A chiral molecule is a molecule that cannot be superimposed on its mirror image. Chiral molecules are optically active in the sense that they rotate the plane of polarized light.
An atom containing carbon forms optical isomers if one or more of the carbon atoms is bonded to four different species.
Louis Pasteur encountered the phenomenon of optical activity in 1843, during his investigation of the crystalline sediment that accumulated in wine casks (a form of tataric acid called paratartaric acid - also called racemic acid, from the Latin racemus, "bunch of grapes"). He used fine forceps to separate two types of crystals identical in shape but mirror images of each other in structure. Both types proved to have all the chemical properties of tartaric acid, but in solution one type rotated plane-polarized light to the left (levorotatory), the other to the right (dextrorotatory). Pasteur later described the experiment and its interpretation:
In isomeric bodies, the elements and the proportions in which they are combined are the same, only the arrangement of the atoms is differetn... We know, on the one hand, that the molecular arrangements of the two tartaric acids are asymmetric, and on the other hand, that these arrangements are absolutely identical, excepting that they exhibit asymmetry in opposite directions. Are atoms of the dextro acid grouped in the form of a right-handed spiral, or are they placed at the apex of an irregular tetrahedron, or are they disposed according to this or that asymmetric arrangement? We do not know.
Now we do know. X-ray crystallographic studies in 1951 confirmed that the levorotatory and dextrorotatory forms of tartaric acid are mirror images of each other at the molecular level and established the absolute configuration of each. The same approach has been used to demonstrate that although the amino acid alanine has two stereoisomeric forms alanine in proteins exists exclusively in on form, the L-isomer (levorotatory).
- From pasteur's lecture to the Societe Chimique de Paris in 1883, quoted in DuBos, R. (1976) Louis Pasteur: Free Lance of Science, p. 95, Charles Scribner's Sons, New York.