Trans Isomer Definition

Trans Isomer
 By Alandb (Own work) [CC0], via Wikimedia Commons

A trans isomer is an isomer where the functional groups appear on opposite sides of the double bond. Cis and trans isomers are commonly discussed with respect to organic compounds, but they also occur in inorganic coordination complexes and diazines. 

Trans isomers are identified by adding trans- to the front of the molecule's name. The word trans comes from the Latin word meaning "across" or "on the other side".​

Example: The trans isomer of dichloroethene (see picture) is written as trans-dichloroethene.

Comparing Cis and Trans Isomers

The other type of isomer is called a cis isomer. In cis conformation, the functional groups are both on the same side of the double bond (adjacent to each other). Two molecules are isomers if they contain the exact same number and types of atoms, just a different arrangement or rotation around a chemical bond. Molecules are not isomers if they have a different number of atoms or different types of atoms from each other.

Trans isomers differ from cis isomers in more than just appearance. Physical properties also are affected by conformation. For example, trans isomers tend to have lower melting points and boiling points than corresponding cis isomers. They also tend to be less dense. Trans isomers are less polar (more nonpolar) than cis isomers because the charge is balanced on opposite sides of the double bond. Trans alkanes are less soluble in inert solvents than cis alkanes.

Trans alkenes are more symmetrical than cis alkenes.

While you might think functional groups would freely rotate around a chemical bond, so a molecule would spontaneous switch between cis and trans conformations, this isn't so simple when double bonds are involved. The organization of electrons in a double bond inhibits rotation, so an isomer tends to stay in one conformation or another.

It is possible to change conformation around a double bond, but this requires energy sufficient to break the bond and then reform it.

Stability of Trans Isomers

In acyclic systems, a compound is more likely to form a trans isomer than the cis isomer because it is usually more stable. This is because having both function groups on the same side of a double bond can produce steric hindrance. There are exceptions to this "rule", such as 1,2-difluoroethylene, 1,2-difluorodiazene (FN=NF), other halogen-substituted ethylenes, and some oxygen-substituted ethylenes. When the cis conformation is favored, the phenomenon is termed the "cis effect".

Contrasting Cis and Trans With Syn and Anti

Rotation is much more free around a single bond. When rotation occurs around a single bond, the proper terminology is syn (like cis) and anti (like trans), to denote the less permanent configuration.

Cis/Trans vs E/Z

The cis and trans configurations are considered examples of geometric isomerism or configurational isomerism. Cis and trans should not be confused with E/Z isomerism. E/Z  is an absolute stereochemical description only used when referencing alkenes with double bonds that cannot rotate or ring structures.

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Helmenstine, Anne Marie, Ph.D. "Trans Isomer Definition." ThoughtCo, Dec. 27, 2017, thoughtco.com/definition-of-trans-isomer-605745. Helmenstine, Anne Marie, Ph.D. (2017, December 27). Trans Isomer Definition. Retrieved from https://www.thoughtco.com/definition-of-trans-isomer-605745 Helmenstine, Anne Marie, Ph.D. "Trans Isomer Definition." ThoughtCo. https://www.thoughtco.com/definition-of-trans-isomer-605745 (accessed May 24, 2018).