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E and Z

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E-Z (Cis-trans) isomers

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Key Questions

  • Answer:

    #E-Z# isomerism is a type of stereoisomerism that exists because of restricted rotation about double bonds.

    Explanation:

    In stereoisomers, the atoms are joined in the same order, but they have a different spatial arrangement.

    In #E-Z# isomers you must have:

    • restricted rotation, often involving a #"C=C"# double bond
    • two different groups on one end of the bond and two different groups on the other end.

    An alkene such as but-2-ene has two different groups on each alkene carbon.

    It can exist as #E-Z# isomers that differ in the positions of the substituents on the double-bonded atoms.

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    The substituents can be given "priorities", with atoms with higher atomic numbers given higher priorities (the Cahn-Ingold-Prelog rules).

    If the highest priority groups for each carbon are on the same side of the molecule, we have the #Z# isomer.

    If the highest priority groups for each carbon are on opposite sides of the molecule, we have the #E# isomer.

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    One way to remember the designations is to think of #Z# as Zame Zide (same side).

    Since #"CH"_3# has higher priority than #"H"#, it is pretty easy to determine that the butene isomers above are #E# and #Z#, respectively.

    But what about the isomers of 2-methylbut-2-enoic acid (below)?

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    #"CH"_3# has higher priority than #"H"#, and #"COOH"# has higher priority than #"CH"_3#.

    Angelic acid has the higher priority groups on the same side, so it is the #Z# isomer.

    Tiglic acid has the higher priority groups on opposite sides, so it is the #E# isomer.

  • Answer:

    #E-Z# isomerism occurs because there is restricted rotation about double bonds.

    Explanation:

    In #E-Z# isomers you must have:

    • restricted rotation, often involving a #"C=C"# double bond
    • two different groups on one end of the bond and two different groups on the other end.

    For example, an alkene such as but-2-ene has two different groups on each alkene carbon.

    It can exist as #E-Z# isomers that differ in the positions of the substituents on the double-bonded atoms.

    www.4college.co.uk

    The substituents can be given "priorities", with atoms with higher atomic numbers given higher priorities (the Cahn-Ingold-Prelog rules).

    If the highest priority groups for each carbon are on the same side of the molecule, we have the #Z# isomer.

    If the highest priority groups for each carbon are on opposite sides of the molecule, we have the #E# isomer.

    www.compoundchem.com

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