Question

Which of the free radical is most stable?

• A. I
• B. II
• C. III
• D. IV

Answer 1

Answer: (C)

III

Answer 2

The stability of free radicals mirrors that of carbocations. Similar to carbocations, tertiary radicals exhibit greater stability compared to secondary and primary radicals. This phenomenon can be elucidated by the electron-donating (+I) effect of alkyl groups bonded to carbon radicals. As the number of alkyl groups attached to the carbon radical increases, there is a corresponding rise in the availability of electrons, leading to enhanced stabilization. Consequently, tertiary free radicals are more stabilized than secondary and primary counterparts.

The overall stability order among free radicals is:
triphenyl methyl > benzyl > allyl $> 3^{\circ} > 2^{\circ} >1^{\circ} >$ methyl > vinyl.

So, the correct option is (C): III