Haloarenes undergo the usual electrophilic reactions of the benzene ring such as halogenation, nitration, sulphonation and Friedel-Crafts reactions. Halogen atom besides being slightly deactivating is o, p-directing; therefore, further substitution occurs at ortho- and para-positions with respect to the halogen atom. The o, p-directing influence of halogen atom can be easily understood if we consider the resonating structures of halobenzene as shown:

Due to resonance, the electron density increases more at ortho- and para-positions than at meta-positions. Further, the halogen atom because of its –I effect has some tendency to withdraw electrons from the benzene ring. As a result, the ring gets somewhat deactivated as compared to benzene and hence the electrophilic substitution reactions in haloarenes occur slowly and require more drastic conditions as compared to those in benzene

Which catalyst is used for the Friedel-Crafts alkylation?

Both option 2 and option 3

The correct answer is option 4. Both option 2 and option 3.

Let us explain why both anhydrous \(AlCl_3\) and \(FeCl_3\) can be used as catalysts for Friedel-Crafts alkylation:

Friedel-Crafts alkylation is a chemical reaction that involves the introduction of an alkyl group into an aromatic ring. The reaction typically uses an alkyl halide and requires a catalyst to generate a reactive intermediate, usually a carbocation.

Role of Catalysts

Formation of Carbocation:

The catalyst facilitates the generation of a highly reactive carbocation from the alkyl halide. This carbocation then reacts with the aromatic ring to form the alkylated product.

Catalysts Used

Anhydrous \(AlCl_3\):

Properties: Anhydrous aluminum chloride \((AlCl_3)\) is a very strong Lewis acid. It effectively accepts electron pairs and is highly efficient in coordinating with the halogen of the alkyl halide.

Mechanism: \(AlCl_3\) reacts with the alkyl halide to form a complex. This complex helps in the cleavage of the \(C-X\) bond (where X is a halogen), generating a highly reactive carbocation.

Usage: It is the most commonly used catalyst for Friedel-Crafts alkylation because it provides the necessary conditions to generate and stabilize the carbocation intermediate.

\(FeCl_3\):

Properties: Ferric chloride \((FeCl_3)\) is also a Lewis acid, but it is generally weaker compared to \(AlCl_3\). However, it is still capable of forming a Lewis acid-base complex with the alkyl halide.

Mechanism: \(FeCl_3\) can also coordinate with the halogen atom of the alkyl halide, leading to the generation of a carbocation. This carbocation then reacts with the aromatic ring.

Usage: While not as effective as \(AlCl_3\) in generating carbocations, \(FeCl_3\) can still serve as a catalyst in Friedel-Crafts alkylation under certain conditions.

Comparison and Usage

Effectiveness: \(AlCl_3\) is preferred in many cases due to its stronger Lewis acidity, which provides a more efficient generation of the carbocation intermediate. This often leads to higher yields and faster reactions.

\(FeCl_3\): While \(FeCl_3\) is less commonly used, it can be a suitable alternative in some scenarios, particularly where AlCl3 might not be available or suitable.

Summary

Both \(AlCl_3\) and \(FeCl_3\) can catalyze Friedel-Crafts alkylation, but \(AlCl_3\) is more frequently used due to its superior effectiveness in generating the reactive carbocation. \(FeCl_3\) can also be used, making option 4 (both \(AlCl_3\) and \(FeCl_3\)) a correct choice for catalysts in this reaction.