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

CONCEPT:

Effect of Hybridization on Bond Dissociation Energy (BDE)

• The bond dissociation energy (BDE) for C-H bonds depends significantly on the hybridization of the carbon atom involved in the bond:
  • sp³ hybridized carbon: The C-H bond in alkyl groups (sp³) has a relatively higher bond dissociation energy. The electron density around the carbon is more spread out due to the tetrahedral arrangement of the bonds.
  • sp² hybridized carbon: The C-H bond in alkenes or aromatic compounds (sp²) has a lower bond dissociation energy. This is because sp² carbon has more s-character, which pulls the electron density closer to the nucleus, weakening the C-H bond.
  • sp hybridized carbon: The C-H bond in alkynes (sp) has an even lower bond dissociation energy due to the high s-character in the sp hybridization, which leads to a stronger bond between the carbon and hydrogen, but lower C-H bond dissociation energy compared to sp² carbon.
• In this problem, the bond dissociation energy varies based on the hybridization of the carbon atoms to which the hydrogen atoms are bonded.

EXPLANATION:

• I: Benzyl group (sp² carbon attached to the aromatic ring) s-character is 33%
• II: sp carbon attached to a CC Triple bond) s-character  is 50%
• III: Alkane (sp³ carbon) s-character  is 25%

Bond Strength \(\propto \) % s-character 

• The bond dissociation energy follows the trend:
  • sp² (Benzyl) < sp  < sp³ (Alkane)

Therefore, the correct order of C-H bond dissociation energy is II > I > III.