2.5: Problems for Chapter 2
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P2.1: Ozone, O3, is an uncharged, non-cyclic molecule. Draw a Lewis structure for ozone. Are the two oxygen-oxygen bonds the likely to be the same length? What is the bond order? Explain.
P2.2: Redraw the structure below, showing the 2pz orbitals that make up the conjugated pi bond system (show the lowest energy molecular orbital).
P2.3: Draw six different resonance contributors on the structure below (the flavin group of flavin adenine dinucleotide (FAD), a biochemical oxidation/reduction molecule) in which the oxygen indicated by an arrow bears a negative formal charge.
P2.4: The structure below shows an intermediate species in a reaction involving the amino acid alanine, attached to pyridoxal phosphate (vitamin B6). Draw a resonance contributor in which the only formal charges are on the oxygens.
P2.5: In problem P1.8, you drew four different amides with molecular formula C3H7NO. One of these constitutional isomers is significantly less soluble in water than the other three. Which one, and why?
P2.6: Draw a MO diagram for a hypothetical He2 molecule, and use this diagram to explain why there is no net lowering of energy when two helium atoms come together (and thus why He2 is not observed). Use the MO diagram of H2 in this chapter as a starting point.
P2.7: Below is the structure of Rimonabant, a drug candidate which is being tested as a possible treatment for alcohol/tobacco dependence and obesity. Draw resonance contributors in which
- there is a separation of charge between the nitrogen indicated by an arrow and the oxygen.
- there is a separation of charge between a chlorine and one of the three nitrogens.
P2.8: For the molecules below, draw resonance contributors in which formal charges are placed on the atoms indicated by arrows. Use curved arrows to show how you are rearranging electrons between resonance contributors.
P2.9: Genepin was recently identified as the active compound in gardenia fruit extract, a traditional Chinese medicine for the treatment of diabetes (Chemical and Engineering News June 12, 2006, p. 34; Cell Metab. 2006, 3, 417). Resonance contributors can be drawn in which the oxygen atom indicated by an arrow bears a positive formal charge. Indicate where the corresponding negative formal charge would be located in the most important of these contributors.
P2.10: Identify any isolated alkene groups in the PAC-1 structure in problem P2.8, and in the genipin structure in problem P2.9.
P2.11: The February 27, 2006 issue of Chemical and Engineering News contains an interesting article on the 100th birthday of Albert Hofmann, the inventor of the hallucinogen LSD. The structure of LSD is shown below. Several resonance contributors can be drawn in which the nitrogen atom indicated by an arrow bears a positive formal charge. Indicate atoms where a corresponding negative formal charge could be located in these contributors.
P2.12: The human brain contains naturally occurring cannabinoid compounds which are related in structure to Δ9-tetrahydrocannabinol, the active compound in cannabis. Cannabinoids are thought to exert an antidepressant effect. Because cannabis itself is not a realistic candidate for the treatment of depression due to its use as a recreational drug, researchers at the University of California, Irvine are studying synthetic compounds, such as the one shown below, which inhibit the degradation of natural cannabinoids in the brain. This compound has been shown to have antidepressant-like effects in rats and mice. (Chemical and Engineering News, December 19, 2005, p. 47; Proc. Natl. Acad. Sci. USA 2005, 102, 18620).
a) Several resonance contributors can be drawn in which the oxygen atom indicated by an arrow bears a positive formal charge. Indicate atoms where a corresponding negative formal charge could be located in these contributors.
b) Answer the same question again, this time with the structural isomer shown below.
P2.13: Give the expected trend (lowest to highest) in boiling points for the following series of compounds :
P2.14: For each pair of molecules below, choose the one that is more water-soluble, and explain your choice.
P2.15: Benzene and toluene (methylbenzene) are both commonly used organic solvents, although benzene is used less frequently in recent years due to its higher toxicity. Benzene has a boiling point of 80oC and a melting point of 5.5 oC. Toluene has a boiling point of 111 oC and a melting point of -93 oC. Use your knowledge of organic structure and noncovalent interactions to rationalize these trends in boiling and melting point.
P2.16: Risperidone is a drug used for the treatment of schizophrenia and bipolar disorder. Draw two resonance contributors in which the nitrogen and oxygen atoms indicated both bear formal charges.
P2.17: In each of the structures below, several resonance contributors can be drawn in which the atoms indicated by an arrow bear a positive formal charge. Circle all atoms which could bear the corresponding negative formal charges.
P2.18:In each of the structures below, several resonance contributors can be drawn in which the atoms indicated by an arrow bear a negative formal charge. Circle all atoms which could bear the corresponding positive formal charges.
P2.19:In each of the structures below, circle all atoms to which the positive formal charge can be delocalized by resonance.