Cis and trans isomers are stereoisomers which have the same molecular formulas but different orientations in three-dimensional space. In a cis isomer, the functional groups are on the same side of the plane as each other, while in a trans isomer, the functional groups are on opposite sides of each other. The terms “cis” and “trans” come from Latin words, where cis means “on this side” and trans means “on the other side.” Cis and trans isomerism is the same as “geometric isomerism,” but the IUPAC strongly favors the use of the terms cis and trans.
How Cis and Trans Isomers Work
Cis and trans isomers occur in both organic and inorganic molecules.
In organic molecules, functional groups are on the same side of the plane of the carbon chain in cis configuration and on opposing sides of the carbon chain in the trans configuration. Here, cis and trans isomers typically occur in compounds containing double bonds. In linear compounds (like alkenes) the substituents freely rotate around a single bond and don’t form isomers. Aromatic compounds, on the other hand, can have cis and trans isomers with single bonds because the ring restricts motion around the bond.
A double bond is rigid. A molecule can’t switch between its cis and trans isomer without breaking the bond first. The reason you don’t see cis and trans isomers around triple bonds is because the bond uses up the bonding pairs of electrons, so there aren’t two functional groups attached on either side.
Cis and trans isomerism occurs in some inorganic molecules. Examples include the diazenes (which contain N=N bond), diphosphenes (which contain the P=P bond), and coordination compounds.
Examples of Cis and Trans Isomers
Here are some examples of cis and trans isomers:
- cis-but-2-ene and trans-but-2-ene (alkenes)
- cis-1,2-dichlorocyclohexane and trans-1,2-dichlorocyclohexane (aromatic hydrocarbon)
- cis-platin and trans-platin (inorganic)
When naming these isomers, write cis and trans first (in italics). Then, write a dash and then the compound name. (Sometimes the italics and the dash are omitted.)
Difference Between Physical Properties of Cis and Trans Isomers
Cis and trans isomers commonly display different physical properties. In some compounds, the differences are slight. In others, they are more pronounced. The reason for the different properties is because the overall dipole moment changes depending on whether the functional groups are on the same side of the molecule or one opposite sides. So, molecules with polar functional groups are more affected by their configurations.
With some exceptions, trans alkenes have lower boiling points and higher melting points than cis alkenes. This is because the trans isomer is more symmetrical and less polar. Essentially, the dipoles in trans alkenes cancel each other out.
Trans alkenes are less soluble in inert solvents and tend to be more stable than cis alkenes. Having both substituents on the same side of the bond causes some steric interaction in the cis alkenes, usually making them less stable. However, there are notable exceptions. For example, the cis isomers of 1-2-difluoroethylene and 1,2- difluorodiazene are more stable than the trans isomers.
Cis and Trans vs E/Z Notations
Cis-trans and E/Z notations are two different notations that describe isomer. In the E/Z notation, the “E” comes from the German word entgenen (meaning “opposed”) and “Z” comes from the German zusammen (meaning “together”). While cis and trans only apply when there are at most two different substituents, E/Z notation describes the position of additional functional groups. For example, E/Z notation finds use in trisubstituted and tetrasubstituted alkenes. The priority of the group is according to the Cahn-Ingold-Prelog priority rules, where an atom with a higher atomic number has a higher priority.
Usually, Z corresponds to a cis isomer and E corresponds to a trans isomer. But, there are exceptions! For example, trans-2-chlorobut-2-ene is (Z)-2-chlorobut-2-ene. The two methyl groups are trans to each other, but it’s a Z isomer because the C1 and C4 carbon atoms are opposite each other, with the chlorine and C4 together.
References
- IUPAC (1997). “Geometric isomerism.” Compendium of Chemical Terminology (2nd ed.) (the “Gold Book”). Blackwell Scientific Publications. ISBN 0-9678550-9-8. doi:10.1351/goldbook
- 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. ISBN 978-0-12-802444-7. doi:10.1016/B978-0-12-802444-7.00004-5
- Williams, Dudley H.; Fleming, Ian (1989). Spectroscopic Methods in Organic Chemistry (4th rev. ed.). McGraw-Hill. ISBN 978-0-07-707212-4.