Stereo electronic effects

Stereoelectronic effects are spatial orientations of molecules or certain courses of a chemical reaction, which result from the spatial arrangement of molecular orbitals. The preference for these follows from donor-acceptor interactions of these orbitals.

Basics

The fundamentals of stereoelectronic effects follow from molecular orbital theory . If two orbitals interact, they split into one orbital with a lower energetic position than the starting orbitals, and one with a higher energetic position than the starting orbitals. Not only two atomic orbitals but also two molecular orbitals can interact and thus achieve an energy gain.

Such an interaction is stronger the closer the interacting orbitals are in their energy, and the better the spatial arrangement enables such an overlap. The size of the overlap can be calculated using the overlap integral of the two orbitals.

In contrast to atomic orbitals, however, molecular orbitals do not interact with two single orbitals, but rather a filled or n-orbital interacts with an unfilled * orbital. The tendency to bring the two electrons partly into this new interaction is stronger, the less electronegative the substituent of the electrons of the contributing bond (electron-donor bond) is. This results in the following series for the donor strength of bonds: H ₃ C-CH ₃ > H ₃ C-H> H ₃ C-NH ₂ > H ₃ C-OH> H ₃ C-F. This results from the fact that the position of the donor orbital is lower (and therefore worse for the interaction), the greater the electronegativity of the atoms involved. ${\ displaystyle \ sigma}$ ${\ displaystyle \ sigma}$

It looks the same for the tendency of non-bonding orbitals to give up the electrons. The more electronegative the atom on which the orbital is located, the lower the energy of the orbital and the worse the donor property. This results in the following series: H ₃ P> H ₂ S> H ₃ N> H ₂ O> HF.

With the acceptor property of * orbitals, it is basically the other way around. The higher the electronegativity of the atoms involved, the higher the acceptor property of the * orbital. This also results in a ranking: H ₃ C-H <H ₃ C-CH ₃ <H ₃ C-NH ₂ <H ₃ C-OH <H ₃ C-F. ${\ displaystyle \ sigma}$ ${\ displaystyle \ sigma}$

The gauche effect

Fig. 1: The Gauche effect in 1,2-difluoroethane

If there are two substituents with high steric demands (i.e. a large spatial extent) on adjacent carbon atoms, they are preferably arranged in an antiperiplanar fashion. The Newman projection is used to illustrate this clearly . In the case of certain compounds, on the other hand, a synclinal (old also gauche) arrangement of the substituents is preferred. This occurs above all when substituents with high electronegativity and thus a strong * -acceptor bond are involved. ${\ displaystyle \ sigma}$

1,2-Difluoroethane is always in the synclinal or old in the gauche arrangement. The greatest overlap between one and one * orbital is found in an antiperiplanar arrangement of the bonds (see Fig. 1). Since the CF bond is both a very bad donor and a very good acceptor, the antiperiplanar arrangement for better CH donor bonds is much more favorable and the two fluorine atoms are arranged synclinally. ${\ displaystyle \ sigma}$ ${\ displaystyle \ sigma}$

The anomeric effect

Main article: Anomeric effect

The anomeric effect occurs in saturated six-membered rings that contain a heteroatom (i.e. an atom other than carbon) in the ring that has a non-bonding orbital. In addition, there is a bond on the neighboring carbon atom with strong -acceptor properties. The best overlap results when the non-bonding orbital is antiperiplanar to the bonding of the atom with acceptor properties. In the case of a tetrahydropyran , this means that the substituent is arranged outside the plane of the ring (axially). ${\ displaystyle \ sigma ^ {*}}$

literature

Kirby: Stereoelectronic Effects, Oxford University Press, Oxford 1996
Clayden, Greeves, Warren, Wothers: Organic Chemistry, Oxford University Press, Oxford 2001

Individual evidence

↑ Alabugin, Zeidan, J. Am. Chem. Soc., 2002, 124, 3175-3185.

[1] Alabugin, Zeidan, J. Am. Chem. Soc., 2002, 124, 3175-3185.