Carbanion stability

[youngxgold]

Okay so I know for carbocations, it's better if the Carbon with the plus is substituted to more carbon groups. Thus, a 3 degree substituted is the most stable.

Is it the opposite for carbanions? The most stable kind of carbon with 3 sigma bonds and a lone pair (-1 formal charge) is not substituted/attached to any other R groups at all correct?

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[venacontracta]

Similar figures for carbocations and radicals.

 

[PiBond]

i think about it like this: a carbanion has a surplus of electrons and thus has a negative charge. in organic chemistry we don't like localized charges. since alkyl groups are electron donating they destabilize carbanions because they are further contributing to the localized charge.

therefore, if we had to choose the most stable carbanion between having alkyl groups attached or H's, then the H's win

 

[orgohacks]

Nice pics David. Did you make those yourself?

 

[Isoprop]

nice picture, but can someone explain why the cyclopropyl anion is where it is? It's a secondary carbanion but more stable than flouromethyl. Something to do with ring strain I'm guessing.

Edit--Now that I think of it, is that carbanion sp2 hybridized? That would explain it with the ring strain.

 

[PiBond]

the carbanion in cyclopropyl isn't sp2, so I'm guessing the reason why it's more stable than fluoromethyl has to do with the fact that the fluorine can donate-in one of its lone pairs which destabilizes the carbanion.

the ring strain on the cyclopropyl would make is less stable.

 

[Isoprop]

Hmm, so then why would a cyclopropyl carbanion be significantly more stable than other secondary carbanions?

 

[PiBond]

no clue, maybe it has something to do with the geometry because a cyclopropyl ring has a wicked amount of ring strain.

orgoshacks would know better than me

 

[orgohacks]

Hmm, so then why would a cyclopropyl carbanion be significantly more stable than other secondary carbanions?

Pibond is right about the ring strain being a factor.

The bonds in cyclopropanes have a higher amount of s-character than normal sp3 bonds. the electron density is outside the ring. sometimes these are referred to as "banana bonds" or bent bonds. the angles are around 120 degrees, making the hybridization on cyclopropyl bonds closer to sp2, hence the stabilization (and similarity to vinyl)

http://en.wikipedia.org/wiki/Bent_bond

hope this helps

 

[Isoprop]

Pibond is right about the ring strain being a factor.

The bonds in cyclopropanes have a higher amount of s-character than normal sp3 bonds. the electron density is outside the ring. sometimes these are referred to as "banana bonds" or bent bonds. the angles are around 120 degrees, making the hybridization on cyclopropyl bonds closer to sp2, hence the stabilization (and similarity to vinyl)

http://en.wikipedia.org/wiki/Bent_bond

hope this helps

Thanks for the info.

 

[SaintJude]

Similar figures for carbocations and radicals.

I need this post.

 

[syoung]

I need this post.

Props to david. that's pretty wicked.