Problems. Chapter Problems

Transcription

1 Chapter ,3-Dienes reveal the effects of conjugation by their relative stability (compared to nonconjugated systems) and a relatively short internal bond (1.47 Å). 7. Electrophilic attack on 1,3-dienes leads to the preferential formation of allylic cations. 8. Extended conjugated systems are reactive because they have many sites for attack and the resulting intermediates are stabilized by resonance. 9. enzene has special stability because of cyclic delocalization. 10. The Diels-lder reaction is a concerted stereospecific cycloaddition reaction of an s-cis diene to a dienophile; it leads to cyclohexene derivatives. It follows the endo rule. 11. Conjugated dienes and trienes equilibrate with their respective cyclic isomers by concerted and stereospecific electrocyclic reactions. 12. Polymerization of 1,3-dienes results in 1,2- or 1,4-additions to give polymers that are capable of further cross-linking. Synthetic rubbers can be synthesized in this way. Natural rubber is made by electrophilic carbon carbon bond formation involving biosynthetic five-carbon cations derived from 3-methyl-3-butenyl pyrophosphate. 13. Ultraviolet and visible spectroscopy gives a way of estimating the extent of conjugation in a molecule. Peaks in electronic spectra are usually broad and are reported as L max (nm). Their relative intensities are given by the molar absorptivity (extinction coefficient) e. Problems 32. Draw all resonance forms and a representation of the appropriate resonance hybrid for each of the following species. j š (e) š 33. For each species in Problem 32, indicate the resonance form that is the major contributor to the resonance hybrid. Explain your choices. 34. Illustrate by means of appropriate structures (including all relevant resonance forms) the initial species formed by breaking the weakest C bond in 1-butene; treating 4-methylcyclohexene with a powerful base (e.g., butyllithium TMED); heating a solution of 3-chloro-1-methylcyclopentene in aqueous ethanol. 35. Rank primary, secondary, tertiary, and allylic radicals in order of decreasing stability. Do the same for the corresponding carbocations. Do the results indicate something about the relative ability of hyperconjugation and resonance to stabilize radical and cationic centers? 36. ive the major product(s) of each of the following reactions. Conc. r 2 r CPC 2 C2 C 2 I C (e) C 2 I KSC3, DMS (f) C3N2, 37. Formulate detailed mechanisms for the reactions in Problem 36 (a, c, e, f ). 38. Rank primary, secondary, tertiary, and (primary) allylic chlorides in approximate order of decreasing S N 1 reactivity; decreasing S N 2 reactivity.

2 / 662 Chapter 14 Delocalized Pi Systems 39. Rank the following six molecules in approximate order of decreasing S N 1 reactivity and decreasing S N 2 reactivity. (e) (f) 40. ow would you expect the S N 2 reactivities of simple saturated primary, secondary, and tertiary chloroalkanes to compare with the S N 2 reactivities of the compounds in Problem 39? Make the same comparison for S N 1 reactivities. 41. ive the major product(s) of each of the following reactions. 3 C I 2 NS, C4, RR NS, C4, RR (S)- C 2 CCPC 2 C 2 CPC C 2 C 2C 2C 2Li, TMED (e) Product of 1. C3C, TF 2., 2 (f) ( ) 2 CPC i C f 3 C r KSC3, DMS 42. Write a detailed step-by-step mechanism to show how each of the products arises from the reaction in Problem 41, part. 43. The following reaction sequence gives rise to two isomeric products. What are they? Explain the mechanism of their formation. 1. Mg 2. D2 44. Starting with cyclohexene, propose a reasonable synthesis of the cyclohexene derivative shown in the margin. 45. ive a systematic name to each of the following molecules. r [ r } 46. Compare the allylic bromination reactions of 1,3-pentadiene and 1,4-pentadiene. Which should be faster? Which is more energetically favorable? ow do the product mixtures compare? NS, RR, C 4 C 2 P C C P C uuuuuvy NS, RR, C 4 C 2 P C C 2 P C C 2 uuuuuvy

