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Isomers Part II


In this lesson we're going to finish our discussion of isomers by talking about optical isomers and their subtypes. And this is a 5b topic. So the last group of isomers we're going to look at are the optical isomers. This subtype of configurational isomers is so-called because the main difference between these isomers is that polarized light passing through a pure sample of different optical isomers will be rotated in different directions.

Light bounces off different functional groups differently. And it will bounce the opposite directions if the functional groups are placed in the opposite position relative to each other. Back in the video on stereochemistry, we talked about the property of chirality, or handedness, which means that just like how your hands are similar but you can't lay one flat on top of each other and have them match, molecules too, can be similar, but not superimposable.

Look at these two molecules. No matter how much we rotate it around that carbon-carbon bond, we'd never come up with a version that could be perfectly superimposed on the other. The And the NH2 are just on opposite sides. And actually, if we flipped one of these molecules around, and rotated the end with the functional groups, we would find them to be mirror images.

Two carbons with the same substituents but different chirality will have mirrored symmetry when we compare them to each other. Now isomers with the exact opposite chirality will bend polarized light, passing through their solutions by the same amount. But in opposite directions. These are called the dextro and levorotatory forms, which are also known as the D and L or plus and minus forms.

It can be hard to predict which form of the molecule will rotate like which direction just by looking at it. So we use a more precise system called the S/R system, to talk about the absolute configuration, we go into this in a lot more depth in the stereochemistry and projections video. But the short version is, we apply the con-angled create a lot of rules like we did for the geometric isomers to assign a priority number substituent of a given carbon, with 1 being the highest, then draw an arrow going from the 1,2,3 atoms.

Next, we sort of mentally rotate so that we're looking at it head on then label it R for rectus if it's clockwise and S for sinister if it's counterclockwise. These molecules will therefore be named according to IUPAC rules as respectively R1 amino 222 dimethalproponal and (S)-1-amino-2-2-dimethyl-propanol. There's a few other things I ought to talk about before we get a bit more in depth on optical isomers.

First, there's a good mathematical formula to know called the Van't Hoff Rule. This rule states that if n is the number of chiral centers in the molecule, then the molecule will have 2 to the n potential R/S combinations. Now remember a carbon can only be in chiral center if it has four different single bond substituents. So this molecule here has two chiral centers and it would have two squared or four potential different stereoisomers based on the S and R configuration because it could be RR, RS, SR, or SS.

Sometimes the potential optical isomers are actually the same molecule though, and we'll talk about this sort of special case in a bit. One more thing, if we have multiple centers in atom. We would label the compound with numbered S's and R's, starting with whichever carbon is first according to IUPAC rules. Or, if that carbon isn't chiral, then we start with whichever carbon is chiral after that.

So. This molecule here is S on the left side and R on the right side. So the name of this molecule will be 2S-3R-3-amino-2-cholo-3-fluoro-butan-2-ol. This convention also applies to E and Z, isomers with multiple double bonds by the way. I should also clarify one more thing, which is that the lower-case l and d notation is not the same as the capital L and D notation.

Capital L and D is used exclusively for organic molecules and is a kind of an older system that isn't used as much anymore. In the capital L and D system, we put a molecule in a Fischer projection with the most oxidized carbon at the top. In this case of glucose that's this aldehyde carbon here. Then we look at the last chiral center if the Is on the right it's D glucose and if it's on the left it's an L glucose.

This system is only really used to refer to sugars and amino acids. All sugars used by the body are in capital D. While all amino acids are capital L. So back to optical isomers have several subtypes as well. One type that you should be intimately familiar with is what is called as enantiomer.

Enantiomers are exact mirror images of each other. They have opposite chirality on every possible bond,, so here we see we have two molecules with identical constituents. One is RSS and the other is SRR. They have the exact opposite chirality at every chiral center. Making them enantiomers of each other.

Enantiomers will bend light the exact same amount but in the opposite direction from each other. Enantiomers will also look like the mirror image of each other if you arrange them across from each other like I have here. Now enantiomers will differ only in respect to the rotation of polarized light.

