Radical Stability "Achiever #77"

[heatwheat]

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This is #77 from Achiever Test 1.

Which of the following sets of hydrocarbons correctly displays free radicals in descending order of stability?



A. CH3H2C. > CH2=CHĊH2 > (CH3)3C. > (CH3)2HC.
B. CH2=CHĊH2 > (CH3)3C. > (CH3)2HC. > CH3H2C.
C. CH3H2C. > CH2=CHĊH2 > (CH3)3C. > (CH3)2HC.
D. (CH3)2HC. > CH3H2C. > CH2=CHĊH2 > (CH3)3C.
E. (CH3)3C. > CH2=CHĊH2 > (CH3)2HC. > CH3H2C.

The correct answer is B, but I thought that the most stable radical was a tertiary radical, making the answer E. Could someone please explain to me why I'm wrong? Thanks.

 

[dentalplan]

The radical (or a carbocation) is most stable when in a conjugated system.

The tertiary readical is very stable, yes. But the conjugated radical is even more stable. This is because the lone electron can diffuse through a conjugated allylic system. Studies have shown that a conjugated radical is more stable than the tertiary radical.

 

[JBarr29]

when you hear radical and stability, think resonance structures first. the one with the most resonance will probably be the most stable

 

[heatwheat]

Ok, I got it. Thanks.