3 Chapter We learned in Section 14-6 that electrophilic additions to conjugated dienes at low reaction temperatures give kinetic product ratios. Furthermore, these kinetic mixtures may change to mixtures with thermodynamic product ratios when the temperature is raised. Do you think that cooling the thermodynamic product mixture back to the original low reaction temperature will change it back to the original kinetic ratio? Why or why not? 48. Compare the addition of 1 to 1,3-pentadiene and 1,4-pentadiene (see Problem 46). Draw the structures of the products. Draw a qualitative reaction profile showing both dienes and both proton addition products on the same graph. Which diene adds the proton faster? Which one gives the more stable product? 49. What products would you expect from the electrophilic addition of each of the following reagents to 1,3-cycloheptadiene? I; r 2 in 2 ; IN 3 ; 2 S 4 in C 2. (int: For b and c, see Exercise 14-13, specifically drawings C). 50. ive the products of the reaction of trans-1,3-pentadiene with each of the reagents in Problem What are the products of reaction of 2-methyl-1,3-pentadiene with each of the reagents in Problem 49? 52. Write a detailed step-by-step mechanism to show how each of the products arises from the reactions in Problem 51, above. 53. ive the products expected from reaction of deuterium iodide (DI) with 1,3-cycloheptadiene; trans-1,3-pentadiene; 2-methyl-1,3-pentadiene. In what way does the observable result of reaction of DI differ from that of reaction of I with these same substrates [compare with Problems 49, 50, and 51]? 54. rrange the following carbocations in order of decreasing stability. Draw all possible resonance forms for each of them. C 2 CC 1 2 C 2 C 1 C 1 2 C CC 1 (e) C 2 CC CC Sketch the molecular orbitals for the pentadienyl system in order of ascending energy (see Figures 14-2 and 14-7). Indicate how many electrons are present, and in which orbitals, for the radical; the cation; the anion (see Figures 14-3 and 14-7). Draw all reasonable resonance forms for any one of these three species. 56. Dienes may be prepared by elimination reactions of substituted allylic compounds. For example, Catalytic 2S4, 3 CCPCC 2 2CPCCPC2 LD, TF 3 CCPCC2 Propose detailed mechanisms for each of these 2-methyl-1,3-butadiene (isoprene) syntheses. 57. ive the structures of all possible products of the acid-catalyzed dehydration of vitamin (Section 14-7). 58. Propose a synthesis of each of the following molecules by Diels-lder reactions. [ ~ 3 C C [ ) C r romoconduritol 59. The haloconduritols are members of a class of compounds called glycosidase inhibitors. These substances possess an array of intriguing biological activity, from antidiabetic and antifungal to activity against IV virus and cancer metastasis. Stereoisomeric mixtures of bromoconduritol (margin) are commonly used in studies of these properties. ne recent synthesis of these compounds proceeds via the bicyclic ethers and. Identify the starting materials for a one-step preparation of both of these ethers via a Diels-lder reaction. Which of the starting molecules in your answer is the diene, and which is the dienophile? This Diels-lder reaction gives an 80 : 20 mixture of and. Explain.

4 664 Chapter 14 Delocalized Pi Systems 60. ive the product(s) of each of the following reactions. C N P š N C Dimethyl azodicarboxylate 3-Chloro-1-propene (allyl chloride) 1 Na cis-2-utene 1 NS, peroxide (RR) 3-romo-1-cyclopentene 1 LD trans,trans-2,4-exadiene 1 (e) trans,trans-2,4-exadiene 1 r 2, 2 (f) 1,3-Cyclohexadiene 1 methyl propenoate (methyl acrylate) (g) 1,2-Dimethylenecyclohexane 1 methyl propenoate (methyl acrylate) 61. Propose an efficient synthesis of the cyclohexenol in the margin, beginning exclusively with acyclic starting materials and employing sound retrosynthetic analysis strategy. [int: Diels-lder reaction may be useful, but take note of structural features in dienes and dienophiles that permit Diels-lder reactions to work well (Section 14-8).] 62. Dimethyl azodicarboxylate (see margin) takes part in the Diels-lder reaction as a dienophile. Write the structure of the product of cycloaddition of this molecule with each of the following dienes. 1,3-utadiene; trans,trans-2,4-hexadiene; 5,5-dimethoxycyclopentadiene; 1,2-dimethylenecyclohexane. Ignore the stereochemistry at nitrogen in the products (amines undergo rapid inversion, as we shall see in Section 21-2). 63. icyclic diene reacts readily with appropriate alkenes by the Diels-lder reaction, whereas diene is totally unreactive. Explain. 64. Formulate the expected product of each of the following reactions. 3 C hv D D hv 3 C 3 C 65. Which of the reactions shown below will occur under the influence of heat? Light? Explain the following reaction sequence. r 1. Pd(C ) 2, R 3 P, K 2 C CP C CC 2 CC ive abbreviated structures of each of the following compounds: (E)-1,4-poly-2-methyl-1,3- butadiene [(E)-1,4-polyisoprene]; 1,2-poly-2-methyl-1,3-butadiene (1,2-polyisoprene); 3,4- poly-2-methyl-1,3-butadiene (3,4-polyisoprene); copolymer of 1,3-butadiene and ethenylbenzene

