What are cis and trans isomers

The cis-trans isomerism or (Z)-(E.) Isomerism or. Configuration isomerism In chemistry, denotes a special form of isomerism in which the molecules differ only in terms of the relative position of two substituents with respect to a reference plane. If there are more than two substituents, those two are considered which have the highest priority according to the Cahn-Ingold-Prelog convention.

  • From one cis- or. (Z) Arrangement is spoken when both substituents are on the same Side of the reference plane.
  • From one trans- or. (E.) Arrangement is spoken when both substituents are on opposite Sides of the reference plane.

A reference plane can be, for example, a double bond or a ring system.

Cis-trans isomers differ in their chemical and physical properties such as melting temperature, boiling temperature and binding enthalpy.

cis-trans isomerism in double bonds: (Z) - (E) isomerism

cis-trans isomerism occurs when both atoms of a double bond have different substituents:

  • When both substituents on the same page lying, one speaks of a in IUPAC parlance (Z)-Isomer (Z stands for Together).
  • When the two substituents on opposite sides the double bond, one speaks of a (E.)-Isomer (E. stands for opposite).

For the correct naming of an isomer according to the IUPAC nomenclature, the corresponding prefix "(Z) - "or" (E.) - “prefixed the name of the connection. The letters that were initially disregarded in the alphabetical classification of names are set in italics.

Due to earlier recommendations, however, the prefixes "cis-" For (Z) Isomer and "trans-" For (E.) Isomers in use, which are also set in italics and are only taken into account in alphabetical order when names are identical.

An example of a connection with (Z)-(E.) Isomerism is ethylene dicarboxylic acid and its isomers (Z)-Ethylenedicarboxylic acid (maleic acid) and (E)-Ethylenedicarboxylic acid (fumaric acid):

cis-trans isomerism in ring systems

Cis-trans isomerism also occurs in cyclic compounds, for example in monocycles that have substituents on different carbon atoms, or in bicycles. To name ring systems according to IUPAC rules, the prefixes "cis-" and "trans- “is placed in front of the connection name in italics and is only used for alphabetical classification if the name is the same.

An example of a monocyclic compound with cis-trans isomerism is 1,2-dimethylcyclopentane: the two methyl substituents can either be on the same side of the molecular plane (cis-Form) or on opposite sides (trans-Shape).

An example of a bicyclic compound is decalin, as the two six-membered rings can be linked differently: the two hydrogen atoms at the points of attachment can either be on the same side of the decalin molecule (cis-Form) or on opposite sides (trans-Shape).

Literature / sources

  • CD Römpp Chemie Lexikon - Version 1.0, Stuttgart / New York: Georg Thieme Verlag 1995

Category: Chemical Bond