Tartaric acid is naturally occurring in grapes. Hi, this is Dr. Cook here, Camp 240 Instructor. Let's take a look at the next video. In the previous video, we talked about symmetry and asymmetric carbons and the fact that molecules can exist as mirror image isomers that are not the same or not superimposable. This is what we would refer to as enantiomers. And I want to mention a little bit of the history and properties of these types of stereo isomers. Enantiomers actually have identical physical properties. If you take two mirror image isomers, the melting points, boiling points, spectroscopic properties, all of the properties of their molecule are identical except the direction that they rotate plane polarized light when it passes through a sample of those molecules. And it was in the 17th century when the Dutch astronomer Christian Huggins discovered plane polarized light. So if you take light, which is traveling in waves in all 360 degrees, and you pass it through a polarizing filter, you get light which is polarized in one direction only. It was in 1859 when another scientist discovered that certain natural compounds rotated plane polarized light. And that's an interesting observation. So it actually turns out that mirror image isomers will rotate plane polarized light the same amount but in different directions. This is an intrinsic property of chiromolecules. So if you have one mirror image isomer, it might rotate that plane of the polarized light to the left a certain number of degrees. And if it's the opposite mirror image isomer, it will rotate it to the right a certain number of degrees. That is the only physical difference between anantimers is the direction of plane polarized light that it bends. Now this is what we refer to as optical activity. In the 18th century, tautaric acid, which is an interesting molecule, was isolated from the sediments from red wine. This is the crystals that form because tautaric acid is naturally occurring in grapes. And you'll notice tautaric acid has two stereogenic carbons associated with it. Louis Pasteur was doing research on tautaric acid and its various salts. And he discovered that the crystalline salts that formed from sodium ammonium tarterate actually formed these crystals that had shapes that were exact mirror images. And he actually used a pair of tweezers and separated them. Interestingly, what Louis Pasteur found was that these molecules, when he isolated tarterates, sometimes they rotated plane polarized light to the right and sometimes to the left. And so what he did was he crystallized these and carefully separated these crystals that were mirror images and put them into two different piles. One that all had one shape . . . rotated plane polarized light in opposite directions. This was the first time enanti-mers had actually been separated from a mixture. And this observation led Louis Pasteur to do a lot more research in this area. So today we can characterize chiral molecules by its optical activity and we have formalized and standardized this so that we can directly compare different molecules and different isomers to each other. Now it makes sense that if enanti-mers only rotate light in opposite directions that if you have a 50-50 mixture of mirror image isomers, then what you would observe is no optical activity. That is there should be no rotation of light because if you have equal amounts of both enanti-mers as the light passes through, they will cancel each other out. So one molecule as the light hits it will rotate it a little bit to the left and then it'll hit the mirror image isomer and it'll rotate it the same amount to the right. So if the sum of that, the net sum of that rotation would be zero if you have exactly a 50-50 mixture. And this is what we refer to as a Rhesemic mixture. A Rhesemic mixture is a 50-50 mixture of enanti-mers and that we don't observe the optical activity in this case.