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Because the cis–trans and E–Z systems compare different groups on the alkene, it is not strictly true that Z corresponds to cis and E corresponds to trans. For example, trans -2-chlorobut-2-ene (the two methyl groups, C1 and C4, on the but-2-ene backbone are trans to each other) is ( Z )-2-chlorobut-2-ene (the chlorine and C4 are together ...
In larger rings (8 or more atoms), cis–trans isomerism of the double bond may occur. This stability pattern forms part of the origin of Bredt's rule, the observation that alkenes do not form at the bridgehead of many types of bridged ring systems because the alkene would necessarily be trans in one of the rings.
The difference between cis-and trans-isomers. More generally, cis–trans isomerism will exist if each of the two carbons of in the double bond has two different atoms or groups attached to it. Accounting for these cases, the IUPAC recommends the more general E–Z notation, instead of the cis and trans prefixes.
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]
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.
Alitretinoin. For organic molecules with multiple double bonds, it is sometimes necessary to indicate the alkene location for each E or Z symbol. For example, the chemical name of alitretinoin is (2E,4E,6Z,8E)-3,7-dimethyl-9-(2,6,6-trimethyl-1-cyclohexenyl)nona-2,4,6,8-tetraenoic acid, indicating that the alkenes starting at positions 2, 4, and 8 are E while the one starting at position 6 is Z.
But-2-ene is an acyclic alkene with four carbon atoms. It is the simplest alkene exhibiting cis/trans-isomerism (also known as (E/Z)-isomerism); that is, it exists as two geometric isomers cis-but-2-ene ((Z)-but-2-ene) and trans-but-2-ene ((E)-but-2-ene). It is a petrochemical, produced by the catalytic cracking of crude oil or the dimerization ...
Trans-alkenes are about 1 kcal/mol more stable than cis-alkenes. An example of this effect is cis- vs trans-2-butene. The difference is attributed to unfavorable non-bonded interactions in the cis isomer. This effects helps to explain the formation of trans-fats in food processing. In some cases, the isomerization can be reversed using UV-light.