Geometric isomers are stereoisomers with two states, commonly found in molecules with carbon-carbon double bonds. They have different structures due to limited rotation in double bonds, and behave differently from each other. For example, cis and trans but-2-ene have different boiling and melting points.
Also called a cis-trans isomer, a geometric isomer is a type of stereoisomer that has two states. Isomers are molecules that have the same molecular formula but differ in their molecular structure. A stereoisomer is a type of isomer where the atoms are in the same order, but are still structurally different. In the case of geometric isomers, the molecules form a ladder or U shape.
Commonly found in molecules with carbon-carbon double bonds, geometric isomers can occur in any substance that 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 own axis, called free spin. Free rotation allows for different structural configurations of molecules even though the atoms are bonded in exactly the same way. Molecules with central atoms bonded by a single bond that exhibit different structures might appear to be geometric isomers, but are in fact simply the same molecule that has become entangled in its bond.
Double bonds, created by two atoms sharing two electrons, do not allow for free rotation. Because their rotation is limited, molecules with double bonds cannot twist to form different shapes, so those that appear 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 former, the group of atoms is attached to the central atom at the same point for both atoms, so each side of the molecule is a mirror image of the other. This creates the U-shaped molecule or cis isomer. In the second structure, the atomic groups are bonded at opposite points on the central molecules, creating a ladder structure.
For example, the simplest geometric isomers are but-2-ene, isomers of the gas butene, which is found in petroleum. 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 CH3 molecule. In cis-but-2-ene, the CH3 molecules are on the same side of each carbon atom, forming a U shape. In trans-but-2-ene, the CH3 molecules are on opposite sides, forming a U shape. to scale.
Geometric isomers behave differently from each other. For example cis and trans but-2-ene, they have different boiling and melting points. The temperature at which the 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 will melt at -157°F (-105°C).
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