Chirality - Online Tutor, Practice Problems & Exam Prep

• Chirality refers to a property where mirror images of molecules are non-superimposable, often involving a carbon atom connected to four unique groups.
• Optical isomers, also known as enantiomers, are chiral molecules with one or more chiral centers and are optically active, meaning they can rotate plane-polarized light.
• A molecule is achiral if it does not have a carbon connected to four different groups, as shown in the example where a carbon is connected to two CH3 groups making it achiral.
• To draw an enantiomer, one method involves drawing the molecule as it would appear in a mirror, maintaining the central carbon and substituents but reversing the spatial orientation of the bonds.
• Another method, called the inversion method, keeps the molecule stationary and inverts the bonds at the chiral center, changing dashed bonds to wedged and vice versa.

Chirality

So when we use the term chirality, this is just a property of molecules in which mirror images of molecules are nonsuperimposable. 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 you were to take that dog out of the mirror and slide it over 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 U exactly because the spot is over here. Lining this dog U 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 four unique groups. And if you don't have four unique groups, then you're classified as being a chiral. So if we take a look here at this molecule on the left, it is a chiral. And it's a chiral because if we take a look, this carbon is connected to what? An OH an NH₂, but then it's connected to two CH₃'s. It is not connected to four different or unique groups.

The molecule on the right is chiral because this H is connected to what in OH and NH₂ and H and a CH₃4 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 one and or go 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 four different or unique groups.

Chirality Example

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 four 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 four bonds. It's making 1, 2, 3 bonds that we see. That fourth 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 triple bond for 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 three 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 one hydrogen that's invisible. So what groups is it connected to? Well, it's connected to this hydrogen, this OH group, this entire benzene, and then this CH₃ group. How many unique groups is that? That's four unique groups, which means that this carbon here is a chiral carbon, which means a 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

In this video we're going to talk about the methods we can use in drawing in an antumer. Now, when drawing enantiomers of a chiral molecule, there are two methods available. Now with method one, 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 in this molecule is looking into it, it would see back its own reflection.

Now what would this look like? Well, we still have this carbon here in the center. You would see this NH₂ still at the top and what else would it see? Well, it's looking in the mirror, so it see these two over here looking back at it. So we'd have the H with still the dash wedge bond, and we'd have our chapter three group, so we'd have it like this. Remember, we want to show the connection between the carbon carbon's, 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, enantiomer is the mirror image.

Now this one method one is a little bit tricky because you have to look into the mirror and you have to draw it kind of like backwards. An easier method would be method 2. With method 2, all you'd have to do is you change your solid wedge to a dashed wedge and your dash wedge to a solid wedge on the chiral center. You Kee the molecule in place. The way it is O 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 wedge bond and it has the H now. And then this wedge bond becomes a dashed bond and it has the Chapter 3 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, right? So we could call this the inversion method for method two, right? So these are the two different ways we can draw the enantiomer of our original chiral molecule.

Chirality Example

So here it says drawing an entrance for each given Chiro molecule using method one. 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. You would see this. Oh, back in the mirror we'd still have this methyl group here and then we have these two 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 two carbon 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 branch here in the back and then finally our New Hampshire 2 here.

So this would represent our second mirror image or second Enanti mirror for this chiral molecule in option two, 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.