Hyperconjugation

Under hyperconjugation is understood in organic chemistry, the electronic interaction between a fully occupied orbital to a σ - bond (usually a CH or CC bond ) and an adjacent unoccupied or singly occupied molecular orbital. The overlap of the two orbitals allows additional delocalization of the electrons from the σ - bond, resulting in an additional resonance stabilization results. The hyperconjugation is thus a form of dative bond, the electrons involved do not come from a free occupied (not binding ) orbital but from the orbital of a covalent bond.

Applications

By hyperconjugation can the stability order of alkyl radicals and alkyl cations explain (3 ° > 2 ° > 1 °). Also can be explained by hyperconjugation example, the directing effect of alkyl substituents in electrophilic aromatic substitution explain. Also, the anomeric effect, which, inter alia, occurs when sugars, can be attributed to hyperconjugation. The hyperconjugation contributes, moreover, that the staggered conformations of the alkanes are lower in energy than the Ecliptic: The overlap of a σ - molecular orbital of a CH bond (or a CC bond ) with an unoccupied anti -binding σ * molecular orbital of an adjacent CH bond ( or C-C bond ) is maximized only in the staggered conformation; in the eclipsed conformation however, it does little to overlap between these two orbitals. The format preferred and illustrative in older textbooks statement that the eclipsed conformers are disadvantaged by steric repulsion, takes probably too short, even if the influence of the steric repulsion is still controversial.

Normal hyperconjugation

In the positive hyperconjugation, the electron density of a σ bond by an adjacent empty or only partially filled non-bonding p- orbital or anti-/nicht-bindendes π orbital is reduced by partial delocalization. This electron deficiency compensation effect is stronger the more interactions are geometrically possible. This allows the stability order of the alkyl radicals explain:

• Primary radical < secondary radical < tertiary radical.

As an example, the compound B ( CH3) 3 are: The electrons are moved here from the σ (CH) bond into the empty p orbital on boron ( see chart at right).

The directing effect of an alkyl substituent in an electrophilic aromatic substitution, can also explain in a π * MO of the aromatic system by an electron transfer from the σ - molecular orbital of the CH bond. The aromatics are destabilized and more reactive towards electrophiles.

Negative hyperconjugation

From negative hyperconjugation occurs when electron density is shifted in the opposite way to normal hyperconjugation. This means that the electron density of a p- orbital in example an empty or partially occupied σ * - or d- orbital can be moved. The negative Hyperconjugation contributes to stabilization. How strong is the influence of the d orbitals in this model, is discussed in professional circles yet. Theoretical calculations that take the d- orbitals as polarization functions into account, however, to achieve good results, so even if a minor effect seems proven. In theoretical chemistry, the basis functions represent but physically meaningless and arbitrary selectable functions dar. Any sufficiently large base ( even those with physically interpretable Gaussian functions) must necessarily reach the basis set limit. Especially in the case of high precision calculations also correlated f - functions or even higher angular functions may be necessary in order to come close to the basis set limit is sufficiently close. Therefore, it is questionable whether the theoretical calculations can help to resolve the dispute, it is also unclear whether the discussion at all has a physical foundation.