If a molecule is non-superimposable on its mirror image, then the molecule is said to have enantiomeric relationship with its mirror image molecule. For example, in 2-chloropropane, the molecule is superimposable with its mirror image, so they are identical molecules, but in 2-chlobutane, the molecule is not superimposable with its mirror image and the two molecules are called enantiomers (Figure 3). Thus, enantiomers are stereoisomers since they differ only in the relative arrangement of the different groups in space but not in bond connectivity. Enantiomers are identical in all physical properties (except optical rotation) and chemical properties and reactivity compared to an achiral reagent in reactivity 

Figure 3

Obviously, the next issue is how to detect and analyse the enantiomers physically. In this respect, in 1801, Haüy, a French mineralogist observed that some quartz crystals rotate polarized light clockwise, while other crystals rotate polarized light to the left. Haüy also noticed that quartz crystals exhibit the phenomenon of hemihedrism (externally, some crystals are non-identical mirror images of other crystals).  This is referred to as optical activity. Followed by this, J. B. Biot observed the optical activity in certain organic compounds and was able to conclude that it is a molecular property. In 1884, Louis Pasteur in an ingenius experiment, crystallized and physically separated two types of crystals of tartaric acid –one of which was hemihedral to the left while the other was hemihedral to the right. When he dissolved the two forms separately and measured their optical rotation, he observed that the crystals having the hemihedral to the left rotated the plane of polarized light to the left and vice-versa. Louis Pasteur thus proposed that the two forms of tartaric acid are mirror image of each other (enantiomers).   

The optical activity of a compound was found to be proportional to

• The concentration of the compound in solution (c)
• The length through light traverses through the solution (l)
• The wavelength used for the measurement (λ) and the temperature at which the measurement is made (t). Usually, the sodium D-line is used for polarimetric measurement.