product Formed: Alkylation produces an alkylbenzene. Acylation produces a ketone.
- Reactivity of Product: The alkyl group is activating, making the product more reactive than the starting material (leading to polyalkylation). The acyl group is deactivating, preventing further reactions on the product.
- Rearrangement: The carbocation electrophile in alkylation can undergo rearrangement, leading to a mixture of products. The acylium ion in acylation does not rearrange, yielding a single product.
5. Why is Friedel-Crafts acylation often preferred over alkylation for preparing alkylbenzenes?
Friedel-Crafts acylation is often preferred for two main reasons, which overcome major limitations of the alkylation reaction:
- No Carbocation Rearrangement: The electrophile in acylation, the acylium ion, is resonance-stabilised and does not rearrange. This ensures that a single, predictable ketone product is formed. In contrast, the carbocation in alkylation can rearrange to a more stable form, leading to a mixture of isomeric products.
- Prevention of Polyalkylation: The acyl group attached during acylation is an electron-withdrawing group, which deactivates the aromatic ring. This makes the ketone product less reactive than the original benzene ring, preventing it from undergoing further acylation. The resulting ketone can then be reduced to the desired alkylbenzene (e.g., via Clemmensen reduction).
6. Why do compounds like nitrobenzene or aniline fail to undergo the Friedel-Crafts reaction?
These compounds fail to react due to the nature of their substituent groups.
- Nitrobenzene: The nitro group (-NO₂) is a very strong electron-withdrawing or deactivating group. It pulls electron density out of the benzene ring, making the ring too electron-poor to be attacked by the electrophile.
- Aniline: The amino group (-NH₂) is a Lewis base. It reacts directly with the Lewis acid catalyst (AlCl₃) and forms a salt. This complex places a positive charge on the nitrogen atom, which strongly deactivates the ring and prevents the reaction from proceeding.
7. What is the problem of polyalkylation in the Friedel-Crafts reaction and how can it be minimised?
Polyalkylation is a common side reaction in Friedel-Crafts alkylation. It occurs because the alkyl group introduced onto the benzene ring is an electron-donating (activating) group. This makes the product, an alkylbenzene, more reactive than the original benzene. Consequently, the product can undergo further alkylation, leading to a mixture of di- and tri-substituted products. This problem can be minimised by using a large excess of the aromatic compound (e.g., benzene) in the reaction mixture, which increases the probability of the electrophile reacting with a starting material molecule rather than an already alkylated product molecule.
8. What is the step-by-step mechanism for the Friedel-Crafts acylation of benzene?
The mechanism for Friedel-Crafts acylation involves three main steps:
- Formation of the Electrophile: The acyl halide (e.g., acetyl chloride, CH₃COCl) reacts with the Lewis acid catalyst (AlCl₃) to form a resonance-stabilised acylium ion electrophile.
- Electrophilic Attack: The positively charged acylium ion attacks the electron-rich benzene ring, breaking one of the double bonds to form a resonance-stabilised carbocation intermediate known as an arenium ion or sigma complex. The ring temporarily loses its aromaticity.
- Deprotonation: A base, typically the [AlCl₄]⁻ complex, removes a proton (H⁺) from the carbon atom that the acyl group is attached to. This restores the aromaticity of the ring, forms the final ketone product, and regenerates the AlCl₃ catalyst.
