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Very often, cis–trans stereoisomers contain double bonds or ring structures. In both cases the rotation of bonds is restricted or prevented. [4] When the substituent groups are oriented in the same direction, the diastereomer is referred to as cis, whereas when the substituents are oriented in opposing directions, the diastereomer is referred to as trans.
Diastereomerism can also occur at a double bond, where the cis vs trans relative positions of substituents give two non-superposable isomers. Many conformational isomers are diastereomers as well. In the case of diastereomerism occurring at a double bond, E-Z , or entgegen and zusammen (German), is used in notating nomenclature of alkenes .
All stereoisomers that are not enantiomers are diastereomers. Diastereomerism also exists in alkenes. Alkenes are designated Z or E depending on group priority on adjacent carbon atoms. E/Z notation describes the absolute stereochemistry of the double bond. Cis/trans notation is also used to describe the relative orientations of groups.
Traditionally, double bond stereochemistry was described as either cis (Latin, on this side) or trans (Latin, across), in reference to the relative position of substituents on either side of a double bond. A simple example of cis–trans isomerism is the 1,2-disubstituted ethenes, like the dichloroethene (C 2 H 2 Cl 2) isomers shown below. [7]
See: cis–trans isomerism. cis (left) and trans (right) configured double bond in maleic acid and fumaric acid. cis (left) and trans (right) isomerism in a ring system. The descriptors cis (lat. on this side of) [2] and trans (lat. over, beyond) [3] are used in various contexts for the description of chemical configurations: [4] [5]
E–Z configuration, or the E–Z convention, is the IUPAC preferred method of describing the absolute stereochemistry of double bonds in organic chemistry.It is an extension of cis–trans isomer notation (which only describes relative stereochemistry) that can be used to describe double bonds having two, three or four substituents.
Simple cis and trans isomers may be indicated with a prefixed cis-or trans-: cis-but-2-ene, trans-but-2-ene. However, cis-and trans-are relative descriptors. It is IUPAC convention to describe all alkenes using absolute descriptors of Z-(same side) and E-(opposite) with the Cahn–Ingold–Prelog priority rules (see also E–Z notation).
The isomers, actually diastereomers, are both chiral. They are the cis and trans isomers arising from the hydrogenation of the terpenes pinene. Both isomers undergo reaction with air (autoxidation) to give 2-pinane hydroperoxides. [1] Partial reduction of these isomers gives 2-pinanol. [2]