Question

ootical for pure substance is +40

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 (@) 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. 2. 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? 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?

Answer 1

g/m2 1. Sample Ali Ig dissolved in lomL 2. 10ml methand has 19 ImL . " 1 g, 1 ml = o.lg. Conc. (c)z oilg/mL (@) = +2.50° hath length (l) = Idm observed rotation (do = (2) С 1 dmx 0.1 g/mL [d] =+25° specific ratation .: Offical rotation for fure = 440° : predominant : fure substance (6)-isom " optical purity = optical rotation Mixture 4 4 fure sample mer

= 25 40 0.625 eeloptical = I purty - t.ee = 62.5%. I hure 62.5% + (100-625)./. racemic mixhare Falenic 37.54. mixture 18.751 لهوية)| 18.75-1. (encantiones) ا ا Total fure = 62.5%. + 18.75%. = 81.25-6 Total enantiomer = 18.756] 2. SAMPLE-3): C = 19 / 10ml - olg/mL [230 = (d) = -3.95 loc Idm yolgime Tedde = -39.51 Offical rotation for Russ - +40° predominant : enantiomer | (e)-isomer. Optical furityfee) = optical rotation Misture " ture =-39.5 YO (eel optical purity = 0.9875 ! ve sign indicate enantiomeris is excess alo ee = 98.75% 1 98.75 enantiomer + (10o-98.75).%. racemic mixture

1.25% racemic mixture 1 I 0.625% 0.6251 (enantiomer) Gure) Totalenantiomer = 93.759% +0.625-1 - 99.375 | Total fure = 10.625 / 7 Thus, mixture components of each Sample were calculated