Sep 2, 2009
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Can someone explain why Substituted Allylic Carbocation is more stable than Tertiary carbocation? Why is Tertiary carbocation more stable than Allylic Carbocation?

I understand that hyperconjugation and conjugation is part of it but unclear as to how these work. Other factors could also be e- delocalization, resonsance structure(?)

What is the difference between hyperconjugation and conjugation on stability of carbocation? I know that hyperconjugation is between empty sigma orbitals and conjugation is between pi orbital, but how does this determine which is more stable?
 

UndergradGuy7

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Its all about resonance I think. In the substituted allylic carbocation if you moved the double bond and did the other structure, you would get a tertiary substituted double bond. This delocalizes the positive charge better than a tertiary carbocation. This is because, not only does the double bond help to delocalize the electrons, but it is also tri-substituted which help. In the tertiary carbocation only the CH3 groups help to delocalize the electrons.

In the allylic carbocation it can do resonance but it will be a primary substituted double bond.

3 substituted double bond > 2 > 1
 
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hyperconjugation and conjugation? Anyone?
 

SaSSyBruin

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Achiever shows that the tert-butyl carbocation is less stable then the allylic one but shows this involving radical stability. Is there is any difference regarding this between carbocation and radical stability ( I dont think so)?

Also, which is more stable, a tertiary carbocation or radical or one that can be delocalized in a conjugated allylic system??? RSD where did this question come from?
 
Mar 20, 2010
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Stability of Carbo-Cations is methyl< primary<secondary=PRIMARY Allylic=PRIMARYBenzylic<tertiary

CH3< 1* < 2* = 1* allylic = 1* benzylic< 3*

The key here is that tertiary is above all Except 2* or 3* allylic and Benzylic, 2*<3* allylic or Benzylic of course.

So the real order is

CH3< 1* < 2* = 1* allylic = 1* benzylic< 3* <2* Allylic= 2* Benzylic<3* Allylic= 3* Benzylic.

More substitution at allylic or benzylic position = more stability

My Chem Teacher pulls this stuff on our test constantly, internal Re-arrangement via H shifts or methyl shifts etc, and if you're not careful you get smoked.
 
Sep 2, 2009
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Stability of Carbo-Cations is methyl< primary<secondary=PRIMARY Allylic=PRIMARYBenzylic<tertiary

CH3< 1* < 2* = 1* allylic = 1* benzylic< 3*

The key here is that tertiary is above all Except 2* or 3* allylic and Benzylic, 2*<3* allylic or Benzylic of course.

So the real order is

CH3< 1* < 2* = 1* allylic = 1* benzylic< 3* <2* Allylic= 2* Benzylic<3* Allylic= 3* Benzylic.

More substitution at allylic or benzylic position = more stability

My Chem Teacher pulls this stuff on our test constantly, internal Re-arrangement via H shifts or methyl shifts etc, and if you're not careful you get smoked.

It still doesn't answer the question why Substituted Allylic is more stable than Tertiary Carbocation. What is the reason, why it is more stable is what I'm interested in. I'm trying to understand it rather than memorizing the facts.

My guess is that In Substituted Allylic Carbocation, there is pi orbital overlap which allows electrons to delocalize through the entire molecule. Where as in Tertiary Carbocation, although there is hyperconjugation, overlap of sigma orbitals and empty p orbital on the positive charge the electrons can only localized around the Positive charge(?) This limitation is why it is less stable compared to Substituted Allylic Carbocation. This is only my take on it. It would be great if someone with insight on this topic can shed some light on the hyperconjugation & conjugation issue.


SaSSyBruin: I got this from an Ochem textbook.
 
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TexasOMFS

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My guess is that In Substituted Allylic Carbocation, there is pi orbital overlap which allows electrons to delocalize through the entire molecule. Where as in Tertiary Carbocation, although there is hyperconjugation, overlap of empty sigma orbitals where electrons can circulate. The electrons are still only localized around the Positive charge(?) This limitation is why it is less stable compared to Substituted Allylic Carbocation. This is only my take on it. It would be great if someone with insight on this topic can shed some light on the hyperconjugation & conjugation issue.

SaSSyBruin: I got this from an Ochem textbook.
You're absolutely correct. Don't forget that the reason the substituted allylic carbocation (1st figure) is more stable than the allylic carbocation (3rd), is that R groups are electron donating and basically help by shoving electron density towards the positive charge. In some cases, allylic carbocation is less stable than a tertiary carbocation. In other cases, it is more stable. It's a matter of the substitution. Btw you don't need to know the concept of hyperconjugation for the DAT. At any rate, it's a THEORETICAL concept. We don't know for sure if it's really happening.