Percent optical purity

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amount of enantiomers in a mixture is often measured by nonpolarimetric methods, use of the term percent optical purity is obsolete, and in general has been replaced by the term percent enantiomeric excess (ee) (equation 2) introduced in 197163, usually equal to the percent optical purity, [/ ] and [5] representing the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. 

The degree of asymmetric induction is often specified by reference to the percent optical purity of the product ... 

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 ... 

Problem 7.20 Optically pure (S)-(-i-)-CH3CHBr-n-CjH,3 has [a] = -1-36.0°. A partially racemized sample having a specific rotation of -i-30° is reacted with dilute NaOH to form (R)-(-)-CH,CH(OH)-n-CsH,3 ([ ] =-5. 97°), whose specific rotation is -10.3° when optically pure, (a) Write an equation for the reaction using projection formulas, (b) Calculate the percent optical purity of reactant and product, (c) Calculate percentages of racemization and inversion, (d) Calculate percentages of frontside and backside attack, (e) Draw a conclusion concerning the reactions of 2° alkyl halides. (/) What change in conditions would increase inversion ... 

Ru(bpy) ZnTMPyP NAD(P) Enzymes Percent Conversion Percent Optical Purity Final Product Concentration (M) Initial Substrate Concentration (M)... 

Percent optical purity = (Mobs/MmaxKlOO) Percent enantiomeric excess = ([/ ] — [S]/[R] + [S])(100)... 

When dealing with a pair of enantiomers, it is essential to have a means of describing the composition of that mixture and the degree to which one enantiomer is in excess relative to its mirror image. One way of describing the composition of a mixture of enantiomers is by its percent optic purity, a property that can be observed directly. Optical purity is the specific rotation of a mixture of enantiomers divided by the specific rotation of the enantiomerically pure substance when they are at the same concentration. 

The specific rotation of a mixture of enantiomers divided by the specific rotation of the enantiomeric y pure substance (expressed as a percent). Optical purity is numerically equal to enantiomeric excess, but experimentally determined. 

Mixtures containing equal quantities of enantiomers are called racemic mixtures Racemic mixtures are optically inactive Conversely when one enantiomer is present m excess a net rotation of the plane of polarization is observed At the limit where all the molecules are of the same handedness we say the substance is optically pure Optical purity or percent enantiomeric excess is defined as... 

Assuming a linear relationship between [a] and concentration, which is true for most cases, the optical purity is equal to the percent excess of one enantiomer over the other ... 

A method for determining optical purities of less than one percent for secondary alcohols has been published, and Mislow has observed a partial resolution which may serve to distinguish between a meso and a racemic host product when a derived inclusion compound is recrystallized from an enantiomerically enriched guest solvent. ... 

Enantiomeric excess (% ee) is the percentage of the major enantiomer minus that of the min or enantiomer optical purity (% op) is the ratio, in percent, of the optical rotation of a mixture of en antiomers to that of the pure enantiomer. 

If one enantiomer is present in excess over the other, then the solution will have a net rotation corresponding in sign (+ or —) to that of the more abundant enantiomer. The composition of the mixture is denoted by the optical purity or the percent enantiomeric excess (ee%). The enantiomeric excess is defined as ee = % major enantiomer — % minor enantiomer and is a measure of the optical purity of the sample. Values range from 100% (pure enantiomer, ee = 100% — 0%) to 0% (racemic mixture or ee = 50% — 50%). A sample which has an optical purity of 92% is thus a mixture of 96% of one enantiomer and 4% of the other enantiomer. 

Because of doubt about the weighing error and the temperature correction, it was necessary to repeat the determination see the results given in Example 1 on p. 56. The new value of [aJo was +152.61 0.23 deg dm (g cm. Evidently the optical purity of the specimen was quite high the difference from the literature value of +152.70 is not significant at the 95 percent confidence level. 

Specific rotation data may assist in the identification of a specific enantiomer, or may be used to determine the optical purity (enantiomeric purity) of a mixture of enantiomers. Optical purity is defined as the percent excess of one enantiomer over another in a mixture and is expressed in Eq. (3) ... 

Enantiomeric excess or % ee (or % optical purity) = [observed specific rotation] divided by [specific rotation of pure enantiomer] X 100. Note that there are examples where the linear relationship between enantiomeric excess and optical rotation fails. A percent enantiomeric excess (% ee) of less than 100% indicates that the compound is contaminated with the other enantiomer. The ratio of enantiomers in a sample of known (measured) optical purity may be calculated as follows fraction of the major isomer = [(% ee) + 0.5 (100 - % ee)]. Thus,... 

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