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Current time:0:00Total duration:5:28

Stereoisomers, enantiomers, and chirality centers

Video transcript

here's a screenshot from the previous video and we're going to use the screenshot to redraw these two molecules and so we start with the molecule on the left and so there's a carbon here and the carbon is bonded to a hydrogen and the hydrogen's going straight up and that bond is in the plane of the page so we draw a carbon and we draw the hydrogen going straight up and we show that this bond is in the plane of the page I decided to use chlorine as being yellow and so therefore this bond is also in the plane of the page so I'll go ahead and draw in 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 and this foreign is going away from us in space and so we can show that with a dash the fluorines going away this molecule on the right we already saw in the previous video this molecule on the right is the mirror image 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 so let's go ahead and draw it once again it has a carbon 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 so I draw in the bromine and then finally this fluorine is going away from us so 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 so let me just move down here and let's look at these three different definitions and we'll start with stereo isomers so stereo isomers are isomers that differ in the three-dimensional arrangement of atoms and so let's let's think about what the word isomer means again so isomer means same parts and so 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 and in terms of what kind of isomer are they well we've talked about structural isomers before or in structural or constitutional isomers but we can't classify these as being structural isomers so let me go ahead and draw one more dot structure this time I'm going to leave out the stereo chemistry so I'm just going to show a carbon bonded to a hydrogen bonded to a fluorine bond to a bromine bonded to a chlorine so I've left out the stereochemistry and you could see that this dot structure I just drew could represent either of these two dot structures that has the stereochemistry shown and so they're all connected in the same way they're all they're all have a carbon directly bonded to a hydrogen of lorina bromine in the chlorine so you can't say that these two isomers are structural isomers of each other you have to say they are stereoisomers they differ in the three-dimensional arrangement of atoms around that central carbon so these are stereoisomers our next definition our it is a Nancy immerse o enantiomers are stereoisomers that are non-superimposable mirror images and so 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 left we couldn't do so so there are different molecules and they are enantiomers of each other which is Greek for opposites finally our last definition here is a chiral Center or a chirality Center or a stereo genic Center or whatever term you'd want to use there it has a tetrahedral it's a tetrahedral carbon so they have two sp3 hybridized alright so if I look at this carbon here in this dot structure this is a tetrahedral arrangement of atoms tetrahedral geometry has four different groups attached to that carbon in this case four different atoms so a hydrogen of fluorine a bromine a chlorine and anytime you have this tetrahedral carbon 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 and so if you're 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 so we have two possible stereoisomers and we could draw we'd write out a little formula here so two to the N where n is the number of Kai roll centers let me go ahead and write this this is the number of chiral centers or chirality centers so 2 to whatever power that is in this case for this dot structure we had one chiral Center so we're going to say 2 to the first power and this is equal to 2 of course and this number tells us how many stereoisomers we have and and so we've already talked about that so one chiral Center gives us two stereoisomers and these two stereoisomers that we drew have are non-superimposable mirror images right so these are non-superimposable mirror images so these two stereoisomers and so there's a special type of type of stereo isomer that we call and Nan tumors and we'll talk much more about number of stereo isomers in a later video and the next video we're going to go in more detail about chiral centers and chirality centers and how to identify the number of chiral centers in a molecule