Test 2:Stereocenter Test - Online Tutor, Practice Problems & Exam Prep

Stereocenters are crucial in determining molecular chirality. A stereocenter is any atom that, when groups are swapped, results in a different molecule. There are two main types: chiral centers, which have four distinct substituents, and trigonal centers, associated with double bonds that can form E/Z isomers. Chiral centers predict chirality, while trigonal centers may not. Understanding these concepts is essential for grasping stereochemistry and its implications in organic synthesis and molecular interactions.

What is a stereocenter?

So this brings us to test 2. We're going to need some kind of reliable way to tell if molecules that aren't rings are chiral or not. And we know that the internal line of symmetry test, or what I call test 1, is ineffective for chains and branches and stuff like that, so we're going to need some kind of test. And it turns out I have just the thing for you guys. What we're going to talk about now is called stereo centers. Okay? It turns out there's another test that we can use, and this test is actually going to be the gold standard for most molecules. It might be a little bit harder to use on some kinds of molecules, but it's going to work every time. How does it work? Well, we have to define what a stereocenter is. A stereocenter is any atom that creates a stereo isomer after swapping groups, and that is called, like I said, a Stereocenter. So, what is that definition? Basically, what I'm trying to say here is that if you swap any two atoms on the same carbon, if you swap their positions and if you wind up getting a different molecule or a molecule that has a different shape afterward, that is called a stereocenter. Here I've given you two different examples.

In this first example, I went ahead, and I had a bromine in the front, and then I swapped it with an H in the back, and what we'll notice is that now the H is in the front and the bromine is in the back. This is a type of stereo center because, after I inverted these two groups, I didn't get the same exact molecule. In one of them, the bromine is in the front. In the other one, the bromine is in the back. Since there's no plane of symmetry here, those are actually different molecules.

Another example would be like this ring. How this ring here has a methyl in the front and an H in the back. And after I swap those two groups, the methyl goes to the back and H goes to the front. Those are also going to be different molecules because, once again, we have a ring, and rings can't rotate easily. So, if it's on the front, it's going to stay on the front. If it's on the back, it's going to stay on the back. So this would also be an example of what we call a stereocenter. The stereo center itself is the atom or the group of atoms that it's attached to.

The difference between chiral and trigonal centers.

Let's define what stereo centers are because it turns out that there are actually 2 different types of stereo centers. So you may hear your professor refer to these as stereogenic centers, but that's just the same thing as a stereocenter. And like I said, the definition is any group that creates stereoisomers after swapping two atoms, or any atom that creates stereoisomers after swapping groups. Here are the two different ones.

The first and most common one that we're going to talk about is called the chiral center. Now, I just want to make this point: a chiral center is a type of stereo center, but they're not exactly the same thing. Because, as I said, there are other types of stereocenters as well. You have to be careful with that because a lot of times I hear students refer to these interchangeably like a chiral center is always a chiral center. No, that's not true. There are some stereo centers that are not chiral centers. So what is a chiral center? A chiral center is going to be any atom that has 4 different substituents. That's it. It's a super easy definition. So if I have an atom in the middle and it's attached to molecule atom A, atom B, and another atom, that is what we call a chiral center and it would be denoted with a star. I would use a little star to say that that is a chiral center. Why is it so special that it has 4 different groups? I'll explain that in a little bit. But that's just the only thing you need to know. If it has 4 different groups, it's a chiral center, and as you guys might have guessed, a chiral center is a predictor of chirality. So if I have a chiral center, that means that that molecule is going to be chiral.

Now let's go on to the next type. The one over here is the trigonal center. A trigonal center is another type of a stereocenter, but it's a little bit different. What this is, it's a double bond, so automatically that's kind of different, a double bond that's capable of making E or Z isomers. Do you guys remember what E or Z is? That's just a fancy way of saying cis or trans, or more technically correct because a lot of these will have more than 2 substituents, but it's the same concept where you have a double bond that can form into 2 different positions or 2 different shapes. When you have that kind of double bond, that is a stereo center. But it's not chiral by itself. So these are actually achiral. It's possible to have a stereo center and for the molecule to be achiral if that stereo center happens to be a trigonal center.

Determining Chirality using Stereocenter

So molecule a, did I have any chiral centers? Meaning did I have any atoms that had 4 different groups on them. Okay? And the answer is no. Okay? The closest thing to that was this carbon right here. What that carbon has is it has a 3 carbon chain on one side. It has an OH on the other side, but then at the bottom, it has 2 H's. Okay. So does it have 4 different groups? No. 2 of the groups that it has on it are exactly the same. So are there any chiral centers? No. Okay.

