1559T_ch14_259-27511/3/059:27Page259 EQA 14 Delocalized Pi Systems:Investigation by Ultraviolet and Visible Spectroscopy This chapter c t:coniugation.Con ation refers to tronic characteristics.stability.chemical reactivity.and spec troscopy.Introductor aspects of all of theseare presented here. Outline of the Chapter 14-2,14-3,14-4 Chemistry of the Allyl System Consequences of conjugation on reaction types you've already seen 145.1 147 Extended Conjugation and Benzene 14-8,14-9 Special Reactions of Conjugated T 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 resu and using molecular orbitals.Both viewpoints offer useful insights into the allyl system.You should pay spe-
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
1559rch14259-27511/3/059:27Pag0260 EQA 260 chapter 14 DELOCALIZED Pi SYSTEMS:INVESTIGATON BY ULTRAVIOLET AND VISIBLE SPECTROSCOPY 14-2,14-3,14-4 Chemistry of the Allyl System diates is now shared by the two carbons at the ends of the allyl system.A reaction sequence involving any al ylic radic usually does give two isomeric products.derive d from attachment of a ognonraGrigard-type what you've leamed earlier about a reaction mechanism directly to a ne that tum up.This is a comerstone of orsanis chemis and predictab bility in ne tuations.You haven't been asked to doa whole lot of this up until now,but you wil skills from now on. deciding just what these molecules are likely to do. Va-s and 6. Conju see the Donds are separate C=C C=C C==c co. C1.45“16ce ns are ap In thei r qualitative chemistry,conjugated dienes be CH2=CH-CH-CH2-E CH:-CH-CH-CH+ECH:-CH-CH-C E usually fastest(kinetic).touh the 1,4 product.highly substituted double bond,isusu ally more stable 14-7.Extended Conjugation and Benzene Further extrapolation on the same themes
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
1559T_ch14_259-27511/3/059:27Pa9e261 EQA Keys ohe Chapter·261 Special Reactions of Conjugated Systems =学8 ately hecause the ment of two or more pairs of electrons in a circle and the simultaneous breaking and forming of and or ions an hings up.Be cause reactive spec polar bondsare not involved,you mightask why here are two reasons: n ha ra special prop uctsar more the starting materials.(That was simplewasn't it?To co yourself of the a ter.take alook at all the of those thermal reactions given in the New Reaction io of the ev Points to take particular note of have to do with stereochemistry:In particular.stereochemical (e..cis- trans)relationships in the starting materials are preserved through the reaction transition states and on into the may nee 050 ems.For instance,for the Diels sions where all the orig inal groups nd up re eto the two ble to follow readily the The in Section 14-9present a more comple situation.where the stereochemistry of 10Po erization of Coniugated Dienes ncomposed of dicnc unitsar sifcbytheayion they are closelyre 14-11.Electronic Spectra:Ultraviolet and Visible Spectroscopy rect extensions of the els involved are best described as molecular orbita and soon in an organic and therefore in structure determination.Much of its past importance has by the developm ecnniques.Ov-v jugated systems in compounds such as compe biomolecu whose NMRand IR spectra are more diffcul to interpret
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
15597.eh14259-27511/3/059:27Page262 262 chapter 14 DELOCALIZED Pi SYSTEMS:INVESTIGATION BY ULTRAVIOLET AND VISIBLE SPECTROSCOPY Solutions to Problems 28 and 29.Major contributing resonance forms are labeled. CH, Cu. Equal contribu CH2 CH -- (he) 一仓 Equal conributor 0一-四-e-0@ 30.(a)CHCHCH-CH CH:CH-CHCH CH3 CHs -d 31.Radicals:allylic>tertiary>secondary>primary Cations:tertiary>allylic secondary >primary
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 primary CH3 CH3 CH3 CH3 CH3 or CH3 CH3CHCH CH2 CH3CH CHCH2 All contributors are equal Equal contributors CH3 CH3 CH3 Major contributor (tertiary radical-like) CH3 CH3 C H C CH2 CH CH3 Major contributor (charge on secondary carbon) CH3 CH3 C H C CH2 CH CH3 CH3 CH3 C H C H2C CH CH3 CH3 CH3 C H C CH3 C CH3 CH3 CH3 C H C CH3 C CH3 Equal contributors CH3 CH3 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
1559T_ch14_259-27511/3/059:27Pa9e263 ⊕ EQA Solutions to Problems.263 cations to exceed resonance-stabilized allylic cations in stability(the reverse of the order for radicals). 32.(a)(CH3)2CHCBr-CH-CH2.(CH,)2CHC-CHCH2Br OH CH; -CHCH2OCHCH3 CH-OCCH (e)Different!S2 not S conditions ⊕ CH CH-SCH: (f)Intramolecular version: H=C CH CH2CH2CH2OH Again,only one product:bond formation at the other end would produce a more strained seven-membered ring. 33.(a CH3 H H (CH3)CH CH-OH (CH3)CH C-C cH 「CH H H c=c (CHa)CH CH produc produc
Solutions to Problems • 263 Hyperconjugation, which is at 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)2CHCBrOCHPCH2, (CH3)2CHCPCHCH2Br A A CH3 CH3 (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 C C H product (CH3)2CH CH2OH CH3 C C H (CH3)2CH CH2 CH3 C C H (CH3)2CH CH2 CH3 C C H (CH3)2CH CH2 OH2 Br Br H product CH2 CH2CH2CH2OH CH CH CH2 CH2CH2CH2OH CH CH H CH2 CH O CH2 I CH3 CH3S CH2SCH3 CH3 This is the only product. O , O OCCH3 CH2OCCH3 CH3 CH2 C CH3 OCH2CH3 CHCH2OCH2CH3 CH CH2 , OH CH3 , OH CH3 1559T_ch14_259-275 11/3/05 9:27 Page 263