Question

Among the hydrogens $H_a,H_b,H_c\text{and}{H}_d$ in the given compound , which of the following has highest bond energy?

• A. $H_a$
• B. $H_b$
• C. $H_c$
• D. $H_d$

Answer 1

The correct option is C $H_c$

(i) By dissociation of $H_a$

The radical formed has $2\alpha -Hydrogens$, hence it is stabilised by hyperconjugation effect.

(ii) By dissociation of $H_b$


The radical formed has $3\alpha -H$ and it is also stabilised by resonance effect. Hence it is the most stabilised radical.

(iii) By dissociation of $H_c$
Here the radical is generated on vinylic carbon. Hence it is the least stable radical.

(iv) By dissociation of $H_d$
The radical formed has no $\alpha -H$ but it is stabilised by resonance effect.

Stability of intermediate,
Resonance effect > Hyperconjugation effect
Hence the order of stability of free radical is
$H_b>H_d>H_a>H_c$
$\text{Bond energy}\propto \frac{1}{\text{Stability of free radical}}$
Hence the order of bond energy is
$H_c>H_a>H_d>H_b$

Option (c) is correct