Question

Sol. Key concept: Compare ordinary alkyl radicals formed by C–H homolysis.

For comparable saturated alkanes, a tertiary carbon radical is generally more stabilised by adjacent alkyl groups and hyperconjugation than a secondary or primary radical. Less energy is therefore required to break the tertiary C–H bond.

The usual bond-dissociation-energy order is primary > secondary > tertiary, so the lowest value is for the 3° C–H bond: option (a). This comparison assumes ordinary alkyl sites; allylic, benzylic and other special environments require separate treatment.