Enantiomer composition determination specific rotation

One of the terms for describing enantiomer composition is optical purity. It refers to the ratio of observed specific rotation to the maximum or absolute specific rotation of a pure enantiomer sample. For any compound for which the optical rotation of its pure enantiomer is known, the ee value may be determined directly from the observed optical rotation. 

For a nonracemic mixture of enantiomers prepared by resolution or asymmetric synthesis, the composition of the mixture was given earlier as percent optical purity (equation 1), an operational term, which is determined by dividing the observed specific rotation (Mobs) of a particular sample of enantiomer with that of the pure enantiomer ( max), both of which were measured under identical conditions. Since at the present, the amount of enantiomers in a mixture is often measured by nonpolarimetric methods, use of the term percent optical purity is obsolete, and in general has been replaced by the term percent enantiomeric excess (ee) (equation 2) introduced in 197163, usually equal to the percent optical purity, [/ ] and [5] representing the relative amounts of the respective enantiomers in the sample. 

Thus a racemic mixture (n1 = n2) has an enantiomeric purity of zero. Any other enantiomeric composition in principle can be determined provided the mixture has a measurable rotation and the rotation of the pure enantiomer, a0, is known. Unfortunately, there is no simple method of calculating a0 in advance. In fact, specific rotations of optically pure compounds are determined most reliably from Equation 19-4 after measurement of enantiomeric purity by independent methods. 

The second approach, the so-called direct method, consists of the direct determination of enantiomers immersed in a chiral environment. A diversity of techniques has been used to determine enantiomeric composition in samples. Polarimetry, NMR using chiral solvating agents (CSAs), or the different chromatographic and related techniques are the most popular. Polarimetry is still broadly applied in organic chemistry laboratories at least as a semiquantitative method. The general, easy-to-perform, and nondestructive characteristics of polarimetry are properties that justify its use. However, limitations such as the need to know the specific optical rotation for the compound of interest, the inherent low sensitivity, and the possible inaccuracies related to the presence of impurities or to the low specific rotation of certain compounds cannot be overlooked. 

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