5. Chirality

Chirality

5. Chirality

Chirality: Study with Video Lessons, Practice Problems & Examples

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Chirality is a key concept in stereochemistry, indicating that a molecule has a non-superimposable mirror image, known as an enantiomer. A chiral compound lacks an internal line of symmetry, while achiral compounds have symmetrical structures. For example, a molecule with a chiral center will display different configurations in its mirror image, affecting its optical activity. Understanding chirality is essential for grasping stereoisomers and their implications in chemical reactions and biological systems.

Chiral is the chemistry term we use to describe objects whose mirror image is different from the original.

The mirror image of any chiral molecule is called an enantiomer.
If a molecule has an internal line of symmetry, it will have the same mirror image (achiral).

concept

What is chirality?

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Video transcript

One of the ways to see if two molecules are going to be related to each other as stereoisomers is to look for something that we call chirality. So what is chirality? Well, what chirality says is this. Chirality says if you look into a mirror and you get a different image than the image of yourself, if you get a slightly different image and opposite image, then that means that you're going to be a chiral compound. That means that that image is going to be chiral and it's going to be different. And what we call that image is called non-superimposable. Non-superimposable is a really confusing way of just saying a different image. Okay? So I can give you a few different examples. For example, I look at my face in the morning. When I see my face in the mirror and I'm brushing my teeth, it looks a lot like the face that I actually have. That's because there's a line of symmetry in my face, so when I get the image back, I see the same exact image. Okay. So that would be something that is called achiral because the image is the same coming off of the mirror or at least it should be mostly the same as my face. Another example of something that is different would be like my hand. So my hand, I put it up against the mirror, the image I get is the opposite image. Okay? So that means that if I were to cut out the image that I see in the mirror and put it over my hand, I would get two opposite images. Okay? That's the idea behind chirality. What chirality says is that if you get an opposite image when you look in the mirror, that is a chiral compound. Now the name for that different image, it has a really funky name that you guys are just going to have to know and it's called an enantiomer. And that can get really fun depending on what your professor's accent is like, but enantiomer just means it's the mirror image of a chiral compound. Alright? So literally, it could be anything. It doesn't have to be a molecule. It could be a person, it could be a car, it could be a cat, whatever. An enantiomer is just the mirror image that's different. Okay? So here, let me show you guys an example with an actual molecule. Here's a molecule and what I want to show you is this fake mirror that I drew. It's just this dotted line here. So imagine that this molecule, like I said, maybe it's like brushing his teeth in the morning and it's looking at itself in the mirror. So it has an eyeball. Obviously, this is just a really cool molecule. So it has an eyeball and what does it see? Well, what it sees is okay, well, a lot of these a lot of the image looks the same. For example, what it sees is that the amine is towards the top. This is let's say that's its hair. So its hair is at the top. He thinks everything's good so far. Alright. And he also sees, for example, notice that I have some wedge and dash notation where the wedge has to do with something being in the front and dash has to do with something being in the back. That means if this is a 3D image, the dash would be behind and the wedge would be upfront. So what he sees is that okay, the hydrogen is still in the back. Cool. So this one is still in the back. Well, you know, everything looks fine, but then he notices something's weird. What he notices is that well, the OH used to be on the right side, but in the image that he's looking back, the OH is actually on the left-hand side of himself. Okay? So it's like his hand or something. His hand's on the other side of his body. Then the same way with the methyl group. The methyl group used to be on the left, but now it's on the right. Now I know this seems normal to you because you're thinking, Oh yes, this is a mirror, so that means everything is going to be flipped, Johnny. This isn't this shouldn't be new to you. This is just what happens when you look into a mirror. That's all your professor wants you to know. What your professor wants you to know is that some molecules when they look in a mirror, they're going to get that opposite image. What non-superimposable means is that if I were to take a cutout of this thing, okay, I'm going to erase some stuff here so it becomes more clear. Erase the eyeball. When I take a cutout of this thing and lay it over this one, is it going to look exactly the same? The answer is no because even though the NH2 would be in the same place and even though the H would be in roughly the same place because they're both facing toward the back, what I would get is that I would get an OH over here and I would get a CH3 over here. And what that means is these are non-superimposable because my groups are in different places if I overlay them over each other. Okay? So it turns out that we have a pretty easy rule that we can follow to test to see if something is going to be chiral or not. This is not the only way to figure it out. But it's just like a rule that I already kind of hinted at, which is this, If a molecule has an internal line of symmetry or anything, if anything has an internal line of symmetry, then it will have the same mirror image. And if you have the same mirror image, that's what we call achiral. That means it's not chiral. It's not a chiral compound.

