Enantiomeric ratio desired values

In the simplest case an achiral substrate is converted to an unequal mixture of the two enantiomers of a chiral product containing only one stereogenic unit. The aim is obviously to achieve the highest possible proportion of the desired enantiomer to maximise the enantioselectivity. The most commonly used measure of the degree of enantioselectivity achieved is the enantiomeric excess (e.e.). This is defined as the proportion of the major enantiomer less that of the minor enantiomer and is cornmonly expressed as a percentage. Thus, for example, if the reduction of acetophenone is carried out asymmetrically to give the enantiomeric alcohols (14) and (15) in a ratio of 90 10, then the e.e. of the process is 80%. Similarly an e.e. of 90% would correspond to an enantiomer ratio of 95 5. The reason for using the enantiomeric excess rather than the enantiomer ratio is that in almost all casesl l it corresponds directly with the optical purity. Thus in the example above (14) has an optical rotation of-120 and (15) of+120°. A sample of 80% e.e. which contains 90% of (14) and 10% of (15) will have a net optical rotation of (0.9 x -120°) +(0.1 x +120°)=-96° which is 80% of the value for the pure major enantiomer. 

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