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Stereoisomers, enantiomers, and chirality centers
The definition of stereoisomers, enantiomers, and chirality centers. How to calculate the number of possible stereoisomers for a structure based on the number of chiral centers. Created by Jay.
Video transcript
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.