5.1 Optical activity
5.1 Optical activity
Light is polarized when it passes through a Nicol prism or polaroid. The passed light is in a certain oscillating plane. The polarized light (polarized by the first Polaroid) passes through the second polaroid when the it is placed in parallel with the first one, but the polarized light completely disappear when the second polaroid is placed perpendicular to the first one (Fig. 5.1).
Figure 5.1 The direction of polaroid and polarized light
Some compounds can rotate the plane of polarized light to a certain amount. This phenomenon is referred to as optical activity, and compounds with such ability is named an optical active substance. Optically active substances exist as a pair ((+) and (-) forms) of enantiomers. If one of the enantiomers rotates the polarized plane clockwise, the other enantiomer rotates the same amount but to the reverse direction. Clockwise rotation is "dextrorotatory", while the counterclockwise rotation is "levorotatory". For dextrorotatory, symbols d or (+), and for levorotatory, symbols l or (-) are frequently used (Fig. 5.2).
Figure 5.2 Rotation of polarized plane by an optically active compound
The magnitude of rotation depends not only the type of compound, but also the concentration of the solution, the length of light path, the solvent, the wavelength of the light employed, and he temperature. The specific rotation ([a]) is defined for a given compound as the rotation (ｰ) induced by 1 g/ml solution 10 cm long under a specified condition (the temperature, wavelength of the light used, the solvent) as shown below. where λ is the wavelength of the light and t is the temperature.
