Question

What is the definition of ‘enantiomeric excess’? How may the enantiomeric excess of a compound be determined using a chiral chromatographic approach?

Answer 1

Hint: You must remember that enantiomers are isomers that are mirror images of each other. Enantiomeric excess gives us the percentage in which one isomer is present in excess in a mixture. Use this to solve the given question.

Complete answer:
Before answering this question, we have to understand what enantiomers are. Optical isomers are compounds having the same atoms and bonds but in different spatial arrangements and will have non-superimposable mirror images. These non-superimposable mirror images are called enantiomers. We generally found enantiomers of compounds having a chiral center and are asymmetric. Now, let us discuss the meaning of enantiomeric excess.
It is also important to remember that meso compounds are also optical isomers, but they contain a plane of symmetry. Therefore, all molecules irrespective of their chirality exhibit optical activity when they are not superimposable on their mirror images
Enantiomeric excess is basically a measurement of purity of a chiral substance. Chiral center is an atom to which four different groups or atoms are bonded so that they can form a non-superimposable mirror image. Enantiomeric excess is written as ‘ee’ and it shows which sample contains one enantiomer in greater amounts than the other. It is given by the difference of enantiomers present in a mixture. A racemic mixture (a mixture of enantiomeric isomers that contains both isomers in equal proportions) has ee of $ 0% $ whereas a pure enantiomer has ee of $ 100% $
Chiral Chromatography: Chemists can use chiral chromatography to determine the $ % $ ee, because the areas of the peaks are proportional to moles of each enantiomer. A team of Brazilian chemists prepared and hydrogenated the $ R $ and $ S $ enantiomers of the sex pheromone of a particular beetle.
 $ ee=|R-S| $ and $ %\left( ee \right)=\dfrac{|R-S|}{R+S}\times 100% $

Note:
It is important to remember here that optical activity does not depend solely on the chirality of a molecule. If the molecule is dissymmetric and has at least one non-superimposable mirror image, it exhibits optical activity and thus possesses enantiomers.