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Homework Help: Cis-trans isomerism

  1. May 29, 2004 #1
    so, i was given a cis-isomer and a trans-isomer. the question is very simple, why this isomer shows cis-trans isomerism.

    i answer it this way, "because the free rotation is prevented by the presence of carbon-carbon double bond"

    is that a correct answer? or maybe a more valid answer?
  2. jcsd
  3. May 30, 2004 #2
    It is true that rotation about a carbon-carbon double bond is rare, due to the fact that it would have to break its pi bond to occur.

    If that is all the analysis this question is asking for, your answer is fine.

    However, if this question is related to a reaction mechanism or anything like that, you may need to state more, and explain exactly WHY both the cis and trans isomers CAN be observed.

    For example, if I told you that R-C(triple bond)C-R' reacts with H2 in Lindlar's catalyst to produce ONLY cis HRC=CR'H, whereas it reacts with Na in liquid NH3 to produce ONLY trans HRC=CR'H, you'd have to look to the mechanism to explain it.

    Point being, your answer depends on the question. What you've stated is correct, but just be aware of exactly what kind of analysis the question is asking for.

    Hope that helps!
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