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Cis/Trans and Z/E

  1. Oct 23, 2012 #1
    What's the difference between Cis/Trans and Z/E? Do they mean different things? And are they attributed differently. From what I understood, Cis/Trans are about Alkenes? Otherwise, when would it be appropriate to use Cis/Trans and when would it be appropriate to use Z/E?

    Also, what would happen if 2 high priority groups were attached to the same Carbon (Let's say we have Br and Cl attached to Carbon #2 while Carbon #1 only has 2 Hydrogen attached to it )Would it be Cis or Trans (Z or E?) since I've been taught to split the molecule parallel to the double bond? And what would happen if both groups were of equal priority (Instead of Br and Cl, it's 2 Br)?
  2. jcsd
  3. Oct 23, 2012 #2


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    Cis/Trans have very limited usage. For example, if the 4 groups attached to sp2 carbons are all different, you can't use Cis/Trans. In this case, you would use Z/E.

    If you are able to define Cis/Trans, do so as they are easy to deduce and imagine, as Z/E are messy and don't click right away, while C/T do.

    In your question, (Br)(Cl)C=CH2, it neither C/T/Z/E. As you can't make any geometrical isomer of this. See, these prefixes are used to distinguish between G.I. but if there's no G.I. you can't forcefully use them.
  4. Oct 23, 2012 #3
    From what I have learned (I'm still in the class):

    Cis/Trans are used when your lowest priority group on both carbons is hydrogen.

    Z means your high priority groups are on the same side (both top or both bottom, if double bond is horizontal...not sure how else to say this). E means the high priority groups are on opposite sides.

    If you have two high priority groups attached to a carbon, the one with the highest atomic number is the highest priority group. In your example, however, you have the rightmost carbon attached to two hydrogens. This has no cis/trans/E/Z nomenclature - no geometrical isomer since the two H's are equal priority. Same deal if you had two Br atoms in place of the hydrogens.
  5. Oct 23, 2012 #4
    This is a good answer, and I'm going to elaborate with some images to better clarify.

    Take for example these structural isomers:

    Notice that when we discuss cis and trans here, we have a Cl atom on either side of the double bond. Cis and trans describes the orientation of the chlorine atoms to the double bond. Essentially, to use cis/trans description, you must have the same atom on either side of the double bond.

    Now take this structure as an example:

    Notice we don't have two of the same atoms on either side of the double bond. So we must assign priority to each atom connected to the sp2 carbons (the carbons that make up the double bond). In this specific example, the highest priority goes to Br and Cl, respectively. Thus we denote them:

    Essentially, I like to remember E,Z kind of like this:
    cis = Z
    trans = E

    However, you may find it more useful to just stick with E and Z notation and abandon cis/trans notation if your teacher allows it (i don't see why they wouldn't...its universally accepted and backed by IUPAC)
    To illustrate, let us reconsider the first example, and instead of naming it cis/trans, lets name it E/Z:

    Consult your book for assigning priorities to atoms. It's very simple, and requires assessing the atomic number, and possibly the mass (if the atomic numbers are the same)
    Last edited: Oct 23, 2012
  6. Oct 23, 2012 #5
    No - first calculating the atomic number. Only when these are equal, then its mass.

    I assume that if the masses differ (BUT atomic numbers do not) then you must not go on to next atoms along bonds?
  7. Oct 23, 2012 #6
    Yikes, you're absolutely correct. Thanks for catching my mistake. I have editted my response and it should be correct now.

    Yes that is correct (i'm assuming you mean "then you MUST GO on to the next atoms along the bonds"), but there is a notable exception when discussing Hydrogen and Deuterium. It should be pretty intuitive though that when Deuterium and Hydrogen share an SP2 carbon, Deuterium gets the highest priority.

    For example:


    Left side is E, right side is Z.
  8. Oct 23, 2012 #7
    No. Sic! I wrote "then you must NOT go on to the next atoms along the bonds".

    Say that on one end of a double bond, one of the groups is methyl, the other ethyl.

    But the methyl group is C-13, while ethyl is C-12.

    Am I right in assuming that since the first atom of the methyl group takes the priority for having higher mass, and then the next atoms of ethyl group are irrelevant?
  9. Oct 23, 2012 #8
    Ok sorry about that. Yes, you are correct. C-13 methyl would have higher priority than the ethyl group. You're looking for the first point of difference between your comparisons. The first point of difference here is C-13 vs. C-12, as you noted. C-13 methyl gets the nod as higher priority than C-12 ethyl.
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