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How do I tell which one is cis/trans? I know the definitions: cis is same side; trans is across; but

1) What am I comparing here.

2) How do I compare/visualize?

enter image description here

enter image description here

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In simple molecules like what you've drawn above, see if the hydrogens are on the same side of the double bond or not. If they are on the same side, then the compound is cis; if the hydrogens are on opposite sides, then the compound is trans. In more complex cases, prioritize the four substituents on the double just as you've been doing with "R\S" nomenclature. If the two highest priority groups are on the same side then the compound is cis, if they are on opposite sides, then the compound is trans. The terms "cis" and "trans" are still used, but especially in the more complex cases they have been replaced with the first letters of the German words "entgegen" (E, opposite) and "zusammen" (Z, together). So cis-2-butene could also be called Z-2-butene.

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  • $\begingroup$ Cool; would the added picture in the OP be an accurate depiction? The first one is a cis; the second one is a trans isomer? $\endgroup$ – Dissenter Jun 4 '14 at 22:43
  • $\begingroup$ Correct, BTW the same terminology can be used for double bonds involving other atoms, like imines, azines, etc. Lone pairs are given a lower priority than hydrogen. $\endgroup$ – ron Jun 4 '14 at 22:45

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