[alexofordummies]

Looking at an allylic free radical, it must be sp2 hybridized otherwise it wouldn't be able to resonate with the C=C next door. Now what if instead of being allylic it has 3 bonds to sp3 or hydrogen species, is it more favorable for that one electron to now sit in an sp3 orbital and act as a lone pair?

[alexofordummies]

Now that I think about allylic carbanions, I believe what I said above should be correct. Because non allylic carbanions, (similar to non allylic free radicals) are normally sp3 hybridized, only becoming sp2 when it allows them to resonate with their neighbors through the pi system.

[alexofordummies]

Could someone still verify my logic is correct?

[azmanam]

carbanions retain sp³ hybridization.  Radicals and carbocations rehybridize to sp² regardless of neighboring groups.  The only exception is when the carbanion can be stabilized by resonance.  then the carbanion rehybridizes to sp².  The cyclopentadienyl anion is an example of an sp³ anion rehybridizing.

[aldoxime_amine]

The cyclopentadienyl anion is an example of an sp³ anion rehybridizing.

Shouldn't that be sp^2?

[azmanam]

no.  in the absence of resonance, the anion would be sp³.  Because it can be stabilized by resonance, the anion rehybridizes to sp².

[Enantiomer]

no.  in the absence of resonance, the anion would be sp³.  Because it can be stabilized by resonance, the anion rehybridizes to sp².

To be fair, wouldn't the chemical almost always be more stable in resonance?  there by making the molecule sp² hybridized?
I remember a question in organic I on a practice test I had, and the question was whether an oxygen anion (connected to a single carbon) was sp² or sp³ and it was always sp² due to the resonance, even when it is in the anion form

[aldoxime_amine]

no.  in the absence of resonance, the anion would be sp³.  Because it can be stabilized by resonance, the anion rehybridizes to sp².

But isn't the molecule always in a resonsnce hybrid? so that the anion is in effect sp2, i agree with the previos poster's answer

[azmanam]

Not necessarily.  Carbon acids with low pk_a usually have a low pk_a because the resulting anion is stabilized by resonance.  In that case, the anion will rehybridize to sp².  But this does not mean every carbanion is resonance stabilized.  In fact, because they are decidedly pseudo tetrahedral (formally trigonal pyramidal), it is possible to create chiral carbanions and use them as nucleophiles to create optically active products.

Read the lede, and this section of wikipedia: http://en.wikipedia.org/wiki/Carbanion#Chiral_carbanions

[aldoxime_amine]

Not sure if i understood much of what was written, but thanks anyway.

[macman104]

I think what aldoxime is asking is if the anion CAN be stabilized by resonance, will it.  I think it's understood that not all anions are capable of resonating, but if they can, they will.

[azmanam]

In that case, we are in agreement.  If it can be stabilized, it definitely will.  Systems always tend to be as stable as possible. 

Sorry if I misunderstood/was unclear:)