Question

Which of the following free radicals is most stable

• A. Primary
• B. Methyl
• C. Secondary
• D. Tertiary

Answer 1

The correct option is D Tertiary

$C{H}_3-\stackrel{\stackrel{C{H}_3}{|}}{\underset{\underset{C{H}_3}{|}}{C^{\bullet }}}>C{H}_3-\stackrel{\stackrel{C{H}_3}{|}}{\underset{\underset{2^o}{\bullet }}{C}}H>C{H}_3-\underset{1^o}{\underset{}{\stackrel{\bullet }{C}{H}_2}}|$

The stability of free radicals is same as that of carbocations. Like carbocations, tertiary radical is more stable than secondary and primary radical. This can be explained by +I effect of the alkyl groups attached to carbon radicals. The more is the number of alkyl groups attached to the carbon radical, the more is the availability of $e^{-}$ and more is the stabilisation.

So tertiary free radical is more stabilised than secondary and primary.