Cis–trans isomerism

The cis -trans isomerism or (Z ) - ( E) isomerism referred to in the chemical form of a special Konfigurationsisomerie at which the molecules differ with respect to a reference plane with respect to only the relative position of two substituents. If there are more than two substituents, one considers that the two having the highest priority according to the Cahn -Ingold-Prelog - Convention.

• From a cis - or (Z)- configuration is when both substituents are on the same side of the reference plane.
• From a trans or (E)- configuration is when both substituents are on opposite sides of the reference plane.

A reference plane may be, for example, a double bond or a ring system. cis -and trans- isomers differ in their chemical and physical properties such as melting point, boiling point, enthalpy of binding. cis -and trans- or (Z ) - and (E) - themselves are referred to as descriptors, preceded by the systematic names of substances.

Cis -trans isomerism in double bonds

Cis -trans isomerism occurs when both atoms of a double bond carry different substituents:

• If both substituents are on the same page, it is called a cis - or (Z )-isomer.
• When two substituents are located on opposite sides of the double bond is referred to as a trans - or (E )-isomer.

An example of (Z ) - ( E) isomerism is the pair of isomers (Z)- butenedioic acid ( maleic acid) and (E) - butenedioic acid ( fumaric acid).

Are attached to the two carbon atoms of a double bond are each a hydrogen atom and a larger substituent bound ( vicinal double bonds), the assignment of the cis and trans is clear and this nomenclature designations should be used. On the other hand are more substituents are present, the (E / Z)- nomenclature must be used for over the sequence usually a clear priorities order has been set.

Cis -trans isomerism in ring systems

Cis -trans isomerism is also the case of cyclic compounds, such as monocycles, which have substituents on different carbon atoms, or bicyclic. To name ring systems according to IUPAC rules, the prefix " cis " and " trans " are the connection name prefixed in italics and are used for the alphabetical arrangement only with the same name.

An example of a monocyclic compound having a cis -trans isomerism is 1,2- dimethylcyclopentane: Both methyl substituents may be either on the same side of the molecular plane are (cis- form) or on opposite sides (trans- form).

At 6-membered rings as in 1,2- dimethylcyclohexane, the cis -and trans- isomers may be difficult to see since the compounds should not be considered as planar rings. Compounds of this type are generally present in the energetically favored chair conformation. Since two chair conformations are present by ring inversion, for example, trans-1 ,2- dimethylcyclohexane and cis -1 ,2- dimethylcyclohexane can not immediately recognize. In trans-1 ,2- dimethylcyclohexane both methyl groups are either equatorial or axial. In the cis forms are each a methyl group are equatorial, before the other axial.

An example of a bicyclic compound is decalin, since the two six-membered rings can be linked in different ways: The two hydrogen atoms at the junctions can be either on the same side of Decalinmoleküls are (cis- form) or on opposite sides ( trans form ).

Interconversion

In a double bond, the cis and trans isomers can be interconverted (cis / trans interconversion ). For this purpose, a temporary resolution of the π - bond is required, which usually only at high temperatures or under influence of radiation proceeds. Such transformations are, for example, a central part of the visual process (see retinal).

Between the N-terminus of L-proline and the C-terminus of another amino acid in the peptides present the amide bond ( peptide bond ) may be present both in the trans and in the cis configuration, since this bond has some double bond character. A change between two configurations takes place only slowly, but can be enzymatically catalyzed by a peptidyl- prolyl cis -trans isomerase.