北京化工大学：《有机化学》课程教学资源（习题与答案）Chapter 14 Delocalized Pi Systems：Investigation by Ultraviolet and Visible Spectroscopy.pdf_大学文库

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 (one 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 electronic characteristics, stability, chemical reactivity, and spectroscopy. Introductory aspects of all of these are presented here. Outline of the Chapter 14-1 The Allyl System An introduction to the system created by overlap of three p orbitals. 14-2, 14-3, 14-4 Chemistry of the Allyl System Consequences of conjugation on reaction types you’ve already seen. 14-5, 14-6 Conjugated Dienes The system made up of four p orbitals. 14-7 Extended Conjugation and Benzene 14-8, 14-9 Special Reactions of Conjugated Systems A new set of mechanisms for ring-forming reactions. 14-10 Polymerization of Conjugated Dienes 14-11 Electronic Spectra: Ultraviolet and Visible Spectroscopy Keys 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 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 special attention to the electrostatic consequences of conjugation as implied by these resonance and molecularorbital 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 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 regiochemical factor, because the reactive character of each of these intermediates is now shared by the two carbons at the ends of the allyl system. A reaction sequence involving any allylic radical, cation, or anion can and usually does give two isomeric products, derived from attachment of a group at 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 mechanistic course of any of these reactions. Learning to understand and handle situations like this requires that you “think mechanistically.” That is, you need to apply what you’ve learned earlier about a reaction mechanism directly to a new type of molecule. You have to follow the mechanism one step at 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 haven’t been asked to do a whole lot of this up until now, but you will 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 other’s behavior. Mechanistically oriented thinking is indispensable in deciding just what these molecules are likely to do. 14-5 and 14-6. Conjugated Dienes With dienes, you see the first situation where interacting functional groups affect chemical behavior. Conjugated 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, and “cumulated” dienes (like allene), 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 applicable. In their qualitative chemistry, conjugated dienes behave very much like alkenes: They readily react with electrophiles in addition reactions. Just as in the case of alkenes, this addition proceeds to give the most stable intermediate. For conjugated dienes, this normally turns out to 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 the 1,4 product, possessing a more highly substituted double bond, is usually more stable. 14-7. Extended Conjugation and Benzene Further extrapolation on the same themes. CH2 CH E CH2 CH CH2 CH E CH CH2 CH2 CH CH E CH2 C C C Cumulated (“1, 2”) Conjugated (“1, 3”) (“1, 4”; “1, 5”; “1, 6”; etc.) (C)n Isolated, n > 1 C C C C C C C C 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 haven’t made any special presentations concerning syntheses of rings, because the ring-forming processes you’ve seen so far were nothing more than 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 movement of two or more pairs 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 don’t need polarized bonds to take place, although dipole–dipole attractions between reacting atoms can speed 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 properties of circularly moving groups of electrons give these transformations low activation barriers, and the products are more stable than the starting materials. (That was simple, wasn’t it?) To convince yourself of the latter, 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 bonds and fewer bonds than the starting material. Points to take particular note of have to do with stereochemistry: In particular, stereochemical (e.g., cistrans) relationships in the starting materials are preserved through the reaction transition states and on into the products. You may need some practice visualizing the reactants to do the problems. For instance, for the DielsAlder cycloaddition, it may be useful to make models of two reacting molecules and to hold them in an arrangement resembling the cycloaddition transition state (Figure 14-9), to see in three dimensions where all the original 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 the 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 diene 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 related to several major classes of biological molecules formally derived from isoprene (2-methyl-1,3-butadiene) 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 atoms, a freshman chemistry topic. Remember how absorption of light by atoms promotes electrons to higher energy levels? Here, you’re seeing the same thing, but with molecules; so, the energy levels involved are best described as molecular orbitals. The experimental techniques for observing these light 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 development 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 conjugated systems in compounds such as complex biomolecules, whose NMR and IR spectra are more difficult to interpret. NaOH Intramolecular “SN2” Br OH Br O (Chapter 6) 1559T_ch14_259-275 11/3/05 9:27 Page 261

(c) (e), (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, 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 102 more reactive than their non-allylic counterparts in SN2 displacements. Therefore, all the primary allylic systems (b, c, d, and f) will be more reactive than a saturated primary halide—even 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) will be similar in reactivity but perhaps a bit slower than a saturated primary— secondary systems are more than 102 slower 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 tertiaries 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), which 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) CH3CH2 CH3 CH3CH2 CH3CH2 Br CH3 CH3CH2 CH3 CH3CH2 CH3 Br CH2CH3 H CH3CH2 CH3CH2 Br H Br CH3 OH CH3CH2 H HO CH3 CH3CH2 H CH3 CH3 OH H CH3CH2 H H OH CH3CH2 H product product CH HOCH2CH3 Br CH2 CH CH2 CH CH2 H H CH CH2 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

42. We figured we’d ask you this question now, so you could take your time 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. In other words, all three species—the two products and the intermediate allylic cation—are 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 processes 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 that 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 faster 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 stability 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 H H I CH2 CH CH2 CH2 (3) Isolated diene (4) Ordinary secondary cation CH CH2 CH CH2 CH3 CH H CH2 CH CH CH3 (1) Conjugated diene (2) Allylic cation, secondary at each end CH CH3 CH CH3 CH CH H 266 • Chapter 14 DELOCALIZED Pi SYSTEMS: INVESTIGATION BY ULTRAVIOLET AND VISIBLE SPECTROSCOPY 1559T_ch14_259-275 11/3/05 9:27 Page 266