- Stereochemistry questions
- Chiral drugs
- Structural (constitutional) isomers
- Chiral vs achiral
- Stereoisomers, enantiomers, and chirality centers
- Identifying chirality centers
- R,S system
- R,S system practice
- Optical activity
- Enantiomers and diastereomers
- Cis–trans isomerism
- E–Z system
- Conformations of ethane
- Conformational analysis of butane
Cis–trans notation can be used to describe the configuration of a double bond with exactly two substituents and two hydrogens. If the two substituents are on the same side of the double bond, the configuration of the bond is cis. If the two substituents are on opposite sides of the double bond, the configuration of the bond is trans. Created by Jay.
Want to join the conversation?
- At3:55, wouldn't it be trans because the isopropyl group is higher priority than the ethyl group?(40 votes)
- In my orgo class, I was taught to assign priorities to the 2 groups on each of the carbons and then look at whether the highest priority groups are on the same side (cis) or different sides (trans). Based on this, I thought that the molecule in the example would be trans. Is the method I am using incorrect?(17 votes)
- At2:57why did you use hydrogens? Aren't the benzene ring and carbonyl groups on opposite sides, making it trans?(4 votes)
- cis and trans refer to the relative positions of two identical groups. If the groups aren't identical, you have to use the Cahn-Ingold-Prelog E/Z nomenclature.(11 votes)
- Doesn't propyl have priority over ethyl? So it should be trans, no?(1 vote)
- Yes, the main chain goes through the isopropyl group (trans), but the two identical groups are cis.
This is why you should use the cis/trans nomenclature only for disubstituted alkenes and the E/Z nomenclature for tri- and tetrasubstituted alkenes.(5 votes)
- Referring to the dramatical ending of this video, is it okay to say that when you have identical groups on the carbon 2x bond, you use cis/trans naming, but you if you have a trisubstituted alkene or a tetrasubstituted alkene, then you use E/Z with different fct groups attached to the 2x bond?(1 vote)
- Yes, you can use cis/trans naming if you have identical groups on each end of the double bond.(4 votes)
- What would the 3rd molecule be called? xxxxx (1,3,5 cyclohexene ?
It can be seen at3:27on the left?
- Its common name is cinnamaldehyde.
Its systematic name would be trans-3-phenylprop-2-enal.(2 votes)
- Can someone name the structure at3:54?(2 votes)
- can triple bonds have cis and trans isomers too?(1 vote)
- No, there's only one other group bonded to a triple bond carbon.(3 votes)
- what is the name of the molecule at3:55distinguished as cis? (the one with 2 ethyl, 1 methyl, and 1 isopropyl groups)(1 vote)
- That's actually a really good question, its systematic name would be (3E)-3-ethyl-2,4-dimethylhex-3-ene.
Identifying the longest carbon chain is a bit tricky here, due to there being 2 different possibilities, but the correct one is to go from the right most carbon over to one of the carbons that is part of the isopropyl. This gives us the most substituents possible.(3 votes)
- The explanation given as to why cis and trans represent different molecules is that double bonds don't have free rotation and so the cis molecule could not rotate to "look like" the trans molecule- is this the same as saying that cis and trans molecules cannot interconvert between eachother?(2 votes)
- Cis and trans molecules do not freely interconvert between each other. However, some interconversion will usually occur due to the dynamic equilibrium of a reaction. After a trans bond is formed the reverse reaction may occur (remaking the reactant) and then the reactant could undergo the reaction again but this time forming the cis bond.(1 vote)
- At approx0:26, why is it 2-butene and not but-2-ene? i always thought you numbered where the double bond was before the ene because the ene indicates the double bond.(1 vote)
- Both are acceptable although but-2-ene is more correct(2 votes)
- [Voiceover] Let's say we were asked to name the molecule on the top left. We would start by numbering our carbons. So this would be carbon one, two, three, and four. Notice we have a double bond starting at carbon two. So the name of this molecule would be 2-butene. Two because we have our double bond starting at carbon two. "But" because we have four carbons and "ene" because we have a double bond presence in the molecule. What about naming the molecule on the right? We number our carbons one, two, three, and four. And once again we have a double bond starting at carbon two. So the name of this molecule would be 2-butene. However these are two different molecules and the reason why is because there's no free rotation around a double bond. Single bonds have free rotation but double bonds don't. So you couldn't rotate the molecule on the left to look like the molecule on the right. Therefore they must be isomers of each other and we need a way to distinguish between our isomers. And so one way to do that is to use cis/trans terminology. So if we look at the molecule on the left, we can see we have two methyl groups. And those two methyl groups are on the same side of our double bond. So if I draw a line in here, it's easier to see those two methyl groups are on the same sides. And we call that the cis isomer. So we put cis in front of our name here. I'm attempting to write it in italics. So this would be cis-2-butene. On the right when we look at those methyl groups, these two methyl groups are on opposite sides of the double bond. So I draw a line in here to make it easier to see those two methyl groups are on opposite sides. And we call that trans. So this is trans isomer. I'm going to write trans here in italics, attempt to anyway. So we have cis-2-butene and trans-2-butene. These are different molecules with different properties. If you want to use cis/trans terminology, you're looking for two identical groups and you are comparing them. So let's look at these next two examples here and figure out which one is cis and which one is trans. We're looking for identical groups. So over here we have an ethyl group attached to our double bond and on the right we have an ethyl group to our double bond. Those two ethyl groups are on the same side of our double bond so this must be the cis isomer. On the right we have this ethyl group and this ethyl group on opposite sides of our double bond. So that must be the trans isomer. All right, let's do some more examples. I'll go down to here. On the left we have cinnamaldehyde molecule. We're looking for two identical groups so we can use cis or trans. You can also use hydrogens, right. You don't have to use a methyl group or an ethyl group so if we look at our double bond we know there's a hydrogen attached to this carbon and we know there's a hydrogen attached to this carbon. And those two hydrogens are on opposite sides of our double bonds. And I'm drawing a line here to make it easier to see. Right, these two hydrogens are on opposite side so we're talking about trans here. Those hydrogens are across from each other. What about the tetra-substituted alkene on the right? We need two identical groups to use our cis/trans and here we have an ethyl group, and here we have an ethyl group. All right over here we have a methyl group and an isopropyl group. But the two methyl groups are on the same side of our double bond. So I draw a line in here and we see that these two groups are on the same side, therefore we're talking about cis here. So this double bond has a cis configuration. Let's compare the drawing on the left to the drawing on the right. The first time you look at these two drawings you might think these are two isomers, and I could use cis/trans terminology to distinguish between them. However, you can't because these are just two ways to represent the same molecule. If you picked up this molecule on the left and you flipped it up, you would get the drawing on the right. So they're not isomers of each other. This is the same molecule. And a fast way to figure that out is to look at this carbon. And you can see you have two identical groups bonded to that carbon. So you can't use cis/trans terminology. That's different from the example we did a minute ago. We had two identical groups, right these two ethyl groups here. However those two ethyl groups weren't bonded to the same carbon. Those two ethyl groups are bonded to different carbons. So this ethyl group is bonded to this carbon, and this ethyl group is bonded to this carbon. So we were able to use cis/trans terminologies. So we looked at our double bond and we said those two ethyl groups are on the same side of our double bond, so this represents a cis configuration of the double bond. So we can't do that up here because while we do have two identical groups, those identical groups are bonded to the same carbon.