JEE Advanced · 2014 · 25 May 2014 · Paper 1 · General Organic Chemistry

The compound is 2,3-dichloro-2,3-dibromobutane: ${\mathrm{CH}}_3\mathrm{C}(\mathrm{Cl})(\mathrm{Br})\mathrm{C}(\mathrm{Cl})(\mathrm{Br}){\mathrm{CH}}_3$.

This molecule has a C-C single bond allowing rotation. We analyze the staggered conformers (most stable):

Newman projections looking down the C2-C3 bond:

1. Anti conformer: Cl and Br on opposite carbons are staggered 180° apart. If the arrangement is asymmetric, dipole moment ≠ 0.
2. Gauche conformers: Two gauche positions (±60°) with different spatial arrangements of Cl and Br.

For non-zero dipole moment, the conformer must be asymmetric (not have a plane or center of symmetry that cancels the dipole).

Systematic analysis of staggered conformers:
• Conformers with Cl-Cl, Br-Br, or Cl-Br eclipsing are unstable.
• Three distinct staggered conformers have non-zero dipole moments due to asymmetric arrangements.

Answer: 3