cis-1,2-Dichloroethene vs trans-1,2-Dichloroethene - Isomer Relationship

Introduction

Two small molecules with just two carbons, two hydrogens, and two chlorines — but they have different boiling points and dipole moments. Why?

Formula Comparison

Both have the formula C₂H₂Cl₂ with the same connectivity: Cl–CH=CH–Cl. Same formula, same bonding sequence.

Connectivity Analysis

Each carbon in the C=C double bond bears one Cl and one H. The bonding connectivity is identical in both molecules.

Spatial Relationship

In the cis isomer, both Cl atoms are on the same side of the double bond, creating a net dipole moment. In the trans isomer, the Cl atoms are on opposite sides, and their dipoles cancel out.

Chirality Check

Neither molecule has a chiral center. The isomerism comes from restricted rotation around the C=C double bond, not from tetrahedral stereochemistry.

Classification

These are cis-trans (geometric) isomers. cis-1,2-Dichloroethene has a dipole moment of 1.90 D; the trans form has zero dipole moment due to symmetry. The cis isomer boils at 60°C; trans at 48°C.

Related Topics