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NEET2025Chemistry
Q.

Among the given compounds I-III, the correct order of bond dissociation energy of CH bond marked with is:

  1. A

    II > I > III

  2. B

    I > II > III

  3. C

    III > II > I

  4. D

    II > III > I

Solution

BDE for CH is inversely related to the stability of the radical formed after homolysis.

I (aryl radical from benzene): sp carbon-centered radical, no good delocalisation pathway, moderately stable.

II (alkynyl radical from phenylacetylene): sp carbon radical, highly s-character, very poorly stabilised → high BDE.

III (cyclopropyl radical): the ring relieves angle strain partially and the radical can stabilise — most stable radical here → lowest BDE.

Radical stability: III > I > II, so BDE order: II > I > III.

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