What is the Relationship Between Isomers? - Online Tutor, Practice Problems & Exam Prep

Understanding isomer relationships is crucial in organic chemistry. Begin by verifying that two compounds have the same atoms and connectivity. If they do, check for chiral centers: zero chiral centers indicate identical molecules, while one chiral center leads to enantiomers if different. For multiple chiral centers, identical configurations yield identical molecules, while all opposite configurations result in enantiomers. A mix of identical and different configurations produces diastereomers. Special cases like meso compounds occur when chiral centers are symmetrical and cancel each other out. Use this systematic approach to analyze stereoisomers effectively.

Different atoms or different connectivity.

Alright. So now we're going to talk about one of the most important types of problems that you guys are going to get in this chapter, and it has to do with identifying the relationship between 2 different types of isomers. Alright, maybe you guys remember this flowchart. I made it when we were talking about constitutional isomers. Remember that we talked about how the very first step is to verify that all the atoms are the same. So we would count the non-hydrogen atoms and the IHD in both compounds. We said if they were not exactly the same, then they were different compounds. Okay. And then we said that if they were the same, then you would go to step 2. And then we would talk about connectivity and we said, are they all connected the same? We talked about that you'd look for a landmark atom. This is all review based on what we learned from constitutional isomers. And then we said if they're not exactly connected the same, then they're constitutional isomers. And then we said if they were, back then we said that if they had the same atoms and if they were connected the same, then we were going to say that they were identical. So usually, for when we were talking about constitutional isomers, we would have "identical" in this blank. But it turns out that now that we have the possibility of stereoisomers, we actually have to go to step 3 now. Instead of just assuming that they're identical, now we have to look at the stereoisomers. We have to say stereo centers. We have to say, is this an R? Is this an S? Stuff like that.

Same atoms, same connectivity, 0 chiral centers.

Now we have to go to step 3. And what step 3 talks about is chiral centers and trigonal centers. So let's go ahead and go for this. Now that we've verified all the atoms are the same and the connectivity is the same, we're going to look for chiral centers. So, if we have 0 chiral or trigonal centers present, that means all the atoms are the same, the connectivity is the same, and there are 0 chiral or trigonal centers, then the two molecules are identical. Okay? So this is that blank that we would have used earlier when we would have said "identical," but now we're just verifying that there are no chiral centers or trigonal centers.

Same atoms, same connectivity, 1 chiral center.

What if you do have one chiral center, which happens all the time? Okay. Well, if you have the same chiral center on both, then they're identical. Okay? If you have different chiral centers for both, then the relationship is going to be enantiomers. Okay? And let me illustrate this with the following molecules. Let's say that I have 2-butanol and I have another 2-butanol. So I've already verified that these two compounds have the same molecular formula, they have the same IHD, everything and they have the same connectivity. They're both secondary alcohols that are butanols. Alright? Then I go ahead and I figure out the configuration of this and I figure out that this one is R, has one chiral center and this one is also R. So what do you think that relationship is? Well, that's going to be identical. Okay. Because they're the same molecule and they have the same chiral center. Now what if I'm comparing it to, instead of R, what if I were comparing it to the same molecule but now my OH is on a dash? Okay. Now instead of being R, this one's going to be S. Okay. What do you think is the relationship between these two guys? Okay. Well, we have one chiral center and they're different, so then these would be enantiomers or mirror images. Does that make sense? That's the way this flowchart works. Basically, we look step by step and say are they the same? Are they different? Etcetera.

Same atoms, same connectivity, 2 or more chiral centers.

So let's go on to discuss what happens when we have 2 or more chiral centers. If we have 2 or more chiral centers and all of them are identical, the molecules will still be identical. For example, if I have a molecule that has 3 chiral centers and the chiral centers are configured as 2R, 3R, 5S. And then I'm comparing it to another molecule that has the same molecular formula, same connectivity, and it happens to be 2R, 3R, and 5S as well. Those are going to be identical.

How about if all centers are precisely different? What if I was comparing it to 2S, 3S, 5R? In this scenario, where every single center is opposite, these molecules are going to be enantiomers as well. We have covered this a bit when I talked about the types of stereo isomers you could have. If everything is completely different, that results in an enantiomer.

But what if not all of them are different but not all of them are the same? Consider the situation where I have 2R, 3R, and then have 5R. Here, I have two centers that are the same, but one is different. What kind of situation would that be? Well, that would fall into the category of neither being the same nor completely different. They are not mirror images, but they still differ. This is a diastereomer. If they're somewhat different but somewhat the same, that would classify as a diastereomer. Does that make sense, guys?

Same atoms, same connectivity, 1 or more trigonal centers.

So then let's go to a few more and then we'll be done. How about if we have 2 chiral centers that are symmetrical and opposite to each other? This is a special case. If we have 2 chiral centers that are symmetrical and opposite to each other, that's going to be meso compounds. Okay. Remember we discussed that meso compounds are kind of an exception where they have 2 chiral centers but they cancel out because they're opposite. Okay. Awesome. So those would be meso compounds.