Draw the mirror image of the following molecule.

example

Drawing Mirror Images and Determining Chirality

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So let's go ahead and do this example where let's just do this as a free response. We just want to work through this together and see if we can figure this out. It says here draw the mirror images of the following molecules and determine if the mirror image is the same or if it's different. Then after we do that, we want to figure out if there is an internal line of symmetry and if there is, let's go ahead and annotate it. So for this first one, what I want to do, you guys can just follow along, I want to draw my fake mirror again. Okay? And that's just going to look like this. It's kind of rough. Okay? So imagine once again that this molecule is looking into the mirror. Alright. It's not even like a perfectly straight mirror or whatever. What is he going to get back? What kind of image is he going to get back? Well, what he's going to find is that okay, there's a 6 membered ring in the mirror. So let's write that down. And then in this first image, the bromines, imagine that the bromines are like something on his face. They are on the right-hand side. But for the mirror image, they would be on the left-hand side. Does that make sense? Okay. So man, I'm not that's a really ugly bond. Okay. So are you guys getting that so far? So that would be our mirror image. Notice that they're both still facing towards the back. Now what I want to know here is this mirror image the same as the original molecule or is it different from the original molecule? What do you guys think? The answer is that it's exactly the same And it's not because it's the reverse. The reverse has nothing to do with it. What it has to do with is that if I flip this around, if I rotate it a little bit, then I'm going to get the same exact thing. Now I told you guys I'm not going to do a lot of rotating. Okay. But for this part, just to illustrate this, I want to show you. So if I rotated this molecule like that, if I rotated it actually a 180 degrees to the right. Okay. I like to call that rotation like a DJ spin. Okay? Where I imagine that's a vinyl record and I'm just spinning it around. Alright? So if I DJ spin that molecule, what I'm going to wind up getting is a molecule that looks like this. All right, so is that the same molecule or is that a different molecule? That is the same molecule. So does that mean that this molecule is chiral or achiral? What do you guys think? What that means is if it says it's the same, that means it's achiral. Now I want you guys to look at if there is an internal line of symmetry in this molecule? Is there a line that you could draw that would split it perfectly in half? And the answer is yes. There's actually an internal line of symmetry right here. If I were to take scissors and I were to cut it in half like that, what I would find is I would get 2 perfectly symmetrical halves. Alright? So remember that I said if it has an internal line of symmetry, then it's going to be the same. Okay. I'm sorry. If it has an internal line of symmetry, then it's going to be the same, so it's going to be achiral. So all of that works out. Cool so far? You may be asked to draw mirror images on your exam.

Draw the mirror image of the following molecule.