Let's talk about trigonal centers. Are there any double bonds that can form cis or trans on here? There's not any double bonds at all, so no. So according to test number 2, would this be chiral or would this be achiral? The answer is that this would be achiral. Why? Because of the fact that it doesn't have any chiral centers and remember that chiral centers are a source of chirality. So we can assume that if it doesn't have any chiral centers, that it's going to be achiral. Isn't that easy? So just say okay, does it have chiral centers? No. Then it's going to be achiral. It's as easy as that. Okay? So now I want you guys to work on molecule b.

Determining Chirality using Stereocenter

So first of all, I want you guys to notice that b is a ring. So technically, I could have used test 1 on this. Remember what test 1 said? Test 1 said test for an internal line of symmetry. So I could have used an internal line of symmetry test, but for the sake of this problem, I want to go ahead and use the second test as well. Okay? But then we'll compare the two and make sure that they agree. All right? So do I have any atoms with 4 different groups on them? No. The closest thing I have is this atom right here. That atom has a chlorine, it has a hydrogen and then it has rings on both sides. It's got this side of the ring on one side and it's got this side of the ring on the other. Okay? The only problem is that even though these carbon groups look kind of different, they're actually exactly the same. So both sides of the ring are perfectly symmetrical, so this is going to count as 3 groups, not 4. Why? Because if I count around, what I'm going to find is that I have the same exact atoms going in both directions. If I count in the green direction, what I'll get is CH2 CH2 CH2. So, it's all the same thing. Then if I go to the blue direction, I get CH2 CH2 CH2. As you can see, there's no difference depending on which path I take. I'm still going to have the same atoms in the same order, so they both count as the same exact group. A lot of times I like to call these r groups here. Those are both r groups, but they would both be the same R group. Okay? So r1, r2, in this case r1 equals r2 because of the fact that they're both exactly the same. That means that I have no chiral centers. Trigonal centers only apply to double bonds, so I'm going to cross that out. So that means that the answer is I don't have any stereogenic centers and this should also be achiral. Does that make sense? And the reason I'm concluding this is achiral is because I have no chiral centers. Alright? Now let's talk about test 1, internal line of symmetry test. Could I have used that test? Yeah. I also could have used that test. What would that test have told me? The same thing. I could have drawn my internal line of symmetry like this. I would have had an internal line of symmetry, so it would have been symmetrical and what does symmetrical say? It says that it's achiral. So I could use both tests in this case, but notice that this test works for both chains and for rings, whereas test 1 only really works for rings. That's the advantage of using test 2 because test 2 works for everything. Okay? So let's move on to the next question.

Determining Chirality using Stereocenter

Molecule C, what did you guys say? This molecule is also achiral. Why? Because let's look at chiral centers. Do I have any chiral centers? Well, the closest thing is that one and once again I only have 3 groups. I have the H, which is obviously 1, the OH, which is obviously different, but then I have once again 2 R groups that are the same. R₁ and R₂ are identical, so that means that there are no chiral centers, no trigonal centers, so this should also be achiral. See how easy this is? Okay, so let's move on to molecule D.

Determining Chirality using Stereocenter

So is the molecule chiral? The answer is yes. This is actually a chiral molecule. Why is that? Well, let's test for chiral centers. So did we have any chiral centers here? And the answer is yes. I actually have more than one. I have 2 chiral centers. I have a chiral center here and I have a chiral center here. How does that work? Well, because notice that I've got a CH₃, then that means in the back here I would have an H. Okay. So I'd have a CH₃ and an H. And then what else? Well, I would have one direction of the ring, let's say that's the green direction. And then I'd have another direction of the ring, let's say that's the blue direction. Okay? So my question to you is, are both sides of this ring exactly the same? For example, if I take the green, is it the same as if I take the blue path? And actually, no, they're completely different. Notice that the green path will get to that methyl group in 2 carbons. Okay? Whereas the blue path, if I take it around, it's going to take 4 carbons to get to the same place. Okay? So, are both sides of this ring perfectly symmetrical with respect to that carbon? No. That carbon has a 2 carbon path and a 4 carbon path. So what that means is that if we have r₁, let's say this is r₁ and this is r₂, this is an example where r₁ does not equal r₂. They're different from each other. Okay? So that means I have 4 different groups, so this would be a chiral center. If I use the same logic, I would find this carbon over here is also a chiral center. So then the answer is I have 2 chiral centers. That's the first time we've gotten 2. What about trigonal centers? No, there's no double bonds. So the question here is, is this going to be chiral or not? Okay. And the answer is yes. This will be chiral. Okay. Because basically just say if we have 1 or more chiral centers, that will be a chiral structure. Okay. We're simplifying that right now. It turns out there's some more rules that we're going to learn later. But for right now I just want you guys to know if it has a chiral center, it's chiral. I don't want to complicate it too much for right now. Is that cool? So I have 2 chiral centers, so it's chiral. I just want you guys to think that way. Cool. So let's move on to molecule E.

