Question

In which of the following pairs, the rate of bromination of first member is more than the second member?

• A. Isobutane, n-butane
• B. n-butane, isobutane
• C. Methane, ethane
• D. None of these

Answer 1

The correct option is A Isobutane, n-butane

For photochemical halogenation (bromination), the rate of the reaction is directly proportional to the stability of the intermediate formed.
The intermediates formed in photochemical halogenation is free radical.
We know that the stability of free radical is in the order,
$\text{Tertiary radical > Secondary radical > Primary radical}$
In (a) and (b), isobutane forms tertiary radical which is stabilised by hyperconjugation. Thus, isobutane undergoes bromination easier than n-Butane.
In (c), ethane undergoes bromination easily than methane because methane radical is not stabilised by hyperconjugation due to absence of $\alpha -$ Hydrogen.
Therefore, option (a) is correct.