Problem Set 4
Chapter 5
Due: Monday, October 2, 2005
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Versions of this symbol date to the time of the Vikings. In the 15th century, it was apparently the symbol of a tripartite alliance of the Milanese families Visconti, Sforza and Borromeo via intermarriage. Break any (wedding?) ring and the others separate, hence the alliance is broken. The rings form a chiral object (left) that are not superimposable on their mirror image. A set of Borremean rings has been used as the logo for a certain refreshment that extols purity, body, and flavor. Is the sense of chirality of the two sets of Borremean rings the same or different? For some other examples, click here.
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Don't forget the Chirality
of Shells (Powerpoint). 1. When (R)-1-chloro-2-methylpentane is
subjected to free radical chlorination, eight dichloro
compounds are possible. a) Draw them and name them with the
appropriate R,S-descriptors. b) Two of the dichlorides are optically
inactive. Which ones are they? Explain and
illustrate. 2. Terpenes are naturally occurring
compounds that are comprised of multiples of the
C5 unit isoprene (it looks like 2-methylbutane).
Limonene is a monoterpene that occurs as both enantiomers in
nature. The (R)-enantiomer has an orange, citrus-like
aroma while the (S)-enantiomer has a harsher, lemony
fragrance. a) Of the limonenes shown on the right,
identify the R and S enantiomers. b) (R)-Limonene (d-limonene) is reported
to have a rotation of [α]D
123.8o. Its enantiomer is reported as [α]D
101.3o. Assume that the enantiomer with the lower
rotation is contaminated with the other enantiomer,
calculate the percent of (+)- and (-)-enantiomers in the
sample. c) When compounds containing double bonds
are treated with H2 in the presence of a noble
metal catalyst, hydrogen is added to the double bond. In the
case of (R)- and (S)-limonene, compounds A and
B are formed, both with the formula
C10H20. Are A and B
formed in the same ratio? d) The energy difference between the
chair conformations of A is greater than the energy
difference of the chair conformations in B. What are
the structures of A and B? What are the energy
differences? Explain and illustrate the equilibria involved.
Show work. [Use values posted on the Bulletinboard.] 3. Steganacin (1), an
optically-active, naturally-occurring lignan lactone, is
active against certain cancer cell lines. Steganacin has
three asymmetric carbons labeled a, b, and c, all of the
(R)-configuration. a) Draw steganacin with its absolute
stereochemistry. b) One might argue that there are eight
possible stereoisomers of 1. There are, in fact,
sixteen because 1 has the property of
atropisomerism.
Draw the two possible stereostructures for 1. Why do
they not interconvert? 4. a) 1,2-Dibromoethane is optically
inactive yet it has a dipole moment . Explain and
illustrate. 5. a) Which of the compounds on the right
are, in principle, capable of resolution? Explain and
illustrate. b) Why is no stereochemistry shown for
the middle bromine in structure 5? 6. A mixture of enantiomers contains
1-1/2 times more of one and shows [α]D
-25.0o. What are the rotations of the pure
enantiomers? Which one is in excess?
b) meso-Tartaric acid exists in three staggered
conformations, none of which has a plane
of symmetry. Yet the compound is
optically-inactive. Explain and illustrate.