[ritwik08]

Why will the Carbanion in the structure not participate in resonance?

CH=CH-C^-=CH

There was no information as what satisfies the valency of the terminal carbons. But the question is that inspite of finding a conjugate structure of C=C-C^-, why doesnt the carbanion take part in resonance?

[azmanam]

thats a very nuanced question.

you'll need to look at the shape and orientation of all the p-orbitals in that structure and compare that to the requirements for resonance

[ritwik08]

thats a very nuanced question.

you'll need to look at the shape and orientation of all the p-orbitals in that structure and compare that to the requirements for resonance

CH₂=CH-C^-=CH₂

I will tell the hybridisation states from left to right:
sp2
sp2
sp/sp2
sp2

The carbanion can change its hybridisation state to facilitate resonance. My teacher told me this. so even if the hybrisdisation state comes to sp2 (2 sigma bond pair and a lone pair), can't it change its hybridisation state to sp to facilitate hybridisation?

[azmanam]

Yes it can change its resonance if it will gain stabilization through resonance, and if it will participate in resonance, it will need to be sp hybridized to place that lone pair in a p-orbital.

now, what is true about the physical orientation of the orbitals in space, and how does that jive with the requirements for resonance

(hint, it might also help to look at the neutral structure from which this anion might have been derived)

[ritwik08]

I still cannot understand, the carbanion will change its hybridisation to sp and participate in resonance. Whats wrong in it?

[azmanam]

list up the requirements for resonance.  then tell me about the p-orbitals on the various carbon atoms and how they are oriented in 3d space.

The key here is going to lie in the fact that the carbanion is on a carbon atom participating in a double bond.  I think we might both agree that in the absence of that double bond on the right, the carbanion would participate in resonance just fine.  So what's different about that double bond and why might it preclude the carbanion from participating in resonance?

[spirochete]

Resonance is not the only factor that determines stability of molecule.  I'd suggest looking up factors affecting alkene stability in your text book.  Most have a page where they detail heats of hydrogenation for different alkenes as a way of indirectly measuring stability.  That should practically give you the answer here.