Chemistry - STEREO ISOMERISM Question with Solution | TestHub

With X = F the compound is 1,2-difluoroethane : the gauche conformer is stabilized by the gauche effect - hyperconjugation from the C-H σ bonds into the low-lying σ* of the C-F bonds, which is geometrically optimal only in the gauche arrangement - so the gauche form wins over anti. For $\mathrm{Cl}$, $\mathrm{Br}$ and I the σ* levels are higher and steric/electrostatic effects dominate, making the anti form more stable.