How do you determine which molecules have a cis-trans isomers?
cis-trans isomerism in organic compounds exists for alkenes. These are classified under geometric isomerism which are a class of stereoisomers wherein the isomerism exists due to the arrangement of groups around the double bond. This is a permanent isomerism and cannot be interchanged from cis to trans or vice versa by mere rotation about the bond.
When two highest priority groups are placed on the same side of the double bond, it is termed as cis- isomer.
Whereas, when two highest priority groups are placed on the opposite sides of the double bond, it is termed as a trans- isomer.
Shown in the picture are cis and trans isomers of 2-butene.
As can be seen when the two highest priority -CH3 groups lies on the same side, it is cis-2-butene
Whereas, when the two CH3- groups are on opposite sides it is a trans-2-butene molecule.
In short, when similar priority groups are placed on the same side, it is a cis isomer and when similar priority groups are placed on the opposite sides, it is a trans isomer. This holds true for all types of compounds.
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