Question

Least stable radical among the following is:
(A)

(B)

(C)

(D)

Answer 1

Hint In order to solve this problem, we need to have an idea about free radicals and then about the stability of free radicals. An atom or group which belong to the same or different elements that behave as a single unit and possess a negative or positive charge is known as a free radical.

Complete step by step solution:
- As we know , the free radical reactions work through homolytic cleavage – that is, the bonds break such that equal numbers of electrons are distributed to each atom and the resulting species are called free radicals.
- As in the case of carbocations, free radicals also are electron-deficient species and the stability of radicals depend on various factors The radicals can be stabilized by the donation of electrons by neighbours and for this reason, the radical stability will increase in the order of methyl < primary < secondary < tertiary.
- Another important factor which determines the stability of radicals will be delocalization. Or in other words the free radicals are stabilized by resonance. As we can observe in option (B) , there is an extended resonance as shown below

Therefore, the least stable radical will be option (B).

Note: Keep in mind that the free radicals are destabilized by the removal of electron density, and the stability decreases with the increase in s-character of the orbital and the radical stability will decrease with increasing electronegativity of the atom and reduces with a decrease in polarity.