Question

Which of the following compound will have the highest $C-H_a$ bond energy?

• A.
• B.
• C.
• D.

Answer 1

The correct option is A

$\text{Bond Energy}\propto \frac{1}{\text{Stability of free radicals}}$
$\text{Stability of free radicals}\propto \frac{+I+R}{-I-R}$
In compound (a), the group $CN$ shows -R effect which will withdraw the electron density toward itself and destabilise the free radical formed.
In compound (b) and (c), we have $Cl$ group. Halide groups have more negative inductive effect compare to the resonance effect. Hence, it will withdraw the electron density toward itself and destabilise the free radical formed.
In compound (c), $OC{H}_3$ will show +R effect and increase the electron density on carbon and stabilise the free radical formed.
Comparing (a), (b) and (c), compound (a) form least stable free radical due to the resonance effect of $CN$
Since, least stable free radical will have highest bond energy, compound (a) has the highest bond energy for $C-H_a$ bond.