Copy of Organic Chemistry Jonh Mc Murry13

Copy of Organic Chemistry Jonh Mc Murry13 - 210 CHAPTER l3...

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Unformatted text preview: 210 CHAPTER l3 Alkenes: Structure and Reactivity 6.49 Addition of HCl to 1-isopropenyl-1-methylcyclopentane yields 1-chloro-1,2,2— 6.50 6.51 6.52 6.53 6.54 trimethylcyclohexane. Propose a mechanism, showing the structures of the intermediates and using curved arrows to indicate electron fl0w in each step. c CH3 + HCI ———~ CH3 CHCH3 3 Vinylcyclopropane reacts with l’lBr to yield a rearranged alkyl bromide. Follow the flow of electrons as represented by the curved arrows. show the structure of the carbocation intermediate in brackets, and show the structure or the final product. .H;l§r r/:__=// —* B" ? Vinylcyclopropane a Calculate the degree of unsatu ration in each of the following formulas: (a) Cholesterol, Calm/,0 (b) DDT, CHI-1905 (C) Prostaglandin E], CZUHEHOS Caffeine, CBH‘ION4OZ (e) COI’USOF‘IC, (221142805 Atropine, CI7H23NO3 The isobutyl cation spontaneously rearranges to the tert—butyl cation by a hydride shift. is the rearrangement exergonic or endergonic? Draw what you think the transition state for the hydride shift might look like according to the Hammond postulate. H c CH H/ \c: ' t ;c\ L H3C CH3 Isobutyl cation tert-Butyl cation Draw an energy diagram for the addition of l—lBr to 1-pentene. Let one curve on your diagram sh0w the formation of 1—bromopentane product and another curve on the same diagram show the formation of 2—bromopentane product. Label the positions for all reactants, intermediates, and products. Which curve has the higher-energy carbocation intermediate? Which curve has the higher; energy first transition state? Make sketches of the transition-state structures involved in the reaction of l-lBr with 1-pentene (Problem 6.53). Tell whether each structure resembles reactant or product. i Assignable in OWL . Key Idea Problems Exercises 21‘ 6.55 liimonene, a fragrant hydrocarbon found in lemons and oranges. is bio- synthesized from geranyl diphosphate by the following pathway. Add curvec arrows to show the mechanism of each step. Which step involves an alkene electrophilic addition? (The ion 01’2064_ is the diphosphate ion, and "'Base' is an unspecified base in the enzyme that catalyzes the reaction.) + OP2054_ \ + 3.. OPZoB _. i _) Base | + Geranyl H Limonene diphosphate 6.56 cpi-Aristolochene, a hydrocarbon found in both pepper and tobacco, is bio- synthesized by the following pathway. Add Curved arrows to show the mecha- nism of each step. Which steps involve alkene electrophilic addition(s], and which involve carbocation rearrangement(s)? (The abbreviation H—A stands for an unspecified acid, and "Base" is an unspecified base in the enzyme.) CH3 CH3 l H—A \ lacid) c\ \\ \f .f + I \V/ H H H H CH3 epi-Aristolochene 6.57 Aromatic compounds such as benzene react with alkyl chlorides in the pres- ence of AICI3 catalyst to yield alkylbenzenes. The reaction occurs through a carbocation intermediate. formed by reaction of the alkyl chloride with AlCl3 (R—Cl + AlCl3 —> W + AlCLf). How can you explain the observation that reaction of benzene with l-chloropropane yields isopropylbenzene as the major product? a3 CHCH3 AICI3 + CH3CH2CH2CI I Assignable in OWL A Key Idea Problems CHAPTER 6 Alkenesz Structure and Reactivity 6.58 .. _CD U" (D Allx’enes can be converted into alcohols by acid-catalyzed addition of water. Assuming that Markovnikov’s rule is valid, predict the major alcohol product from each of the following alkenes. (3) CH3 CH2 [C] CH3 l l CH30H20:CHCH3 CH3CHCH2CH=CH2 Reaction of 2,3-ciimethyl-Lbutene with HBr leads to an alkyl bromide, Cél-lHBr. On treatment of this alkyl hromide with KUH in methanol, elimina— tion of HBI‘ to give an alkene OCCLIFS and a hydrocarbon that is isomeric with the starting alkene is formed. What is the structure of this hydrocarbon, and how do you think it is formed from the alkyl bromide? Key Idea Problems I Assignable in OWL i... i i i s 1-‘ t" GE 13: Thomson Throughout this chapter, sign in at www.thomsonedu.com for online self—study and interactive tutorials based on your level of understanding. Online homeworkforthis chapter may be assigned in Organic OWL "w". Alkenes: Reactions and Synthesis Alkene addition reactions occur widely, both in the laboratory and in living organisms. Although we've studied only the addition of HX thus far, many closely related reactions also take place. In this Chapter. we'll see briefly how alkenes are prepared, we‘ll discuss many further examples of alkene addition reactions, and we'll see the wide variety of compounds that can be made from alkenes. H OH H H \ / \ / - //C—C\\ //C*C\\ K\ PH Ho on \ / Cic _ // \\ Alcohcil Alkane //C C\\ Halohydrin - \ / i 1 2-Diol x x \ 2‘ ,« M R C70 /cic\\ / / \. Carbonyl 1.2-Dihalide / \ compound H X C \ /- C/ \C Cic _ // \\ // \\ Halide Cyclopropane Epoxide WHY THIS CHAPTER? Much of the background needed to understand organic reactions has now been covered, and