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Optical activity of compounds in solution can be measured with a polarimeter, shown schematically in the upper part of the graphic. The angle of rotation of plane polarized light, conventionally the sodium D-line at 589 nm, is determined using two polarizers. (In the graphic, an idealized incident polarizer is reduced to a vertical slit coincident with the plane of polarization.) The angle of polarization of the emerging light is given by ... , where ... is the specific rotation, referred to the sodium D-line and a temperature of 20 ºC, ... is the path length in decimeters (set equal to 10), and ... is the concentration in g/ml. The tartaric acid molecule has three possible stereoisomers: ... (R,R)-tartaric acid is the naturally occurring form. Its mirror-image enantiomer, (S,S)-tartaric acid, as well its diastereoisomer, (2R,3S)-tartaric acid, can also be synthesized. The last is called the meso form and is superposable with its mirror image. Thus the chirality of the two asymmetric carbon atoms cancels and this molecule is not optically active. Thus there is no variation in ... with concentration of (R,S). Equal concentrations of (R,R) and (S,S), called a racemic mixture, also exhibits net cancellation of optical rotation. In this Demonstration, you can choose concentrations for the individual stereoisomers or mixtures of tartaric acid and observe the resultant optical rotation. You can rotate the 3D diagram of (2R,3S)-tartaric acid to show the configurations around the R and S carbon centers.
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