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Cis-Trans Isomers

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Hi I have a couple of questions about cis-trans isomers.


1. When a molecule has one double bond and exhibits one instance of cis-trans isomerism, I put the cis or trans at the very beginning of the name right?


2. When a molecule has two or more double bonds and has 2 or more places of cis-trans isomerism, then where do I put the extra cis or trans?


3. When there are no identical groups around the double bond, do I write cis or trans depending on the orientation of the parent chain? Or do I not write cis or trans at all?


Thank you so much.

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1. yeah, thats right eg. cis-2-chloroethene or trans-2-chloroethene


2.umm, not sure on that one, might want to check the IUPAC site for that


3. again check the IUPAC site but i think it has something to do with the molecular weights of the groups(the two heaviest?)

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for 3, it's either an E or Z conformation, and it you label each chain by priority, based on atomic number of the first atom (if the first atoms are the same, you go to the second, and so on). If the two higher priorities are on the same side, you have a Z conformation. If they're on opposite sides, it's E.

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"yeah, thats right eg. cis-2-chloroethene or trans-2-chloroethene"

Err, that's a rather poor example.

In one case the chlorine is cis to a hydrogen and trans to the other one. In the other case the Cl is trans to a hydrogen and cis to the other. There's only one isomer.

For what it's worth that compound is 1 chloroethene rather than 2chloroethene


Anyway, the answer seems to depend who you ask but, for example, names like "cis,cis,cis,trans-[]-fenestrane" can be found on the 'net.

Here are the "official" rules from people who can't spell sulphur


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