Geometric Isomer Definition (Cis-Trans Isomers)

How Cis-Trans Isomers Work

The rotation of atoms around a bond produce cis and trans geometric isomers.
The rotation of atoms around a bond produce cis and trans geometric isomers. Todd Helmenstine

Isomers are chemical species that have the same chemical formulas, yet are different from each other. For example, learn about geometric isomerization:

Geometric Isomer Definition

Geometric isomers are chemical species with the same type and quantity of atoms as another species, yet having a different geometric structure. Atoms or groups exhibit different spatial arrangements on either side of a chemical bond or ring structure.

Geometric isomerism is also called configurational isomerism or cis-trans isomerism. Note cis-trans isomerism is a different description of geometry than E-Z isomerism.

The terms cis and trans are from the Latin words cis, meaning "on this side". and trans, meaning "on the other side". When substituents are both oriented in the same direction as each other (on the same side), the diastereomer is called cis. When the substituents are on opposing sides, the orientation is trans.

Cis and trans geometric isomers exhibit different properties, including boiling points, reactivities, melting points, densities, and solubilities. Trends in these differences are attributed to the effect of the overall dipole moment. The dipoles of trans substituents cancel each other out, which the dipoles of cis substituents are additive. In alkenes, trans isomers have higher melting points, lower solubility, and greater symmetry than cis isomers.

Identifying Geometric Isomers

Skeletal structures maybe be written with crossed lines for bonds to indicate geometric isomers. The IUPAC does not recommend the crossed line notation anymore, preferring wavy lines connecting a double bond to a heteroatom. When known, the ratio of cis- to trans- structures should be indicated.

Cis- and trans- are given as prefixes to chemical structures.

Examples of Geometric Isomers

Two geometric isomers exist for Pt(NH3)2Cl2, one in which the species are arranged around the Pt in the order Cl, Cl, NH3, NH3, and another in which the species are ordered NH3, Cl, NH3, Cl.

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Helmenstine, Anne Marie, Ph.D. "Geometric Isomer Definition (Cis-Trans Isomers)." ThoughtCo, Jan. 11, 2018, Helmenstine, Anne Marie, Ph.D. (2018, January 11). Geometric Isomer Definition (Cis-Trans Isomers). Retrieved from Helmenstine, Anne Marie, Ph.D. "Geometric Isomer Definition (Cis-Trans Isomers)." ThoughtCo. (accessed January 18, 2018).