Cis vs Trans - Online Tutor, Practice Problems & Exam Prep

Understanding cis and trans isomerism is crucial for studying alkenes and cyclic compounds. Cis refers to groups on the same side of a double bond, while trans indicates groups on opposite sides. For multi-substituted alkenes, the E and Z naming system is used, where E denotes trans and Z denotes cis configurations based on the highest priority groups. This distinction is essential for predicting physical properties and reactivity in organic compounds, as these geometric isomers can exhibit different behaviors due to their spatial arrangements.

How to name different types of double bonds or rings

Now that we know how to name double bonds, there's an extra complication that we have to consider, which is that double bonds, because of the lack of free rotation, are going to be able to arrange themselves in different ways. So now we have to take into account not only the fact that there's a double bond and it has priority, but we're also going to have to take into account the arrangement of that double bond. So let's go ahead and get started. So I'm sure you've heard of the words cis and trans before. Cis and trans are names given to particular arrangements of double bonds or of rings. So, basically, any time that you have no free rotation, you're going to have the possibility for cis and trans. For example, a double bond, if two groups are oriented in the same position, remember that a double bond is pretty like has a lot of regions of overlap, so it can't twist out of shape. So that means the double bond is stuck there. In the same way, a ring has the same problem where if I, for example, have a ring that looks like this and then I have a group facing up and I also have another group facing up. Notice that this is like a 3D structure. Let's say this is one atom and another atom. See how they're both facing the same way? I can't actually rotate this one to the down position without breaking the ring because that ring, in order to move one down, would have to snap. So cis and trans are the words that we use for these arrangements that are stuck together. And, like I said, these isomers exist because free rotation around pi bonds is impossible. So, basically, the way that this works is that when two groups happen to be on the same side of the fence, okay, we're going to I'm going to talk about that in a second. When they're on the same side, that's going to be cis. When they're on different sides of the fence, that's going to be trans. So, what kind of mysterious fence am I talking about? Well, let's say we have a double bond. The way that I like to split it up is I like to draw a dotted line right through the middle of the double bond. That double bond is my fence. And then what I say is how are these groups related to each other on that double bond? So for example, if I wanted to compare and are they on the same side of the fence or different sides? Well, they're both on the same side. They're both on the top side, so the relationship between would be cis. What about the relationship between and What would that relationship be? Well, those are on different sides, so that one would be trans. Got that so far? How about the relationship between a and b? How about if I were to say what kind of relationship is that? A and b actually don't have a cis and trans relationship. The reason is because they're on the same carbon. Cis and trans only applies to atoms on other carbons, not on ones that are on your own carbon. So a and b would actually get a completely different type of name that we're not going to discuss here, but it wouldn't have to do with cis and trans. So cis and trans only has to do with the way that one atom to another relates to. Here I've got two different versions of 2-butene. When I say that, you shouldn't be shocked. You should know what I'm talking about because that means that's a butane with a double bond in the 2 position. Easy. But these are both not both the same molecule. In fact, they're going to have different physical properties. They're going to behave differently and stuff like that. So what would be the names that we would give to these to show that they're distinctly different? The names that we would give would be, well, I would draw my fence, and I would say okay, are these on the same side of the fence or different? Same. This one would be cis-2-butene. And then this one would be trans-2-butene. And these are, later on we're going to talk about them. There's something called stereo isomers which means that they actually are connected the same way but they're shaped differently. But for right now just know that these are molecules that are locked in their position and they can't move.

Why we need to use the E/Z naming system

Now I want to talk about another naming system called E and Z. Okay? I call this the easy naming system and it's actually really easy. It's related to cis and trans. The only thing is that you use the E and Z naming system when you have multi-substituted alkenes. Okay. So I know you can barely see that, multi-substituted. Even you can see it and it still doesn't make a whole lot of sense. My handwriting is terrible. Okay. So multi-substituted alkenes. So let me give you an example. Let's go down to this example here, assign cis and trans to the following alkenes. I don't need names. I don't need full names. I just want to know if it's cis or trans. So let's start off with this first one. Would that one be cis or trans? The answer is that this would be trans. Okay? Because if I drew my fence along here, what you would see is that one group is on one side and one group is on the other. Does that make sense? Cool. How about 2? What would that be? 2 would be cis because both groups are on the same side. Okay? Now lastly, what about 3? What would you say about 3? Okay. So 3 is actually a trick question. There is no cis and trans. Why? Because I have 3 different groups. I have one here. I have one here, so that could be trans. But then I also have this one over here messing things up. Did I tell you how to figure out cis and trans when you have 3 things? No. So basically, the E and Z naming system is going to allow us to give unique names to tri- and tetra-substituted alkenes. What that means is that cis and trans only works if you have 2 substituents. But if you have more than 2, cis and trans breaks down like in example 3. Example 3, there's no way to use cis and trans there. So I have to use E and Z instead.

What does E and Z stand for?

When I'm using e and z, all I do is this. I'm going to choose the highest priority groups on both corners of the double bond. Okay? What does high priority mean? It means highest atomic mass. So I'm going to try to pick the side of the double bond. I'm going to try to pick on each side of the double bond the atom that has the highest atomic mass. That one's going to get my priority and then I'm going to figure out how those two high priority groups are related to each other. If they're related to each other trans, then we're going to assign the letter e. If they're related to each other as cis, then we're going to assign the letter z. So basically z is just a fancy way of saying cis with three or more substituents. E is a fancy way of saying trans with three or more substituents.

