Identify the catalyst in Friedel-Crafts acylation reaction.

Anhydrous AlCl₃

The correct answer is option 4. Anhydrous \(AlCl_3\).

In Friedel-Crafts acylation reactions, anhydrous aluminum chloride \((AlCl_3)\) acts as a catalyst. Here's how it facilitates the acylation of aromatic compounds:

Activation of Acyl Halide: An acyl halide, typically acetyl chloride \((CH_3COCl)\), is used as the acylating agent. \(AlCl_3\) acts as a Lewis acid, which means it can accept a pair of electrons to form a coordinate covalent bond. AlCl3 activates the acyl halide by binding to the chlorine atom, making the carbonyl carbon more electrophilic.

Formation of Acylium Ion: The activated acyl halide loses a chloride ion in the presence of \(AlCl_3\), forming an acylium ion (\( \text{RCO}^+ \)). For instance, acetyl chloride forms the acylium ion \(CH_3CO^+\).

Reaction with Aromatic Ring: The acylium ion \((RCO^+)\) is highly electrophilic and readily reacts with the aromatic ring. The aromatic ring donates electron density to the acylium ion, leading to the formation of an acylated aromatic product.

Regeneration of Catalyst: After the reaction, \(AlCl_3\) remains unchanged and can catalyze further acylation reactions.

Key Points:

Lewis Acid: \(AlCl_3\) acts as a Lewis acid catalyst by accepting electron pairs from the acyl halide, activating it for reaction with the aromatic compound.

Electrophilic Substitution: The mechanism involves electrophilic substitution where the acylium ion adds to the aromatic ring, replacing a hydrogen atom.

Conditions: Anhydrous conditions are crucial because water can hydrolyze \(AlCl_3\), rendering it ineffective as a catalyst.

Conclusion: Anhydrous aluminum chloride \((AlCl_3)\) plays a critical role in Friedel-Crafts acylation reactions by activating acyl halides and facilitating their reaction with aromatic compounds to form acylated aromatic products. Its function as a Lewis acid catalyst ensures efficient and selective acylation under mild conditions.