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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Aravind on September 18, 2012, 11:46:05 AM

Title: why is the order of stability the opposite in the case of carboanion
Post by: Aravind on September 18, 2012, 11:46:05 AM

i studied that the carbocation and free radical have the stability order 3*>2*>1*
this is due to hyperconjugation and inductiive effect stabilisation ,so why isnt the same order there for carboanion ??wont these efects play similiar role and give 3*>2*>1*.I cannot reason this .

Title: Re: why is the order of stability the opposite in the case of carboanion
Post by: discodermolide on September 18, 2012, 12:01:43 PM

The stabilisation of carbanions does not follow the same rules as for carbocations and radicals.
Carbanion stabilisation is governed by the type of neighbouring group it is attached to, for example a ketone or an ester. See picture. Of course the anion can also stabilise itself towards the ester.
So you have resonance stabilisation to give a keto form and an enol form.

Title: Re: why is the order of stability the opposite in the case of carboanion
Post by: fledarmus on September 18, 2012, 01:00:22 PM

The stability of a positive charge is increased if the positive charge isn't quite so large - in other words, if you have adjacent electron donating groups or resonance structures that will move the charge onto a more electropositive atom the positive charge will effectively be somewhat reduced, and the carbocation will be stabilized. However, electron donating groups would de-stabilize a negative charge. On the other hand, electron withdrawing groups, or resonance structures that will move the negative charge onto a more electronegative atom, will stabilize a carbanion but destabilize a carbocation.