Case: Read the passage and answer the following questions

Aldehydes and ketones are generally prepared by oxidation of primary and secondary alcohols. Dehydrogenation of alcohols gives aldehydes and ketones. Ozonolysis alkenes give aldehydes ketones or a mixture of both depending on the substitution pattern of the alkene. Aldehydes are prepared from acyl chloride, nitrile, esters and even from hydrocarbons by different kinds of oxidation. Ketones are prepared from benzene or its derivatives through Friedel-Crafts acylation reaction. Carboxylic acids are prepared from aldehydes and ketones on oxidation. Nitriles are hydrolysed to amides and then to carboxylic acids. It is even prepared using Grignard’s reagent.

What is the reagent used in Friedel-Crafts acylation reaction?

AlCl₃

The correct answer is option 1. \(AlCl_3\)

The reagent used in the Friedel-Crafts acylation reaction is $AlCl_{3}$ (Anhydrous Aluminum Chloride).

In organic chemistry, $AlCl_{3}$ acts as a powerful Lewis acid catalyst. Its primary role is to generate a highly reactive electrophile (the acylium ion) which can then attack the benzene ring.

How the Reaction Works

When an acyl chloride $(R-COCl)$ reacts with benzene in the presence of anhydrous $AlCl_{3}$:

1. Electrophile Generation: The $AlCl_{3}$ pulls the chlorine atom away from the acyl chloride, forming the $[AlCl_{4}]^{-}$ ion and the acylium ion $(R-C^{+}=O)$.
2. Electrophilic Attack: The acylium ion attacks the electron-rich benzene ring.
3. Restoration of Aromaticity: A proton is removed from the ring to restore its stability, resulting in the formation of an aromatic ketone.

Why the other options are incorrect:

• $LiAlH_4$ (Lithium Aluminum Hydride): This is a strong reducing agent. It is used to reduce carbonyl compounds (like ketones or aldehydes) back into alcohols, rather than forming them.
• $CuS_2$: This is not a standard reagent for Friedel-Crafts reactions. Copper-based reagents are more commonly used in coupling reactions like the Ullmann reaction or Sandmeyer reaction.
• $H_2 - C$: This notation likely refers to hydrogenation over a carbon support, which is a reduction process, not an acylation process.