The topic of this video will be enantiomers and their physical properties. Remember that enantiomers are mirror image isomers, and enantiomers have nearly identical physical properties. They will have the same boiling point, the same melting points, the same density, the same refractive indices, but there are a couple of very important differences. Perhaps the most important difference is important to their biochemistry.
Because biomolecules are parallel. they can distinguish between enantiomers. Let's look at some examples. Methadone is an analgesic. The structure of methadone looks like this.
Take a minute and pause the video and see if you can determine which of the centers is the chiral center in methadone. The chiral center in methadone is this one. It is the only sp3 hybridized carbon with four different groups on it.
As I mentioned before, methadone is an analgesic and the R enantiomer is significantly more active than the S enantiomer. Again, pause the video and see if you can draw the R enantiomer of methadone. The R enantiomer of methadone looks like this. Typically, the easiest way to draw the correct enantiomer is to choose one, either draw the methyl group coming towards you or away from you, and then determine the stereocenter designation.
If you're correct, then you can leave it. If you're not correct, you just have to switch the direction that that group is pointing. Let's determine the stereocenter here. Remember that the fourth group that we don't show is hydrogen, and it is pointing away from us.
Nitrogen is our highest priority group. The methyl group is our third highest priority group. And the other carbon-containing group is our second highest priority group.
Rotation here is clockwise, meaning that we have drawn the correct R, an antimer. Let's look at another example. Albuterol is a bronchodilator. Albuterol looks like this.
Pause the video for a moment and see if you can find the chiral center in albuterol. The chiral center in albuterol is here. Only the R enantiomer of albuterol is active. See if you can pause the video now and draw the R enantiomer of albuterol. The R enantiomer of albuterol looks like this.
If you assign the priorities of the groups on the chiral center, you'll see that the hydroxyl group is priority 1. The nitrogen containing group is priority 2, and the arine containing group is priority group 3. Rotation here is clockwise, making this the R in antiamer. If biological systems can distinguish an antiamer, we need to be able to as well. And we do so in the lab using a method called polarimetry.
Polarimetry is a technique that uses plain polarized light. Plain polarized light is light that is composed of waves that vibrate only in a single plane. For example, light that vibrates only in an up and down direction, not left to right, or into or out of the screen.
Because it vibrates in a single direction, we can measure its rotation. Two enantiomers will rotate plain polarized light by the same amount, but in opposite directions. So if we have a pair of enantiomers, one R, one S, and we shine plain polarized light, on samples of each of these enantiomers, each enantiomer will cause that plain polarized light to rotate by the same amount, but in exactly opposite directions.
Substances that rotate plain polarized light are called optically active. Although we sometimes work with a pure mixture of an R or an S enantiomer, it's actually quite common for a chemical synthesis to produce a 50-50 mixture of the R and S enantiomers. This is called a racemic mixture.
Because of a racemic mixture, contains equal amounts of the R and S in antemers, its optical rotation, the amount by which it rotates plane polarized light, is zero. This is because 50% of the mixture is rotating the plane polarized light in one direction and 50% is rotating it in exactly the opposite direction, canceling them out.