Question

Which C-H bond has minimum bond dissociation energy for homolysis

A. IV
B. III
C. II
D. I

Answer 1

The bond dissociation energy is defined as the amount of energy required to break the chemical bond formed between the two atoms in a chemical compound. The compound which forms a stable intermediate will be strong and high energy will be required to break the bond.

Complete step by step answer:
The covalent bond present between the two atoms is broken by two ways. It can be done either by heterolytic cleavage or by homolytic cleavage.
In homolytic cleavage, the product formed are radicals and the energy required for breaking the covalent bond is known as bond dissociation energy.
The homolytic bond dissociation energy is defined as the amount of energy required to break one mole of gases which are covalently bonded into radical pairs. The SI unit of bond dissociation energy is kJ/mol.

In the given compound, there are four hydrogen atoms bonded to carbon atoms by a covalent bond. But the C-H bond present at first position forms a stable intermediate by resonance formed by the radical due to the presence of methyl group which shows positive inductive effect, the double bond and the hyperconjugation due to five hydrogen atoms present in the ring.
Therefore, the C-H bond at (I) position requires minimum bond-dissociation energy to break the bond.

So, the correct answer is Option A.

Note: On comparing the primary C-H bond, secondary C-H Bond and tertiary C-H bond, the tertiary C-H bond requires less bond dissociation energy.