example

Drawing Mirror Images and Determining Chirality

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Video transcript

So let's do this. My mirror would look like this. What I would wind up getting is a 5-membered ring that looks like that. Okay? And I would get that the CH₃ up here is now going this way, and then the one on the dash is now going this way. Cool? Awesome. So now those are why I mirror image. So now I'm wondering the same exact question. I'm wondering if this is the same molecule. So let's go ahead and do the same exact thing again. Let's do a DJ spin, 180 degrees, and figure out if this is the same molecule. So once I do that, what it's going to look like is like this. I'm going to get the ring facing the same way originally, but now notice that this bottom group, that was facing towards the dash, now moved up. So that means that now I have a dash facing up, and notice that the one on the wedge now rotated, so now it must be going down. Alright. So, is this the same molecule as the original one? What do you guys think? It turns out that this is a different molecule. This one is a different molecule. Why? Because there's actually no way that I can rotate this molecule for it to become the other one. I know you're in disbelief. I know some of you guys are thinking, hey, but what if I flip it like this or if I flip it like this or if I rotate it in a bunch of different ways, couldn't it become that? Never. You can actually never turn this one into that one, no matter how hard you try. Okay? So these are different compounds. So that means that after the mirror image is going to be different, so that means that this is a chiral molecule. Does that make sense? And then chiral means that I'm getting a different mirror image. Okay? What is the name of this mirror image? Do you guys remember I told you guys that it has a special name, kind of funky, enantiomer? Okay, you guys are already learning a lot. Okay? And we're like, we haven't even really gotten into it yet. I'm just introducing this. Alright. One more thing. Is there a line of symmetry? Is there a line that if I cut it down the middle, I'm going to have 2 perfectly symmetrical halves on the original molecule? And the answer is no. Because if I went ahead and drew this line of symmetry again, would I get 2 perfect halves? No, I wouldn't, because one side would have a methyl going up, one side would have a methyl going down. They would not match perfectly. So I'm just going to write here, and you guys should put this in your notes. I'm going to say here, no internal line of symmetry. Wow. Okay? And what does that mean if I have no internal line of symmetry? What that means is that it's chiral. Okay? So we can use that simple rule, and we can figure out if something's chiral or not. Now the whole point of this is to show you guys, to prove why the internal line of symmetry is important. Do you guys see that now? How if it does have one, you're going to get achiral. If it doesn't have one, you're going to get chiral. But now that we understand this, do we have to go through this mirror image crap every time? No. Instead, we can just look for a line of symmetry, and that's where I'm going with this. Alright? So let's go ahead and move on to the next topic, and we're going to talk about the line of symmetry.

Do you want more practice?

5. Chirality - Part 1 of 3

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5. Chirality - Part 2 of 3

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5. Chirality - Part 3 of 3

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Learn more about Chirality

Chirality is the name given to the phenomenon where two or more molecules with the same chemical formula and atomic connectivity are non-superimposable. These molecules with the same molecular formula and atomic connectivity but different spatial arrangements are called stereoisomers. Chiral molecules rotate polarized light as a result of the property called optical activity.

What does it take to be chiral?

In order for a carbon to be chiral, it needs to have four unique groups. Let’s say we’ve got a carbon with these groups attached to it: NH2, OH, CH3, and H. That’s four unique groups, so we’d say that carbon is a chiral center (aka stereogenic center). If the groups are NH2, OH, CH3, and CH3 the carbon would not be chiral because it only has three types of groups; two of them are the same.

Chiral and achiral molecules

It’s pretty easy to see how many unique groups are attached to the carbons in the examples above, but what happens when all of the groups are R groups (carbon chains)? As long as the lengths or connectivity of the chains are different, the center carbon will be chiral.

Chiral molecule with only alkyl groups

The center carbon in the above molecule has alkyl groups as all four substituents, but all are different! The substituents attached—in order of 1-4—are isopropyl, propyl, ethyl, and methyl. A molecule, like the one above with just one chiral center, is said to have point chirality.

Identifying chiral centers

Let’s go ahead and see how many chiral centers we can find in the molecule below!

Vitamin D3

Try to find where all the chiral centers are on your own before looking at the answer below. Remember: in order for a carbon to be a chiral center, it needs to have four unique groups.

Vitamin D3 labeled

There are five different chiral centers in Vitamin D3. Each colored dot is a chiral center, and the numbers drawn in the same color are the unique groups. Depending on the wedge and dash information at each chiral center, different stereoisomers can be formed. Stereoisomers are molecules with the same atomic connectivity and molecular formula but with different spatial arrangement.

Number of stereoisomers possible (non-meso)

Once we’ve determined the number of chiral centers, determining the total number of possible stereoisomers is generally pretty simple. Let’s take a look at the blue chiral center with the alcohol above.

Right now the bond to the alcohol is drawn in plane to not imply any stereochemistry, but that OH is either facing toward us (on wedge) or facing away from us (on dash). It’s binary, so all we need to do is take 2 and raise it to the nth power, where n is equal to the number of chiral centers we have. We’ve got five chiral centers, so it would be 2n = 25 = 32 total possible stereoisomers.

Total stereoisomers

Absolute configuration

1-aminoethanol blank

The two molecules above are mirror images of each other. Molecules that are mirror images of each other are called enantiomers. Enantiomers have the exact opposite stereochemistry. Some real-world examples of enantiomeric objects include hands, feet, and shoes. The word chiral is actually derived from the Greek word for hand.