Determining Chirality using Stereocenter

So does the molecule have any stereogenic centers? Yes, it does. Is it chiral? No, it's not because this molecule has no chiral centers. If you'll notice, most of these atoms here don't even have four groups around them. For example, this carbon right there, it only has three groups because it has a bromine, a methyl and then the double bond. So you can never have a chiral center directly on a double bond. That doesn't even make sense. You need a tetrahedron. Okay? But let's say we do have one atom here that has one, two or three, one that kind of has four different groups. It might let's see. It has a double bond on one side, it has a methyl on the other, but then it has two H's. So, anytime you see a CH₂, you can kind of just throw it away. Don't worry about it because there are two H's, so it can't be a chiral center. So the answer is I have no chiral centers here. Now let's look at trigonal centers. Do I have any double bonds capable of forming stereoisomers or cis or trans if I were to swap groups? And the answer is yes, I do because I have a double bond here. That double bond, if I were to switch the position of the bromine and the methyl, let's draw that out and see what happens. Then what that means is that I would have a methyl going up, a bromine going down, and then I would have still the ethyl group and the methyl. Is this equal to this? Are they the exact same thing? No. It turns out they're not. Remember how to do e and z? We would take the highest priority on both sides. So on this side, I would say, okay, my highest priority on the red side, my highest priority must be the Bromine because Bromine has a higher molecular weight. Are you guys cool with that? On the blue side, what's my highest priority? Is it methyl or ethyl? It's going to be ethyl. So would this be e or z? Well, the big groups are cis to each other, so this would be equal to z. Remember that z stands for cis. Then over here, I would have the opposite situation where we have bromine and ethyl, but they're on different sides of the fence. K. So if they're on different sides of the fence, this would be e. By the way, if this is confusing you, if you have never done this before, I already taught this. So go ahead and look back towards the part of a past lesson, and that might help you guys. So the point here is that I will get a different stereoisomer if I swap groups, so this does have one trigonal center. Does that make sense? Cool. So I have zero chiral centers, one trigonal center. Is this chiral? No, it's still achiral. The reason is because I told you guys that trigonal centers by themselves are not chiral. K. They're just a source of stereoisomers. You can make stereoisomers around them, but by themselves they're not chiral. So this is also achiral. Alright? So let's go on to the last question, which is a question.

Here’s what students ask on this topic:

What is a stereocenter in organic chemistry?

A stereocenter in organic chemistry is any atom in a molecule that, when two of its substituents are swapped, results in a different stereoisomer. This means the molecule's spatial arrangement changes, leading to a different configuration. Stereocenters are crucial for determining the chirality of a molecule. There are two main types of stereocenters: chiral centers, which have four different substituents, and trigonal centers, which are associated with double bonds capable of forming E/Z isomers. Understanding stereocenters is essential for studying stereochemistry and its implications in molecular interactions and organic synthesis.

How do you identify a chiral center in a molecule?

To identify a chiral center in a molecule, look for an atom, typically carbon, that is bonded to four different substituents. This unique arrangement means that the molecule cannot be superimposed on its mirror image, making it chiral. Chiral centers are often denoted with an asterisk (*) in molecular diagrams. The presence of a chiral center indicates that the molecule is chiral and can exist as enantiomers, which are non-superimposable mirror images of each other. Recognizing chiral centers is fundamental for understanding molecular chirality and its effects on chemical properties and biological activity.

What is the difference between a chiral center and a trigonal center?

A chiral center is an atom, usually carbon, bonded to four different substituents, leading to a molecule that is chiral and can exist as enantiomers. In contrast, a trigonal center is associated with a double bond that can form E/Z isomers (cis/trans isomers). While a chiral center always indicates chirality, a trigonal center does not necessarily make the molecule chiral. Trigonal centers are achiral by themselves but still create stereoisomers due to the restricted rotation around the double bond. Understanding these differences is crucial for mastering stereochemistry concepts.

Why are stereocenters important in organic chemistry?

Stereocenters are important in organic chemistry because they determine the spatial arrangement of atoms in a molecule, which directly affects the molecule's physical and chemical properties. The presence of stereocenters can lead to the formation of stereoisomers, including enantiomers and diastereomers, which can have vastly different biological activities and reactivities. For example, one enantiomer of a drug might be therapeutically active, while the other could be inactive or even harmful. Understanding stereocenters is essential for designing and synthesizing molecules with desired properties and for studying their interactions in biological systems.

How do E/Z isomers relate to stereocenters?

E/Z isomers, also known as cis/trans isomers, are related to stereocenters through trigonal centers. A trigonal center involves a double bond that can form two different spatial arrangements of substituents, leading to E (entgegen, opposite) or Z (zusammen, together) isomers. These isomers are a type of stereoisomerism where the substituents are positioned differently around the double bond. While E/Z isomers are not chiral, they are still stereoisomers because they have different spatial arrangements. Recognizing E/Z isomers is crucial for understanding the stereochemistry of molecules with double bonds.