Question

Sol. Key concept: Hydrogen abstraction is favoured when it forms the more stable carbon radical.

H_e is on a tertiary allylic carbon, while H_c is on a secondary allylic carbon. Their radicals are resonance-stabilised; the tertiary allylic radical from e is the more stabilised of the two.

H_d is a secondary alkyl hydrogen and H_a is a primary alkyl hydrogen, without allylic resonance stabilisation at those labelled sites. H_b is vinylic: it is bonded directly to a double-bond carbon, and its C–H bond is especially difficult to break.

The intended relative ease of abstraction is e > c > d > a > b, option (c). The identity of the halogen affects selectivity, but this question asks for the usual qualitative radical-stability comparison.

Correction / clarification: The difficulty of abstracting H_b should be attributed to the strong vinylic C–H bond and unfavourable vinyl radical, rather than simply to a −I effect.