Optical activity and chiral molecules

A compound is considered optically active if it rotates the plane of plane-polaxized light, as described in Chapter 5. An explanation for optical activity is that, upon entering a crystal, plane-polarized light is split into two circularly polarized waves, one right-handed and the other left-handed. Since the crystal has a right-handed or left-handed character, the two circularly polarized waves travel through it with different velo- 

FIGURE 14.12. Three-point attachment of a prochiral molecule CX2YZ to a chiral receptor (hatched area). Since the receptor binds the groups X, Y, and Z at specific sites only, it must bind Xb but cannot bind Xa- Note that, after binding, Xa (free in space) would be presumed to have different chemical properties from Xb (bound to the receptor). 

FIGURE 14,13. Mode of three-point attachment of prochiral methylaminomalonate to a chiral surface (a cobalt(III) complex), (a) Chemical formula of complex, (b) stereoview of complex, and (c) stereospecific decarboxylation. The carboxyl group that is hydrogen bonded to the complex is lost rather than the metal-bound carboxylate. 

Eilhard Mitscherlich in 1844 reported that solutions of commercial tartaric acid rotated the plane of polarized light in a clockwise manner, while solutions of racemic acid had no effect on plane-polarized light. This finding was surprising because the chemical formulae of these two acids are the same, HOOC-CH(OH)-CH(OH)-COOH, and their crystals appeared identical. Mitscherlich posed this problem as Mitscherlich s riddle.  

The answer to Mitscherlich s riddle was found by Louis Pasteur in 1848. Because of his great contributions to medical science, it is not always appreciated that he started his scientific career as a crystallog-rapher, and that he solved the enigma posed by Mitscherlich. Pasteur s work on tartaric acid and racemic acid is a testimony to his fine powers of observation, a faculty that stood him in great stead in later years. He wrote at this stage (translated), I am very happy. I shall soon publish a paper on crystallography.  

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