有机化学结构与功能5th答案sg_ch14

1、14 Delocalized Pi Systems: Investigation by Ultraviolet and Visible Spectroscopy This chapter covers an assortment of topics derived from a single concept: conjugation. Conjugation refers to ? overlap of three or more p orbitals on adjacent atoms in a molecule. The allyl systems are the simplest (on

2、e ? bond plus a third p orbital), and conjugated dienes (two adjacent ? bonds ? 4 p orbitals) are next in line. As you will see, conjugation affects the properties of the involved orbital systems, giving rise to modified elec- tronic characteristics, stability, chemical reactivity, and spectroscopy.

3、 Introductory aspects of all of these are presented here. Outline of the Chapter 14-1The Allyl System An introduction to the ? system created by overlap of three p orbitals. 14-2, 14-3, 14-4Chemistry of the Allyl System Consequences of conjugation on reaction types youve already seen. 14-5, 14-6Conj

4、ugated Dienes The ? system made up of four p orbitals. 14-7Extended Conjugation and Benzene 14-8, 14-9Special Reactions of Conjugated ? Systems A new set of mechanisms for ring-forming reactions. 14-10Polymerization of Conjugated Dienes 14-11Electronic Spectra: Ultraviolet and Visible Spectroscopy K

5、eys to the Chapter 14-1.The Allyl System Delocalization generally results in stabilization. The experimental results cited in Section 14-1 illustrate the relative ease of generating allylic radicals, cations, and anions, compared with ordinary 1? radicals, cations, or anions. The origins of allylic

6、stabilization are presented in two different but equivalent ways: using resonance and using molecular orbitals. Both viewpoints offer useful insights into the allyl system. You should pay spe- cial attention to the electrostatic consequences of conjugation as implied by these resonance and molecular

7、- orbital pictures. Electrons can move freely through conjugated ? systems, either toward an electron-deficient atom or away from an electron-rich one. This delocalization obviously is electrostatically desirable and, again, results in overall stabilization. 259 1559T_ch14_259-275 11/3/05 9:27 Page

8、259 260 Chapter 14 DELOCALIZED Pi SYSTEMS: INVESTIGATION BY ULTRAVIOLET AND VISIBLE SPECTROSCOPY 14-2, 14-3, 14-4.Chemistry of the Allyl System The presence of an allyl system gives rise to the possibility of easily formed, stabilized radicals, cations, and anions. It also introduces a new regiochem

9、ical factor, because the reactive character of each of these interme- diates is now shared by the two carbons at the ends of the allyl system. A reaction sequence involving any al- lylic radical, cation, or anion can and usually does give two isomeric products, derived from attachment of a group at

10、either of these two “ends.” Notice that none of these reactions is fundamentally new. All you are seeing is the modified outcome of a nucleophilic displacement, a radical halogenation, or a Grignard-type reaction when the substrate leads to an allylic intermediate as it follows the ordinary mechanis

11、tic course of any of these reactions. Learning to un- derstand and handle situations like this requires that you “think mechanistically.” That is, you need to apply what youve learned earlier about a reaction mechanism directly to a new type of molecule. You have to fol- low the mechanism one step a

12、t a time, see what you get, and analyze the consequences of any unusual new structural types that turn up. This is a cornerstone of organic chemistry, allowing some degree of extrapolation and predictability in new situations. You havent been asked to do a whole lot of this up until now, but you wil

13、l need to develop these skills from now on. Much of what is coming up will involve molecules with multiple functional groups that may affect each others behavior. Mechanistically oriented thinking is indispensable in deciding just what these molecules are likely to do. 14-5 and 14-6.Conjugated Diene

14、s With dienes, you see the first situation where interacting functional groups affect chemical behavior. Conju- gated dienes possess p orbitals on four adjacent atoms. They are more stable than the other two alternatives: isolated dienes, where the double bonds are separated by one or more atoms, an

15、d “cumulated” dienes (like al- lene), where the double bonds share a common atom. As you saw with allyl systems, the presence of conjugation leads to stabilization. The result is lower energy for conjugated dienes relative to the others. Again, both resonance and molecular-orbital explanations are a

16、p- plicable. In their qualitative chemistry, conjugated dienes behave very much like alkenes: They readily react with elec- trophiles in addition reactions. Just as in the case of alkenes, this addition proceeds to give the most stable in- termediate. For conjugated dienes, this normally turns out t

17、o be a resonance stabilized allylic cation: That represents the basic story. The rest of the section deals with details, mainly associated with the fact that the allylic cation can attach a nucleophile at either of two positions. Attachment to give 1,2-addition is usually fastest (kinetic), although

