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JEE Main 2025 Apr 2 Shift 1, Chemistry Q18: Properties of Alkynes

JEE Main2025Apr 2, Shift 1Chemistry
Q.

Consider the following compound (X)

The most stable and least stable carbon radicals, respectively, produced by homolytic cleavage of corresponding C – H bond are :

  1. A

    II, IV

  2. B

    III, II

  3. C

    I, IV

  4. D

    II, I

Solution

Each position generates a different carbon radical when its C–H bond breaks homolytically. Their stability is ranked by hyperconjugation and (especially) resonance/conjugation with the adjacent triple bond:

Position II: The radical sits on the carbon adjacent to the -system. The unpaired electron is stabilised by resonance/conjugation with the triple bond (propargyl-type), making this the most stable.

Position III: Propargylic-type radical (one bond from the triple bond) — moderately stable.

Position IV: Tertiary (or branched secondary) alkyl radical — stabilised mainly by hyperconjugation.

Position I: Acetylenic radical on the terminal -carbon. No resonance, no hyperconjugation, and the orbital holds the unpaired electron tightly close to the nucleus — this is the least stable.

Most stable = II, least stable = I, which is option (4).

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