Why do certain molecules show geometric isomerism?

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

Geometric isomerism, also known as cis-trans isomerism, is a form of stereoisomerism. This type of isomerism arises from the restricted rotation around a double bond or a ring structure, which creates different spatial arrangements of the atoms or groups attached to the carbon atoms involved in the double bond or ring.

In a molecule with a double bond, the pi bond formed by the sideways overlap of p orbitals prevents free rotation. This means that the groups or atoms attached to the carbon atoms of the double bond are 'locked' in place, leading to different geometric arrangements. For example, in but-2-ene, the methyl groups (CH3) can be on the same side of the double bond (cis-isomer) or on opposite sides (trans-isomer).

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

It's important to note that geometric isomerism can significantly affect the physical and chemical properties of the molecules. For instance, cis-isomers usually have higher boiling points than trans-isomers due to the presence of a dipole moment in the molecule. This is because the similar groups in cis-isomers are on the same side, creating an area of higher electron density and thus a dipole. In contrast, in trans-isomers, the similar groups are on opposite sides, cancelling out the dipole moment.

In summary, geometric isomerism is a fascinating aspect of molecular structure that arises from the restricted rotation in certain molecules. It not only leads to different spatial arrangements of atoms or groups, but can also significantly influence the properties of the molecules.