18、 the 1,4 product, possessing a more highly substituted double bond, is usu- ally more stable. 14-7.Extended Conjugation and Benzene Further extrapolation on the same themes. CH2CH2CHE?CH CH2CH2CHECH ? CH2CHCH E CH2 ? ? CCC Cumulated (“1, 2”) Conjugated (“1, 3”)(“1, 4”; “1, 5”; “1, 6”; etc.) (C)n Iso

19、lated, n 1 CC CC CCCC 1559T_ch14_259-275 11/3/05 9:27 Page 260 Keys to the Chapter 261 14-8 and 14-9.Special Reactions of Conjugated ? Systems Up until now we havent made any special presentations concerning syntheses of rings, because the ring-forming processes youve seen so far were nothing more t

20、han intramolecular versions of ordinary reactions, such as Now, however, a new set of ring-forming reactions are presented separately because they represent a totally new mechanistic class, sometimes collectively called pericyclic reactions. Mechanisms for these involve move- ment of two or more pai

21、rs of electrons in a circle and the simultaneous breaking and forming of ? and ? bonds. They are therefore examples of concerted processes. These generally do not involve radicals or ions and dont need polarized bonds to take place, although dipoledipole attractions between reacting atoms can speed

22、things up. Because reactive species like radicals, ions, or polar bonds are not involved, you might ask why these reactions should happen at all. There are two reasons: kinetic and thermodynamic. Certain special prop- erties of circularly moving groups of electrons give these transformations low act

23、ivation barriers, and the prod- ucts are more stable than the starting materials. (That was simple, wasnt it?) To convince yourself of the lat- ter, take a look at all the examples of those thermal reactions given in the New Reaction section of the text. In every case the products contain more ? bon

24、ds and fewer ? bonds than the starting material. Points to take particular note of have to do with stereochemistry: In particular, stereochemical (e.g., cis- trans) relationships in the starting materials are preserved through the reaction transition states and on into the products. You may need som

25、e practice visualizing the reactants to do the problems. For instance, for the Diels- Alder cycloaddition, it may be useful to make models of two reacting molecules and to hold them in an arrange- ment resembling the cycloaddition transition state (Figure 14-9), to see in three dimensions where all

26、the orig- inal groups will wind up relative to the two newly formed ? bonds. You should be able to follow readily the positions of the atoms during the course of the reaction of a molecule like 1,3-cyclopentadiene. The electrocyclic reactions in Section 14-9 present a more complex situation, where t

27、he stereochemistry of the process is a function both of the reaction conditions (heat or light) and of the number of electrons involved. The details are beyond the scope of the course, so only introductory material has been presented. 14-10.Polymerization of Conjugated Dienes Polymers composed of di

28、ene units are significant for two reasons. Just like polymers of simple alkenes, they are industrially important (and have been for a much longer time, by the way). In addition, they are closely re- lated to several major classes of biological molecules formally derived from isoprene (2-methyl-1,3-b

29、utadiene) as the monomeric unit. Some of the variety in this biochemistry is illustrated in this section. 14-11.Electronic Spectra: Ultraviolet and Visible Spectroscopy The principles behind electronic spectroscopy are very simple and, in fact, are really direct extensions of the spectroscopy of ato

30、ms, a freshman chemistry topic. Remember how absorption of light by atoms promotes electrons to higher energy levels? Here, youre seeing the same thing, but with molecules; so, the energy lev- els involved are best described as molecular orbitals. The experimental techniques for observing these ligh

31、t absorptions are straightforward. UV-vis spectroscopy (as it is often abbreviated) was once very important in determining the presence or absence of conjugation, and so on, in an organic molecule, and therefore in structure determination. Much of its past importance has been reduced by the developm

32、ent of sophisticated NMR equipment and techniques. UV-vis spectroscopy is used to confirm structural assignments made on the basis of NMR and IR spectroscopy and to identify con- jugated systems in compounds such as complex biomolecules, whose NMR and IR spectra are more difficult to interpret. NaOH

33、Intramolecular “SN2” BrOHBrO ? ? (Chapter 6) 1559T_ch14_259-275 11/3/05 9:27 Page 261 Solutions to Problems 28 and 29. Major contributing resonance forms are labeled. (a) (b) (c) (d) (e) 30. (a) (b) (c) 31. Radicals: allylic ? tertiary ? secondary ? primary Cations: tertiary ? allylic ? secondary ?

