Chirality: Study with Video Lessons, Practice Problems & Examples

Chirality refers to molecules whose mirror images are non-superimposable, exemplified by optical isomers or enantiomers, which have one or more chiral centers—carbon atoms bonded to four unique groups. To draw enantiomers, one can use two methods: creating a mirror image or inverting the spatial orientation of bonds. Chiral molecules are optically active, meaning they rotate plane polarized light, a concept more relevant in advanced organic chemistry. Understanding chirality is crucial for grasping stereochemistry and its implications in chemical reactions and biological systems.

Chirality Concept 1

So when we use the term chirality, this is just a property of molecules in which mirror images of molecules are non superimposable. If you go back and take a look at my videos on isomers, you'll see where I did this analogy with these dogs. So here, this dog is looking in the mirror. In the mirror, it sees its mirror image. Now if we were to take that dog out of the mirror, and slide it over to this dog, they wouldn't match up, they wouldn't line up. That's because if we slip this dog over out of the mirror, this spot here would not line up exactly because the spot is over here. Lining this dog up over here would mean that its spot would be on this side. This means that they are mirror images of each other. Now, Optical Isomers. Optical Isomers are also called enantiomers. These are chiral molecules, and they possess one or more chiral centers. Now what the heck is a chiral center? Well, a chiral center is where a carbon is connected to 4 unique groups. And if you don't have 4 unique groups, then you're classified as being achiral. So if we take a look here at this molecule on the left, it is achiral. And it's achiral because if we take a look, this carbon is connected to what? An OH, an NH₂, but then it's connected to 2 CH₃s. It is not connected to 4 different or unique groups. The molecule on the right is chiral because this H is connected to what? An OH, an NH₂, an H, and a CH₃. Four unique groups. So this carbon here is chiral. Now chiral molecules, we say, are optically active. So that's why we say optical, they're optical stereoisomers, that's because they're optically active. All that means is that they rotate Plane Polarized Light. In this level of chemistry, you won't have to worry too much about that at all. That's reserved more for real organic chemistry when you take Orgo 1 and Orgo 2. But for right now, just realize they're called optical isomers because they rotate plane polarized light. So this is the definitional explanation of that. Right? So just remember, when we're talking about chirality, we're talking about mirror images, we're talking about a carbon atom within a molecule connected to 4 different or unique groups.

Chirality Example 1

Here in this example question, it says, identify the following molecule as chiral or achiral. Now here's a huge hint. When it comes to chiral centers, remember, the carbon must be connected to 4 unique groups. But if you're a double bonded or triple-bonded carbon, it's not possible. If we take a look at this double-bonded carbon for instance, carbon must make 4 bonds. It's making 1, 2, 3 bonds that we see. That 4th bond is that invisible hydrogen. Now how many groups would that carbon be connected to? 1, 2, 3. Can't get to 4. So if you have a double bond or a triple bond for a carbon, it can't be chiral. So that means we're ignoring all these carbons within this benzene ring, and we're focusing on this carbon here, and this carbon here. The carbon on the far right, it's making one bond, so it has 3 hydrogens we don't see. Definitely not chiral. But this carbon here, it's making 1, 2, 3 bonds that we see, so it has 1 hydrogen that's this entire Benzene, and then this CH₃ group. How many unique groups is that? That's 4 unique groups, which means that this carbon here, is a chiral carbon. Which means the molecule overall would be chiral. So here, we're going to say the following molecule is a chiral molecule.

Identify chiral centers in the provided optical isomers.

• A.
• B.
• C.
• D.

 Identify molecule(s) capable of rotating plane polarized light.

• A. A and D
• B. B and C
• C. A and C
• D. C and D
• E. A, C, and D

Drawing Enantiomers Concept 2

In this video, we're going to talk about the methods we can use in drawing an enantiomer. Now when drawing enantiomers of a chiral molecule, there are 2 methods available.

Now, with method 1, we're going to draw an image the molecule sees in the mirror. So let's just imagine this blue dotted line here is our mirror, and this molecule was looking into it. It would see back its own reflection. Now, what would this look like? Well, we'd still have this carbon here in the center, it would see this NH2 still at the top, And what else would it see? Well, it's looking in the mirror, so it'd see these 2 over here, looking back at it, so we'd have the H with still the dash wedge bond, And we'd have our CH3 group. So we'd have it like this. Remember we want to show the connection between the carbon carbons. So it's best to draw it this way. And then we'd have the OH back here. This new image that I've just drawn is the mirror image of my original molecule, or its enantiomer.

