Science, Tech, Math › Science Trans Isomer Definition Share Flipboard Email Print In a trans isomer, functional groups are on opposite sides of a double bond or ring plane. By Alandb (Own work) [CC0], via Wikimedia Commons Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated June 05, 2019 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 is written as trans-dichloroethene. Key Takeaways: Trans Isomer A trans isomer is one in which functional groups occur on opposite sides of a double bond. In contrast, the functional groups are on the same side as each other in a cis isomer.Cis and trans isomers display different chemical and physical properties.Cis and trans isomers share the same chemical formula, but have different geometry. 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. History Friedrich Woehler first notice isomers in 1827 when he discerned silver cyanate and silver fulminate share the same chemical composition, but displayed different properties. In 1828, Woehler discovered urea and ammonium cyanate also had the same composition, yet different properties. Jöns Jacob Berzelius introduced the term isomerism in 1830. The word isomer comes from the Greek language and means "equal part." Sources Eliel, Ernest L. and Samuel H. Wilen (1994). Stereochemistry of Organic Compounds. Wiley Interscience. pp. 52–53.Kurzer, F. (2000). "Fulminic Acid in the History of Organic Chemistry". J. Chem. Educ. 77 (7): 851–857. doi:10.1021/ed077p851Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002). General chemistry: principles and modern applications (8th ed.). Upper Saddle River, N.J: Prentice Hall. p. 91. ISBN 978-0-13-014329-7.Smith, Janice Gorzynski (2010). General, Organic and Biological Chemistry (1st ed.). McGraw-Hill. p. 450. ISBN 978-0-07-302657-2.Whitten K.W., Gailey K.D., Davis R.E. (1992). General Chemistry (4th ed.). Saunders College Publishing. p. 976-977. ISBN 978-0-03-072373-5.