Why are enantiomeric excess % and specific rotation considered?

I was wondering, when working in a lab in an area related to organic compounds, when and why would a chemist consider the enantiomeric excess % and how would a chemist used that information?? Same Thing goes with specific rotation, why, how, and when would a chemist use this? I was just curious because I saw this in my organic chemistry book and they relate the problems to lab work and I want to get a good grasp of tis context and it made me wondered. 




Enantiomeric excess is very important concept in organic chemistry because it clearly states the purity of one isomer over the other. For example, have a look at the structure of Thalidomide which is racemic (50:50 —R:S isomers) in nature. Between the two form, one is having very important medical property where as another form is sedative. In these cases, synthesizing enantiomerically pure (>99%) form is mandatory and in January 1996, the FDA announced it would consider further incentives for developing single isomer drugs, owing to their better pharmacokinetics prosperity, safety, and tolerability.
<http://en.wikipedia.org/wiki/Thalidomide>


These are the reason which makes chemists to target enantimerically pure form using latest available technologies like, Chiral GC or HPLC. 


Specific optical rotation is very old concept to judge the enatiopurity of the compounds but its not accurate because it depends on various factors like concentration of the sample, solvent which we use, temperature, etc. Even small impurity in the sample gives drastic change in the rotation value. Thats why chemists adapt latest technique like HPLC of GC, nevertheless, chemists still carry out specific optical rotation to make sure that the enantiomerically excess compound has some rotation.


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To be specific, the enantiomeric excess is useful because it gives a limit on yield based on recrystallization given certain assumptions. If you assume that the enantiomers themselves crystallize better than the racemate, the excess is the theoretical maximum yield because every bit of the opposite enantiomer reduces the amount of the one you can get back.


This is historically why e.e. is used over other measurements of purity like e.r. (enantiomeric ratio).


Specific rotation is not so useful these days because we have other analytical tools. With appropriate reference, chiral chromatography, like HPLC, will tell you what you have. Specific rotation was originally used to help identify which of the two enantiomers you had. This doesn't tell you about the internal structure, since there's no easy way to figure out the rotation based on the structure (unless you use computational chemistry, but that wasn't available back then either).


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Measurement of how much one enantiomer is present in excess of the racemic mixture is known as enantiomeric excess(optical purity). It is often denoted by ee.
ee=% of one enantiomer -% of another enantiomer.
It is important in the production of chiral drugs and discovery of asymmetric catalysts. It can be used to measure the purity of a compound. Also, enantiomeric excess is important when determining the optical activity of a mixture of two enantiomers. 


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