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can undergo inversion.) Indicate which one of the conformations is most stable and
Ph N chair ip N
H3C CH3 CH3
Ph inversion O inversion O
CH3 Ph N
CH3 O CH3 chair ip H3C
H3C N CH3
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
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.
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 $
& = 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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- Spring '08