Topic: Bond fission, free radicals, carbocations, carbanions and their stability· Some Basic Principles of Organic Chemistry

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

I: benzene with starred ring C-H; II: phenylacetylene with starred terminal alkyne C-H; III: cyclopropane with starred C-H. Options listed below structures

1. II > I > III

2. I > II > III

3. III > II > I

4. II > III > I

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Answer: 1. II > I > III

Key idea: the more s-character in the carbon, and the less stable the radical left behind, the stronger the C-H bond (higher BDE).

II: sp carbon (50% s character), BDE is the highest (about 133 kcal/mol).
I: sp² aryl C-H (about 112 kcal/mol); the aryl radical is not stabilised by resonance.
III: the marked \(\mathrm{CH_2}\) of cyclopropene is allylic, so the C-H bond is the weakest.

II > I > III: option (1).

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report

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

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