The Molecular Structure of Chiral Substances

As already stated optical rotation was first discovered by Jean Baptiste Biot in 1815 but the basic theory of sterioisomerism was established by Pasteur in 1848 as a result of his work on sodium ammonium tartrate. Pasteur found that sodium ammonium tartrate contained two kinds of crystals, one being the mirror image of the other. With the aid of a lens and a pair of tweezers he laboriously separated the two kinds of crystals and found that one kind rotated the plane of polarized light to the right (that is clockwise, dextro (d)) and the other to the left (anti clockwise, laevo (l).

Figure 7. The Structure of Lactic Acid

The bonds associated with the carbon atom are assumed to take the positions of the apexes of a regular tetrahedron with the carbon atom situated at the center. Consider lactic acid which has four different atoms or groups associated with a specific carbon atom and consider it situated in front of a mirror as depicted in figure 7. It is clear that the molecules are not superimposable and are not interconvertable and it follows that the two substances are different although their formulas are the same. The two isomers are optically active and these specific stereoisomers are called enantiomers. 2 methyl butanol and sec butyl chloride, shown in figure 8, are two further examples. The chiral atom, chiral center or sterogenic center are often depicted in the manner shown in figure 8.

Figure 8. Structures of 2-Methyl-Butanol and sec Butyl Chloride Depicted in the Convention that Identifies the Chiral Center and the Direction of the Bonds to the Chiral Carbon.