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 one another. Geometric isomers are chemical species with the same type and quantity of atoms as one another, yet having different geometric structures. In geometric isomers, 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.

Geometric or Cis-Trans Isomers

  • Geometric or cis-trans isomerism describes the spatial arrangement of atoms within molecules that have the same chemical formulas.
  • Geometric isomers are compounds that contain either double bonds or else ring structures that prevent functional groups from freely rotating around a chemical bond.
  • In a cis isomer, the functional groups are on the same side of a chemical bond.
  • In a trans isomer, the functional groups are on opposite or transverse sides of a bond.

Cis and Trans Geometric Isomers

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. (Note that cis-trans isomerism is a different description of geometry than E-Z isomerism.)

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, while 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 International Union of Pure and Applied Chemistry (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.

In cis-1,2-dichloroethene, the two chlorine atoms are the functional groups and they are both on the same side of the carbon-carbon double bond. In trans-1,2-dichloroethene, the chlorine atoms are on opposite sides of the double bond. In this example, the cis isomer has a boiling point of 60.3 °C. The trans isomer has a boiling point of 47.5 °C.

E-Z Isomerism

Cis-trans notation has certain limitations. For example, it does not work with alkenes when there are more than two substituents. In such cases, E-Z notation is preferable. E-Z notation identifies the structure of a compound using absolute configuration based on the Cahn-Ingold-Prelog priority rules.

In E-Z notation, the E comes from the German word entgegen, which means "opposed", and Z comes from the German word zusammen, meaning "together". In the E configuration, the higher priority groups are trans to each other. In the Z configuration, the higher priority groups are cis to each other.

However, the cis-trans and E-Z systems compare different groups so Z does not always correspond to cis and E does not always correspond to trans. For example, trans-2-chlorobut-2-ene has the C1 and C4 methyl groups trans to each other, but the compound is (Z)-2-chlorobut-2-ene because the chlorine and C4 groups are together and the C1 and C4 are opposite.

Sources

  • Bingham, Richard C. (1976). "The stereochemical consequences of electron delocalization in extended π systems. An interpretation of the cis effect exhibited by 1,2-disubstituted ethylenes and related phenomena". J. Am. Chem. Soc. 98 (2): 535–540. doi:10.1021/ja00418a036
  • IUPAC (1997). "Geometric isomerism". Compendium of Chemical Terminology (2nd ed.) (the "Gold Book"). Blackwell Scientific Publications. doi:10.1351/goldbook.G02620
  • March, Jerry (1985). Advanced Organic Chemistry, Reactions, Mechanisms and Structure (3rd ed.). ISBN 978-0-471-85472-2.
  • Ouellette, Robert J.; Rawn, J. David (2015). "Alkenes and Alkynes". Principles of Organic Chemistry. doi:10.1016/B978-0-12-802444-7.00004-5. ISBN 978-0-12-802444-7.
  • Williams, Dudley H.; Fleming, Ian (1989). "Table 3.27". Spectroscopic Methods in Organic Chemistry (4th rev. ed.). McGraw-Hill. ISBN 978-0-07-707212-4.
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Helmenstine, Anne Marie, Ph.D. "Geometric Isomer Definition (Cis-Trans Isomers)." ThoughtCo, Mar. 2, 2022, thoughtco.com/definition-of-geometric-isomer-cis-trans-604481. Helmenstine, Anne Marie, Ph.D. (2022, March 2). Geometric Isomer Definition (Cis-Trans Isomers). Retrieved from https://www.thoughtco.com/definition-of-geometric-isomer-cis-trans-604481 Helmenstine, Anne Marie, Ph.D. "Geometric Isomer Definition (Cis-Trans Isomers)." ThoughtCo. https://www.thoughtco.com/definition-of-geometric-isomer-cis-trans-604481 (accessed May 28, 2022).