Remember, an enantiomer is the mirror image. Now this one, method 1, is a little bit tricky, because you have to look into the mirror, and you have to draw it kind of like backwards. And your dashed wedge to a solid wedge on the chiral center. You'll keep the molecule in place the way it is. So here carbon would still be here, this NH2 would still be here, this OH would still be over here. And all we're doing here is we're inverting the bonds. So now, this dashed bond becomes a wedged bond, and it has the H now. And then this wedged bond becomes a dashed bond, And it has a CH3 connected to it. In this method, we keep the molecule stationary in the same spot, and we're just changing the bonds that show spatial orientation.

Alright. So we could call this the inversion method for method 2. Right. So these are the 2 different ways we can draw the enantiomer of our original chiral molecule.

Drawing Enantiomers Example 2

So here it says, drawing answers for each given chiral molecule using method 1. Alright. So here we're going to imagine this is our mirror, and our molecule looks into it. When it does, it sees back its own reflection. So here we have our carbon, it would see this OH back in the mirror, we'd still have this methyl group here, and then we have these 2 in the back. Make sure you're showing the connections correctly, carbon to carbon. And then here connected to the H. So this would be the enantiomer or mirror image of our original molecule.

For the second one, we imagine there's a mirror here. We'd have these 2 carbons still connected to each other. And we're looking at our reflection in the mirror. So we'd have that H there, we'd have this H here, and this H here. And in the back, we have this H still here. This Br here in the back. And then finally, our NH₂ here. So this would represent our 2nd mirror image or second enantiomer for this chiral molecule in option 2. Right. So this is how you would show both of our mirror images or enantiomers of our original chiral molecules.

Provide the enantiomer using method 2. (Hint: chiral center is circled in red.)

• A.
• B.
• C.
• D.

Predict enantiomer for thalidomide compound given below.

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

What is chirality in chemistry?

Chirality in chemistry refers to a property of molecules where their mirror images are non-superimposable. This means that the molecule and its mirror image cannot be aligned perfectly, much like how your left and right hands are mirror images but cannot be superimposed on each other. Chiral molecules typically have one or more chiral centers, which are carbon atoms bonded to four unique groups. These molecules are optically active, meaning they can rotate plane-polarized light. Understanding chirality is essential for studying stereochemistry and its implications in chemical reactions and biological systems.

What is a chiral center?

A chiral center, also known as a stereocenter, is a carbon atom within a molecule that is bonded to four different groups. This unique arrangement allows for the existence of non-superimposable mirror images, or enantiomers. For example, if a carbon atom is bonded to an OH group, an NH₂ group, a hydrogen atom, and a CH₃ group, it is considered a chiral center. The presence of chiral centers in a molecule makes it optically active, meaning it can rotate plane-polarized light.

How do you draw the enantiomer of a chiral molecule?

To draw the enantiomer of a chiral molecule, you can use two methods. Method 1 involves drawing the mirror image of the molecule. Imagine a mirror placed next to the molecule and draw what the molecule would look like in the mirror. Method 2, known as the inversion method, involves keeping the molecule stationary and inverting the spatial orientation of the bonds at the chiral center. For example, if a bond is represented as a dashed wedge, it becomes a solid wedge in the enantiomer, and vice versa.

What are optical isomers?

Optical isomers, also known as enantiomers, are a type of stereoisomer where the molecules are non-superimposable mirror images of each other. These isomers have one or more chiral centers and are optically active, meaning they can rotate plane-polarized light. The direction in which they rotate light can be either clockwise (dextrorotatory) or counterclockwise (levorotatory). Optical isomers are crucial in many biological processes and chemical reactions, as their different spatial arrangements can lead to different properties and activities.

Why are chiral molecules optically active?

Chiral molecules are optically active because they can rotate plane-polarized light. This optical activity arises from the asymmetry of the chiral center, where a carbon atom is bonded to four different groups. When plane-polarized light passes through a solution of chiral molecules, the light's plane of polarization is rotated. The direction and degree of this rotation depend on the specific arrangement of the groups around the chiral center. This property is significant in advanced organic chemistry and is used to study the behavior and interactions of chiral molecules.