Gauche effect

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Main article: alkane stereochemistry

The term "gauche" refers conformational isomers (conformers) where two vicinal groups are separated by a 60° torsion angle.

In stereochemistry, gauche interactions hinder bond rotation. For example, sighting along the C2-C3 bond in staggered butane, there are two possible relative potential energies. The two methyl groups can be in an anti-bonding relationship, or offset at sixty degree dihedral angles. In the latter configuration, the two methyls are said to be in a gauche relationship, and the relative potential energy of each methyl-methyl gauche interaction is 0.9 kilocalories per mole. In general a gauche rotamer is less stable than an anti rotamer.

[edit] Gauche effect

The Gauche effect characterizes any gauche rotamer which is actually more stable than the anti rotamer [1]. This effect is present in 1,2-difluoroethane (H2FCCFH2) for which the gauche conformation is more stable by 2.4 to 3.4 KJ/mole. Another example is 1,2-dimethoxyethane.

Gauche effect in 1,2-difluoroethane, left: anti and right: gauche

Key in the explanation of the gauche effect in difluoroethane is the increased p orbital character of both C-F bonds due to the large electronegativity of fluorine. As a result electon density builds up above and below to the left and right of the central C-C bond. The resulting reduced orbital overlap can be partially compensated when a gauche conformation is assumed, forming a bent bond.


The molecular geometry of both rotamers can be obtained experimentally by high resolution infrared spectroscopy augmented with in silico work [1]. In accordance with the model described above, the carbon - carbon bond length is higher for the anti rotamer (151.4 pm vs. 150). The steric repulsion between the fluorine atoms in the gauche rotamer causes increased CCF bond angles (by 3.2°) and increased FCCF dihedral angles (from the default 60° to 71°).

In the related 1,2-difluorodiphenylethane (two protons replaced by phenyl) the threo isomer is found (by X-ray diffraction and from NMR coupling constants) to have an anti conformation between the two phenyl groups and the two fluorine groups and a gauche conformation is found for both groups for the erythro isomer [2]. According to in silico results this conformation is more stable by 0.21 kcal/mole.

A gauche effect has also been reported for a molecule featuring 4 successive fluor syn substituents, the last one introduced by deoxofluorinating agent bis(2-methoxyethyl)aminosulfur trifluoride [3] :

Synthesis of an All-syn Four Vicinal Fluorine Motif

A related effect is the cis effect in alkenes.

[edit] References

  1. ^ a b Contribution to the Study of the Gauche Effect. The Complete Structure of the Anti Rotamer of 1,2-Difluoroethane Norman C. Craig, Anthony Chen, Ki Hwan Suh, Stefan Klee, Georg C. Mellau, Brenda P. Winnewisser, and Manfred Winnewisser J. Am. Chem. Soc.; 1997; 119(20) pp 4789 - 4790; (Communication) DOI:10.1021/ja963819e
  2. ^ The vicinal difluoro motif: The synthesis and conformation of erythro- and threo- diastereoisomers of 1,2-difluorodiphenylethanes, 2,3-difluorosuccinic acids and their derivatives O'Hagan D, Rzepa H, Schuler M, Slawin A Beilstein Journal of Organic Chemistry, 2006 2:19 ( 2 October 2006 ) DOI:10.1186/1860-5397-2-19
  3. ^ Enantioselective Synthesis of an All-syn Four Vicinal Fluorine Motif Luke Hunter, David O'Hagan, and Alexandra M. Z. Slawin J. Am. Chem. Soc.; 2006; 128(51) pp 16422 - 16423; (Communication) DOI:10.1021/ja066188p
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