Question

The specific rotation for optically pure (S)-(+)- phenylsuccinic acid is reported to be [ α]D =+171 degrees (4.0, acetone). Under the same conditions, the specific rotation for a lab sample of phenylsuccinic acid was found to be +130 degrees. Calculate the optical purity (%ee) of the lab sample and the ratio of (R) and (S) enantiomers.

Answer 1

Solution :-

Specific rotation of pure (S)-(+)- phenylsuccinic acid = [ α]D =+171

Observed specific rotation =+130

Lets calculate the optical purity

Optical purity (ee) = [observed rotation / specific rotation of pure]*100%

                         =[(+130) / (+171)]*100%

                          = 76.0 %

Now lets calculate the ratio of the R and S enantiomers

First we need to find the totla percent of the S enantiomer in the sample

% S = [(100 % - 76.0 %)/2]+76.0%

       = 88 %

Therefore we have 88 % S enantiomer and (100%-88%) = 12 % R enantiomer