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 is the energy required to break a bond, and it depends on the stability of the bond. In the case of C-H bonds, the following factors affect their bond dissociation energy:

• Aromatic C-H bond (I): The C-H bond in an aromatic compound is stabilized by resonance and the delocalization of electrons in the aromatic ring. This results in a relatively high bond dissociation energy.

• Aliphatic C-H bond (II): The C-H bond in a typical aliphatic compound (like an alkane) is weaker compared to the C-H bond in an aromatic system because there is no such resonance stabilization.
  Thus, the bond dissociation energy will be moderate.

• C-H bond in a cyclopropane-like structure (III): The C-H bond in a strained structure like cyclopropane is weaker due to the angle strain, making it easier to break. Hence, this bond has the lowest bond dissociation energy.

Thus, the correct order of bond dissociation energy is II>I>III.