Relative stability of allyl radical

In summary, the beginning of the chapter on Conjugated Unsaturated Systems discusses the relative stability of the allylic radical in comparison to other compounds. It states that the allylic radical has a DH° of 369 kJ*mol-1, followed by 3° radical at 400 kJ*mol-1, 2° radical at 413 kJ*mol-1, 1° radical at 423 kJ*mol-1, and vinyl radical at 465 kJ*mol-1. This is a measure of the bond strength in a chemical bond. The order of stability for the radicals is the same as for carbocations. It is also noted that the number of resonance structures for the ally
  • #1
themonk
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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.
 
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  • #2
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
Thank you DDTea.
 

Related to Relative stability of allyl radical

1. What is the relative stability of an allyl radical?

The relative stability of an allyl radical refers to its energy level compared to other similar chemical species. It is known to be more stable than a vinyl radical, but less stable than a propyl radical.

2. What factors influence the relative stability of an allyl radical?

The factors that influence the relative stability of an allyl radical include the number of resonance structures, the presence of electron-donating or electron-withdrawing groups, and the steric hindrance of substituents attached to the allyl group.

3. How does resonance affect the stability of an allyl radical?

Resonance refers to the delocalization of electrons within a molecule. In an allyl radical, the presence of three resonance structures leads to increased stability as the unpaired electron is distributed over three carbon atoms rather than just one.

4. How does the presence of electron-donating or electron-withdrawing groups impact the stability of an allyl radical?

Electron-donating groups, such as methyl or hydroxyl groups, can stabilize an allyl radical by providing additional electron density to the radical center. On the other hand, electron-withdrawing groups, such as nitro or carbonyl groups, can destabilize an allyl radical by withdrawing electron density from the radical center.

5. Can the relative stability of an allyl radical be predicted?

While there are general trends that can help predict the relative stability of an allyl radical, it ultimately depends on the specific molecular structure and its interactions with surrounding groups. Experimental data is often needed to accurately determine the relative stability of an allyl radical in a given system.

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