Optical purity definition

This definition refers to an enantiomeric mixture produced in an asymmetric synthesis. In some cases where a diastereomeric mixture is produced, the definition has to be altered accordingly. Percent optical purity is an operational term that depends on optical rotation measurements. It is not necessarily equal to the percent enantiomeric purity (13), which is a more meaningful term and is the extent to which one enantiomer is formed in excess over the other ... 

As stated earlier, this technique relies essentially on the formation of covalently bonded diastereomers derived from a pair of chiral analytes (SAs pair of enantiomers) which have been converted to a pair of diastereomers using an optically pure chiral derivatizing agent (CDA) which, in this case, serves as a chiral selector (SO). In this context the definition of " optical purity of the CDA is critical (see Section 3.2.1.2.) and has to be evaluated by complementary methods. 

In its broadest terms the discussion of HPLC detection for chiral species must include the analysis of mixtures with achiral substances as well as the quality testing of, for example, the enantiomeric purity of a chemically pure drug form. The distinction between the definitions of chemical purity versus optical purity can not be overemphasized. In an efficient chiral HPLC system the latter problem is trivial, and if retention times are significantly different then any conventional detector such as RI, electrochemical, absorption, etc., could be used. Co-elutions are a major experimental concern in separations of mixtures and at this juncture it is not only prudent but absolutely necessary to involve a chiroptical detector to preferentially identify the chiral analyte. 

An optically active Grignard reagent has the ability to differentiate between the two enantiofaces of a carbonyl compound such as 45 [64], In the example shown, the (S)-enantiomer of the product alcohol, 46, is obtained with a high degree of optical purity (= specific rotation of mixture- - specific rotation of one pure enantiomer X 100 for definitions of other terms used in this work, see [65]). 

As an example of the use of SC-CO2 in an enzymatic reaction, the lipase-catalyzed esterification of oleic acid with racemic ( )-citronellol should be mentioned. At 31 °C and 8.4 MPa, the (—)-(5)-ester is formed enantioselectively in SC-CO2 with an optical purity of nearly 100% [924]. The reaction rate is enhanced by increasing pressure, i.e. by increasing the solvation capability or solvent polarity of SC-CO2. A linear correlation has been found between reaction rate and the solvatoehromie solvent polarity parameter 1(30) see Section 7.4 for the definition of t(30). 

The term chiral is also used to describe a sample of a substance. When used in this sense, it is not necessary that every molecule in the sample has the same handedness see the definitions below for optical purity, enantiopurity, etc. A racemic sample is one containing (statistically) equal numbers of right-handed and left-handed enantiomers, and therefore showing zero optical activity at all wavelengths. A sample can also be chiral, nonracemic i.e., it contains an excess of one enantiomer. 

Finally the most definitive evidence for a cage reaction was the observation that when an equimolar mixture of the peroxide (48) and the peroxide-dio (48-Djo) were decomposed, no crossover products were obtained only equal amounts of fully protonated hydrocarbon (49) and the hydrocarbon-d (49-0 ) were formed. The optical purity of the isolated ( + )-(5)-1-methyl-2,2-diphenylcyclopropane (49) was found to be 31-37 % with a net retention of configuration. Thus, when the lifetime of the rapidly inverting g radical is sufficiently great to permit diffusion out of the solvent cage the product formed by the... 

THC (27). [a]D +114° with absolute optical purity (Srebnik et al. 1984). These enantiomers have now been tested in human volunteers (Hollister et al., in press). Subjects received progressively increasing intravenous doses of (lS)-delta-3-THC (26) and (lR)-delta-3-THC (27), beginning with 1 mg (in 1 ml ethanol) followed by progressive doubling of doses until, definite effects were observed. Cannabimimetic effects with the (IS) epimer were noted at doses of 8 mg or higher. No effects were noted with the (IR) epimer. (lS)-delta-3-THC (26) is thus estimated to have a potency from one-third to one-sixth that of delta-1-THC. 

Recovery of the unreacted enantiomer affords much higher optical purities than that obtainable for the product of the reaction for a given yield. For a discussion and definition of the E value see C. S. Chen, C.J. Sih, Y. Fujimoto, G. Gir-daukas, J. Am. Chem. Soc. 1982, 104, 7294. 

The optical purity is usually, but not always, equal to enantiomer excess. In order for the two to be equal, it is necessary that there be no aggregation. It is possible, for example, that a homochiral or heterochiral dimer (see Glossary, Section 1.6, for definitions) would refract the circularly polarized light differently than the monomer (or each other). In 1968 [19] Krow and Hill showed that the specific rotation of (S)-2-ethyl-2-methylsuccinic acid (85% ee) varies markedly with concentration, and even changes from levorotatory to dextrorotatory upon dilution. In 1969 [20], Horeau followed up on Krow and Hill s observation, and showed that the optical purity (at constant concentration) and enantiomer excess of (5)-2-ethyl-2-methylsuccinic acid were unequal except when enantiomerically pure or completely racemic. This deviation from linearity is known as the Horeau effect, and its possible occurence should be remembered when determining enantiomeric purity by polarimetry. 

The capacity of solutions of the above polymers to rotate the plane of polarized light excluded complete racemization, but was not a definite proof of complete stereospecificity. It was then of interest to determine the extent of possible inversion at the monomer asymmetric center. This evaluation has been performed either by controlling the optical purity of the nonpolymerized monomer at different conversions or, when possible, by cleavage of the macromolecules into low molecular weight fragments under relatively mild conditions. 

Because of the high purity, the excellent morphological definition, and the good optical properties of the microcrystals, the surface and catalytic properties of ZnO powders prepared by Zn combustion have been investigated extensively by spectroscopic techniques. The results of these investigations are well suited to comparison with results obtained with single crystals and less well-defined samples. 

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