Negative hyperconjugation

In organic chemistry, negative hyperconjugation is the donation of electron density from a filled π- or p-orbital to a neighboring σ^*-orbital.^[1] This phenomenon, a type of resonance, can stabilize the molecule or transition state.^[2] It also causes an elongation of the σ-bond by adding electron density to its antibonding orbital.^[1]

A schematic representation of negative hyperconjugation. In real systems, several of the hydrogens are replaced with other functional groups.

Negative hyperconjugation is seldom observed, though it can be most commonly observed when the σ^*-orbital is located on certain C–F or C–O bonds.^[3][4]

In negative hyperconjugation, the electron density flows in the opposite direction (from a π- or p-orbital to an empty σ^*-orbital) than it does in the more common hyperconjugation (from a σ-orbital to an empty p-orbital).

See also

• Negative hyperconjugation in silicon
• Conjugated system

References

1. "Negative Hyper-Conjugation" (PDF). Old.iupac.org. Archived from the original (PDF) on 2012-03-24. Retrieved 2013-10-23.
2. "2.2.3 Negative Hyperconjugation for Ebooksclub.org Molecular Orbitals and Organic Chemical Reactions Student Edition". Scribd.com. 2011-04-29. Retrieved 2012-08-20.
3. "Negative hyperconjugation of some fluorine containing groups – New Journal of Chemistry (RSC Publishing)". Pubs.rsc.org. Retrieved 2012-08-20.
4. Karni, Miriam; Bernasconi, Claude F.; Rappoport, Zvi (2007-08-09). "Role of Negative Hyperconjugation and Anomeric Effects in the Stabilization of the Intermediate in SNV Reactions". The Journal of Organic Chemistry. 73 (8): 2980–2994. doi:10.1021/jo7017476. PMID 18376875.