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Definitions: Rotation of Light


Achiral molecules do not rotate light. Also, if there are equal amounts of d and l enantiomers (a racemic mixture) there will be no net rotation of light. This is because the rotation of light in one direction (from one enantiomer) will be exactly the same magnitude as the rotation of light (from the other enantiomer) in the other direction, and these amounts will cancel each other out.

If there are unequal amounts of enantiomers, there will be some net rotation, less than that of a pure sample of one enantiomer. This net rotation will determine the enantiomeric excess, or ee, of one enantiomer over the other. The following formula can be used to calculate enantiomeric excess:

ee = ([R] - [S])/([R] + [S]) * 100%


R and S are stereochemical abbreviations that will be discussed in more detail in the Assigning Priorities section of this tutorial. These are used to distinguish enantiomers from one another, similar to d and l. The brackets - [x] - denote concentration of a particular enantiomer.

This formula can be simplified as follows:
ee = %R - %S or ee = %S - %R


The first formula should be used if there is more of the R enantiomer, and the second formula should be used if there is more of the S enantiomer.

For example, if there is 75% of the S enantiomer and 25% of the R enantiomer present in a mixture, the enantiomeric excess would be 50% for the S enantiomer. This makes sense, as 25% of the S enantiomer is cancelled out by the 25% of the R enantiomer, leaving 50% of the S enantiomer.


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