Cis–trans isomerism

Cis-2-butene

Trans-2-butene

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Cis-1,2-dichlorocyclohexane-2D-skeletal

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Cis–trans isomerism

Cis–trans isomerism, also known as geometric isomerism or configurational isomerism, is a form of stereoisomerism describing the orientation of functional groups within a molecule. This type of isomerism is particularly significant in organic chemistry and coordination chemistry.

Overview

Cis–trans isomerism occurs when two substituents are attached to either side of a double bond or a ring structure. The terms "cis" and "trans" are derived from Latin, where "cis" means "on this side" and "trans" means "on the other side."

• Cis isomer: The substituents are on the same side of the double bond or ring.
• Trans isomer: The substituents are on opposite sides of the double bond or ring.

Examples

Alkenes

In alkenes, cis–trans isomerism arises due to the restricted rotation around the carbon–carbon double bond. For example, in 2-butene, the cis isomer has both methyl groups on the same side of the double bond, while the trans isomer has the methyl groups on opposite sides.

Cycloalkanes

In cycloalkanes, the ring structure restricts the rotation of substituents, leading to cis–trans isomerism. For instance, in 1,2-dichlorocyclohexane, the cis isomer has both chlorine atoms on the same side of the ring, whereas the trans isomer has them on opposite sides.

Coordination Compounds

In coordination chemistry, cis–trans isomerism is observed in square planar and octahedral complexes. For example, in cisplatin, a square planar complex, the two chloride ligands are adjacent to each other in the cis isomer, while in the trans isomer, they are opposite each other.

Properties and Applications

Cis and trans isomers often have different physical and chemical properties. These differences can affect the boiling points, melting points, solubilities, and reactivities of the isomers. For example, cis-2-butene has a higher boiling point than trans-2-butene due to stronger intermolecular forces. In biochemistry, cis–trans isomerism plays a crucial role in the function of biomolecules. For instance, the cis-trans isomerization of proline residues in proteins can influence protein folding and function.

Nomenclature

The IUPAC nomenclature system uses the terms "cis" and "trans" to describe the relative positions of substituents. However, for more complex molecules, the E–Z notation is often used, which is based on the Cahn–Ingold–Prelog priority rules.

See Also

• Stereochemistry
• E–Z notation
• Chirality (chemistry)
• Conformational isomerism
• Tautomerism