Gauche effect

Typically studied examples include 1,2-difluoroethane (H2FCCFH2), ethylene glycol, and vicinal-difluoroalkyl structures.

The resulting reduced orbital overlap can be partially compensated when a gauche conformation is assumed, forming a bent bond.

[5][6] The molecular geometry of both rotamers can be obtained experimentally by high-resolution infrared spectroscopy augmented with in silico work.

A gauche effect has also been reported for a molecule featuring an all-syn array of four consecutive fluoro substituents.

The reaction to install the fourth one is stereoselective:[8] The gauche effect is also seen in 1,2-dimethoxyethane[citation needed] and some vicinal-dinitroalkyl compounds.

The gauche effect is illustrated by the fact 1,2-difluoroethane (left) adopts a gauche conformation whereas 1,2-diiodoethane (right) adopts an anti conformation. The F–C–C–F dihedral angle is 68°, whereas the I–C–C–I dihedral angle is 180°. [ 1 ]
Relative conformation energy diagram of butane as a function of dihedral angle. [ 3 ] A: antiperiplanar, anti or trans. B: synclinal or gauche. C: anticlinal or eclipsed. D: synperiplanar or cis. [ 4 ]
In 1,2-difluoroethane, shown in two conformations (top) and corresponding Newman projections (bottom), the anti conformation (left) is less stable than the gauche conformation (right).
Hyperconjugation model for explaining the gauche effect in 1,2-difluoroethane
Synthesis of an All-syn Four Vicinal Fluorine Motif
Synthesis of an All-syn Four Vicinal Fluorine Motif