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Topic: stability of carbocations  (Read 26653 times)

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Offline orgoclear

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stability of carbocations
« on: May 14, 2009, 04:43:39 AM »
Arrange the following carbocations in decreasing order of their stability:
Benzylic, Allylic and tertiary.
Assume that none of them are substituted
I had this doubt because i say many sites and books writing

Benzylic>Allylic>Tertiary
Benzylic~Tertiary>Allylic
Tertiary>Benzylic~Allylic

I personally feel that the first one is the correct order because the resonance structures of benzylic carbocation are more in number than in allylic carbocation. And generally resonance superseeds inductive effect and hyperconjugation. So, tertiary carbocation will be the least stable one of the three

Anyone who can help me out would be most welcome

Offline Arctic-Nation

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Re: stability of carbocations
« Reply #1 on: May 14, 2009, 06:42:57 AM »
You're very correct in your assessment. Conjugation (allylic) is good, more conjugation (benzylic) is even better, and hyperconjugation helps but is nowhere near as good as normal conjugation.

But keep in mind that substitution can radically alter stability, for better or for worse.

Offline orgoclear

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Re: stability of carbocations
« Reply #2 on: May 15, 2009, 12:03:48 AM »
morrison boyd (which i consider a very good and reliable book)
claims the order to be
tert>benzylic~secondary~allylic>primary>methyl

Offline Arctic-Nation

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Re: stability of carbocations
« Reply #3 on: May 15, 2009, 05:08:25 AM »
Now this is getting interesting. I've done a little more research, and there seems to be quite some disagreeing between sources. While I don't have my entry-level organic textbook (McMurry) with me, which I believe has a nice list with all relevant data, in Clayden's Organic Chemistry it is strongly implied that allylic and benzylic cations are more stable than tertiary cations, on the assumption that real conjugation is better than hyperconjugation.

On the other hand, simple googling will reveal other possibilities, such as: tertiary ~ benzylic > secondary ~ allylic; others will say that the extra conjugation in benzylic cations doesn't make a difference compared to allylic cations.

So yeah, at this moment I'm not really sure myself anymore. :-\

Offline StarvinMarvin

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Re: stability of carbocations
« Reply #4 on: May 15, 2009, 06:23:09 AM »
In my humble opinion, J. Clayden seems to be the most convincing. It cannot be denied that charge delocalization increases its stability, therefore, it would be benzyl>allyl>tert>sec>primary. What is more, as far as I remember, Boyd and Morrison's book had a lot of mistakes (depending on the edition) and I wouldn't rely on that book that much. Maybe I will come up with something more once I try to look it up in March's book (this evening).

Offline Squirmy

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Re: stability of carbocations
« Reply #5 on: May 15, 2009, 07:47:10 AM »
Quote
But keep in mind that substitution can radically alter stability, for better or for worse.

Agreed! I would prefer a more detailed order...something like:

tertiary benzyllic > secondary benzylic  ~ tertiary allylic > tertiary ~  primary benzylic ~ secondary allylic > secondary >/~ primary allylic > primary > methyl

This depends greatly on where you make your breaks between "~" and ">".

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