Louis Pasteur Discovers Molecular Chirality
There's an easy way to understand chirality.
Hold out your hands, palms facing each other. Imagine that each hand is the chemical structure of a molecule. Most complex molecules are chiral. Like your hands, the two structures of chiral molecules - in sugars, they're referred to as D and L, from the Latin dexter and laevus - differ only in the arrangement of their elements. Put your hands together and they seem to match exactly. In the same way, the common sugar D-glucose is the mirror image of L-glucose, its rare counterpart. But put your hands down one on top of the other, both facing down, and you'll see that they're not identical at all; they're what chemists call non-superimposable.
Pasteur carefully observed the paratartaric acid under a microscope. Looking at the tiny crystals, he noticed two different types. While almost identical, they were actually mirror images of each other. Pasteur's next step required incredibly meticulous work. Again, working with the microscope, he separated the two types of crystals into two piles. After separating the crystals, Pasteur made two solutions—one with each of the piles—and tested how they interacted with polarized light. He found that both solutions rotated the light— but in opposite directions. When the two types of crystals were together in the solution of paratartaric acid the effect of rotation of the light was canceled.