Chemistry - STEREO ISOMERISM Question with Solution | TestHub

The situation in $\mathrm{ClC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{Cl}$ is the reverse of $\mathrm{FC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{F}$ : the anti conformer is found to be more stable than the gauche conformer.

gauche form (less stable) anti form (more stable)

Two effects compete. The hyperconjugative gauche effect (σ(C-H) donation into σ*(C-X)) weakens sharply down the group, because σ*(C-Cl) lies much higher in energy than σ*(C-F). Meanwhile chlorine is much larger than fluorine, so steric repulsion in the gauche form is significant, and the two C-Cl bond dipoles repel each other in the gauche arrangement while they cancel in the anti form. Steric and electrostatic effects therefore win for $\mathrm{ClC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{Cl}$, and the anti conformer is the more stable.