Ch20 - Chapter 19: Enolate Anions and Enarnines S olutions...

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Solutions Chapter 19: Enolate Anions and Enarnines 502 Problem 19.80 The following molecule undergoes an intramolecular reaction in the presence of pyrrolidium acetate, the protonated form of pyrrolidine. Draw the product of this reaction, assuming that a dehydration reaction takes place. o cfMe o Pyrrolidinium acetate Me ~j o 0 Me Me Intramolecular reactions are favored that produce five or six-membered rings, as in this example. Problem 19.81 Organocuprates predominantly react to give lA-addition products with a.~-ui1saturated carbonyl species, while Grignard reagents often add to the carbonyl, in a process referred to as 1,2-addition. To increase the yield of 1,4-addition products CuI is added to convert an easily prepared Grignard reagent into a organocuprate reagent in situ (during the reaction). Predict the major product and stereochemistry of the following reaction, assurJ1ing the more stable chair product predominates. 1. /"MgBr CuI, THF, -30°C .. 2. Equilibration to more stable chair product in base The predominant product chair conformation has both the methyl group a.,nd the alkyl group equatorial. Raccoon buncrllyfish ChaclOdon JunuJa Kauai, Hawaii. Chapter 20: Conjugated Systems Solutions 503 CHAPIER2Q SolutWns to the Problems Problem 20.1 Which of these terPenes (Section 5.4) contains conjugated double bonds? OH (c) (a) ( b) HO Geraniol Lirnonene An aggre gati ng pheromone of bark beetles Only the a.ggregating pheromone of bark beetles (c) has conjugated double bonds. Problem 20.2 Estimate the stabilization gained due to conjugation when I,4-pentadiene is converted to trans-l ,3-pentadiene. Note that the answer is not as simple as comparing the heats of hydrogenation of 1,4-pentadiene and tTlins-l,3-pentadiene. Although the double bonds are moved from unconjugated to conjugated, the degree of substitution of one of the double . bonds is also changed; in this case from a monosubstituted double bond to a trans disubstituted double bond.'To answer .this question, you must separate the effect that is the result of conjugation from that caused by a change in the degree of substitution. . . 1,4.Pentadiene trans.1,3-Pentadiene As stated in the question, there are actually two important issu~s to be addressed. It is best to isolate the two different interactions by considering analog~)Us systems in Table 20.1. First, trans-l,3-pentadiene has an internal double bond while 1,4-pentadiene does not. Internal double bonds are lower in energy because they are more highly substituted. A good estimate of the stabilization energy provided by having an internal yersus terminal double bond is 12 kJ/mol, calculated as the difference in energy between I-butene (MIo = -127 kJ/mol) and trans-2-butene (MIo = -115 kJ/mol). The overall difference in energy between 1,4-pentadiene
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This note was uploaded on 07/09/2011 for the course CHE 301 taught by Professor Diver during the Spring '08 term at SUNY Buffalo.

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Ch20 - Chapter 19: Enolate Anions and Enarnines S olutions...

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