Certain molecules exhibit geometric isomerism due to restricted rotation around a double bond or within a ring structure.

Geometric isomerism, often referred to as cis-trans isomerism, is a specific type of stereoisomerism. This phenomenon arises from the limited rotation around a double bond or a ring structure, resulting in distinct spatial arrangements of the atoms or groups attached to the carbon atoms involved.

In molecules containing a double bond, the formation of a pi bond—resulting from the sideways overlap of p orbitals—prevents free rotation. Consequently, the groups or atoms attached to the carbon atoms of the double bond become ‘locked’ in position, leading to various geometric configurations. For instance, in but-2-ene, the methyl groups ($\text{CH}_3$) can be oriented on the same side of the double bond, resulting in the cis-isomer, or on opposite sides, yielding the trans-isomer.

Similarly, in cyclic structures such as cycloalkanes, the inherent rigidity of the ring restricts rotation as well. This can lead to different spatial arrangements of substituents, depending on whether they are positioned on the same side of the ring (cis-isomer) or on opposite sides (trans-isomer).

It is crucial to recognize that geometric isomerism can have a significant impact on the physical and chemical properties of molecules. For example, cis-isomers generally exhibit higher boiling points than their trans counterparts. This difference is attributed to the presence of a dipole moment in cis-isomers, where similar groups are situated on the same side, creating an area of higher electron density and, consequently, a dipole. In contrast, in trans-isomers, the similar groups are positioned on opposite sides, effectively canceling out the dipole moment.

In summary, geometric isomerism is a captivating aspect of molecular structure that arises from restricted rotation within certain molecules. This phenomenon not only leads to diverse spatial arrangements of atoms or groups but also significantly influences the properties of the molecules involved.