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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Read the stereoisomers module in the StudyAids and do the exercises. There is no need to record answers on your homework.
Don't forget the Chirality
of Shells (Powerpoint).
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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. |
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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. |
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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. |
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4. a) 1,2-Dibromoethane is optically
inactive yet it has a dipole moment . Explain and
illustrate. |
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