Question

In Friedel-Crafts reaction, benzene reacts with isopropyl bromide in the presence of aluminium trichloride to give:

• A. $ \text{Benzophenone} $
• B. $ \text{Acetophenone} $
• C. $ \text{Isopropyl benzene} $
• D. $ \text{n-Propyl benzene} $

Hint:

In Friedel–Crafts alkylation, secondary carbocations like isopropyl are stable and commonly formed. Always consider carbocation stability and rearrangement.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Friedel-Crafts reactions come in two types, alkylation, which attaches an alkyl group to benzene using an alkyl halide, and acylation, which attaches an acyl group using an acyl halide. Isopropyl bromide is an alkyl halide, so this reaction is an alkylation.

Step 2: Key Formula or Approach:
In Friedel-Crafts alkylation, the Lewis acid catalyst pulls the halide off the alkyl halide to form a carbocation.
\[ (\text{CH}_3)_2\text{CHBr} + \text{AlCl}_3 \to (\text{CH}_3)_2\text{CH}^+ + \text{AlCl}_3\text{Br}^- \]
This carbocation is the electrophile that attacks the benzene ring.

Step 3: Detailed Explanation:
The carbocation formed from isopropyl bromide is a secondary carbocation, since the positive carbon is attached to two other carbons, and secondary carbocations are reasonably stable and do not need to rearrange further.
This isopropyl carbocation attacks the benzene ring through electrophilic aromatic substitution, replacing one ring hydrogen and attaching the isopropyl group directly to the ring.
A straight chain n-propyl carbocation would be primary and much less stable, so the reaction does not proceed through that pathway, ruling out n-propyl benzene, and benzophenone and acetophenone both need an acyl halide reagent instead of an alkyl halide, so they are not possible from isopropyl bromide either.

Step 4: Final Answer:
The reaction of benzene with isopropyl bromide and aluminium trichloride gives isopropyl benzene, also known as cumene.