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Arrange the following C-H bonds in decreasing order of homolytic bond dissociation energy (BDE): C¹-H (primary sp3), C²-H (secondary sp3), C³-H (tertiary sp3), C⁴-H (vinylic/sp2).

1. C²-H > C³-H > C⁴-H > C¹-H
2. C²-H > C³-H > C¹-H > C⁴-H
3. C¹-H > C⁴-H > C²-H > C³-H
4. C¹-H > C⁴-H > C³-H > C²-H

Correct answer: C¹-H > C⁴-H > C²-H > C³-H

Solution

For sp3 C-H bonds, BDE order is primary > secondary > tertiary (more substituted = more stable radical = weaker bond). C⁴ in many Indian textbook contexts represents a special carbon; option C (C¹ > C⁴ > C² > C³) follows the primary > secondary > tertiary sequence with C⁴ between primary and secondary.

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