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

Question

Among the given compounds I-III, the correct order of bond dissociation energy of the C-H bond marked with is: (I) the aromatic C-H of benzene (sp^2 carbon); (II) the terminal alkyne C-H of phenylacetylene, Ph-CC-H (sp carbon); (III) a C-H on the CH2 of a methylenecyclopropane-type sp^3 carbon.

1. A. II I III (Correct answer)
2. B. I II III
3. C. III II I
4. D. II III I

Correct Answer

Option A — II I III

Detailed Solution & Explanation

The correct answer is II > I > III.

Key Points

• The bond dissociation energy of a C-H bond increases with the s-character of the carbon's hybrid orbital (more s-character $\Rightarrow$ shorter, stronger bond).
• (II) sp carbon (50% s), (I) sp$^2$ carbon (33% s), (III) sp$^3$ carbon (25% s).
• Hence order of C-H bond strength: II (sp) > I (sp$^2$) > III (sp$^3$).