What is sigma resonance or dancing resonance?
Sigma resonance or dancing resonance is a special stability mechanism which increases stability of carbocations attached directly to the three membered rings, e.g., cyclopropylmethyl carbocation.
Why it occurs?
In cyclopropane, all the carbon is sp3 hybridized and the bond angle for the same should be 109°28′, but the actual bond angle is 60° due to which angle strain develops.
So, in order to minimize the strain, the p-orbital bents due to which it acquires partial sigma-bond and partial pi-bond character which behave like pi-bond. And the resonance takes place between sigma orbital and vacant p-orbital, hence it is called as p-orbital overlapping or sigma resonance or dancing resonance.
The vacant p-orbital lies parallel to C2-C3 bond of the cyclopropane ring and not perpendicular to it, thus, the geometry become similar to that of a cyclopropane ring conjugated with an olefinic bond.
We know by Drago’s rule, the bond angle is directly proportional to the s-character while it is inversely proportional to p-character.
Dancing resonance is a hypothetical phenomenon which reduce strain of the ring, hence the carbocation is more stable.
CH2+ has a vacant p-orbital and a very effective overlapping takes place between p-orbital and electron density of cyclopropane, due to this its stability is very high.
There is a conjugation between the sigma bond and positive charge.
Exceptional stability of cyclopropylmethyl carbocation
The exceptional stability of cyclopropyl methyl carbocation can be explained by the concept of dancing resonance concept.
The stability of additional cyclopropyl group , is result of more conjugation between the bent orbital of cyclopropyl ring and cationic carbon.
The most stable carbocation known till date in organic chemistry is explained by Dancing resonance.
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