Is the molecule below an E or Z isomer? From what I believe I would say E isomer because the two priority halides Cl and Br are on opposite sides of the double bond. But I'm not 100% sure. Because they are on the same side of the double bond if I look at it form another perspective.

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    $\begingroup$ You don't chose two priority substituents out of four. Instead, you choose a priority group on one end of the double bond, then another group on the other end. $\endgroup$ – Ivan Neretin Jun 1 '17 at 12:40
  • $\begingroup$ I find that the site below has a good explanation on how to approach E/Z classification. chemguide.co.uk/basicorg/isomerism/ez.html $\endgroup$ – J. Ari Jun 1 '17 at 12:46
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    $\begingroup$ @IvanNeretin so Br is the highest on one side and F is the highest on the other and as they are both the same side it makes this a Z ? $\endgroup$ – Aaron Jun 1 '17 at 13:42
  • $\begingroup$ Yes, that's right. $\endgroup$ – Ivan Neretin Jun 1 '17 at 13:51

Just adding on to Ivan's comment, you have to split the double bond "into half" such that both halves have two sets of double bonds not having "to split and remain with single bond". After that assign priorities.

I don't know if I explained well, but here is a better picture:

enter image description here

Considering each of the double-bond carbons separately, look at the two substituents attached and rank them according to the atomic number of the first atom in each. An atom with higher atomic number ranks higher than an atom with lower atomic number.

  • $\begingroup$ So it is "Z" ? because the Br and the F are the highest relative to their sides of the C=C double bond $\endgroup$ – Aaron Jun 1 '17 at 13:44
  • $\begingroup$ Precisely its Z $\endgroup$ – xavier_fakerat Jun 1 '17 at 14:03

Firstly take one carbon and decide seniority sequence of groups on that carbon, which will be done according to CIP rule (for alkenes here) which states that:

  1. the group with higher atomic number will be senior.
  2. bond pair is always senior to lone pair.
  3. the doubly bonded group will be treated as two same groups are attached to that carbon( for eg. =Cl is equivalent to two single bonded -Cl while deciding the seniority).

Now, after deciding the seniority groups on one carbon (i.e.on one side if the double bond) do the same for the other doubly bonded carbon. Now if the higher ranked groups are on same side of the double bond then the notation is "Z" otherwise it will be "E".


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