And then finally we've been talking about chiral centers. What about trigonal centers? That's kind of its own thing. So for trigonal centers, if I have 1 or more trigonal center and both of them are the same, then that's going to be identical. So an example of that would be 2-butene versus 2-butene. Notice that I'm pinning I'm doing a cis and a cis and I'm comparing them. If they both have the same arrangement, cis or trans, then they're just going to be identical. But what if I'm comparing it to that one versus the trans-2-butene? What's that relationship going to be? It turns out that these are definitely stereoisomers. Right? They look different but they're not mirror images. One is not the mirror image of the other, so these are actually going to be diastereomers. And that is always the case when you have double bonds that switch cis and trans configurations. You're always going to get diastereomers as a product, not enantiomers. So don't think of enantiomers because enantiomers are mirror images. But these, basically, this one here and this one up here are definitely not mirror images of each other. They're diastereomers. That's their relationship. Does that make sense? Cool.

So now I want to teach you guys a little secret here. I've given you all of these rules. This is your flow chart. I really want you guys to use this a lot. Commit it to memory and also just use it as when you're doing your practice problems, have this out for reference.

When to use R and S, when you don’t have to.

Something that's going to help is that this whole time, I've been comparing R and S. So that implies that every single time you have to figure out R and S. Okay. But it turns out that the same and the different part can actually work without finding R and S. So, for example, if I had a molecule that, you know, if I have 2 molecules that are exactly the same, except that the wedges and dashes are different, I don't need to actually calculate cis and R and S. I can just instead say, are they the same or are they different. But that only works if my molecules haven't been rotated. If my molecules are rotated, meaning that your molecules are rotated into different positions when you're comparing them, then you do have to figure out R and S. Okay. So what I'm trying to say here is that R and S, if you figure that out, you always get it right. That's always the fail-proof way to do it. But a lot of times, we're going to cheat and, instead of using R and S, we're just going to look and say, 'Hey, are the molecules rotated?' No, they're exactly in the same position. The only thing that's changed is the bond being towards the front or the back. And in that case, I would just say are they the same or are they different and that's going to save me a lot of time. All right? So with that said, let's go ahead and move on to the next page and see if we can figure out these relationships.

Identify the relationship between the following organic compounds: 

• A. Constitutional Isomers
• B. Diastereomers
• C. Enantiomers
• D. Identical

Identify the relationship between the following organic compounds: 

• A. Constitutional Isomers
• B. Diastereomers
• C. Enantiomers
• D. Identical

Identify the relationship between the following organic compounds: 

• A. Constitutional Isomers
• B. Diastereomers
• C. Enantiomers
• D. Identical

Identify the relationship between the following organic compounds: 

• A. Constitutional Isomers
• B. Diastereomers
• C. Enantiomers
• D. Identical

Here’s what students ask on this topic:

What is the difference between constitutional isomers and stereoisomers?

Constitutional isomers, also known as structural isomers, have the same molecular formula but differ in the connectivity of their atoms. This means the atoms are connected in different orders, leading to different structures. Stereoisomers, on the other hand, have the same molecular formula and the same connectivity of atoms but differ in the spatial arrangement of atoms. Stereoisomers can be further classified into enantiomers (non-superimposable mirror images) and diastereomers (not mirror images). Understanding these differences is crucial for identifying and categorizing isomers in organic chemistry.

How do you determine if two molecules are enantiomers?

To determine if two molecules are enantiomers, follow these steps: First, ensure that the molecules have the same molecular formula and connectivity. Next, identify the chiral centers in each molecule. If each chiral center in one molecule has the opposite configuration (R vs. S) compared to the corresponding chiral center in the other molecule, then the molecules are enantiomers. Enantiomers are non-superimposable mirror images of each other. For example, if one molecule is R at a chiral center and the other is S at the same center, they are enantiomers.

What are diastereomers and how do they differ from enantiomers?

Diastereomers are stereoisomers that are not mirror images of each other. They have the same molecular formula and connectivity but differ in the spatial arrangement of atoms at one or more chiral centers. Unlike enantiomers, which are non-superimposable mirror images, diastereomers have different physical and chemical properties. For example, if two molecules have multiple chiral centers and at least one, but not all, of the chiral centers have different configurations, they are diastereomers. This distinction is important for understanding the behavior and reactivity of different isomers.

What are meso compounds and how are they identified?

Meso compounds are a special type of stereoisomer that contain multiple chiral centers but are optically inactive due to an internal plane of symmetry. This symmetry causes the chiral centers to cancel each other out, resulting in a molecule that is superimposable on its mirror image. To identify a meso compound, look for a molecule with chiral centers that has an internal plane of symmetry, making it achiral overall. For example, a molecule with two chiral centers that are mirror images of each other across a plane of symmetry is a meso compound.

How do cis and trans isomers relate to diastereomers?

Cis and trans isomers are a type of diastereomer found in compounds with double bonds or ring structures. In cis isomers, substituents are on the same side of the double bond or ring, while in trans isomers, they are on opposite sides. These isomers have different physical and chemical properties but are not mirror images of each other, classifying them as diastereomers. For example, cis-2-butene and trans-2-butene are diastereomers because they differ in the spatial arrangement of substituents around the double bond.