Stereoisomers, enantiomers, and chirality centers

Voiceover: Here's a screen shot from the previous video. We're going to use this screen shot to redraw these two molecules, so we start with the molecule on the left. There's a carbon here. The carbon is bonded to a hydrogen. The hydrogen is going straight up and that bond is in the plane of the page, so we draw our carbon, and we draw the hydrogen going straight up. We show that this bond is in the plane of the page. I decided to use chlorine as being yellow. Therefore, this bond is also in the plane of the page. I go ahead and draw my chlorine like that. I made bromine red, and this bromine is coming out at us in space, so we use a wedge to show the bromine coming out at us. I decided to make fluorine green. This fluorine is going away from us in space, so we can show that with a dash, so the fluorine is going away. This molecule on the right, we already saw on the previous video, this molecule on the right is the mirror image to the one on the left, but you can't superimpose the molecule on the right with the one on the left, therefore, it's a different molecule. Let's go ahead and draw it. Once again, it has a carbon in the center bonded to a hydrogen that's going up, so I can draw that in there. It's also bonded to a chlorine with the bond in the plane of the page. This time, the chlorine is going to the left. The bromine is still coming out at us in space. I'll draw in the bromine. Finally, this fluorine is going away from us. I can go ahead and draw in the fluorine going away from us in space. Let's use these images here to talk about three definitions. Let me just move down here and let's look at these three different definitions. We'll start with stereoisomers. Sterioisomers are isomers that differ in the three dimensional arrangement of atoms. Let's think about what the word isomer means again. Isomer means same parts. These two different molecules are composed of the same parts. Each of these molecules contains one carbon, one hydrogen, one fluorine, one bromine and one chlorine. In terms of what kind of isomer are they, we've talked about structural isomers before, structural or constitutional isomers. We can't classify these as being structural isomers. Let me go ahead and draw one more dot structure. This time, I'm going to leave out the stereochemistry. I'm just going to show a carbon bonded to a hydrogen, bonded to a fluorine, bonded to a bromine, bonded to a chlorine. I've left out the stereochemistry. You can see that this dot structure I just drew, could represent either of these two dot structures that has the stereochemistry shown. They're all connected in the same way. They all have a carbon directly bonded to a hydrogen, a fluorine, a bromine and a chlorine. You can't say that these two isomers are structural isomers of each other. You have to say that they're stereoisomers. They differ in the three dimensional arrangement of atoms around that central carbon. These are stereoisomers. Our next definition is enantiomers. Enantiomers are stereoisomers that are non-superimposable mirror images. Once again, we saw in the previous video, that this molecule on the right is the mirror image to the one on the left, but when we tried to superimpose the one on the right on the one on the left, we couldn't do so. They are different molecules. They are enantiomers of each other, which is Greek for opposites. Finally, our last definition here is, chiral center, or a chirality center, or a stereogenic center, or whatever term you'd want to use there. It has a tetrahedral carbon, so I think it's SP3 hybridized. When I look at this carbon here in this dot structure, this is a tetrahedral arrangement of atoms, tetrahedral geometry. It has four different groups attached to that carbon. In this case, four different atoms. So a hydrogen, a fluorine, a bromine, a chlorine. Any time you have this tetrahedral carbon that has four different groups attached to it, you create a chiral center, so this carbon right here is a chiral center, or a chirality center. If you're starting without stereochemistry, if you start with this dot structure right here, and you identify that you have one chiral center present in this dot structure. We've just seen one chiral center means two possible stereoisomers. We have two possible stereoisomers. We could write out a little formula here, two to the n, where n is the number of chiral centers. Let me go ahead and write this. This is the number of chiral centers, or chirality centers. Two to whatever power that is. In this case, for this dot structure, we had one chiral center. We're going to say two to the first power. This is equal to two, of course, and this number tells us how many stereoisomers we have. We've already talked about that. One chiral center gives us two stereoisomers. These two stereoisomers that we drew, are non-superimposable mirror images. These are non-superimposable mirror images. These two stereoisomers. They're a special type of stereoisomer that we call enantiomers. We'll talk much more about number of stereoisomers in a later video. The next video, we're going to go into more detail about chiral centers and chirality centers, and how to identify the number of chiral centers in a molecule.