% Optical purity 39, 351. %(s) 30,3251 %(R) 09: 75 CONCLUSIONS: Briefly comment
ID: 556739 • Letter: #
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% Optical purity 39, 351. %(s) 30,3251 %(R) 09: 75 CONCLUSIONS: Briefly comment on how well the experiment went and the yield and optical purity of your resolved amine. POSTLAB QUESTIONS 1. As the new chemist working at "Drugs Us", you are given the task of resolving racemic phenylethyl- amine into its individual enantiomers using (S)-malic acid as resolving agent. After you carried out the resolution, you labeled the products you isolated "Sample A" and "Sample B". You subjected both samples to polarimetry ( = 589 nm (sodium D-line), 1-dm cell) and obtained the following results: Sample A: 1.00 g dissolved in 10.0 mL methanol gave an observed rotation (a) of +2.50°. Sample B: 1.00 g dissolved in 10.0 mL methanol gave an observed rotation (a) of -3.95° For samples A and B, calculate the specific rotation for each sample and identify the predominant enantiomer formed in each. Also, calculate the optical purity (enantiomeric excess) of each sample as well as the % of each enantiomer present in each sample. Show your calculations. 9 Note that the product amine salts are not enantiomers; they are diastereomers. In this experiment the (R)-aminium-(S)-malate diastereomeric salt preferentially crystallized as the less soluble crystals that are easily isolated by vacuum filtration. Where are the (S)-aminium-(S)-malate diastereomers (more soluble crystals) and how could you obtain them from the mixture of diastereomeric salts? What would be the principal problem encountered in doing so? 2. 3. Instead of trying to isolate the more soluble (S)-aminium-(S)-malate diastereomers from the mixture of diastereomeric salts, what would be a better way of ensuring that the (S)-(-)-amine diastereomeric salt crystallizes preferentially as the less soluble crystals? Hint: what is the enantiomer of the (R)-aminium-(S)-malate diastereomer and how could it be formed? 70Explanation / Answer
1. Specific rotation = observed rotation/concentration (g/ml) x path length (dm)
For sample A,
Specific rotation = +2.50/(1g/10ml)(1dm) = +25.0 degrees
R-enantiomer is in excess
specific rotation of pure R-enantiomer = +40 degrees
%optical purity = +25 x 100/+40 = 62.5%
%R-enantiomer = 81.25%
%S-enantiomer = 18.75%
For sample B,
Specific rotation = -3.95/(1g/10ml)(1dm) = -39.5 degrees
S-enantiomer is in excess
specific rotation of pure S-enantiomer = -40 degrees
%optical purity = -39.5 x 100/-40 = 98.75%
%S-enantiomer = 99.375%
%R-enantiomer = 0.625%
2. The more soluble (S)-aminium-(S)-malate remained in solution as a racemic mixture. This could be obtained from the solution by sucessive crystallization and removing the less soluble diastereomer first. Then at the end we could do a kinetic resolution to separate the more soluble diastereomer from the solution. The method is tedious and probability of getting pure diasteromer is very low.
3. Epimerization of the chiral center in the (R)-aminium-(S)-malate would give us the formation of (S)-aminium-(S)-malate distereomer. This would enrich the solution with the desired diastereomer and we could obtain it as major component from the solution.
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