What are Geometric Isomers?

Also called a cis-trans isomer, a geometric isomer is a type of stereoisomer which has two states. Isomers are molecules which have the same molecular formula but differ in their molecular structure. A stereoisomer is a type of isomer in which atoms occur in the same order, but are still structurally different. In the case of geometric isomers, molecules either form a stair shape or a U shape.

Commonly occurring in molecules with carbon-carbon double bonds, geometric isomers may occur in any substance which has two central atoms with a double or triple bond. Single bonds, created by two atoms sharing an electron, allow molecules to spin on their axis, called free rotation. Free rotation allows for different structural configurations of molecules even though the atoms are bonded in exactly the same way. Molecules with single bonded central atoms that show different structures might seem to be geometric isomers, but are in fact simply the same molecule that has twisted in its bond.

Double bonds, created by two atoms sharing two electrons, do not allow for free rotation. Since their rotation is restricted, molecules with double bonds cannot twist to form different shapes, so those which seem to have a different structural arrangement are structurally different molecules. In geometric isomers, an atom — or more often a group of atoms — is bonded to each of the central atoms. This creates two types of structures.

In the first, the group of atoms is attached to the central atom in the same place for both atoms, so each side of the molecule is a mirror image of the other. This creates the U shape molecule, or the cis isomer. In the second structure, the atom groups are bonded at opposite places on the central molecules, creating a stair structure.

For example, the simplest geometric isomers are but-2-ene, isomers of the gas butene, which is found in oil. Butene, C₄H₈ has four isomers, two of which are geometric isomers. The central atoms in but-2-ene are both carbon, joined by a double bond. Attached to each carbon atom is a hydrogen atom and a molecule of CH₃. In cis-but-2-ene, the CH₃ molecules are located on the same side of each carbon atom, forming a U shape. In trans-but-2-ene, the CH₃ molecules are located on opposite sides, forming stair shape.

Geometric isomers behave differently from each other. For example cis and trans but-2-ene, have different boiling and melting points. The temperature at which cis isomer boils is 39.2°F (4°C) and its melting temperature is -218°F (-139°C), while the trans isomer will boil at 33.8°F (1°C) and melt at -157°F (-105°C).