Cis and Trans

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Cis and Trans, E and Z Nomenclature - Double Bond Isomerism
11:00 — by Jules B.

Tip: This isn't the place to ask a question because the teacher can't reply.

Key Questions

  • When Pi bonds are involved.

    When a compound only have sigma bonds (single bonds) there is allowance for free rotation therefore no cis or trans isomers.

    When you add in pi bonds (double bonds) there is a lot strain on the carbon atoms holding them in place, so there is no free rotation about the bond. And the one or both doubled bonded carbon atoms have another group attached to them other than hydrogen,

    Cis isomers are when the hydrogen atoms are on the same side, trans isomers are when the hydrogen atoms are on either side. Reference to the hydrogen atoms that are bonded to the carbon with the double bond.

    google images

    Black= Carbon
    White= Hydrogen
    Green= Any molecule or compound (R)

    Good Luck:)

  • Answer:

    yes !


    suppose you have given the compound 2- butene and told to draw the structure ,
    there are two structures possible as you can see in the image
    so which structure will you draw??
    hope you got the point........

  • Answer:

    If similar groups are on the same side of a double bond, they are cis to each other; if they are on opposite sides of a double bond, they are trans to each other.


    The cis-trans definition is unambiguous only when you have two different groups on one of the alkene carbons and the same two groups on the other carbon, as in but-2-ene.

    Then the two identical methyl groups are either cis or trans to each other, and the two identical hydrogen atoms are either cis or trans to each other.

    The ambiguity comes from the definition of "similar groups".

    Consider a molecule like 3-methylpent-2-ene

    Some chemists argue that the structure above is trans-3-methylpent-2-ene because the identical methyl groups are on opposite sides of the double bond.

    Other chemists argue that this is cis-3-methylpent-2-ene because the main chain passes through the double bond with its similar alkyl groups (methyl and ethyl) on the same side of the double bond.

    Note that the cis and trans prefixes are always in italics and they are joined by a hyphen to the name of the molecule.

    The Cahn-Ingold-Prelog convention removes this ambiguity.