Question

Rank the bond dissociation energies of the bonds indicated with the arrows. (from smallest to largest)

• A. 1<2<3
• B. 3 < 2 < 1
• C. 2 < 3 <1
• D. 3 < 1 < 2

Answer 1

Answer: (B)

3 < 2 < 1

Answer 2

The bond dissociation energy (BDE) of a C-H bond is inversely related to the stability of the radical formed. Tertiary radicals are more stable than secondary radicals due to hyperconjugation. Bond 3 is a tertiary C-H bond, forming a tertiary radical, hence it has the lowest BDE. Bonds 1 and 2 are secondary C-H bonds. Comparing the stability of the secondary radicals formed from bonds 1 and 2, the radical from bond 2 is more stable because its adjacent carbons provide more alpha-hydrogens (3) for hyperconjugation compared to the radical from bond 1 (2 alpha-hydrogens). Therefore, BDE(3) < BDE(2) < BDE(1).