Percentage enantiomeric excess

Many workers use the equivalent term percentage enantiomeric excess rather than optical purity ... 

In a stereoselective reaction, one stereoisomer is formed in a major amount than another. When the stereoisomers are enantiomers the selectivity is known as enantioselectivity. The degree of enantiomeric purity of a solution is measured by its enantiomeric excess, or ee. The percentage enantiomeric excess is found by dividing the observed optical rotation by the optical rotation of pure enantiomer in excess and multiplying by 100. 

The 0% ee means 50 50 racemic mixture, while 50% ee means 75 25 mixture. Thus, enantiomeric excess or ee is a measure for how much of one enantiomer is present compared to the other. For example, in a sample with 40% ee in R, the remaining 60% is racemic with 30% of R and 30% of S, and so the total amount of R is 70%. Thus, the percentage enantiomeric excess is also written as ... 

Determine the percentage enantiomeric excess As457 (actual)/As457 (theoretical) x 100... 

By judicious choice of chiral auxiliary-reagent pairs, it has been possible to extend this chemistry to the enantioselective synthesis of p-hydroxy-a-methyl-carboxylic acid derivatives having either anti or syn stereochemistry (Schemes 24 and 25). For example, the boron azaenolate obtained upon reaction of (65) with diisopinocamphenylboryl triflate reacts with a series of aldehydes to provide adducts that are readily converted to the anti methyl esters (66) in good overall yields (Scheme 24). The anti.syn ratios for these reactions are typically >9 1, and the percentage enantiomeric excesses for the anti adducts are in the range of 77-85%. On the other hand, the boron azaenolate derived from oxazoline (61c) and 9-borabicyclononane triflate reacts with aldehydes to give adducts that can be converted into the methyl esters of the jyn-carboxylic acids (67 Scheme 25). The symanti ratios in these reactions are typically... 

When you prepare a sample of an enantiomer by a resolution method, the sample is not always 100% of a single enantiomer. It frequently is contaminated by residual amounts of the opposite stereoisomer. If you know the amount of each enantiomer in a mixture, you can calculate the optical purity. Some chemists prefer to use the term enantiomeric excess (ee) rather than optical purity. The two terms can be used interchangeably. The percentage enantiomeric excess or optical purity is calculated as follows ... 

Enantiomeric excess (Section 7 4) Difference between the per centage of the major enantiomer present in a mixture and the percentage of its mirror image An optically pure material has an enantiomenc excess of 100% A racemic mixture has an enantiomeric excess of zero... 

The enantiomeric excess of the product according to this second Chen equation (Figure 2.8) shows a very unpleasant featnre The first percentages of product already show an enantiomeric excess <100%, and during the conversion a decrease occurs. Only for very high values of E 100% enantiomeric excess is obtained. Typical practical values of E are 5-50. When >98% enantiomeric excess is desired, it is therefore better to concentrate on compounds which are remaining substrates of a reaction than on compounds which are products. 

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. 

Finally, when working in the field of asymmetric synthesis, the organic chemist needs to quote both the chemical yield and the optical yield. The percentage optical yield or optical purity [enantiomeric excess (ee) %], is calculated thus ... 

The enantiomeric excess (e.e.) is a similar method for expressing the relative amounts of enantiomers in a mixture. To compute the enantiomeric excess of a mixture, we calculate the excess of the predominant enantiomer as a percentage of the entire mixture. For a chemically pure compound, the calculation of enantiomeric excess generally gives the same result as the calculation of optical purity, and we often use the two terms interchangeably. Algebraically, we use the following formula ... 

When talking about compounds that are neither racemic nor enantiomerically pure (usually called enantiomerically enriched or, occasionally, scalemic) chemists talk not about ratios of enantiomers but about enantiomeric excess. Enantiomeric excess (or ee) is defined as the excess of one enantiomer over the other, expressed as a percentage of the whole. So a 98 2 mixture of enantiomers consists of one enantiomer in 96% excess over the other, and we call it an enantiomerically enriched mixture with 96% ee. Why not just say that we have 98% of one enantiomer Enantiomers are not like other isomers because they are simply mirror images. The 2% of the wrong enantiomer makes a racemate of 2% of the right isomer so the mixture contains 4% racemate and 96% of one enantiomer. 96% ee. 

As described in this chapter, there are many reactions that can be performed by chemists to create new chiral centers. When these reactions are performed in such a way as to create one enantiomer in greater amounts than the other the process is called asymmetric or stereoselective synthesis. The term en-antioselectivity refers to the efficiency with which the reaction produces one enantiomer. This efficiency is quantitatively described as the enantiomeric excess (ee) of the product, which is the percentage by which one enantiomer is produced in excess of the other. Thus a 45 8 mixture of two enantiomers will have an enantiomeric excess of [(45 - 8)/(45 + 8)] X 100, which equals 70%. It should be noted that if neither the startingmaterial or reaction system is chiral and non-racemic, then the product will be formed as an equal mixture of the enantiomers (i.e., a racemate). 

Enantiomeric purity and enantiomeric excess (ee) are usual terms used in the determination of enantiomers. Enantiomeric purity is defined as the measured ratio (expressed as a percentage) of the detected enantiomers, whereas ee-values describe the relative difference of the separated enantiomers (expressed as a percentage). Usually quantifications are given in ee-values, but one should note, that convincing results can be concluded only for baseline-resolved enantiomers (cRs > 1.50). Exact calculations of partially resolved mirror images, as frequently happened in the current literature, remain unintelligible in view of differences in sensory qualities and odour thresholds of enantiomers Eig. 6.25, [1-9]. 

The chemical industry as we currently know it would be markedly different without transition metal catalysts, as these play roles in a wide range of processes. The key task of a catalyst is to accelerate a reaction by effectively lowering the activation barrier for the reaction. Apart from acceleration, a catalyst may also be able to induce optical activity in an organic product if it includes a chiral ligand. The success of an asymmetric catalyst is defined by the enantiomeric excess, which is the difference in percentage yields of the major and minor enantiomers of the product. If 90% of one optical isomer forms and 10% of the other, the enantiomer excess is 80% obviously, the closer that this value is to 100% (which means stereospecificity is achieved) the better. Asymmetric synthesis in industry depends fully upon transition metals as the active site of the catalysis. 

The composition of a mixture of enantiomers is given by the enantiomeric excess, abbreviated e.e, which is the percentage excess of the major enantiomer over the minor enantiomer ... 

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