The carbanion which is most stable is


The answer is 1. But why won't the answer be 2 because it shows resonance, whereas 1 only shows I effect and resonance effect is more dominant?

  • $\begingroup$ did you miss out (3).. carbanion is most stabilised in option 3 due to presence of phenyl ring $\endgroup$ – glucose Apr 14 '19 at 14:04
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    $\begingroup$ now that you've pointed out I'm confused between (2) and (3) but the answer is (1) $\endgroup$ – saurya yadav Apr 14 '19 at 16:16
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    $\begingroup$ I am not able to make out why answer is 1... but will surely let you know once I am able to find out some excerpt on it... that is the reason I commented it and not answered ;) $\endgroup$ – glucose Apr 14 '19 at 17:26
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    $\begingroup$ Think of the pKa, as the answer by glucose has suggested. One way is to think about which bases it reacts with. Acetylene is deprotonated by even bases like ammoniacal silver nitrate (a.k.a. Tollen's reagent) Which is not a very strong base. But toluene is very hardy and can't be deprotonated even by very strong bases like sodamide. $\endgroup$ – S R Maiti Apr 14 '19 at 20:45

Some points to check while comparing stability of a carbanion:

  • Inductive effect: preferentially $\ce{-I}$ effect
  • Resonance: preferential $\ce{-M}$ effect
  • Hybridization: preferentially attached to carbon atom having more s-character

... many more: stability

Now the problem is option 1 and 3 are in itself very 'nice' (w.r.t. stability) possible cases of Hybridization-stabilization and Resonance-stabilization respectively.

So the natural (what I thought) way out here is comparing $\ce{pKa}$ 's of acetylene and toluene respectively for option 1 and 3.

from here :

$\ce{pKa}$ of acetylene is about 25.

$\ce{pKa}$ of toluene is about 43.

Less $\ce{pKa}$ denotes how (much more) feasible is for the acid to dissociate by deprotonating itself.

So, acetylene carbanion is more stable than toluene carbanion


You can read up this link and widen your concept. https://chemistry.stackexchange.com/a/10703/73527 I liked the fact what @glucose stated ,so my answer is just a continuation. You can opt out option 2& 4

4 option

You should know that it has a +I effect

3 option

because the no.of congujating structures is just two .. Not enough for stability. right! Now coming to the real answer Why pka of toluene is more than acetylene? pka facts shows that acetylene can deprononate quite easily but why? Why hybridisation precedes resonance?

With toluene , the methyl group is electron-donating onto the benzene ring, so toluene extends the p orbital system all throughout the system, but sometimes the electrons are outside of the ring. Since the conjugation extends outside of the ring, not all of its resonance structures contribute to a stabilization of the ring itself.

So we can see why hybridisation precedes. Greater the electronegativity more it can carry negative charge stability

 whereas 1 only shows I effect 

No, it doesn't show only inductive effect but hybridisation hence electronegativity also play a role. Greater the s character greater the electronegativity and hence greater the charge stability.


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