It’s pretty easy to identify mirror images of each other when they’re rotated so their groups-in-common are facing each other, but molecules rotate all the time! How on Earth can we tell which enantiomer we’re looking at when the molecules rotate or if there’s just one given? Enter the Cahn-Ingold-Prelog system for assigning absolute configuration. We’ll just call it assigning R and S for short.

The first step is to identify a chiral center. Once we’ve found our four unique groups, it’s time to order them in terms of the mass of the atom directly attached to the center carbon. The heaviest atom gets priority 1, and the lightest atom gets priority 4. 1-aminoethanol with labeled priorities

1-aminoethanol with labeled priorities

Oxygen is the heaviest atom attached to the carbon, so it gets priority 1. Nitrogen gets priority 2, carbon gets 3, and hydrogen gets 4. In both cases, the group with the lowest priority is already in the back (on dash), so all we have to do now is trace a circle around priorities 1, 2, and 3 in that order. If it’s clockwise, the chiral center is said to be “R”; if it’s counter-clockwise, it’s said to be S. R and S of 1-aminoethanol

R and S of 1-aminoethanol

Check it out! The mirror images of 1-aminoethanol have opposite R and S configurations. Okay, it’s not so bad when the lowest-priority group is in the back. What happens when it’s not on dash? Do we have to re-draw it to put it there? Nope! All we have to do is assign the priorities just like before, but there’s a bit of a trick. 1-aminoethanal

1-aminoethanal

The molecule on the left has the hydrogen in the back, so it’s easy to solve. The one on the right, though, has the oxygen in the back. What we need to do from here is a) swap the priority values {1 and 4 here}, b) trace a circle around 1, 2, and 3, and c) take the opposite result from the trace. For example: if, after the swap, the trace is counterclockwise the actual configuration is an R.

S-1-aminoethanol and R-1-aminoethanol

Check it out! After swapping priorities, the circle traced is counterclockwise. It looks like an S, but it’s actually an R because we swapped priorities.

Fischer projection

Before we get into stereoisomers beyond enantiomers, let’s learn how to find the R and S of chiral centers in Fischer projections. Good news: it’s actually easier!

Fischer projection

All we have to do is, just like before, assign our priorities based on atomic mass and trace around the top three priorities. If the lowest-priority group is on the vertical like in the molecule on the left, the stereochemistry is as it looks; if the lowest-priority group is on the horizontal like in the molecule on the right, stereochemistry is flipped! Let’s see what it looks like before trying on our own.

Fischer projection R and S

Not bad at all, right? The chiral center on top is an S for the molecule on the left and an R for the molecule on the right. Why don’t we apply this new skill on this molecule right below. There are three chiral centers, and we’re going to have to use the playoff system to assign the priorities since many atoms directly attached are carbon.

3-chloro-2,3,4,5-tetrahydroxypentanal without stereochemistry

3-chloro-2,3,4,5-tetrahydroxypentanal with stereochemistry

From top down, the chiral centers are R, S, and R. Including stereochemistry, the name of this molecule is (2R,3S,4R)-3-chloro-2,3,4,5-tetrahydroxypentanal.

Stereoisomers

Enantiomers

We’ve already talked a bit about enantiomers, but that was only with molecules with only one chiral center. If a molecule has more than one chiral center, all of them swap wedge/dash information (R and S) between the enantiomers. Let’s use 3-chloro-butan-2-ol as our example molecule.

Enantiomers

Notice that both the alcohol and chlorine switch from wedged bonds to dashed bonds. Both chiral centers switched from R to S. What happens if not all chiral centers are swapped? Well, that’s when we end up with diastereomers.

Diastereomers

In this case, only one of our chiral centers swapped R and S configuration. That leaves us with diastereomers. Something to think about: the enantiomer of the molecule on the right is still a diastereomer of the molecule on the left. Check it out:

Relationships between stereoisomers

Meso compounds

Here’s a riddle for you: what’s got at least two chiral centers but is itself achiral? Here’s a hint: it’s got diastereomers but no enantiomer; it’s actually superimposable on its mirror image. Nope, it’s not a vampire! Meso compounds have a plane of symmetry and an even number of chiral centers with opposite R & S configurations.

