Question

Identify reactant (A) used in the following conversion.
Chlorobenzene + A $\xrightarrow{\text{anhydrous AlCl}_3}$ 1-Chloroacetophenone + 4-Chloroacetophenone

• A. used in the following conversion.
• B. Acetophenone
• C. Acetic acid
• D. Acetyl chloride

Hint:

When you see anhydrous $\text{AlCl}_3$ and the product is a ketone attached to a benzene ring, look immediately for an acid chloride (like acetyl chloride) in the options!

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks us to identify the missing reactant "A" that reacts with chlorobenzene in the presence of anhydrous aluminum chloride to yield chloroacetophenones.

Step 2: Key Formula or Approach:
The presence of anhydrous $\text{AlCl}_3$ as a catalyst strongly indicates a Friedel-Crafts reaction.
Since the products are acetophenones (which contain an acyl group, $\text{-COCH}_3$), this is specifically a Friedel-Crafts acylation.
An acyl group is introduced using an acid chloride or an acid anhydride as the reagent.

Step 3: Detailed Explanation:
Chlorobenzene undergoes electrophilic aromatic substitution. The chlorine atom is ortho/para directing.
To attach the acetyl group ($\text{CH}_3\text{CO-}$) to the benzene ring to form acetophenones, we must use acetyl chloride ($\text{CH}_3\text{COCl}$).
The mechanism begins with $\text{AlCl}_3$ reacting with acetyl chloride to generate the highly electrophilic acylium ion ($\text{CH}_3\text{C}\equiv\text{O}^+$).
This electrophile attacks the ortho and para positions of chlorobenzene, yielding a mixture of the ortho isomer (often referred to as 1-(2-chlorophenyl)ethanone or 2-chloroacetophenone) and the para isomer (4-chloroacetophenone).

Step 4: Final Answer:
The required reactant is acetyl chloride, matching option (D).