Summary
Highlights
Geometric or cis-trans isomerism is a type of spatial isomerism defined by the different arrangements of atoms relative to the plane of the double bond.
Isomerism, a key reason for the diversity of organic substances, occurs because the carbon skeleton is flexible. Isomers are substances with the same composition but different molecular structures. This phenomenon was discovered in 1824 by Justus von Liebig and Friedrich Wöhler, who found two substances with identical composition but different properties: cyanic acid and fulminic acid. The term 'isomer' was coined by Jöns Jacob Berzelius in 1830.
Isomers are divided into two main classes: structural and spatial. Structural isomers differ in molecular structure and the order of atom connectivity. Organic molecules exhibit three types of structural isomerism: hydrocarbon skeleton isomerism, isomerism due to the different positions of a substituent or multiple bond, and interclass isomerism, where isomers belong to different classes of organic compounds with different functional groups.
Spatial isomerism, particularly geometric (cis-trans) isomerism, is characteristic of alkenes and other compounds with double bonds. This type of isomerism arises from the restricted rotation around the double bond. The first cases of geometric isomerism were observed by Johannes Wislicenus in the 1870s with fumaric and maleic acids.
The systematic nomenclature of alkenes derives from the names of corresponding alkanes by replacing the suffix '-ane' with '-ene'. The main chain is chosen to include the double bond, even if it's not the longest chain. Numbering starts from the end closest to the double bond, and the position is indicated after the '-ene' suffix. Common historical names like 'ethylene' (ethene) and 'propylene' (propene) are also used.
Geometric isomerism in ethene compounds is caused by the absence of free rotation around the double bond. For cis-trans isomers to exist, each carbon atom involved in the double bond must not have identical atoms or groups of atoms attached to it. If they do, cis and trans isomers cannot form.