Question

Which of the following is correct order of stability of free radicals?

• A. allyl > 3° > 2° > 1° > CH₃°
• B. 3° > 2° > 1° > allyl > CH₃°
• C. CH₃° > 3° > 2° > 1° > allyl
• D. 3° > 2° > 1° > CH₃° > allyl

Answer 1

The Correct Option is A

The stability of free radicals is an important concept in organic chemistry. Let's analyze the stability order of various free radicals presented in the options. 

1. Allyl Radical: The allyl radical (“allyl”) is highly stabilized due to resonance. It is an extended conjugation system which can delocalize the unpaired electron over adjacent carbon atoms, leading to enhanced stability.
2. Tertiary Free Radical (3°): These radicals are stabilized through hyperconjugation and inductive effects. Here, the unpaired electron on the carbon is stabilized by electron-releasing alkyl groups, making it more stable than secondary and primary radicals.
3. Secondary Free Radical (2°): These radicals are less stable than tertiary but more stable than primary radicals, due to fewer hyperconjugative structures and inductive effects than tertiary radicals.
4. Primary Free Radical (1°): These radicals have even less hyperconjugation and inductive effects to stabilize the radical, making them less stable than secondary and tertiary radicals.
5. Methyl Radical (CH₃°): This is the least stable due to the absence of hyperconjugation and inductive stabilization.

Now, putting these concepts together, the correct order of stability for these free radicals is:

allyl > 3° > 2° > 1° > CH₃°

This order reflects the increasing stabilization mechanisms such as resonance, hyperconjugation, and inductive effect that influence free radical stability.