Cyclohexa-1,3-diol

Chiral molecules rotate polarized light. Their enantiomers (aka optical isomers) rotate light in equal magnitude and opposite direction. If we know how much a molecule rotates light (specific rotation), we can actually determine the concentration of each enantiomer based on the observed rotation of the light as it passes through the solution.

Rotation of light

If two enantiomers are known to be in solution and polarized light is not rotated as it passes through, the solution is racemic—that is, it has equal concentrations of each enantiomer.

Quick question: would a meso compound rotate polarized light? It’s got chiral centers, so the answer should be yes, right? Nope, they won’t due to their overall achirality. Here’s a nice way to think about it: meso compounds have a plane of symmetry with respect to atomic connectivity and two chiral centers with opposite R and S configurations. That means that if the “R” chiral center rotates light just a bit one way, the “S” chiral center will rotate it back to its initial starting point immediately after. That happens because enantiomers (molecules with opposite R and S configurations) rotate light in opposite directions with equal magnitude.

Why does chirality matter?

Back in the 1960s, anti-morning sickness drug caused widespread birth defects in the Germany and across Europe. The drug thalidomide, a molecule with just one chiral center, was prepared and sold as a racemic mixture of S-thalidomide and R-thalidomide.

Thalidomide

Soon after it was released as an over-the-counter drug, it was discovered that one of the enantiomers was a mild sedative safe enough for use during pregnancy that helped with nausea while the other enantiomer was extremely toxic to the forming embryos. More than 10,000 children were born with severely deformed limbs as a result of the use of thalidomide.

Chirality is also important in less horrid ways. Get this: all of the amino acids that your body (and the rest of the eukaryotic world) uses are left-handed! They're all, except for achiral glycine, of the S or "L" absolute configuration. Amino acids and sugars use D and L in place of R and S, but they mean the same thing.

Here’s what students ask on this topic:

What is chirality in organic chemistry?

Chirality in organic chemistry refers to a property of a molecule that makes it non-superimposable on its mirror image. This means that the molecule and its mirror image are different, much like how your left and right hands are mirror images but not identical. A chiral molecule typically has a chiral center, often a carbon atom bonded to four different groups. This lack of symmetry results in two enantiomers, which are the two non-superimposable mirror images. Understanding chirality is crucial for studying stereoisomers and their effects on chemical reactions and biological systems.

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How can you determine if a molecule is chiral?

To determine if a molecule is chiral, check for the presence of a chiral center, usually a carbon atom bonded to four different groups. Additionally, ensure the molecule lacks an internal plane of symmetry. If the molecule has an internal line of symmetry, it is achiral. Another method is to visualize the molecule's mirror image and see if it is superimposable on the original. If the mirror image is non-superimposable, the molecule is chiral. This property is essential for understanding the molecule's optical activity and behavior in chemical reactions.

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What are enantiomers in organic chemistry?

Enantiomers are a type of stereoisomer in organic chemistry. They are non-superimposable mirror images of each other, much like your left and right hands. Enantiomers have identical physical properties, such as melting points and boiling points, but they differ in the way they interact with polarized light and other chiral environments. This difference in interaction is crucial in biological systems and pharmaceuticals, where one enantiomer may be active and the other inactive or even harmful.

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What is the significance of chirality in biological systems?

Chirality is highly significant in biological systems because many biomolecules, such as amino acids and sugars, are chiral. The specific three-dimensional arrangement of these molecules affects how they interact with enzymes, receptors, and other biomolecules. For example, only one enantiomer of a drug may be therapeutically active, while the other could be inactive or cause adverse effects. Understanding chirality helps in the design and development of pharmaceuticals, ensuring that the correct enantiomer is used for desired biological activity.

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What is the difference between chiral and achiral molecules?

Chiral molecules are those that have non-superimposable mirror images, meaning their mirror image is different from the original molecule. They lack an internal plane of symmetry and usually have a chiral center. Achiral molecules, on the other hand, have superimposable mirror images, meaning their mirror image is identical to the original. Achiral molecules possess an internal plane of symmetry. This distinction is important for understanding the molecule's optical activity and its behavior in chemical and biological systems.

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