Pasteur & van't Hoff
From WikidChem
Slide 7: This slide shows the various isomers of tartaric acid. Notice the different melting points and amounts of optical rotation for each isomer. Also, Pasteur (in 1848) noticed that racemic acid is a 50:50 mixture of the two tartaric acid isomers (called l and d tartaric acid respectively), which he separated by growing crystals of each type and then noticing that the two types of crystals are mirror images of each other. A consequence of the fact that these two isomers are mirror images of one another is that they rotate light the same amount in opposite directions. For racemic acid, which is a mixture of the two, the light from the l and d tartaric isomers cancel out leading to a total optical rotation of 0 for racemic acid (recall the two polar filters from lecture). For more on how the two isomers rotate light, take a look at slide 29 from the lecture on 11/15/06. However, this explains the behavior of three of the four isomers - the only one left is mesotartaric acid. We will learn later what "meso" means and what that entails in terms of configuration. Slide 8: Shows a picture at the lab in Bonn in 1872. A young van't Hoff was a student of Kekule's. Because of the recent discoveries of optical rotation and the polarization of light, (these discoveries were more than 50 years old, what was relatively new was knowledge of the constitution of the compounds, due to work of Wislicenus, among others - JMM) he theorized that atoms that rotated light were possibly located on a spiral. (Pasteur had mused about a spiral 24 years previously - van't Hoff proposed the "asymmetric" tetrahedral carbon. It is interesting that, as a student in Bonn, he must have seen Kekulé's lecture model of benzene, which had tetrahedral carbon atoms.)
Slide 9: This slide gives us background information on J.H. van't Hoff. Van't Hoff was not very big on carrying out actual experiments. He was more into theories. He is one of three individuals responsible for inventing physical chemistry in the last 20 years of the 19th century, and he won the first Nobel Prize in 1901 for his work on the laws of chemical dynamics and osmotic pressure in solutions. He has a romantic turn of mind. The trait that he most admired was imagination, and his 'hero' was Lord Byron.
LY 11/28 6:51 pm DC 11/28 12:33 pm ZS 11/29 10:54 pm
