Atropisomers: Videos & Practice Problems

Atropoisomers are chiral compounds without chiral centers, exemplified by allenes and substituted biphenyls. Allenes can be chiral if they form E or Z isomers, while substituted biphenyls are chiral if their ortho substituents are different. The inability to freely rotate due to structural constraints leads to chirality in these unusual molecules. Understanding these exceptions is crucial for mastering stereochemistry and recognizing the nuances of chiral compounds in organic chemistry.

Recognizing chiral molecules with zero chiral centers.

Hey everyone. So now that I know in general terms how to predict what types of stereoisomers I'm going to get based on the number of chiral centers, I'm going to start teaching you some of those exceptions to the rules. And the first of those exceptions is a chiral compound that doesn't have any chiral centers at all, and these are called atropoisomers. Okay. So atropoisomers are these unusual compounds that contain no chiral centers, yet they actually are chiral due to their inability to freely rotate. Now it really depends, class by class, professor by professor, how much emphasis they put on these compounds because they are unusual and they're not ones that we deal with a whole lot in Orgo 1. But sometimes your professor might just throw these in as a trick so that you'll forget about these kinds of compounds that can't rotate. Okay? So here I have just a few. In fact, there's a few more, but these are the really common ones. So allenes. As you can see, allenes, they're two double bonds together. They can't rotate because double bonds can't rotate. Substituted biphenyls. I know you don't know what that is yet, but a biphenyl is just two phenyl groups that are attached in one spot and they're substituted, meaning that they can't really rotate because those substituents that you can see sticking out kind of act as teeth. They lock it together, so it can't fully rotate. Okay. Trans cyclooctene. This is actually a molecule that I was just recently talking about how it can form the cis version and the trans version. It turns out the trans version, because it can't rotate, it can actually form a right-handed version and a left-handed version of this twisted, messed-up thing, and is chiral because of the lack of rotation. Then finally, BINAP, which I don't even remember exactly what it stands for, but it's very similar to the substituted biphenyl where except in this case, one of an entire phenyl group or an entire benzene ring is acting as one of those teeth that are not going to let it rotate across that single bond. So, all of these are going to be chiral even though they lack chiral centers. These are, like I said, this is not the usual. This is not the norm. These are unusual molecules that happen to display chirality.

Determining if allenes are chiral or not.

So what I want to do is teach you guys tests for the two most common ones, which are allenes and substituted biphenyls, and the other ones don't really need rules, so you're fine. You can just always assume that they're going to be chiral. However, it turns out allenes and substituted biphenyls can be chiral, or they cannot be chiral depending on the rules they follow. So let's look at this. We are simply going to use a modified version of test two, which is the one for stereo centers, to identify trigonal centers in the allene. But you might wonder about the two double bonds. Well, here is the tricky part: we are going to visualize the allene as just one big double bond. Maybe squint a little bit and try to see it; try to ignore the carbon in the middle and just pretend it's one big double bond. By the way, this is just my way of solving these, but I think it has helped a lot of students, and I believe it will help you too.

If it can form E or Z isomers after visualizing it as a big double bond, that means it's chiral. Remember when I taught you guys about trigonal centers, I mentioned that they're actually achiral if they pass the test. But allenes are different, so just think about allenes as their own thing. Allenes are going to be chiral if they can form E or Z through this weird long double bond. So, I'll do the first one with you guys, and then I'll let you guys do b and c on your own.

Here's what I would do: look at this compound and ignore the "c" in the middle, just pretending it's one big double bond. Then, I would ask myself if this double bond is able to form E and Z, or cis and trans. Since I have two of the same atom on one carbon, on one side, it actually cannot form E and Z, or cis and trans. This means that no matter how much I switch these two groups, I'm never going to be able to get E and Z. So, this one would be achiral. Now, I want you guys to solve further and use this knowledge to determine the chirality in other cases.

Is the following allene chiral?

Alright, so it is going to be chiral. Okay. And the reason it's going to be chiral is because if I visualize this as a big double bond, okay, then what I have is that none of the carbons have two of the same group on them. So that means that if I were to draw my fence here, my methyl groups could arrange in two different ways. This would be that they're both on the same side, so this would be the cis conformation, but I could also have a situation where my allene looks like this, where I have my CH₃ and my H, but then my CH₃ is over here and my H is back here. Okay? And that would be the transversion. Now like I said, is this chemically accurate? Not really. But it's going to help you guys to figure out which ones are chiral or achiral. So this one would be chiral. Go ahead and try to do.

Is the following allene chiral?

