Question

When chlorobenzene is treated with acetyl chloride in the presence of anhydrous AlCl\(_3\), 4-Chloroacetophenone is formed as the major product. It is an example of

• A. Nucleophilic substitution
• B. Electrophilic substitution
• C. Free radical substitution
• D. Nucleophilic addition
• E. Electrophilic addition

Hint:

Friedel-Crafts acylation introduces a ketone group onto an aromatic ring and is an electrophilic substitution reaction.

Answer 1

The Correct Option is B

Step 1: Understanding the Reaction:
The reaction involves an aromatic ring (chlorobenzene) reacting with an acyl chloride (acetyl chloride) in the presence of a Lewis acid catalyst (anhydrous AlCl\(_{3}\)). This is a characteristic reaction of aromatic compounds where a hydrogen atom on the ring is replaced by another group.
Step 2: Identifying the Reaction Mechanism:
The reaction described is a Friedel-Crafts Acylation. The general mechanism involves three steps:
1. Generation of an electrophile (an acylium ion) by the reaction of the acyl chloride with the Lewis acid catalyst.
\[ \text{CH}_3\text{COCl} + \text{AlCl}_3 \rightarrow [\text{CH}_3\text{C}=\text{O}]^+ + [\text{AlCl}_4]^- \] 2. Attack of the \(\pi\)-electron system of the aromatic ring on the electrophile to form a resonance-stabilized carbocation (sigma complex).
3. Loss of a proton (H\(^+\)) from the sigma complex to restore the aromaticity of the ring.
Step 3: Classifying the Reaction:
In this reaction, the aromatic hydrogen of chlorobenzene is substituted by an acetyl group (\(-COCH_{3}\)).
- The species attacking the electron-rich benzene ring is an electrophile (the acylium ion, CH\(_{3}\)CO\(^+\)).
- A hydrogen atom on the ring is replaced, or substituted, by this electrophile.
Therefore, the reaction is an electrophilic substitution reaction.
The other options are incorrect:
- Nucleophilic substitution involves a nucleophile attacking an electron-deficient center. Aromatic rings are electron-rich and do not undergo nucleophilic substitution easily.
- Free radical substitution is characteristic of alkanes reacting with halogens in the presence of UV light.
- Addition reactions (nucleophilic or electrophilic) would result in the loss of aromaticity, which does not happen here. Addition reactions are characteristic of alkenes and alkynes.
Step 4: Final Answer:
The Friedel-Crafts acylation of chlorobenzene is a classic example of an electrophilic aromatic substitution reaction. This corresponds to option (B).