Problem Set 5
Chapter 5
Due: Monday, February 13, 2006
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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 discourses on chirality, see:
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Don't forget the Chirality
of Shells (Powerpoint). 1. When (R)-1-chloro-2-methylbutane is
subjected to free radical chlorination, five dichloro
constitutiona isomers 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 why and
illustrate. 2. An optically-active compound A
(C10H16, [α]D
= +124o) reacts with H2 in the
presence of a catalyst to provide compounds B and
C. [Note: Hydrogen adds to double bonds under
these conditions. Thus, ethylene is converted into
ethane.] Both B and C show no optical
rotation, even when the solutions are diluted, both
compounds have the formula C10H20, and
both compounds are 1,4-disubstituted cyclohexanes. Compound
B has an energy difference of 3.9 kcal/mol between
its two chair conformations. a) Determine the structures of B
and C. Show your reasoning. b) What is the energy difference between
the two chair conformations of C? Show
work. c) The structure of A cannot be
determined from the information provided. There are eight
structures?, exclusive of which enantiomer is
dextrorotatory, that fit the other data. Two of them are
diastereomers. Draw the structures with at least one center
of the (R)-configuration. d) A sample of A is isolated from
mandarin oranges and it is found to have [α]D
= +31o. How much of the l-enantiomer is present
in the sample? Show work. 3. Enriched A has an optical
purity of 20% while B has an enantiomeric excess of
50%. Both compounds are rich in the (R)-enantiomer.
Compounds A and B react to form two
diastereomers
ARBR:ASBS
and
ARBS:ASBR.
One diastereomer is 64% optically pure while the other one
is only 33% optically pure. a) How much of each enantiomer is present
in A and B? Show work. b) What are the percentages of each
diastereomer? Show work. c) Show how the ee of each diastereomer
is obtained. d) What is the optical purity of each
diastereomer? Show work. 4. a) 1,2-Dibromoethane is optically
inactive yet it has a dipole moment . Explain and
illustrate. President Affiliation (D = Democrat; R
= Republican) Handedness Chirality Truman D L Eisenhower R R Kennedy D R Johnson D R Nixon R R Ford R "R" Carter D R Reagan R R Bush 41 R L Clinton D L Bush 43 R R
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.
5. Which of the compounds on the right are, in principle,
capable of resolution? Explain and illustrate.
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?
7. On the Lighter Side: The last eleven US Presidents
(33-43) are shown below in order of service in addition to
their party affiliation and their handedness.
[Handedness is "officially" designated as how you write.
Ford throwing out the "first pitch" indicates he is a
rightie and not a southpaw (leftie). By the way, no photo
has been converted to its mirror image by me but I found
Truman as both "enantiomers". ] Everyone of these photos
has something in common. Place that something (L or R) in
the last column and then separate the last two columns as
diastereomers and enantiomers. If you can correlate their
diastereoisomerism with party affiliation, you are better
than I am.