Question

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

• A. I $>$ II $>$ III
• B. III $>$ II $>$ I
• C. II $>$ I $>$ III
• D. I $>$ III $>$ II

Hint:

Resonance stabilization (like in aromatic compounds) usually increases the bond dissociation energy, while strain (like in cyclopropanes) decreases it.

Answer 1

The Correct Option is C

Bond dissociation energy quantifies the energy needed to sever a chemical bond, its magnitude influenced by the bond's stability. For C-H bonds, the following factors dictate their dissociation energy:

• Aromatic C-H bond (I): Resonance and electron delocalization within the aromatic ring confer stability to the C-H bond in aromatic compounds, resulting in a comparatively high bond dissociation energy.

• Aliphatic C-H bond (II): Lacking the resonance stabilization present in aromatic systems, the C-H bond in typical aliphatic compounds (e.g., alkanes) is weaker. Consequently, its bond dissociation energy is moderate.

• C-H bond in a cyclopropane-like structure (III): Strained ring systems, such as cyclopropane, exhibit a weaker C-H bond due to angle strain, facilitating bond cleavage. This leads to the lowest bond dissociation energy among the examples.

Therefore, the order of bond dissociation energy, from highest to lowest, is II > I > III.