So hopefully you said that C is achiral. Okay? Because C, if you visualize it as a big double bond, has 2 of the same group on one side. So that means that no matter how much I rotate these guys here, no matter how much I flip them, I'm always going to have the same compound. So this one again, since that's 2 of the same thing, just like this situation over here, which is a bad situation, they're both going to be achiral because they can't form cis and trans. Alright, so that was easy. Right? So now what I want to do is move on to substituted biphenyls.

Determining if substituted biphenyls are chiral or not.

So these guys are going to have a pretty weird rule too. Basically, for substituted biphenyls, what we look at is the substituents in the ortho position. Now, ortho is a word we're not really going to use until organic chemistry too very much, but just as a heads-up, what it means is next to or adjacent to. Okay. So basically what we're looking at is the substituents that are next to that single bond or that sigma bond that it's rotating around. Okay? And what I want to do is I want to figure out if any of the rings have 2 of the same group. If they have 2 of the same group, then it's not chiral. But if it has 2 of the same group, then that's going to be achiral. Okay? But if it has 2 different groups, then it's fine, then it would be chiral, similar to the way that we did it for alanines. Okay. So let's go ahead and do together, and then I'll have you guys do B and C on your own. So for A, do I have 2 of the same groups on any of the rings in the ortho position? The answer is no. These groups are different, and these 2 groups are different. So that means this is going to be a chiral substituted biphenyl because none of the rings have 2 of the same exact thing on one side. So go ahead and try to figure out B and C.

Is the following substituted biphenyl chiral?

B is achiral. Why? Because it has two of the same group on one of the benzene rings. Okay? Just so you guys know, this is a group that we're not going to deal with a lot in organic chemistry 1. This is called a nitro group. Okay. Just introducing it in case you guys are probably going to see it in organic chemistry 2. Okay. So, a nitro group, but there are two of the same ones, so it doesn't matter how unusual it is. If there are two of the same thing, it cannot form cis and trans. That would be achiral. Let's move on to C.

Is the following substituted biphenyl chiral?

So I'm going to take myself out of the screen here, so you guys can see everything. So in this case, do I have 2 of the same group on this molecule or on any of these rings and the answer is no. This one, I have 2 different groups. This one, I have 2 different groups. So all I'm looking at is I don't have 2 of the same ones, so this one would be chiral. So hopefully that made sense to you guys, and let me know if you have any questions on atrope isomers. Remember that these are like a specific subset of molecules, but are not the norm. So you should really use the general rules that I told you about. If it does not have a chiral center, assume that it's achiral unless it's one of these 4 guys that I talked about. Alright, so let's go ahead and move on to the next topic.

Here’s what students ask on this topic:

What are atropisomers in organic chemistry?

Atropisomers are a type of stereoisomer found in organic chemistry. Unlike typical chiral compounds that have chiral centers, atropisomers are chiral due to restricted rotation around a bond, often because of steric hindrance. This restricted rotation prevents the molecule from interconverting between different conformations, leading to stable, distinct isomers. Common examples include allenes, substituted biphenyls, trans-cyclooctene, and BINAP. Understanding atropisomers is crucial for mastering stereochemistry and recognizing the nuances of chiral compounds.

How can you determine if an allene is chiral?

To determine if an allene is chiral, visualize the allene as one big double bond, ignoring the central carbon. If this 'double bond' can form E or Z isomers, the allene is chiral. This is because the presence of different substituents on the terminal carbons of the allene prevents free rotation, leading to distinct isomers. If the substituents on one of the terminal carbons are identical, the allene will be achiral as it cannot form E or Z isomers.

What makes substituted biphenyls chiral?

Substituted biphenyls are chiral if the substituents in the ortho positions (adjacent to the single bond connecting the two phenyl rings) are different. This difference in substituents prevents free rotation around the single bond, leading to stable, distinct isomers. If any of the rings have two identical substituents in the ortho positions, the biphenyl will be achiral as it can freely rotate and interconvert between conformations.

Why can't trans-cyclooctene freely rotate?

Trans-cyclooctene cannot freely rotate due to the ring strain and steric hindrance in its eight-membered ring structure. The trans configuration forces the molecule into a twisted, non-planar shape, creating significant steric hindrance that prevents free rotation. This lack of rotation leads to the formation of distinct right-handed and left-handed isomers, making trans-cyclooctene chiral.

What is BINAP and why is it chiral?

BINAP (2,2'-Bis(diphenylphosphino)-1,1'-binaphthyl) is a chiral ligand used in asymmetric synthesis. It consists of two naphthyl rings connected by a single bond, with each ring having a diphenylphosphino group. The steric hindrance from the bulky substituents prevents free rotation around the single bond, leading to stable, distinct isomers. This restricted rotation makes BINAP chiral, even though it lacks traditional chiral centers.