34、primary CH3CH3 ? ? ? ? ? CH3CH3CH3 or ? CH3 CH3CHCHCH2CH3CHCHCH2 ? ? ? ? ? All contributors are equal ? ? ? ? ? ? Equal contributors CH3CH3CH3 ? Major contributor (tertiary radical-like) CH3CH3 C H C CH2 CH ? CH3 Major contributor (charge on secondary carbon) CH3CH3 C H C CH2 CH ? CH3 CH3CH3 C H C H

35、2C CH ? CH3 ? CH3CH3 C H C CH3 C ? CH3 CH3CH3 C H C CH3 C ? CH3 Equal contributors CH3CH3 C H C CH3 C ? CH3 ? 262 Chapter 14 DELOCALIZED Pi SYSTEMS: INVESTIGATION BY ULTRAVIOLET AND VISIBLE SPECTROSCOPY 1559T_ch14_259-275 11/3/05 9:27 Page 262 Solutions to Problems 263 Hyperconjugation, which is at

36、least partially responsible for the tertiary ? secondary ? primary stabilization order, is more important for cations than for radicals. The effect is large enough for tertiary cations to exceed resonance-stabilized allylic cations in stability (the reverse of the order for radicals). 32. (a) (CH3)2

37、CHCBrOCHPCH2,(CH3)2CHCPCHCH2Br AA CH3CH3 (b) (c) (d) (e) Different! SN2, not SN1 conditions: (f) Intramolecular version: Again, only one product; bond formation at the other end would produce a more strained seven-membered ring. 33. (a) CH3 CC H product (CH3)2CHCH2OH CH3 CC H (CH3)2CHCH2 CH3 CC H (C

38、H3)2CHCH2 CH3 CC H (CH3)2CH CH2OH2 BrBr H? ? ? ? product ? CH2CH2CH2CH2OH CHCH CH2CH2CH2CH2OH CHCH ? ? ?H? CH2 CH O CH2I CH3 CH3S CH2SCH3 CH3 This is the only product. ? O , O OCCH3 CH2OCCH3 CH3 CH2 CCH3 OCH2CH3 CHCH2OCH2CH3 CHCH2 , OH CH3 , CH3 OH 1559T_ch14_259-275 11/3/05 9:27 Page 263 (c) (e), (

39、f) See answers to Problem 32. 34. (a) Tertiary ? allylic ? secondary ? ? primary (order of cation stability) (b) Allylic ? primary ? secondary ? ? tertiary 35. SN1 reactivity: e (allylic and tertiary) ? a (allylic and secondary) ? d (forms same cation as e, but requires ionization at primary carbon,

40、 so will be slower) ? c ? b ? f (these follow cation stability order) SN2 reactivity: Steric hindrance predominates, so f ? b ? d ? c ? a ? e 36. SN2 reactivities: Data in this chapter (Section 14-3) reveal that allylic halides are about 102more reactive than their non-allylic counterparts in SN2 di

41、splacements. Therefore, all the primary allylic systems (b, c, d, and f) will be more reactive than a saturated primary halideeven the branched system (c) will possess higher reactivity, because branching reduces reactivity only by about a factor of 20 (Table 6-9). The secondary allylic system (a) w

42、ill be similar in reactivity but perhaps a bit slower than a saturated primary secondary systems are more than 102slower than primaries (Table 6-8), so the steric hindrance of the greater substitution just about cancels the acceleration due to the allylic system. Both allylic and saturated tertiarie

43、s are quite dead to SN2 displacement. SN1 reactivities: Follow cation stabilities together with nature of the position of the leaving group. So (e) is fastest, followed by a saturated tertiary halide. Then comes (a), followed by the primary allylic systems (probably in the order d, c, b, and f), whi

44、ch are comparable to the saturated secondary. Saturated primary halides do not react by the SN1 mechanism. 37. Write all possible allylic isomers in each case and pay attention to stereochemistry. (a) (b) CH3CH2CH3CH2CH3 CH3CH2Br CH3CH3CH2CH3CH3CH2CH3 CH2CH3Br H CH3CH2CH3CH2 Br H Br CH3OH CH3CH2H CH

45、3HO CH3CH2H CH3CH3 OH H CH3CH2H HOH CH3CH2H productproduct CH HOCH2CH3 Br CH2 CHCH2CHCH2 ?H?H? ? ? ? ? CHCH2 CH O O H H CH2 CH2CH3 CH2CH3 HOCH2CH3 264 Chapter 14 DELOCALIZED Pi SYSTEMS: INVESTIGATION BY ULTRAVIOLET AND VISIBLE SPECTROSCOPY 1559T_ch14_259-275 11/3/05 9:27 Page 264 Solutions to Proble

46、ms 265 CH3CH3 AA (c) CH3CH2COCHPCH2(racemic)CH3CH2CPCHCH2Br A Br (d) CH3CHOHCH3CHOH(All possible AA stereoisomers for (e) CH3CHCHPCHCH2CH3? CH3CHPCHCHCH2CH3each structure) (f) 38. 39. 40. (a) cis-2-trans-5-Heptadiene, or (2E, 5Z)-2,5-heptadiene (b) 2,4-Pentadien-1-ol (c) (5S, 6S)-5,6-Dibromo-1,3-cyc

47、looctadiene (d) 4-Ethenylcyclohexene H A m 41. CH2PCHOCHOCHPCH21,4-Pentadiene has the weakest COH bond (arrow), a bond that is doubly allylic (DH? ? 77 kcal mol?1); this isomer will therefore be brominated fastest. Because only a very weak COH bond needs to be broken, its first propagation step has

