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Optical Purity

Article ID: 138
Last updated: 25 Jan, 2013
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Check out this interactive stereochemistry tutorial on the ChemEdDL

The optical purity or the enantiomeric excess (ee%) of a sample can be determined as follows:

Optical purity = % enantiomeric excess = % enantiomer1 - % enantiomer2
                     = 100 [a]mixture / [a]pure sample
  where [a] is the specific rotation

ee%  =  100 ([R]-[S]) / ([R]+[S])

where [R] = concentration of the R-isomer
          [S] = concentration of the S-isomer

Diasteromeric substances can have different rotations both in sign and in magnitude. 

Look at some problems like these more in depth.
Consider that (S)-bromobutane has a specific rotation of +23.1o and (R)-bromobutane has a specific rotation of -23.1o

Question 1: Determine the optical purity of a racemic mixture?

Question 2: Determine the enantiomeric excess of the racemic mixture?

Question 3: Which isomer is dominant and what is the optical purity of a mixture, or (R)- and (S)-bromobutane, whose specific rotation was found to be -9.2 degrees?

Question 4: What is the percent composition of the mixture?

Blank Question

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Department of Chemistry & Chemical Biology