Q27 / 79 · NEST (NISER) 2026 · Chemistry
tert-Butylbenzene reacts with 1-chloro-2-methylpropane in the presence of anhydrous $\text{AlCl}_3$. The major product in this reaction is:
- A
- Correct answerB
- C
- D
Solution
The reaction is a Friedel-Crafts alkylation.
First, 1-chloro-2-methylpropane reacts with anhydrous $AlCl_3$ to form a primary carbocation, the isobutyl cation:
$(CH_3)_2CH-CH_2-Cl + AlCl_3 \rightarrow (CH_3)_2CH-CH_2^+ + AlCl_4^-$
This less stable primary carbocation undergoes a 1,2-hydride shift to form a more stable tertiary carbocation, the tert-butyl cation:
$(CH_3)_2CH-CH_2^+ \rightarrow (CH_3)_3C^+$
The tert-butyl group on the starting material, tert-butylbenzene, is an ortho/para directing group.
Due to severe steric hindrance caused by the bulky tert-butyl group already present on the benzene ring, the incoming bulky tert-butyl electrophile predominantly attacks at the less hindered para position.
Therefore, the major product is 1,4-di-tert-butylbenzene, which corresponds to the structure in option (2).
Answer :
First, 1-chloro-2-methylpropane reacts with anhydrous $AlCl_3$ to form a primary carbocation, the isobutyl cation:
$(CH_3)_2CH-CH_2-Cl + AlCl_3 \rightarrow (CH_3)_2CH-CH_2^+ + AlCl_4^-$
This less stable primary carbocation undergoes a 1,2-hydride shift to form a more stable tertiary carbocation, the tert-butyl cation:
$(CH_3)_2CH-CH_2^+ \rightarrow (CH_3)_3C^+$
The tert-butyl group on the starting material, tert-butylbenzene, is an ortho/para directing group.
Due to severe steric hindrance caused by the bulky tert-butyl group already present on the benzene ring, the incoming bulky tert-butyl electrophile predominantly attacks at the less hindered para position.
Therefore, the major product is 1,4-di-tert-butylbenzene, which corresponds to the structure in option (2).
Answer :
Hence, the correct answer is option: 'B'.