Difference Between Geometrical and Optical Isomerism
Isomers are compounds that have the same molecular formula but differ in their structural arrangement. There are various types of isomerism, including geometrical and optical isomerism. These two types of isomerism exhibit different characteristics and have distinct differences. In this article, we will explain the difference between geometrical and optical isomerism.
Geometrical isomerism, also known as cis-trans isomerism, occurs when two different groups or atoms are bonded to a central atom, resulting in possible different spatial arrangements. This type of isomerism arises due to the presence of a double bond or a ring structure. Geometrical isomers cannot freely rotate around the bond that connects them, resulting in different spatial arrangements of the substituent groups.
The key difference between geometrical and optical isomerism lies in how they arise. Geometrical isomerism arises due to the restricted rotation around a bond, while optical isomerism arises due to the presence of a chiral center in a molecule.
Optical isomerism, also known as enantiomerism, occurs when a molecule has a chiral center. A chiral center is an atom that is bonded to four different substituent groups. This chiral center leads to two mirror-image isomers that are non-superimposable, just like our left and right hands. These two mirror-image isomers are called enantiomers, and they have the same physical and chemical properties except for their interaction with plane-polarized light.
One of the main differences between geometrical and optical isomerism is their geometry. Geometrical isomers have different spatial arrangements due to the restricted rotation around a bond, whereas optical isomers have mirror-image arrangements due to the presence of a chiral center.
Another difference lies in their representation. Geometrical isomers are typically represented using cis-trans notation, while optical isomers are represented using the R/S notation. The cis-trans notation indicates the relative position of substituent groups on a double bond or a ring structure, while the R/S notation represents the absolute configuration of a chiral center.
The next difference between geometrical and optical isomerism is their physical properties. Geometrical isomers have similar physical properties, such as melting point and boiling point, due to their identical chemical formula and bonding. However, they may have different chemical reactivity due to their different spatial arrangements. On the other hand, optical isomers have almost identical physical properties, except for their interaction with plane-polarized light. This optical activity arises from the ability of the enantiomers to rotate the plane of polarized light in opposite directions.
The differences between geometrical and optical isomerism are also evident in their naming conventions. Geometrical isomers are named using cis and trans prefixes to indicate their spatial arrangements. The cis isomer refers to the substituent groups on the same side of a double bond or a ring structure, while the trans isomer refers to the substituent groups on opposite sides. Optical isomers, on the other hand, are named using R and S configurations, which depend on the priority of the substituent groups around the chiral center.
The next difference lies in their occurrence. Geometrical isomerism is commonly observed in compounds with double bonds or ring structures. For example, alkenes can exhibit geometrical isomerism when they have substituent groups on both sides of the double bond. Additionally, cyclic compounds with substituent groups can also exhibit geometrical isomerism. Optical isomerism, on the other hand, can be seen in compounds with a chiral center. Chiral centers are often found in organic compounds containing carbon atoms bonded to four different groups.
Furthermore, the interconversion of geometrical isomers is generally easier than the conversion of optical isomers. Geometrical isomers can interconvert under various conditions, such as rotation around the bond or a reaction that breaks the bond temporarily. Optical isomers, however, cannot be interconverted without breaking and reforming the chiral center. This interconversion requires an external influence, such as a chemical reaction.
In conclusion, geometrical and optical isomerism are two different types of isomerism with distinct characteristics and differences. Geometrical isomerism arises due to the restricted rotation around a bond, while optical isomerism arises due to the presence of a chiral center. Geometrical isomers have different spatial arrangements, while optical isomers have mirror-image arrangements. Geometrical isomers are represented using cis-trans notation, while optical isomers are represented using the R/S notation. They have different physical properties, naming conventions, occurrence, and interconversion. Understanding these differences can help in distinguishing between geometrical and optical isomers and explaining their unique properties.