Question

In which of the following pairs, the bromination of first member is easier 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) reactivity of hydrogen atom is $3^{\circ }H>2^{\circ }H>1^{\circ }H$
Differences in reactivity can only be attributed to differences in $C-H$ bond dissociation energies. The weaker bonds are broken more easily than the strong bonds. The difference in $C-H$ bond dissociation energy reported for primary$\left(1^o\right)$>secondary$\left(2^o\right)$>tertiary$\left(3^o\right)$ sites agrees with the halogenation observations. The reactivity order of hydrogen attached to central C for substitution reaction is $M{e}_{3}CH>M{e}_{2}C{H}_2>MeC{H}_3$
​It is due to radical formed by the reactive hydrogen elimination, which is stabilized by hyperconjugation with adjacent hydrogen atoms.