UPSC Civil Services (Mains), Chemistry optional · 2026 · Paper I

Q5(b). cis- and trans-dichloroethylene from dipole moments

Question

Distinguish between cis- and trans-dichloroethylene by using dipole moments.

The cis isomer is polar (μ>0) because its two C–Cl bond moments add; the trans isomer has μ=0 because they cancel.

Each C–Cl bond moment points from C to Cl. The molecular dipole is their vector sum: μᵣₑₛ≈2μ₁1/2θ for two equal moments at angle θ.
the C–Cl bonds are on the same side, about 60° apart. Components along C=C cancel, those across it add: μ≈√3 μ_(C–Cl), polar.
the two C–Cl moments are parallel and opposite (θ=180°), so μₜᵣₐₙₛ=0. The molecule has a centre of inversion (C₂ₕ), and no molecule with an inversion centre can be polar.
μ is measured from the relative permittivity at several temperatures (Debye equation). The isomer with a measurable μ is cis; the one with μ=0 is trans.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

Each C–Cl bond moment points from C to Cl. The molecular dipole is their vector sum: μᵣₑₛ≈2μ₁1/2θ for two equal moments at angle θ.
cis: the C–Cl bonds are on the same side, about 60° apart. Components along C=C cancel, those across it add: μ≈√3 μ_(C–Cl), polar.
trans: the two C–Cl moments are parallel and opposite (θ=180°), so μₜᵣₐₙₛ=0. The molecule has a centre of inversion (C₂ₕ), and no molecule with an inversion centre can be polar.
Test: μ is measured from the relative permittivity at several temperatures (Debye equation). The isomer with a measurable μ is cis; the one with μ=0 is trans.

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