Answers to PS3
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Answers to PS3

Course Number: CH 242, Fall 2009

College/University: Colby

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Chem 242 Spring 2008 Answers to Problem Set 3 Question 1. Provide reagents (by each arrow) that will accomplish the following transformations. Yes, this is a review of last semesters material! H3C CH3 1. BuLi (or equivalent strong base) 2. CH3I H3C H Hg(OAc)2, H2O H3C O CH3 H2, Lindlar's catalyst or Na, NH3 1. BH3 2. H2O2 H3C H3C OH 1. O3 2. H2O2 O HO CH3 H3C H2SO4, NaCl or TsCl followed by NaCl Cl Chem...

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242 Spring Chem 2008 Answers to Problem Set 3 Question 1. Provide reagents (by each arrow) that will accomplish the following transformations. Yes, this is a review of last semesters material! H3C CH3 1. BuLi (or equivalent strong base) 2. CH3I H3C H Hg(OAc)2, H2O H3C O CH3 H2, Lindlar's catalyst or Na, NH3 1. BH3 2. H2O2 H3C H3C OH 1. O3 2. H2O2 O HO CH3 H3C H2SO4, NaCl or TsCl followed by NaCl Cl Chem 242 Spring 2008 Question 2. Label the indicated hydrogen atoms as homotopic, enantiotopic, or diastereotopic. H H H homotopic H Me H Me H CH3 F CH3 diastereotopic homotopic H3C H H CH3 diastereotopic Question 3. Rank the following carbocations from most to least stable. Most stable resonance between 1 and 3 carbocations resonace between 1 and 2 carbocations localized 1 carbocation Least stable a vinyl cation. This has no resonance stabilization and the cation is forced into an sp2 orbital Chem 242 Spring 2008 Question 4. a) Provide the major product of the reaction shown below. Assume that only one molecule of HCl adds to the cyclohexadiene. CH3 Cl HCl CH3 b) Draw all the possible cationic intermediates for the reaction. Which of these are resonance forms? Which structure is most stable? Least stable? CH3 2 carbocation, not resonance stabilized, disubstituted olefin Least stable Chloride attack here gives the major product CH3 CH3 resonance between a 3 and a 2 carbocation Most stable CH3 CH3 CH3 resonance two between 2 carbocations CH3 2 carbocation, not resonance stabilized, trisubstituted olefin (If you said this was the least stable, you are close enough) Chem 242 Spring 2008 Question 5. Crotonaldehyde (A) is much more stable than methyl ketene (B). O H H CH3 CH3 O C A B a) Draw molecular orbitals, including nodes and orbital phases, for crotonaldehyde. Note: these orbitals are drawn looking down from the "top" (they are all porbitals) O 4 H CH3 O 3 H CH3 O 2 H CH3 O 1 H CH3 b) Draw the major resonance form(s) of methyl ketene. Are the double bonds in methyl ketene considered conjugated? Why or why not? O C H CH3 H O C CH3 The double bonds are not conjugated, they are orthogonal (twisted 90 relative to each other) Chem 242 Spring 2008 c) Explain briefly the difference in stability between A and B. Crotonaldehyde contains a 4-electron conjugated -system, methyl ketene has no conjugation between -bonds. Also, the resonance structure of methyl ketene shown on the right in part (b) is not very stable due to the vinyl carbocation. Therefore, the oxygen gains very little electron density from the adjacent carbon atom. Question 6. Show how you can accomplish the following multistep transformation (yup, this is a hard one, but somewhat similar to your lab experiment...). O O H O H H O O H O O O Diels-Alder O O H O H H H O O H H RCO3H (such as CF3CO3H or mCPBA) O H H O O O O O endo product
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