Free Radical Stability: Free radical stability is primarily determined by the degree of substitution. More substituted free radicals are more stable. This is due to hyperconjugation (overlap of the singly occupied p-orbital with sigma bonds on adjacent carbon atoms) and inductive effects (electron donation from alkyl groups).
Radical I: Radical I is a secondary allylic free radical. The allylic stabilization from the adjacent double bond makes it more stable than simple secondary radicals.
Radical II: Radical II is a tertiary free radical. It is more stable than secondary or primary radicals due to hyperconjugation. Since it's also benzylic, with the unpaired electron delocalized across the benzene ring, it has greater stability. Benzyl radicals are more stable than allyl radicals. Hence this is most stable.
Radical III: Radical III is a primary free radical. The primary radicals are the least stable because there are fewer alkyl groups that can stabilize the carbon atom with the unpaired electron through hyperconjugation or inductive effects.
Because radical II benefits from tertiary and benzylic stabilization, is most stable; radical I is only stabilized through allylic and secondary carbons, and radical III is only stabilized through a primary carbon, it follows that the order of stability is II > I > III.
Correct Answer: II > I > III
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