JEE Main 2025 Apr 7 Shift 2, Chemistry Q17: Properties of Alkyl Halides (Halo Alkanes) & Aryl Halides (Halo Arenes)

Statement I: cis-1,2-dichloroethene vs cis-1,2-dibromoethene.

• Both isomers share the same bent (cis) geometry, so bond dipoles reinforce in each.
• Chlorine has vacant 3d orbitals that accept electron density from the C=C $\pi$ system via $p\pi$-$d\pi$ back-bonding, which introduces a partial C=Cl double-bond character:

$\ddot{\text{Cl}}-\text{CH}=\text{CH}-\ddot{\text{Cl}}⟷\stackrel{+}{\mathrm{Cl}}=\text{CH}-\text{CH}=\stackrel{-}{\mathrm{Cl}}$

• This delocalisation enhances the C-Cl bond dipole. Bromine's 4d orbitals are too diffuse and high in energy to participate effectively, so no comparable enhancement occurs in the dibromo compound.

$\therefore$ cis-dichloro is more polar than cis-dibromo. Statement I is correct.

Statement II: trans-1,2-dibromoethene ($\mu =0$) vs cis-1,2-dibromoethene ($\mu \ne 0$).

• cis-dibromo: $\mu \ne 0\Rightarrow$ stronger dipole-dipole forces $\Rightarrow$ higher boiling point.
• trans-dibromo: $\mu =0\Rightarrow$ weaker forces $\Rightarrow$ lower boiling point.

Statement II claims the trans isomer is simultaneously lower boiling and more polar than the cis isomer - self-contradictory, since higher polarity would raise, not lower, the boiling point. Statement II is incorrect.

$\therefore$ Correct choice: Statement I is correct but Statement II is incorrect.