Question

The relative stability of the following radicals is

• A. i > ii > iii
• B. ii > iii > i
• C. i > iii > ii
• D. iii > i > ii

Answer 1

The correct option is C i > iii > ii

The stability of free radicals depends on various factors like resonance effect, hyperconjugation effect, inductive effect etc.,
In (i), the free radical is allylic radical. Allylic radical undergo delocalisation of electron to form resonating structure. Hence, allylic radical is the most stable radical among the given radicals.
In (ii), the free radical is vinylic radical. These radical are highly unstable due to the radical on highly electronegative $s{p}^2$ hybridised carbon. Thus, it is the least stable one.
In (iii), the free radical is secondary radical. This comoaritevely stable than (ii) due to the hyperconjugation effect.

Thus, the order of relative stability of radicals is
(i) > (iii) > (ii)