5 Chapter (styrene, C 6 5 C P C 2, SR, used in automobile tires); (e) copolymer of 1,3-butadiene and propenenitrile (acrylonitrile, C 2 P C, latex); (f) copolymer of 2-methyl-1,3-butadiene (isoprene) and 2-methylpropene (butyl rubber, for inner tubes). 68. The structure of the terpene limonene is shown in the margin (see also Exercise 5-29). Identify the two 2-methyl-1,3-butadiene (isoprene) units in limonene. Treatment of isoprene with catalytic amounts of acid leads to a variety of oligomeric products, one of which is limonene. Devise a detailed mechanism for the acid-catalyzed conversion of two molecules of isoprene into limonene. Take care to use sensible intermediates in each step. Two molecules of isoprene may also be converted into limonene by a completely different mechanism, which takes place in the strict absence of catalysts of any kind. Describe this mechanism. What is the name of the reaction? E C N 3 C C 2 Limonene 69. The carbocation derived from geranyl pyrophosphate (Section 14-10) is the biosynthetic precursor of not only camphor, but also limonene (Problem 68) and a-pinene (Chapter 4, Problem 46). Formulate mechanisms for the formation of the latter two compounds. 70. What is the longest-wavelength electronic transition in each of the following species? Use molecular-orbital designations such as nyp*, p 1 yp 2, in your answer. (int: Prepare a mole cular-orbital energy diagram, like that in Figure 14-16, for each.) 2-Propenyl (allyl) cation; 2-propenyl (allyl) radical; formaldehyde, 2 C P ; N 2 ; (e) pentadienyl anion (Problem 55); (f) 1,3,5-hexatriene. 71. Ethanol, methanol, and cyclohexane are commonly used solvents for UV spectroscopy because they do not absorb radiation of wavelength longer than 200 nm. Why not? 72. The ultraviolet spectrum of a M solution of 3-penten-2-one exhibits a pyp* absorption at 224 nm with and an nyp* band at 314 nm with Calculate the molar absorptivities (extinction coefficients) for these bands. 73. In a published synthetic procedure, acetone is treated with ethenyl (vinyl) magnesium bromide, and the reaction mixture is then neutralized with strong aqueous acid. The product exhibits the 1 NMR spectrum shown below. What is its structure? When the reaction mixture is (improperly) allowed to remain in contact with aqueous acid for too long, an additional new compound is observed. Its 1 NMR spectrum has peaks at d (s, 3 ), 1.79 (s, 3 ), 2.25 (broad s, 1 ), 4.10 (d, J 5 8 z, 2 ), and 5.45 (t, J 5 8 z, 1 ) ppm. What is the structure of the second product, and how did it get there? 1 NMR ( ) 4 Si Mz 1 NMR spectrum ppm ( δ )

6 666 Chapter 14 Delocalized Pi Systems 74. Farnesol is a molecule that makes flowers smell good (lilacs, for instance). Treatment with hot concentrated 2 S 4 converts farnesol first into bisabolene and finally into cadinene, a compound of the essential oils of junipers and cedars. Propose detailed mechanisms for these conversions. 3 C Farnesol 3 C ^ ( ) 2 C ^ 3 C isabolene Cadinene 75. The ratio of 1,2- to 1,4-addition of r 2 to 1,3-butadiene (Section 14-6) is temperature dependent. Identify the kinetic and thermodynamic products, and explain your choices. 76. Diels-lder cycloaddition of 1,3-butadiene with the cyclic dienophile shown in the margin takes place at only one of the two carbon carbon double bonds in the latter to give a single product. ive its structure and explain your answer. Watch stereochemistry. This transformation was the initial step in the total synthesis of cholesterol (Section 4-7), completed by R.. Woodward (see Section 14-9) in This achievement, monumental for its time, revolutionized synthetic organic chemistry. Team Problem 77. s a team, consider the following historic preparation of a tris(1,1-dimethylethyl) derivative of Dewar benzene,, by the photochemical isomerisation of 1,2,4-tris(1,1-dimethylethyl)benzene by van Tamelen and Pappas (1962). does not revert to via either a thermal or a photochemical electrocyclic mechanism. Formulate a mechanism for the conversion of to and explain the kinetic robustness of with respect to the regeneration of. Dewar benzene Preprofessional Problems 78. ow many nodes are present in the LUM (lowest unoccupied molecular orbital) of 1,3- butadiene? Zero; one; two; three; (e) four 79. rrange the following three chlorides in decreasing order of S N 1 reactivity. C 2 C 2 2 CPCC C 2 C C.. C;. C. ;.. C; C...

7 Chapter When cyclopentadiene is treated with tetracyanoethene, a new product results. Its most likely structure is CC 2 C 2 CC C NC 81. Which common analytical method will most clearly and rapidly distinguish from? IR spectroscopy; UV spectroscopy; combustion analysis; visible spectroscopy.