The key difference between optical and geometrical isomerism is that optical isomers are pairs of compounds which appear as mirror images of each other whereas geometrical isomers are pairs of compounds containing the same substituents attached to a carbon-carbon double bond differently.
Optical isomers and geometrical isomers are two types of stereoisomers. Stereoisomers are organic compounds containing the same molecular formula, but a different spatial arrangement of the atoms. The sequence of chemical bonds is also the same for these isomers.
CONTENTS
1. Overview and Key Difference
2. What is Optical Isomerism
3. What is Geometrical Isomerism
4. Side by Side Comparison – Optical vs Geometrical Isomerism in Tabular Form
5. Summary
What is Optical Isomerism?
Optical isomers are stereoisomers that have the same chemical formula and the same connectivity of atoms but a different spatial arrangement. They come in pairs and are related to each other by reflection. This means these compounds appear as mirror images of each other. These mirror images are non-superposable. Human hands are analogous to this type of isomers.
Figure 01: Optical Isomerism result in Mirror Images
The optical isomers of the same compound have similar physical properties except for the property of rotation of plane-polarized light. Here, one isomer rotates the plane-polarized light into one direction while its optical isomer rotates the same plane-polarized light beam to the opposite direction. Therefore, optical isomers can show different biological effects on the same biological system as well.
What is Geometrical Isomerism?
Geometrical isomers are stereoisomers that have the same substituents attached differently to a carbon-carbon double bond. This type of isomers occurs because the double bond between carbon atoms avoid the ability to rotate around the axis of the double bond and this, in turn, gives fixed positions to the isomer. However, in order to be named as a geometrical isomer, the compound should have different substituents attached to both sides of the double bond. If one side of the double bond has the same substituents attached to the carbon atom in that side, then there cannot be a geometrical isomer to that compound.
Figure 02: Geometrical Isomerism in Stilbene
Geometrical isomers are also called cis-trans isomers because these isomers come in pairs, and we name them as cis-isomer and trans-isomer, considering the relative positions of the substituents. Cis-isomers have similar substituents on the same side while trans-isomers have different substituents on one side.
What is the Difference Between Optical and Geometrical Isomerism?
Optical isomers and geometrical isomers are two types of stereoisomers. The key difference between optical and geometrical isomerism is that optical isomers are pairs of compounds which appear as mirror images of each other, whereas geometrical isomers are pairs of compounds containing the same substituents attached to a carbon-carbon double bond differently. Thus, optical isomers are non-superposable mirror images, whereas geometrical isomers have a difference in the connectivity of substituents to the double bond.
Moreover, another difference between optical and geometrical isomerism is that optical isomers have similar physical properties, but geometrical isomers have different physical properties.
Summary – Optical vs Geometrical Isomerism
Stereoisomers are organic compounds containing the same molecular formula but a different spatial arrangement of the atoms. Optical isomers and geometrical isomers are two types of stereoisomers. The key difference between optical and geometrical isomerism is that optical isomers are pairs of compounds which appear as mirror images of each other, whereas geometrical isomers are pairs of compounds containing the same substituents attached to a carbon-carbon double bond differently.
Reference:
1. “Optical Isomerism in Organic Molecules.” Chemistry LibreTexts, Libretexts, 5 June 2019, Available here.
Image Courtesy:
1. “Énantiomères octaèdriques” By Tpa2067 – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Cis-trans-stilbene” By Pancrat – Own work (CC BY-SA 3.0) via Commons Wikimedia