In nomenclature, what does "cis" and "trans" refer to?

"Cis" and "trans" are terms used to describe the geometric arrangement of substituents around a double bond or within a ring structure in organic chemistry, specifically relating to isomerism. In this context, "cis" indicates that similar or identical substituents are on the same side of the double bond or ring, while "trans" denotes that they are on opposite sides. This different spatial arrangement can significantly affect the physical and chemical properties of the compounds, such as boiling points and reactivity.

For example, in the case of 2-butene, the cis isomer has both methyl groups positioned on the same side of the double bond, potentially creating steric hindrance, whereas the trans isomer has the methyl groups on opposite sides, leading to a more extended molecular conformation. This differentiation is crucial for applications in fields such as pharmaceuticals, where the activity of a compound can depend heavily on its isomeric form. Hence, the correct identification of "cis" and "trans" relates directly to isomerism based on substituent positions.