Question

Questions: Rank the indicated hydrogen atoms in order of increasing ease of abstraction in a radical halogenation reaction.

Short answer

Answer

The increasing order of the abstraction of the hydrogen atom is as follows:

H_c< H_d<H_a<H_b.

Step-by-step solution

Radical

The atom that has an electron being unpaired is called a radical. The tertiary radical has high stability, whereas the methyl radical has the lowest stability.

The allylic carbon radical has a double bond at the alpha position.

Explanation for stability

H_b : This hydrogen gets easily abstracted from the C-H bond cleavage that requires low energy due to allylic carbon radical stabilized by resonance.

Formation of radical from H_b

H_a : C-H bond cleavage requires less energy due to 3⁰C-H cleavage that forms a 3⁰ carbon radical.

Formation of radical from H_a

H_d : The C-H bond cleavage requires lesser energy than 1⁰ C-H bond cleavage.

So, the correct order is H_c<H_d<H_a<H_b .