Midtermexamsolutions3 - Chemistry 124 Third Examination...

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Chemistry 124 Third Examination Answers November 14, 2008 Statistics mean 1/3 > 2/3 > this exam 76.7 86 71 sum of 3 exams 229.9 247 219 1. Over the past three and a half years the American Chemical Society has honored 181 different compounds as “Molecule of the Week.” This week (November 10, 2008) it belatedly honored L-(+)-Tartaric acid (shown in the figures to the right). A) (1.5 min) Give common names for three other forms (or configurational isomers) of tartaric acid with sharp melting points. racemic acid [ John s Acid from the Vosges according to Berzelius] meso-tartaric acid [originally pyrotartaric acid] l-tartaric acid [first isolated by Pasteur] NOTE: the ACS has chosen to show d-tartaric acid in a very unlikely “eclipsed” conformation. We ll return to this point within a few lectures. B) (2 min) Write CIP priority numbers (1 is high) on the substituents of one of the stereogenic carbons in the ball-and-stick formula and label it as R or S. C) (1.5 min) Explain whether L-(+)-Tartaric acid should be denoted d- , or l- , or whether the designation is uncertain? Since d - means the same as (+), and l - means the same as (-), it should be -tartaric acid. D) (2 min) In the left margin draw the Fischer Projection of L-(+)-Tartaric acid. 2. (5 min) Briefly describe two different ways to separate 50:50 mixtures of enantiomers. Try to be specific. The methods must NOT involve the type of conglomerate used by Pasteur in his 1848 preparation of “unnatural” tartaric acid. The key to separating enantiomers that do not crystallize as conglomerates (mixtures of enantiomerically pure solids) is to separate diastereomers formed by (temporary) interaction with a chiral, resolved substance. One method is chromatograpy using a natural solid (such as starch) as the stationary phase. This was used by Eisai scientists in a resolution during the synthesis of the drug candidate E-7389. A second method is to prepare a salt from a racemic sample of acid and a single enantiomer of a natural alkaloid, such as brucine, separate the diastereomeric salts on the basis of their solubility, and recover the resolved acids from their salts by treating with mineral acid. This was used by Kohler, Walker, and Tishler in resolving an allene derivative to confirm van t Hoff s hypothesis that it should be chiral. [A different approach to obtaining a single enantiomer, which does not involve
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Midtermexamsolutions3 - Chemistry 124 Third Examination...

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