Relative stability of allyl radical

  • Thread starter themonk
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  • #1
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Main Question or Discussion Point

I am reading the beginning of the chapter about Conjugated Unsaturated Systems and it is discussing relative stability of the allylic radical in comparison with other compounds.

This is what it tells me for the DH°:
Allylic Radical - 369 kJ*mol-1
3° Radical - 400 kJ*mol-1
2° Radical - 413 kJ*mol-1
1° Radical - 423 kJ*mol-1
Vinyl radical - 465 kJ*mol-1

DH° is the dissociation energies (a measure of the bond strength in a chemical bond). Now it is saying that Allyl> 3° > 2° > 1° > Vinyl. So if Vinyl has the highest bond strength, how is it less stable? OR is it saying that once the H is removed from Vinyl that it really wants something to replace it, so it is least stable?

What makes Vinyl so much less stable than the others? (radical's vicinity to double bond?)

Just want to make sure my logic is still working, since this seems like a very simple problem.
 

Answers and Replies

  • #2
133
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Comparing allyl radical to the vinyl radical, how many resonance structures can you draw for each? I can draw two for allyl, one for vinyl.

If you notice, this is the same order of stability for carbocations. Radicals are funny, because in some ways they are like carbanions and in others they are like carbocations. In this case, the order of stability parallels carbocations.
 
  • #3
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Thank you DDTea.
 

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