Cis–Trans Isomerism
Same formula, different geometry — cis and trans isomers share a sequence of bonded atoms but differ in 3D orientation, producing distinctly different properties.
Cis–trans isomerism describes a phenomenon that multiple molecules have an identical chemical formula but different arrangements of atoms. cis means that the functional groups are on the same side of some plane, while trans means that they are on opposite sides. Cis–trans isomers are stereoisomers that have the same molecular formula and sequence of bonded atoms, but differ only in the three-dimensional orientations of their atoms in space.
A simple example of cis–trans isomerism is but-2-ene, shown as follows. The cis-2-butene has a lower melting point than the trans-2-butene, presumably because the former has a smaller contact surface to form van der Waals bonds with its kins than the latter.
The two fatty acids, Oleic acid and elaidic acid, provide another example: Oleic acid, the cis isomer, has a melting point of 17 °C, whereas the trans isomer, elaidic acid, has a higher melting point of 44 °C. This is due to the fact that the straighter trans isomers are more likely to form more van der Waals interactions among them.