Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

Benzene reacts with \(\mathrm{CH}_{3} \mathrm{COCl}\) in the presence of anhydrous \(\mathrm{AlCl}_{3}\) to give (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}_{2} \mathrm{Cl}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{3}\)

Step-by-Step Solution

Answer

The reaction produces (d) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{3} \).

Step 1: Understand the Reaction Type

This reaction is a type of electrophilic aromatic substitution known as Friedel-Crafts acylation. In this process, an acyl group is introduced into the benzene ring.

Step 2: Identify the Acylating Agent

The acylating agent in this reaction is acyl chloride, specifically acetyl chloride, denoted by \( \mathrm{CH}_{3} \mathrm{COCl} \). This reagent contains an acyl group (CH₃CO-), which is key to the acylation reaction.

Step 3: Recognize the Role of the Catalyst

Anhydrous \( \mathrm{AlCl}_{3} \) serves as a Lewis acid catalyst. It facilitates the formation of a more reactive acylium ion from the acyl chloride, essentially activating the acyl group for the attack on the benzene ring.

Step 4: Form the Acylium Ion

The \( \mathrm{AlCl}_{3} \) interacts with \( \mathrm{CH}_{3} \mathrm{COCl} \) to form an acylium ion (\( \mathrm{CH}_{3} \mathrm{CO}^{+} \)). This ion is stabilized by resonance and is highly electrophilic, making it suitable to substitute onto the benzene ring.

Step 5: Electrophilic Attack and Product Formation

The acylium ion \( \mathrm{CH}_{3} \mathrm{CO}^{+} \) attacks the electron-rich benzene ring, substituting one of its hydrogen atoms. This results in the formation of acetophenone, represented as \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{3} \).

Step 6: Verify the Product Match

Compare the generated product, \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{3} \), with the options given. The correct answer is (d) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{3} \).

Key Concepts

Electrophilic Aromatic Substitution

In organic chemistry, electrophilic aromatic substitution is a basic but essential reaction mechanism. This process involves the replacement of a hydrogen atom in an aromatic compound, such as benzene, with an electrophile. The aromatic ring acts as a nucleophile, which means it seeks positively charged particles (electrophiles) to react with. Friedel-Crafts acylation is a specific type of electrophilic aromatic substitution. This reaction adds an acyl group, denoted as RCO-, onto the benzene ring. In the process, a hydrogen atom on the benzene is replaced by the acyl group. As a result, the structural integrity and stability of the aromatic ring are preserved.
One of the attractive features of electrophilic aromatic substitution is its ability to introduce functional groups into aromatic rings, enhancing their chemical utility. Friedel-Crafts acylation follows a series of well-defined steps: acylium ion formation, followed by an electrophilic attack on the aromatic ring. These steps assure that the aromaticity of the ring is retained, which is crucial for the resonance stability of benzene.

Acylium Ion Formation

Acylium ion formation is a pivotal step in the Friedel-Crafts acylation reaction. This step involves the conversion of an acyl chloride, such as acetyl chloride (\(\mathrm{CH}_{3}\mathrm{COCl}\)), into a more reactive entity called the acylium ion (\(\mathrm{CH}_{3}\mathrm{CO}^{+}\)). The transformation equips the acyl group to act as an effective electrophile.
Usually, this conversion requires a catalyst, and anhydrous \(\mathrm{AlCl}_{3}\) serves this purpose. When acetyl chloride interacts with \(\mathrm{AlCl}_{3}\), the chlorine atom detaches, forming the acylium ion and a complex with \(\mathrm{AlCl}_{4}^{-}\). What makes the acylium ion fascinating is its resonance stabilization. It can be represented as a resonance hybrid, making it both a stable and highly reactive electrophile. Its electromagnetic nature makes it perfectly suited for attacking the electron-rich aromatic rings, leading to the effective substitution of the hydrogen atom.

Lewis Acid Catalysis

A Lewis acid is a chemical species that can accept an electron pair to form a coordinate covalent bond. In the context of Friedel-Crafts acylation, \(\mathrm{AlCl}_{3}\) acts as a Lewis acid catalyst. The role of \(\mathrm{AlCl}_{3}\) extends beyond simple facilitation. It has the important task of converting acyl chlorides into more reactive acylium ions by accepting a lone pair from the chlorine atom.
The process involves the bonding of \(\mathrm{AlCl}_{3}\) with the chlorine atom of acyl chloride, allowing the carbonyl group to develop a positive charge, forming the acylium ion (\(\mathrm{CH}_{3}\mathrm{CO}^{+}\)). The resultant acylium ion is sufficiently electrophilic to attack the benzene ring. After the acylation process, \(\mathrm{AlCl}_{3}\) is regenerated and can continue to act as a catalyst in the reaction. This regeneration underscores one of the key characteristics of a catalyst: it is not consumed in the reaction it catalyzes. \(\mathrm{AlCl}_{3}\), through Lewis acid catalysis, significantly enhances the efficiency and efficacy of Friedel-Crafts acylation, making it a staple reaction in organic synthesis.