Assigning E/Z

So I'm going to go through the first one by myself. What I mean by myself is with you guys, just guiding you through it. And then we'll see. Okay. So first of all, let's look at f. When I say that I have to identify the highest priority groups on both corners, what is a corner? I'm just talking about this being one corner and this being another. Okay. So let's look at the green one first. Is there a side of this double bond that is going to have the higher priority? Is there one of those substituents that's going to have a higher priority? And the answer is yes. Fluorine is going to have a higher priority over this group here. Why? Because even though this group down here looks bigger, the Fluorine has a higher atomic mass right away. So what that means is Fluorine beats carbon. Fluorine is further down the periodic table than carbon is, so this is going to be my high priority up here. Oops. Whatever. This is to be my high priority up here. Now let's move to red. So for red, is there a way to assign which one is higher priority? And actually, these are both exactly the same. They're both CH3. So this is an example where I actually have 4 different substituents, but since I have 2 of the same substituent on a corner, I can't assign cis and trans or e and z. Why? Because there's no way to distinguish these from each other. They're both the same thing. So actually, this one is going to be not available. I can't assign cis and trans unless there's different things on both sides. Okay? So what I'm trying to say is that there's no way for me to know if this is cis and trans because this CH3 is always going to be cis to the f and trans to the ethyl. This CH3 is always going to be cis to the ethyl and trans to the f, so it doesn't really matter. This one does not get a designation.

Assigning E/Z

Hey everyone. Now that we've done example a, let's take a look at example b. So here when we're looking at this alkene, the first thing we should do is find our corner carbons. Remember our corner carbons are just our alkene carbons. We're going to make this one our blue corner carbon and this one our black corner carbon. The next thing we're going to do is let's look at the blue one. What are the 2 groups that are attached to it? If we take a look we know there is a carbon here that's invisible and carbon has to make 4 bonds, so there is a hydrogen that's also present. If we look at carbon and hydrogen, which has higher priority? Carbon would have higher priority, so we're going to highlight that. Now let's take a look at our black corner carbon. Here, what 2 groups is it attached to? Well, it's attached to an oxygen here and a carbon here. Of these 2, which has higher priority? Oxygen would have higher priority. So now that we determined the 2 higher priority groups, let's draw our fence. And we see that our 2 higher priority groups are on the same side. Now remember, if we're dealing with a tri-substituted or tetra-substituted alkene, then we use the e or z designation. Since this is tri-substituted, we're going to use e or z. And they're on the same side, so that would be cis, and cis is related to z. So this is a z alkene. Now, echoing back to my French roots, a way that I remember what z means is z is the same side. Okay? So that's a way that I tend to try to remember what is a z alkene, how it's related to cis. Remember, if you were a z alkene, the opposite of that would be an e alkene. Alright. So just keep this in mind when taking a look at any of these types of alkene structures. So determine if they do in fact have an e or z designation.

Determine the IUPAC name of the following molecule 

• A. 5-bromo-5-fluoro-3-heptene
• B. 3-bromo-3-ethyl-3-fluoro-5-hexene
• C. 4-bromo-2-ethyl-4-fluoro-1-hexene
• D. 5-bromo-5-fluoro-3-methyleneheptene

Determine the IUPAC name of the following molecule 

• A. (E)-5-isopropyl-3,6-dihydroxy-2-heptene
• B. (Z)-3-isopropyl-5-hepten-2,5-diol
• C. (E)-3-isopropyl-2,5-dihydroxy-5-heptene
• D. (Z)-5-isopropyl-2-hepten-3,6-diol

Here’s what students ask on this topic:

What is the difference between cis and trans isomers?

Cis and trans isomers are types of geometric isomers found in alkenes and cyclic compounds. In cis isomers, the substituent groups are on the same side of the double bond or ring structure. In trans isomers, the substituent groups are on opposite sides. This difference in spatial arrangement can lead to variations in physical properties such as boiling points and melting points, as well as chemical reactivity. The lack of free rotation around the double bond or within the ring structure is what causes these isomers to exist.

How do you determine if a molecule is cis or trans?

To determine if a molecule is cis or trans, first identify the double bond or ring structure. Draw a dotted line through the double bond or across the ring. If the substituent groups are on the same side of this line, the molecule is cis. If the substituent groups are on opposite sides, the molecule is trans. This method works well for alkenes with two substituents. For more complex alkenes with three or more substituents, the E/Z naming system is used instead.

What is the E/Z naming system in organic chemistry?

The E/Z naming system is used for alkenes with three or more substituents. In this system, the highest priority groups on each carbon of the double bond are identified based on atomic mass. If the highest priority groups are on the same side, the isomer is designated as Z (from the German word 'zusammen', meaning together). If they are on opposite sides, the isomer is designated as E (from the German word 'entgegen', meaning opposite). This system provides a clear way to name complex alkenes where cis/trans nomenclature is insufficient.

Why can't cis and trans nomenclature be used for multi-substituted alkenes?

Cis and trans nomenclature is limited to alkenes with only two substituents. When an alkene has three or more substituents, the simple same-side/opposite-side distinction becomes ambiguous. For example, with three substituents, it is unclear which two groups should be compared. The E/Z naming system resolves this by prioritizing groups based on atomic mass, allowing for a clear and unambiguous naming convention for multi-substituted alkenes.

How do cis and trans isomers affect the physical properties of a compound?

Cis and trans isomers can have significantly different physical properties due to their spatial arrangements. For example, cis isomers often have higher boiling points than trans isomers because the same-side arrangement can create a dipole moment, leading to stronger intermolecular forces. Conversely, trans isomers tend to have higher melting points because their more symmetrical shape allows for better packing in the solid state. These differences are crucial for understanding the behavior and reactivity of organic compounds.