Hyperconjugation

from Wikipedia, the free encyclopedia

In organic chemistry, hyperconjugation is understood to mean an electronic interaction between a fully occupied orbital of a σ bond (usually a CH or CC bond) and an adjacent unoccupied or singly occupied molecular orbital. The overlapping of the two orbitals enables an additional delocalization of the electrons from the σ bond, which results in additional mesomerism stabilization . Hyperconjugation is thus a form of dative bond , whereby the electrons involved do not come from a free occupied (non-binding) orbital, but from the orbital of a covalent bond.

Applications

Hyperconjugation can explain the stability sequence of alkyl radicals and alkyl cations ( tertiary more stable than secondary > primary > methyl ). Hyperconjugation can also explain, for example, the directing effect of alkyl substituents in electrophilic aromatic substitution . Also the anomeric effect , which u. a. occurs in sugars , can be attributed to hyperconjugation. The hyperconjugation also contributes to the fact that the staggered conformations of the alkanes are lower in energy than the ecliptic ones: the overlap of a σ molecular orbital of a CH bond (or a CC bond) with an unoccupied antibonding σ * molecular orbital of an adjacent CH bond ( or CC bond) is here only maximized in the staggered conformation; In contrast, in the ecliptic conformation there is hardly any overlap between these two orbitals. The clear explanation, often used in older textbooks, that the ecliptic conformers are disadvantaged by steric repulsion, does not go far enough, even if the influence of steric repulsion is still controversial.

Normal hyperconjugation

Hyperconjugation (red arrow)

In the case of positive hyperconjugation, the electron density of a σ bond is reduced by means of a neighboring, empty or only partially filled non-binding p orbital or anti / non-binding π orbital by means of partial delocalization . This electron deficiency compensation effect is stronger, the more interactions are geometrically possible. This is how the stability sequence of the alkyl radicals can be explained:

  • primary radical <secondary radical <tertiary radical.

The compound B (CH 3 ) 3 can serve as an example : The electrons are shifted here from the σ (CH) bond into the empty p orbital on boron (see diagram on the right).

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

Negative hyperconjugation

Negative hyperconjugation is when the electron density is shifted in the opposite way to normal hyperconjugation. This means that the electron density from a p orbital in z. B. an empty or partially occupied σ * or d orbital can be moved. The negative hyperconjugation also contributes to stabilization. How strong the influence of the d orbitals is in this model is still being discussed in specialist circles. Theoretical calculations that take the d orbitals into account as polarization functions, however, achieve good results, which is why an effect, albeit a minor one, seems to be proven. In theoretical chemistry, however, the basic functions represent physically meaningless and arbitrarily selectable functions. Every sufficiently large basis (even those with physically non-interpretable Gauss functions) must necessarily reach the basis set limit. In the case of high-precision, correlated calculations in particular, f-functions or even higher angle functions may be necessary to get sufficiently close to the limit of the basic set. It is therefore questionable whether the theoretical calculations can contribute to the solution of the dispute; it is also unclear whether the discussion has a physical foundation at all.

literature

  • Eberhard Breitmaier, Günther Jung: Organic chemistry . 4th edition, Thieme, Stuttgart 2001, p. 50, ISBN 3-13-541504-X .
  • Paula Y. Bruice: Organic Chemistry , 5th ed., Pearson Studium, Munich 2007, pp . 122, 196, 559, ISBN 978-3-8273-7190-4 .

Individual evidence

  1. ↑ V. Pophristic, L. Goodman: Hyperconjugation not steric repulsion leads to the staggered structure of ethane . In: Nature . tape 411 , no. 6837 , 2001, p. 565-568 , doi : 10.1038 / 35079036 .
  2. ↑ Peter R. Schreiner: The "correct" teaching: a lesson from the misunderstood origin of the rotational barrier in ethane . In: Angewandte Chemie . tape 114 , no. 19 , 2002, pp. 3729-3731 , doi : 10.1002 / 1521-3757 (20021004) 114: 19 <3729 :: AID-ANGE3729> 3.0.CO; 2-7 .
  3. ↑ FM Bickelhaupt, EJ Baerends: The Case for Steric Repulsion Causing the Staggered Conformation of Ethane . In: Angewandte Chemie . tape 115 , no. 35 , 2003, p. 4315-4320 , doi : 10.1002 / anie.200350947 .