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    Hi!

    Can someone explain when to use cis/trans and when to use E/Z? Thanks!
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    Cis and trans are used when the groups on the two carbon atoms of the double bond are the same. For example in but-2-ene each carbon in the double bond is bonded to one hydrogen atom and one methyl group.You call it cis when the two methyl groups are together (both above or below the double bond) and trans for when they on is above and one is below the double bond. E/Z is used when the groups are different for example. 2-bromo-1-chloropropene where on one carbon in the double bond you have a chlorine atom and a hydrogen atom and on the other you have a methyl group and a bromine atom. In this case the bromine and chlorine take priority so whether these are 'together' or not will determine whether you use E or Z.
    Hope this helps
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    And priority is based on the atomic mass number, so if there is a Br- to Cand H to the same C, the one with the higher Mr has priority. By labeling priorities, if both on top are the same (equal 1-1 or 2-2) then it's Z isomerism. If diagonally across, then it's E isomerism.

    (Original post by Monodon)
    Cis and trans are used when the groups on the two carbon atoms of the double bond are the same. For example in but-2-ene each carbon in the double bond is bonded to one hydrogen atom and one methyl group.You call it cis when the two methyl groups are together (both above or below the double bond) and trans for when they on is above and one is below the double bond. E/Z is used when the groups are different for example. 2-bromo-1-chloropropene where on one carbon in the double bond you have a chlorine atom and a hydrogen atom and on the other you have a methyl group and a bromine atom. In this case the bromine and chlorine take priority so whether these are 'together' or not will determine whether you use E or Z.
    Hope this helps
 
 
 
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