Geometric isomer

Geometric isomerism is a form of stereoisomerism found in organic chemistry and inorganic chemistry describing the spatial arrangement of groups within molecules. This type of isomerism arises due to the restricted rotation around double bonds or because of the presence of a rigid structure, such as a cyclic compound, which prevents the free rotation of the molecule. Geometric isomers have the same molecular formula but differ in the spatial orientation of their atoms or groups.

Overview

Geometric isomerism is most commonly encountered in compounds that contain double bonds (e.g., alkenes) or ring structures. The most familiar examples are the cis-trans isomers of alkenes. In these compounds, the atoms or groups attached to the carbon atoms involved in the double bond can be arranged in two different ways: on the same side (cis) or on opposite sides (trans) of the double bond. This difference in arrangement can significantly affect the physical and chemical properties of the isomers.

Types of Geometric Isomerism

There are two main types of geometric isomerism:

• Cis-trans isomerism: This occurs when two identical or similar groups are either on the same side (cis) or on opposite sides (trans) of a double bond or a ring structure.
• E-Z isomerism: A more comprehensive system used to describe isomers of molecules with more than two substituents. The E (from the German Entgegen, meaning "opposite") and Z (from the German Zusammen, meaning "together") nomenclature is based on the sequence rules of Cahn-Ingold-Prelog priority rules to determine the priority of the groups attached to the double-bonded carbons.

Importance of Geometric Isomerism

Geometric isomerism plays a crucial role in the physical and chemical properties of molecules. Isomers can have different boiling points, melting points, densities, and even different biological activities. For example, the cis and trans isomers of a substance may have different colors, solubilities, and reactivities. In biochemistry, the geometric isomerism of compounds can be essential for the biological activity of molecules, including drugs, where one isomer may be therapeutically active, and the other may be less active or inactive.

Examples

• 1,2-dichloroethene: This molecule can exist as either a cis or trans isomer, with the chlorine atoms either on the same side or opposite sides of the double bond.
• Retinal: A key molecule in the vision process, exists in different geometric isomers (cis and trans forms), which are interconverted in the eye, triggering the visual process.

See Also

• Stereochemistry
• Isomer
• Optical isomerism
• Conformational isomerism