Negative hyperconjugation

{{Short description|Phenomenon in organic chemistry}}

File:NegativeHyperconjugation-8.svg

In organic chemistry, negative hyperconjugation is the donation of electron density from a filled π- or p-orbital to a neighboring σ^*-orbital.{{cite web |url=http://old.iupac.org/goldbook/NT07081.pdf |title=Negative Hyper-Conjugation |publisher=Old.iupac.org |accessdate=2013-10-23 |url-status=dead |archiveurl=https://web.archive.org/web/20120324085532/http://old.iupac.org/goldbook/NT07081.pdf |archivedate=2012-03-24 }} This phenomenon, a type of resonance, can stabilize the molecule or transition state.{{cite web|url=https://www.scribd.com/doc/54173058/46/Negative-Hyperconjugation |title=2.2.3 Negative Hyperconjugation for Ebooksclub.org Molecular Orbitals and Organic Chemical Reactions Student Edition |publisher=Scribd.com |date=2011-04-29 |accessdate=2012-08-20}} It also causes an elongation of the σ-bond by adding electron density to its antibonding orbital.

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.{{cite web|url=http://pubs.rsc.org/en/content/articlelanding/2008/NJ/b718430a |title=Negative hyperconjugation of some fluorine containing groups – New Journal of Chemistry (RSC Publishing) |publisher=Pubs.rsc.org |date= |accessdate=2012-08-20}}{{cite journal|title=Role of Negative Hyperconjugation and Anomeric Effects in the Stabilization of the Intermediate in SNV Reactions |journal=The Journal of Organic Chemistry |volume=73 |issue=8 |pages=2980–2994 |date=2007-08-09 |doi=10.1021/jo7017476 |pmid=18376875 |last1 = Karni|first1 = Miriam|last2=Bernasconi |first2=Claude F. |last3=Rappoport |first3=Zvi }}

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).