Q30 / 155 · WBJEE 2026 · Chemistry
Indicate the major product of the following reaction:
- A
- B
- Correct answerC
- D
Solution
The reaction of a secondary alkyl halide with a strong base ($\text{CH}_3\text{ONa}$) proceeds via an E2 elimination mechanism.
E2 elimination in a cyclohexane ring requires an anti-periplanar (trans-diaxial) arrangement of the leaving group and the $\beta$-atom being abstracted.
In the reactant, the -Br group is on a wedge (up). For anti-periplanar elimination, the $\beta$-atom must be on a dash (down).
At the upper $\beta$-carbon, the -$\text{CH}_3$ group is on a dash, but it cannot be eliminated. The -H atom is on a wedge, so it is syn-periplanar and cannot be eliminated.
At the lower $\beta$-carbon, the -D atom is on a dash, making it anti-periplanar to the -Br group. The -H atom is on a wedge.
Therefore, the base abstracts the anti-periplanar -D atom, leading to the elimination of DBr and the formation of a double bond at the lower position.
The resulting product retains the -H atom on the newly formed vinylic carbon and the original stereochemistry at the upper allylic carbon, which corresponds to option (C).
Answer:
E2 elimination in a cyclohexane ring requires an anti-periplanar (trans-diaxial) arrangement of the leaving group and the $\beta$-atom being abstracted.
In the reactant, the -Br group is on a wedge (up). For anti-periplanar elimination, the $\beta$-atom must be on a dash (down).
At the upper $\beta$-carbon, the -$\text{CH}_3$ group is on a dash, but it cannot be eliminated. The -H atom is on a wedge, so it is syn-periplanar and cannot be eliminated.
At the lower $\beta$-carbon, the -D atom is on a dash, making it anti-periplanar to the -Br group. The -H atom is on a wedge.
Therefore, the base abstracts the anti-periplanar -D atom, leading to the elimination of DBr and the formation of a double bond at the lower position.
The resulting product retains the -H atom on the newly formed vinylic carbon and the original stereochemistry at the upper allylic carbon, which corresponds to option (C).
Answer:
Hence, the correct answer is option: 'C'.