48、a much smaller Earelative to 1,3-pentadiene, where a stronger methyl COH bond needs to be broken. However, both will give identical product mixtures because identical radicals are formed from each: CH2CH2CH2CHCHCHCH2CHCHCH CH2CH2CHCHCHCH2CH2CHCHCH or CH3CH2CH2CH2Li, TMEDA NBS, h?, CCl4 Li, THF (Grig

49、nard reagent is also okay.) THF Li Br CCH3CH3 OH 2. H?, H2O 1. CH3CCH3, THF O CH3 MgCl CH3 D and CH3 MgCl DOD ?DO? DOD ?DO? CH3 D (CH3)2CSCH3 C CH3 H CH CH3CHCHCHCH2CH3CH3CHCHCHCH2CH3 Li? 1559T_ch14_259-275 11/3/05 9:27 Page 265 42. We figured wed ask you this question now, so you could take your ti

50、me and figure out the right answer instead of maybe getting it wrong on an exam. Have a look at Figure 14-8 in the text. At high temperature, an equilibrium mixture exists because there is enough energy for molecules to “move” from any location on the reaction coordinate to any other location on it.

51、 In other words, all three speciesthe two products and the intermediate allylic cationare interchanging rapidly, and at any given time the relative quantities of each are governed by their relative thermodynamic stabilities. That being the case, if the temperature were to drop, the interconversion p

52、rocesses would slow down because fewer molecules would contain sufficient energy to pass over the activation barriers. This would mainly affect conversion of the two product molecules into the intermediate carbocation because those processes possess the highest activation barriers. The result is tha

53、t the thermodynamic ratio of products originally established at high temperature would remain pretty much unchanged (frozen) upon cooling of the reaction mixture. It will not revert to the kinetic ratio! 43. (1) is more stable than (3), and (2) is more stable than (4). Reaction (1) ? H?n (2) is fast

54、er and leads to the more stable cation. Note: When the text says that allylic and secondary cations are similar in energy, it is referring to the ease of formation of the simplest allylic cation, ? CH2OCHPCH2, which is primary at each end. Additional alkyl groups on allylic cations increase their st

55、ability and their ease of formation, as you might expect. 44. Expect 1,2- and 1,4-addition to occur in each case. Note that the 1,2-additions in (b) and (c) might be expected to show anti stereochemistry, similar to additions to ordinary alkenes. (a) I 1, 2 and 1, 4 products are the same! I HH I CH2

56、CH2CH2CH (3) Isolated diene(4) Ordinary secondary cation CHCH2CH3CH2CH ? CH H? CH2CH3CHCH (1) Conjugated diene(2) Allylic cation, secondary at each end CHCH3CH3CH ? CHCH H? 266 Chapter 14 DELOCALIZED Pi SYSTEMS: INVESTIGATION BY ULTRAVIOLET AND VISIBLE SPECTROSCOPY 1559T_ch14_259-275 11/3/05 9:27 Pa

57、ge 266 Solutions to Problems 267 (b)(c) (d) 45. Addition of the electrophile will always be at C1, generating the best allylic cation. The 1,2-addition product is given first. I A (a) CH3OCHOCHPCHOCH3(cis and trans) OHOH AA (b) BrCH2OCHOCHPCHOCH3andBrCH2OCHPCHOCHOCH3(cis and trans) N3N3 AA (c) ICH2O

58、CHOCHPCHOCH3andICH2OCHPCHOCHOCH3(cis and trans) (d) CH3OCHOCHPCHOCH3(cis and trans) A OCH2CH3 II AA 46. (a) (CH3)2COCHPCHOCH3(cis and trans) and(CH3)2CPCHOCHOCH3 (b), (c) Same answers as Problem 45, but with a methyl group added to C2 in each case. (d) (CH3)2COCHPCHOCH3(cis and trans) and(CH3)2CPCHO

59、CHOCH3 AA OCH2CH3OCH2CH3 47. (a) (b) (c) CH2 CH3 CCHCHCH3 DI CH2 CH3 CCHCHCH3 DI and CH2CHCHCHCH3 DI CH3CHCHCHCH2 ID D I I D and OCH2CH3 From both 1, 2- and 1, 4-additions N3 N3 II and OH Br and Br OH 1559T_ch14_259-275 11/3/05 9:27 Page 267 With DI it is easy to distinguish between 1,2- and 1,4-addition in the case of the cyclic diene in Problem 44 and the unbranched acyclic diene in Problem 45. 48. (e) CH2PCHOCHPCHO ? CH2 ? (d) CH3OCHPCHO ? CHOCH3 (secondary allylic at both ends) ? (a) ? CH2OCHPCH2? (c) ? (b) 49. See th