E and Z isomers

[Leester]

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Could someone explain this concept to me? It came up on a practice test and I am still a bit unsure about it. Does E = trans and Z = cis? And do these designations only apply to alkenes?

Thanks

 

[Triflate]

yes, you are correct. E = trans and Z = cis. The E/Z system only works for alkenes. Cis/trans can be used for most organic compounds with multiple stereogenic centers, however. For example - think 1,2-cyclohexanediol. That compound could be represented as cis or trans depending on the stereochemistry. R and S can be used as well. Also, keep in mind that all cis/trans and E/Z isomers are diasteriomers.

 

[Leester]

Thank you

 

[KyoPhan]

cis/trans and e/z applies only to alkenes.
EDIT: My above statement about cis/trans/e/z only applying to alkenes is incorrect as pointed out.

In the cis/trans, there are 2 similar groups are attached to different carbons in the alkene bond. If they are on the same side, it's cis. If it's on opposite side, it's trans.

In the E/Z naming system, 2 groups attached to the carbon do not necessarily have to be the same. They can all be different in fact. E/Z works with "Priority." You may remember the word priority from figuring out if something is R or S in the whole chiral problems. This is similar in that you have to figure out the priority of the attached groups to each carbon. If high priority is on the same side as the other high priority, it's "Z". If the high priority is on opposite sides, it's "E"

You should youtube it for more information and practice problems.

Cis is not always Z, and Trans is not always E.

A good example is seen here:
http://chemistry14d.websitetoolbox.com/post/cis-is-not-always-z-and-trans-is-not-always-e-5538265

Also, R/S does not work with alkenes. They are used for alkanes that are chiral.

 

[Triflate]

I didn't say R/S worked with alkenes. I said the E/Z system only works for alkenes. And yeah, cis/trans and E/Z are not always synonymous. Maybe like 99% of the time they will be. There are exceptions to everything in organic chemistry. I figured the OP wasn't looking for meticulous information.

And for the record, alkanes can be cis/trans. Disubstituted cycloalkanes ( for ex. - 1,2-cyclohexanediol ) would be capable of cis/trans isomerism.