% Optical purity 39: 35 : 1: 96(S). CONCLUSIONS: Briefly comment on how well the
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% Optical purity 39: 35 : 1: 96(S). 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. 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? 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? 2. 3. 70Explanation / Answer
1. Specific rotation = observed rotation/concentration (g/ml) x path length (dm)
for sample A,
specific rotation = +2.50/(1g/10ml) x 1 dm = +25 degrees
R-enantiomer is predominant
optical purity = +25 x 100/+40 = 62.5%
% S-enantiomer = 18.75%
% R-enantiomer = 18.75 + 62.5 = 81.25%
for sample B,
specific rotation = -3.95/(1g/10ml) x 1 dm = -39.5 degrees
S-enantiomer is predominant
optical purity = -39.5 x 100/-40 = 98.75%
% R-enantiomer = 0.625%
% S-enantiomer = 0.625 + 98.75 = 99.375%
2. The (S)-aminium-(S)-malate diastereomer is in solution as a racemic mixture with the other diastereomer. This can be obtained by chiral resolution of racemic mixture. We can keep removing the major diastereomer from the system continuosly until majority of it removed. At this point only the (S)-aminium-(S)-malate diastereomer would be present as the major isomer in solution. It would be a time consuming and expensive procedure to follow.
3. If we could epimerise the (R)-aminium-(S)-malate to its enantiomer (S)-aminium-(S)-malate form by adding a base, then we could preferentially enhance the concentration of the minor isomer in solution and be able to crystalize this from the solution.
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