Gauche effect

Main article: alkane stereochemistry

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

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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 (4 kJ/mol). In general a gauche rotamer is less stable than an anti-rotamer.

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 in the gas phase. Another example is 1,2-dimethoxyethane.

There are two main explanations for the gauche effect: hyperconjugation and bent bonds. In the hyperconjugation model, the donation of electron density from the C–H σ bonding orbital to the C–F σ^* antibonding orbital is considered the source of stabilization in the gauche isomer. Due to the greater electronegativity of fluorine, the C–H σ orbital is a better electron donor than the C–F σ orbital, while the C–F σ^* orbital is a better electron acceptor than the C–H σ^* orbital. Only the gauche conformation allows good overlap between the better donor and the better acceptor.

Key in the bent bond 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 electron 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. Of these two models, hyperconjugation is generally considered the principal cause behind the gauche effect in difluoroethane.^[2][3]

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 ^[4]. According to in silico results this conformation is more stable by 0.21 kcal/mol (880 J/mol).

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 ^[5] :

The gauche effect is very sensitive to solvent effects, due to the large difference in polarity of the two conformers. For example, 2,3-dinitro-2,3-dimetylbutane exists only the gauche conformation in the solid state, prefers the gauche conformer in benzene solution by a ratio of 79:21, but in carbon tetrachloride it prefers the anti conformer by a ratio of 58:42.^[6] Another case is trans-1,2 difluorocyclohexane, which has a larger preference for the eq/eq conformer in more polar solvents.^[3]

A related effect is the cis effect in alkenes.

See also

• Anomeric effect

References