Z and E

[Leonardo_VegaHidalgo_2E]

How do you determine whether a molecule is cis or trans from a line structure?

[Emma_Green_2C]

Cis is when the substituents are on the same side of a double bond and trans is when they are on opposite sides. This should be apparent on the line structure just by looking at it. Z corresponds with cis and E corresponds with trans. You determine Z and E by looking at the atomic number (whichever is highest) of the groups and seeing if the highest priority groups are on the same side (Z) or opposite side (E). Hope this helps!

[AnkitaNair1E]

Hi Leornardo!

So I think it's important to distinguish that there is a slight difference between cis and trans and a Z and E molecule. Yes, both are used to establish whether molecules are on the same or opposite side, but we use cis/trans in our class to distinguish hydrocarbon chains and we use Z and E to to distinguish other molecules with different atomic weights and when we have double bonds. If this sounds confusing to you, don't worry. I'm about to provide you some examples.

For example: here us cis 1-2 dipropylcyclohexane

cis-1,2-diethylcyclohexane.JPG (8.37 KiB) Viewed 1797 times

Notice how the propyl group on carbon 1 is axial up. And that the propyl group on carbon two is equatorial up. Well because both substituents are facing upwards, we know that this molecule is a cis molecule (i.e. both substituents are have the same direction. It doesn't really matter if one is axial or equatorial.) If we had one molecule axial up for example and another molecule equatorial down, then we would say that the molecule is trans because they face different directions (up/down).

Here is another example. Consider Cis 1,2 methylcyclohexane.

images.png (2.81 KiB) Viewed 1797 times

This molecule is also Cis because the line structure shows that both methyls are coming out of the page (hence the filled in line) so we know that the two have the same direction and are cis.

So what about "E" and "Z" ?

Well usually this applies when you're looking at a molecule with a double bond that will have either functional groups or halogens like Chlorine and Bromine attached. In other words you may have a double bond with molecules that have different atomic masses.
Consider 1-bromo-2-chloroethene

Cs.png (4.93 KiB) Viewed 1797 times

In this scenario we have two halogens attached to the molecule. As you may know, Bromine and Chlorine have different atomic masses. The way we figure out if a molecule is "Z" or E is to look at each side of the double bond. So in this example, on the right side, chlorine is the heaviest atom and is located below Carbon 2. On the left side, bromine is the heaviest atom and is located below Carbon 1. Since the heaviest atom on each side is located on the bottom (i.e. they are on the same side) we identify this molecule as a "Z" molecule because both heavy molecules (chlorine and bromine) are on the same side. So wewould call it Z 1-bromo-2-chloroethene.

Here's another view of Z 1-bromo-2-chloroethene

Unknown.png (1.19 KiB) Viewed 1797 times

[AnkitaNair1E]

And here's a version of E 1-bromo-2chlorethene.

4575447.png (3.68 KiB) Viewed 1795 times

Notice how the Bromine and Chlorine are on opposite sides of the double bond? i.e.:Chlorine is above the carbons and Bromine is below. This would be a version of "E" or a molecule where the heaviest molecules are on different sides of the double bonds.

In conclusion, use cis and trans to describe hydrocarbon substituents and use "E" and "Z" to describe situations where you have halogen substituents/functional groups (molecules with different atomic masses) attached to a double bond.

Hope this helped!

[Chem_Mod]

Hi Leornardo!

So I think it's important to distinguish that there is a slight difference between cis and trans and a Z and E molecule. Yes, both are used to establish whether molecules are on the same or opposite side, but we use cis/trans in our class to distinguish hydrocarbon chains and we use Z and E to to distinguish other molecules with different atomic weights and when we have double bonds. If this sounds confusing to you, don't worry. I'm about to provide you some examples.

For example: here us cis 1-2 dipropylcyclohexanecis-1,2-diethylcyclohexane.JPG

Notice how the propyl group on carbon 1 is axial up. And that the propyl group on carbon two is equatorial up. Well because both substituents are facing upwards, we know that this molecule is a cis molecule (i.e. both substituents are have the same direction. It doesn't really matter if one is axial or equatorial.) If we had one molecule axial up for example and another molecule equatorial down, then we would say that the molecule is trans because they face different directions (up/down). The reason we refer to this as "cis" instead of "Z" or "E" is because essentially the Carbons are only bound to carbons and hydrogens so there is no real difference in atomic weight.

Here is another example. Consider Cis 1,2 methylcyclohexane.
Cs.png

This molecule is also Cis because the line structure shows that both methyls are coming out of the page (hence the filled in line) so we know that the two have the same direction and are cis.

So what about "E" and "Z" ?

Well usually this applies when you're looking at a molecule with a double bond that will have either functional groups or halogens like Chlorine and Bromine attached.
Consider 1-bromo-2-chloroethene
10421353.png

In this scenario we have two halogens attached to the molecule. As you may know, Bromine and Chlorine have different atomic masses. The way we figure out if a molecule is "Z" "cis" or E "Trans" is to look at each side of the double bond. So in this example, on the right side, chlorine is the heaviest atom and is located below Carbon 2. On the left side, bromine is the heaviest atom and is located below Carbon 1. Since the heaviest atom on each side is located on the bottom (i.e. they are on the same side) we identify this molecule as a "Z" or "cis" molecule and would call it Z 1-bromo-2-chloroethene.

Here's another view of Z 1-bromo-2-chloroethene
Unknown.png

Please keep in mind Z does not correlate to cis and E does not correlate to trans. They are based off entirely different naming systems.
viewtopic.php?f=88&t=21070

[AnkitaNair1E]

Thank you for pointing out my error. I fixed my explanation so hopefully it will better help students understand.

[Chem_Mod]

Thank you for pointing out my error. I fixed my explanation so hopefully it will better help students understand.

No worries at all! I just want to make sure everyone is learning correctly :)