Their physical properties are going to be otherwise identical. They also usually have similar reactivity. But take note, when a reactions depends on chirality, which is usually because we have a chiral-catalyst like an enzyme, then they will not react the same way. Enzymes will usually work best with one enantiomer since their substrate binding site will be shaped to accommodate one isomer and not the other.

Now diastereomers are another subtype of isomer. Diastereomers have different stereochemistry from each other but are not opposite on every chiral center. We have here some examples, RSS, SRS, RRS, SSS. These guys are all diastereomers of each other.

They share chirality on at least one chiral center, and so they are not exact mirror images. Diastereomers do not bend light exactly the opposite way from each other. And in fact it can be hard to predict how they will be different optically since each functional group can bend light a different way by a different amount. Additionally, because not all chiral centers are reversed into diastereomers, they will have often have different physical properties, like dipole moments, densities, and boiling points, and they may have a different chemical reactivity as well.

It's important to make clear that enantiomer and diastereomer are relative terms. An isomer can be an enantiomer to one isomer and diastereomer to a different one, which has its own enantiomers. Here's a diagram showing the four possible isomers of this compound. 1-chloropropane-1,2-diol.

We see that the SS and RR forms are enantiomers of each other and the SR and RS forms are also enantiomers of each other. But SS is a diasteromers to SR and RS. As they are to it in turn. Enantiomers come in pairs but the number of diasteromers is limited only by the number of chiral centers.

One special type of optical isomer is what we call a Meso Compound. A Meso Compound is an optical isomer of an optically active molecule with chiral centers, that does not rotate plane-polarized light. Why does this happen? Well, remember each of a functional group can rotate the course of polarized light passing through it by a certain amount, and groups around a chiral center combine their effects to make a net rotation caused by that chiral center.

If light rotated by one chiral center is rotated all the way back by another, these effects can actually cancel each other out. We do need a few conditions for this to happen. First, we need more than one chiral center. Generally an even number. Second, the chiral centers must have matching substituents.

If they had different constituents, they bend light by different amounts and the rotations would not be able to cancel each other. So every chiral center must have a counterpart with the same functional groups. Third, the molecule must be bisceptible by an internal mirror plane. What this means is that if you cut this molecule in half, like along the orange line here, and then mirrored it, it would super-imposable on the other half of the molecule.

Other chiral centers and other structures like double bonds can also potentially bend light. So in order for light bent by one side of the molecule to be exactly unbent by the other, the two sides need to be exact mirror images of each other, with reverse chirality. This mirror image requirement means that meso compounds will have opposite chilarity on their chiral centers.

So we see here that the top molecule is SR, while the bottom one is RS. Another implication of this is that meso compounds are actually achrial, because, while they have local chilarity, on the whole they don't have handedness, because they are internally symmetric, because remember, we can draw this mirror plane. And actually take a look here, these two molecules that we have drawn here, the SR and the RS, they're actually the same molecule.

If we take the RS form on the bottom here, rotate it 180 degrees. Around the middle of that carbon carbon-carbon bond, then that chlorine would whine up point upward and now the left side would be S and the right side R. Just like the top molecule here. The RS and the SR isomers of this molecule are actually the exact same molecule. All you need to do to switch is just rotate that molecule around.

So this compound here that we've drawn would actually only have three different potential optical isomers. It could have the SS isomer, the RR isomer, and the combined SR/RS isomer. Which are actually the same molecule. So to sum things up optical isomers which are configurational isomers and therefore also stereoisomers have different chirality or handedness.

Two isomers are enantiomers if they are exact mirror images which bend polarized light is equal and opposite directions. And have similar properties. While diastereomers have different chirality, but are not mirror images and may not have similar properties. Meso compounds are optical isomers that do not actually bend polarized light.

Because they have internally mirrored stereochemistry. We also don't wanna forget that we can find the number of optical isomers of a molecule by using the formula 2 to the n, where n is the number of stereo centers. We can also actually modify this with minus m, where m is the number of meso compound pairs, since a pair of meso compounds are actually the same molecule.

Therefore they would only count for one isomer.

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