Although geometrical isomers have completely different physical and chemical properties (for example cis and trans 2 – butene, have different boiling points and densities), optical isomers (also called enantiomers) differ in only one characteristic their interaction with plane polarized light.
Physical properties of geometrical isomers depend on
1. The polarity of the molecules.
2. The shape or symmetry of molecules.
The polarity strongly influences the relative boiling points as it determines the strength of the intermolecular forces.
For example, cis – 1, 2 dichloroethene has a net dipole moment and dipole-dipole attractions between its molecules in addition to the van der Waal’s forces, whereas trans 1, 2 dichloroethene which is non-polar has only van der Waal’s forces. The boiling point of the cis-isomers is therefore higher. Melting point is generally, more influenced by the symmetry of the molecules as this affects the packing in the solid state. The trans isomers are able to pack more closely due to their greater symmetry, so the intermolecular forces are more effective than in the ch – isomer. The melting point of the trans-isomer is therefore higher.