Determining Stereochemical Relationships for Cyclic Structures Using the R and S Method
Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial the molecules in question, and the manner in which these relationships influence the reactivity of the molecules in question (dynamic stereochemistry). Flow chart for determining the relationship between isomers. ISOMERS if different .. Accordingly, the index of refraction (speed of light in the solution relative to. Mar 16, anti (A,X) — (1) Describes the relative stereochemistry of two substituents on (3 ) Describes the stereochemical relationship of two groups on.
Assign an R or S orientation to the stereocenters above. First consider the molecule on the left. Priorities can be assigned to the groups, using the priority rules. This is shown above.R,S system - Stereochemistry - Organic chemistry - Khan Academy
Furthermore, the stereocenter can be assigned an S orientation. In order to correctly assign the orientation of a Sawhorse Projection, the molecule must be viewed such that the fourth group is either oriented toward you or away from you.
The fourth priority group in this molecule, the hydrogen, is on the right side of the molecule. Therefore, view the Sawhorse Projection from its right-hand side in order to assign an orientation. This effectively makes the hydrogen "wedged," and the other groups on that stereocenter dashes or sticks.
So then the mirror image, you would have a hydrogen that's pointed out, and then you would have the carbon, and then you would have the fluorine being further away. And same thing in the mirror image here.
You would have the chlorine coming closer since this chlorine is further back, closer to the mirror, and then you would have the hydrogen pointing outwards like that. And then, obviously, the rest of the molecule would look exactly the same. And so this mirror image that I just thought about in white is exactly what this molecule is: You might say, wait, this hydrogen is on the right, this one's on the left.
This is actually saying that the hydrogen's pointing out front, the fluorine is pointing out back, hydrogen up front, fluorine back, chlorine out front, hydrogen back, chlorine out front, hydrogen back.
So these are actually mirror images, but they're not the easy mirror images that we've done in the past where the mirror was just like that in between the two. This one is a mirror image where you place the mirror either on top of or behind one of the molecules. So this is a class of stereoisomers, and we've brought up this word before. We call this enantiomers. So if each of these are an enantiomers, I'll say they are enantiomers of each other.
They're made up of the same molecules, so that they have the same constituents. They also have the same connections, and not only do they have the same connections, that so far gets us a steroisomer, but they are a special kind of stereoisomer called an enantiomer, where they are actual mirror images of each other. Now, what is this one over here in blue? Just like the last one, it looks like it's made up of the same things.
You have these carbons, these carbons, these carbons and hydrogens up there. Same thing over there.
You have a hydrogen, bromine, hydrogen and a bromine, hydrogen, chlorine, hydrogen, chlorine, hydrogen, chlorine, hydrogen, chlorine. So it's made up of the same things. They're connected in the same way, so they're definitely stereoisomers.
Well, we have to make sure they're not-- well, let's make sure they're not the same molecule first.
Here, hydrogen's in the front. There, hydrogen's in the back. Here, hydrogen is in the back. Here, hydrogen is in the front.
So they're not the same molecule. They have a different three-dimensional configuration, although their bond connections are the same, so these are stereoisomers. Let's see if they're enantiomers. So if we look at it like this, you put a mirror here, you wouldn't get this guy over here. Then you would have a chlorine out front and a hydrogen. So you won't get it if you get a mirror over there.
Stereoisomers, enantiomers, diastereomers, constitutional isomers and meso compounds
But if we do the same exercise that we did in the last pair, if you put a mirror behind this guy, and I'm just going to focus on the stuff that's just forward and back, because that's what's relevant if the mirror is sitting behind the molecule. So if the mirror's sitting behind the molecule, this bromine is actually closer to the mirror than that hydrogen.
So the bromine will now be out front and then the hydrogen will be in back. This hydrogen will be in the back. I'm trying to do kind of a mirror image if it's hard to conceptualize. And then that would all look the same. And then this chlorine will now be out front, and this hydrogen will now be in the back in our mirror image, if you can visualize it.
And then we have another one. And this chlorine is closer to the mirror that it's kind of been sitting on top of. So in the mirror image, it would be pointing out, and then this hydrogen would be pointing back. Now let's see, is our mirror image the same as this? So the mirror image, our bromine is pointing in the front, hydrogen in the back there.
Then we have hydrogen in-- then in our mirror image, we have the hydrogen in back, chlorine in front. So far, it's looking like a mirror image. And then in this last carbon over here, chlorine in front, hydrogen in back. But here, we have chlorine in the back, hydrogen in front.
Stereochemistry - Wikipedia
So this part, you could think of it this way. This is the mirror image of this, this is the mirror image of this part, but this is not the mirror image of that part. So when you have a stereoisomer that is not a mirror, when you have two stereoisomers that aren't mirror images of each other, we call them diastereomers.
I always have trouble saying that. Let me write it. These are diastereomers, which is essentially saying it's a stereoisomer that is not an enantiomer. That's all it means: A stereoisomer's either going to be an enantiomer or a diastereomer. Note that a cyclic molecule must have at least one stereocenter and must not have a plane of symmetry in order to be a chiral molecule.
If a cyclic molecule has no stereocenters, the molecule is achiral. Two cyclic molecules with the same connectivity and no stereocenters are identical. Determine whether the stereocenters are R or S. Assign an R or S orientation to the stereocenters above. Priorities can be assigned to the groups on both molecules, using the priority rules.
The priorities for each stereocenter bearing a chlorine are colored in teal, and the priorities for each stereocenter bearing a bromine are colored in pink.
This is shown above.