ex02anss2012

B draw all four possible chair conformations of a

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Unformatted text preview: mine can undergo inversion.) Indicate which one of the conformations is most stable and why. O O CH3 H3C CH3 Ph N chair ip N H3C CH3 CH3 O Ph inversion O inversion O CH3 Ph N CH3 O CH3 chair ip H3C H3C N CH3 O Ph This form is most stable (fewest axial substituents) O MCMP 204 / Exam 2 / Spring 2012 ANSWERS Page 7 *7. (11 points) The natural form of the amino acid aspartic acid (L-Asp, specific rotation [a]20 = +5 degrees mL g–1 dm–1) is used either alone or in combination with the amino acid D arginine as a nutritional supplement. (The unnatural form is also used as a supplement, but only by men, because there is some evidence that it increases testosterone levels—therefore it is not recommended for use by women or children.) Over time in the environment or in aqueous solution, amino acids can racemize (i.e. they chemically convert from their pure enantiomer forms to their racemic forms) through an intramolecular reaction, the details of which we won’t consider here. O O OH H OH H O O H3N H3N O O aspartic acid (Asp) The local coroner’s office, knowing your expertise in organic chemistry, comes to you with an old skull sample that was excavated from a building site. You suggest that you might be able to date the sample by determining the amount of racemization in the sample. Going to the literature (J. Forensic Sci. 2002, 47(1), pp. 32–36), you find that the rate of racemization of aspartic acid found in skull samples follows the equation ! 1+ r $ –4 –1 ln # & = kt , where k = 6.24 ' 10 yr and t = time " 1– r % In this equation, r is the ratio [D-Asp]/[L-Asp]. You isolate Asp from the sample and find (by a special type of chromatography) that the Asp in the sample is 6% D-Asp and 94% LAsp. (a) How old is the sample? Show your work. r = 6/94 = 0.0638 (1 + r)/(1 – r) = 1.0638/0.9362 = 1.136; the ln of this is 0.1275. 0.1275/(6.24 × 10–4 yr–1) = 204 yr. This the sample age. (b) Curious to see whether you could have determined the age by optical rota...
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