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JEE Main 2025 Jan 28 Shift 2, Chemistry Q13: Preparation of Alkyl Halides (Halo Alkanes) & Aryl Halides (Halo Arenes)

JEE Main2025Jan 28, Shift 2Chemistry
Q.

The major product of the following reaction is :

  1. A

    6-Phenylhepta-2,4-diene

  2. B

    2-Phenylhepta-2,5-diene

  3. C

    6-Phenylhepta-3,5-diene

  4. D

    2-Phenylhepta-2,4-diene

Solution

Excess alcoholic KOH on a vicinal/1,3-dibromide carries out two consecutive E2 eliminations, removing both Br atoms together with adjacent -hydrogens to generate a conjugated diene whenever possible (thermodynamic, Saytzeff product).

For the substrate shown, the two eliminations place the double bonds at the 2,3 and 4,5 positions of a 7-carbon chain, with the phenyl at C-2. Numbering the chain so that the phenyl-bearing alkene gets the lowest locant gives 2-phenylhepta-2,4-diene, a fully conjugated diene that also benefits from conjugation with the aromatic ring.

Concept behind this question

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