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Author Topic: Stability of carboanions.  (Read 6512 times)

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katehovey

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Stability of carboanions.
« on: May 21, 2007, 06:49:39 AM »

What is the order of primary, secondary and tertiary anions? I know that an acetylide anion is more stable than a vinylic anion, which is more stable than an alkyl anion. From that I'd guess that a less substituted carboanion would be more stable than a higher substituted one. Is this right?
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sjb

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Re: Stability of carboanions.
« Reply #1 on: May 21, 2007, 07:45:49 AM »

What is the order of primary, secondary and tertiary anions? I know that an acetylide anion is more stable than a vinylic anion, which is more stable than an alkyl anion. From that I'd guess that a less substituted carboanion would be more stable than a higher substituted one. Is this right?

Not entirely, it would depend on what the substituents on the carbonanion were.

Consider what makes carbocations more stable, and see if you can apply other trends.

S
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katehovey

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Re: Stability of carboanions.
« Reply #2 on: May 21, 2007, 08:04:35 AM »

Well, I know with carbocations, on a more substituted carbon, hyperconjugation and the inductive effect stabalise the cation. So with a less substituted carboanion, would there be less of both these, having less effect on the lone pair of electrons?
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refid

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Re: Stability of carboanions.
« Reply #3 on: May 21, 2007, 01:14:02 PM »

I think a tert-butyl carbanion would not be stable since the 3 methyl  group would donate electron density towards the anion (less stable). And in primary carbanion (grignards) are stable due to less of this donation of electron density

Sam (NG)

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Re: Stability of carboanions.
« Reply #4 on: May 21, 2007, 01:16:11 PM »

I think a tert-butyl carbanion would not be stable since the 3 methyl  group would donate electron density towards the anion (less stable). And in primary carbanion (grignards) are stable due to less of this donation of electron density

Correct.  (n-BuLi is much more stable than t-BuLi)
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edwinksl

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Re: Stability of carboanions.
« Reply #5 on: May 29, 2007, 10:45:02 PM »

I would say in general, there is an inverse correlation between the stabilities of carbocations and carbanions. Whatever makes a carbocation more stable also makes a carbanion less stable.
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