Hyperconjugation

In organic chemistry, hyperconjugation (σ-conjugation or no-bond resonance) refers to the delocalization of electrons with the participation of bonds of primarily σ-character. Usually, hyperconjugation involves the interaction of the electrons in a sigma (σ) orbital (e.g. C–H or C–C) with an adjacent unpopulated non-bonding p or antibonding σ* or π* orbitals to give a pair of extended molecular orbitals. However, sometimes, low-lying antibonding σ* orbitals may also interact with filled orbitals of lone pair character (n) in what is termed negative hyperconjugation.[1] Increased electron delocalization associated with hyperconjugation increases the stability of the system.[2][3] In particular, the new orbital with bonding character is stabilized, resulting in an overall stabilization of the molecule.[4] Only electrons in bonds that are in the β position can have this sort of direct stabilizing effect — donating from a sigma bond on an atom to an orbital in another atom directly attached to it. However, extended versions of hyperconjugation (such as double hyperconjugation[5]) can be important as well. The Baker–Nathan effect, sometimes used synonymously for hyperconjugation,[6] is a specific application of it to certain chemical reactions or types of structures.[7]

Hyperconjugation: orbital overlap between a σ orbital and π* orbital stabilizes alkyl-substituted alkenes. The σ orbital (solid color) is filled, while the π* orbital (grayed) is an unpopulated antibonding orbital. Ref. Clayden, Greeves, Warren
  1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "hyperconjugation". doi:10.1351/goldbook.H02924
  2. ^ John McMurry. Organic chemistry, 2nd edition. ISBN 0-534-07968-7
  3. ^ Alabugin, I.V.; Gilmore, K.; Peterson, P. (2011). "Hyperconjugation". WIREs Comput Mol Sci. 1: 109–141. doi:10.1002/wcms.6. S2CID 222197582.
  4. ^ The mixed orbital of antibonding character is, in fact, raised in energy compared to the original antibonding orbital. However, since the antibonding orbital remains unpopulated in most cases, this does not usually affect the energy of the system.
  5. ^ Alabugin, I. V. (2016) Remote Stereoelectronic Effects, in Stereoelectronic Effects: A Bridge Between Structure and Reactivity, John Wiley & Sons, Ltd, Chichester, UK. doi:10.1002/9781118906378.ch8
  6. ^ Cite error: The named reference Deasy-CR-1945 was invoked but never defined (see the help page).
  7. ^ Madan, R.L. (2013). "4.14: Hyperconjugation or No-bond Resonance". Organic Chemistry. Tata McGraw–Hill. ISBN 9789332901070.