Dancing Resonance
Also known as Sigma-Bond Resonance or Super-Hyperconjugation.
"Dancing Resonance" is a special type of conjugation observed in the Cyclopropylmethyl (CPM) Carbocation. It provides exceptional stability to the molecule, making it even more stable than the benzyl cation, despite the ring strain.
1. Mechanism of Stabilization
Why is it called Dancing Resonance?
In the cyclopropyl ring, the bond angles are $60^\circ$ (instead of the ideal $109.5^\circ$ for $sp^3$). To minimize strain, the orbitals bend outward, forming Bent Bonds or Banana Bonds.
These bent $\sigma$-orbitals (rich in p-character) are perfectly aligned parallel to the empty p-orbital of the cationic carbon ($-\overset{+}{C}H_2$). This allows for effective overlap and delocalization of the $\sigma$-electrons into the empty p-orbital.
2. Stability Comparison
The extent of overlap in dancing resonance is massive, leading to extraordinary stability.
CPM vs Benzyl Cation
Although the Benzyl cation is stabilized by extensive resonance (4 canonical structures), the Cyclopropylmethyl (CPM) cation is more stable.
Reason: The orbital overlap in dancing resonance is stronger and more effective than the $\pi-\pi$ overlap in benzene resonance.
3. Important Facts
- This effect is strictly observed in Carbocations. It is generally not significant in free radicals or carbanions.
- The bond angle strain ($60^\circ$) is the driving force that pushes the electron density out, making it available for conjugation.
- Multiple cyclopropyl rings attached to the cationic center have an additive effect on stability.
Knowledge Check
Test your understanding of Dancing Resonance
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