What is the Difference Between Optical and Geometrical Isomerism?

The main difference between optical and geometrical isomerism lies in the way they differ in structure and properties. Here are the key differences between the two types of isomerism:

1. Structural Difference: Optical isomers have the same molecular formula and the same sequence of chemical bonds, but their atoms are arranged differently in 3D space, making them non-superimposable mirror images. Geometrical isomers, also known as cis-trans isomers, have the same molecular formula but differ in the arrangement of groups at a single atom, at double bonds, or in rings.
2. Physical Properties: Optical isomers have similar physical properties, while geometrical isomers have different physical properties.
3. Optical Activity: Optical isomers are optically active, meaning they can polarize or change the direction of plane-polarized light. In contrast, only cis-isomers in geometrical isomerism show optical activity, while trans-isomers are optically inactive.
4. Criteria: Optical isomerism requires planes of symmetry, non-superimposable mirror images, and chirality. Geometrical isomerism requires a carbon-carbon double bond in the molecule and two unlike atoms or groups linked to each doubly bonded carbon atom.

In summary, optical isomers are non-superimposable mirror images with similar physical properties, while geometrical isomers have different physical properties and are based on the arrangement of substituents around a double bond or in rings.

Comparative Table: Optical vs Geometrical Isomerism

Optical isomerism and geometrical isomerism are two types of stereoisomerism, which occur when molecules have the same molecular formula and sequence of chemical bonds but differ in their spatial arrangement. Here is a table comparing the differences between optical and geometrical isomerism:

In summary, optical isomers are stereoisomers with the same molecular formula and connectivity but differ in the placement of substituted groups around one or more atoms, while geometrical isomers are stereoisomers with the same substituents attached differently to a carbon atom. Optical isomers often occur in biologically important molecules, such as DNA helices, while geometrical isomers are observed in compounds with double bonds between carbon atoms, like alkenes.