What is a defining feature of cis-trans isomerism?

Cis-trans isomerism, also known as geometric isomerism, is characterized by the relative positions of functional groups within a molecule that has restricted rotation around a bond, typically in alkenes or some cyclic compounds. This form of isomerism is indeed a special case of E/Z isomerism, where molecules have certain groups or substituents that can be oriented differently based on their spatial arrangement.

In the case of cis-trans isomers, if two identical groups are on the same side of the double bond or ring structure, the isomer is referred to as "cis." Conversely, if these groups are on opposite sides, it is labeled "trans." This definition emphasizes that cis-trans isomerism is specifically focused on cases where there is symmetry or identical groups involved, thereby creating distinct molecular geometries that can lead to different physical and chemical properties.

Recognizing that cis-trans isomerism is specifically highlighted in cases where groups are identical helps clarify why this choice is the defining feature. Other forms of isomerism, such as structural isomerism, do not necessarily focus on the spatial arrangement of identical groups in this manner, which distinguishes cis-trans as a specialized consideration within the broader classification of isomers.