Name: 8. (10 pts) Calculate the enantiomeric excess of a mixture of lactic acid

Question

Name: 8. (10 pts) Calculate the enantiomeric excess of a mixture of lactic acid enantiomers using the following information: (S)-lactic acid (shown below) has a specific rotation of +18.5º. The observed rotation of the enantiomeric mixture is -8.5º. Which isomer (R or S) is present in enantiomeric excess? What is the percent enantiomeric excess of the solution? What percentage of the solution is racemic? What percentage of the solution is the Senantiomer and what percentage of the solution is the R enantiomer? Answer these questions in the boxes provided below. Make sure that you show your calculations in the space provided below: OH (S)-Lactic acid Isomer (R or S) found in enantiomeric excess % enantiomeric excess Tagx100 (542 772 x 33% % of the solution that is racemic % of the solution that is (S)-lactic acid % of the solution that is (R)-lactic acid

Explanation / Answer

from the given information, R enantiomer is excess,

enantiomer excess = observed rotation / rotaion of pure S-enantiomer

                   = 8.5 / 18.5*100

      %ee        = 46%

racimic mixture = 100-46 = 54%

% of S-isomer = 54/2 = 27%

% of R-isomer = 27+46 = 73%

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