The topic of this lecture will be chirality and what it means for a molecule to be chiral. Chiral molecules are stereoisomers. Remember that we've talked about stereoisomers previously. Stereoisomers are molecules that have the same bonding sequence but differ in the orientation of their atoms in space.
And the type of stereoisomer we talked about previously are geometric or cis-trans isomers. Geometric isomers differ in the orientation of their atoms in space due to the fact that there can be no rotation around a double bond or around single bonds that are in a ring. Chiral molecules are another type of stereoisomer. Specifically though, chiral molecules are stereoisomers that have non-superimposable mirror images. What we mean by non-superimposable mirror images is that those two mirror images, itself and its mirror image, cannot be overlapped in such a way that all of the pieces overlap exactly.
A good example of this is your right hand and your left hand, which are mirror images of one another, but there's no way to place those two hands over one another so that everything overlaps. You can have the fingers overlap, not the right side of your hand, right, the back versus the front, or you can have the back and the front line up, but then your fingers don't line up properly. This is a really good example of a chiral item.
Chiral molecules have non-superimposable mirror images. A-chiral molecules have superimposable mirror images. Their mirror images can be overlapped over one another perfectly. The easiest way to recognize a-chiral molecules is to look for a plane of symmetry, because any object or molecule with a plane of symmetry is a-chiral.
So when we're discussing chirality, it's important to be able to find and recognize mirror planes of symmetry. And so that's what we're going to practice with next. So when we're looking for an internal mirror plane of symmetry, you're looking for a place where you could place a mirror, and the reflection of the mirror would look like the half of the molecule or the half of the object that you're hiding with the mirror. So for example, if you look at your hands, there's nowhere that you could place a mirror that the reflection in that mirror would look the same as the portion of your hand that you're blocking with the mirror. An example of an object that does have a mirror plane is a spoon.
A spoon has an internal mirror plane of symmetry along its longer axis. So if you were to place a mirror down the middle of the spoon, long ways, the reflection of what you see in that mirror would look exactly the same as what you're seeing as you're blocking behind it. Molecules have the same property, either containing or not containing an internal mirror plane of symmetry.
So let's look at some of those molecules now. This molecule, cis-1,2-dichlorocyclopentane, has an internal mirror plane of symmetry. It goes right down the middle of the molecule.
If we were to hold up a mirror and cover half of the molecule along this dotted line, the reflection that we see in the mirror would look exactly like the molecule, the portion of the molecule that we're blocking with the mirror. This molecule contains an internal mirror plane of symmetry and therefore is achiral. Trans-1,2-dichloro-cyclopentane does not contain an internal mirror plane of symmetry.
There is nowhere that you can draw a line down this molecule where you could hold a mirror and see in the mirror the portion of the molecule that you're blocking. This molecule does not have a mirror plane of symmetry. What about this molecule? Pause the video for a moment and see if you can find a mirror plane of symmetry in this molecule. Making this molecule with a modeling kit can be especially helpful in determining whether or not it contains a mirror plane of symmetry.
This molecule does not contain a mirror plane of symmetry. What about this molecule? Again, take a moment to pause the video and see if you can find a mirror plane of symmetry in this molecule. You may find it helpful to make a model of this molecule.
Hopefully you've been able to see that this molecule does not contain a mirror plane of symmetry. We'll do one last example. Consider this molecule. This molecule consists of two fused cyclopentane rings.
Again, you may find it useful to pause the video now and make a model of this molecule to determine whether or not it contains a mirror plane of symmetry. This molecule does contain a mirror plane of symmetry, making it achiral. This is easiest to see in the drawing if we rotate the molecule.
If we rotate this molecule so that this bond goes to the top and this bond to the bottom, it looks like this. Now it's much easier to see the mirror plane of symmetry that goes right down the center of this molecule. this molecule achiral. Hopefully these examples help you to understand how to look for that internal mirror plane of symmetry.
You should practice some more examples on your own until you're comfortable determining whether a molecule has an internal mirror plane of symmetry. This is useful because if it contains a mirror plane of symmetry, it must be achiral. Now that we've learned to determine whether a molecule is achiral, what makes a molecule chiral? Most chiral molecules contain a chiral carbon atom. We call this chiral atom a chiral center.
In organic chemistry, a chiral center is an sp3 hybridized atom bonded to four different groups. Most often in organic chemistry, this is a carbon atom, although it doesn't always have to be. Let's look at an example. This molecule, 2-bromobutane, contains a chiral center.
The chiral center is the sp3 hybridized carbon that is bonded to four different groups. In this case, this carbon, which is bonded to a methyl, a bromine, an ethyl, and a hydrogen. Let's draw this out in three dimensions to make it more obvious. Here is our chiral center drawn out in perspective. We know this is a chiral center because when we draw its mirror image, that mirror image is non-superimposable.
Because 2-bromobutane has non-superimposable mirror images, it is chiral. These two non-superimposable isomers that I've drawn are called enantiomers. Enantiomers are mirror image isomers. This is another type of stereoisomer, just like the cis-trans isomers.
They differ in the arrangement of atoms in three-dimensional space. A chiral center is one type of stereocenter. A stereocenter is an atom at which the interchange of any two groups creates a stereoisomer. A chiral center is a type of stereocenter.
What this means is that all chiral centers are stereocenters, but not all stereocenters are chiral centers. Let's look at some examples so that we learn to differentiate between stereocenters and chiral centers. First, we'll look at this molecule.
This molecule has two stereocenters. These two atoms are stereocenters because the exchange of any two groups on the stereocenter makes it into a different isomer, specifically a different stereoisomer. For example, if we interchange the methyl and the hydrogen on the rightmost carbon, we get a stereoisomer. Specifically, the interchange of these two groups gives us the cis-isomer instead of the trans-isomer, which is a stereoisomer of the first molecule. This means that the cis-stereoisomer also has two stereocenters, shown here.
Neither of these molecules is chiral. Both of them are achiral. Pause the video now.
and determine where the mirror planes of symmetry are in these molecules that make them achiral. The stereocenters in each of these molecules cannot be chiral centers, because remember, chiral centers have to be sp3 hybridized, and these stereocenters are sp2 hybridized. So the stereocenters that are marked in these molecules are stereocenters but not chiral centers. Let's look at another example.
This molecule contains a chiral center. The nitrogen is a chiral center because it's sp3 hybridized and contains four different groups that it's bonded to. Because it's a chiral center, it is also a stereocenter.
Let's look at one final example. This molecule contains neither a chiral center nor a stereocenter. It cannot contain a chiral center because the carbon, even though it's written out in perspective, is sp3 hybridized but does not bond into four different groups.
Two of them. are the same. This carbon is also not a stereocenter because the exchange of any two groups on it does not give a different molecule.
For example, if we exchange the two groups here and here, we would get the same molecule, meaning that this is also not a stereocenter. This molecule contains a plane of symmetry and is achiral.