it’s time to begin a systematic description of the major func- tional groups. Both in this chapter on alkenes and in future chapters on other 213 214 CHAPTER? Alkenes: Reactions and Synthesis 7.1 functional groups, we’ll discuss a variety of reactions but try to focus on the general principles and patterns of reactivity that tie organic chemistry together. There are no shortcuts: you have to know the reactions to under- stand organic chemistry. Preparation of Alkonos: A Preview of Elimination Reactions Before getting to the main subject of this chapter—the reactions of alkenes— let‘s take a brief look at how alkenes are prepared. The subject is a bit complex, though, so we'll return in Chapter 11 for a more detailed study. For the present, it’s enough to realize that alkenes are readily available from simple precursorsi usually alcohols in biological systems and either alcohols or alkyl halides in the laboratory. Just as the chemistry of allcenes is dominated by addition reactions, the preparation of alkenes is dominated by elimination reactions. Additions and eliminations are, in many respects, two sides of the same coin. That is, an addi- tion reaction might involve the addition of I-lBr or HZO to an alkene to form an alkyl halide or alcohol, whereas an elimination reaction might involve the loss of l—lBr or H20 from an alkyl halide or alcohol to form an alkene. Atlciillriri \ / x\ /\' c:c + xe-v ,cic / \ "/ \ El |lllE-".‘lt)1i The two most common elimination reactions are Liv/n’rlrolinlogeianion—the loss of HX from an alkyl halideiand (1(fll'l'rll't'lfimlilhe loss of water from an alcohol. Dehydrohalogenation usually occurs by reaction of an alkyl halide with strong base such as potassium hydroxide. For example, bromocyclohexane yields cyclohexene when treated with KOH in ethanol solution. H I /\;/Br H k/L l + so. + Hair CH3CH20H L l H H H Bromocyclohexane Cyclohexene (81%) Dehydration is often carried out by treatment of an alcohol with a strong acid. For example, loss of water occurs and l-methylcyclohexene is formed Problem 7.1 Problem 7.2 72 Thomson Click Organic Interactive to use a web-based palette to predict products of the addition of halogens to alkenes. 7.2 Addition of Halogens to Alkenes 215 when i-methylcyclohexanol is warmed with aqueous sulfuric acid in tetra- hydrofuran (TH 13) solvents CH3 W + H90 V. CH3 ' [J H THE 50 ‘ 1-Methylcyclohexanol 1-Methylcyclohexene (91%) O l 7 Tetrahydrofuran [THFl—a common solvent in biological pathways, dehydrations rarely occur with isolated alcohols but instead normally take place on substrates in which the —OH is positioned two carbons away from a carbonyl group. In the biosynthesis of fats, for instance, fi-hydroxybutyryl ACP is converted by dehydration to tmnscrotonyl ACP, where .=—\CP is an abbreviation for my! carrierprorein. We’ll see the reason for this requirement in Section 1110. HQ H i._I H ,I | l /C\ / \ ——' /C<\ / :\ H3C ,C\ ACP H3C ACP + ilEO ll H H B—Hydroxybutvryl ACP trans-Crotonyl ACP One problem with elimination reactions is that mixtures of products are often formed. For example, treatment of 2—bromo—2—methylbutane with KOH in ethanol yields a mixture of two alkene products. What are their likely structures? How many alkene products, including 5,2 isomers, might be obtained by dehydra tion or 3-methyl~3-hexanol with aqueous Sulfuric acid? on CH3CHZCH2cCHZCH3 lfi§9£4 ? CH3 3-lVlethyI-3ehexanol Addition of Halogens to Alltenes Bromine and chlorine add rapidly to alkenes to yield 1,2-dihalides, a process called lmlogenntion. For example, approximately 6 million tons per year of 1,2-dichloroethane (ethylene dichloride) are synthesized industrially by addition 216 CHAPTER 7 Alkenes: Reactions and Synthesis of C12 to ethylene. The product is used both as a solvent and as starting material for the manufacture of poly(vin)rl chloride), PVC. Fluorine is too reactive and dif- ficult to control for most laboratory applications, and iodine does not react with most alkcnes. H H Cl CI \ / i l C:C + Cl; —> Hicec—H / \ l I H H H H Ethylene 1,2ADichloroethane (ethylene dichloride) BaSed on what we've seen thus far, a possible mechanism for the reaction of bromine with alkenes might involve electrophilic addition of Br+ to the alkene, giving a carbocation that could undergo further reaction with Br‘ to yield the dibromo addition product. ,r Br- I J, f, | l H l H t' '~ H r i“ - \ / /, . Possnble Czci H #C _CT H_C_C_H Poseible mechamsm? / I \ l i mechanism? H H H H H H Although this mechanism seems plausible, it’s not fully consistent with known facts. In particular, it doesn't explain the ster‘mi'hemisn't-r ot' the addi- tion reaction. That is, the mechanism doesn’t tell which product stereoisomet is formed. When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only the trans stcreoisomet of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected it a planar carbocation intermediate were involved. We say that the reaction occurs with anti stereochemistry, meaning that the two bromine atoms come from opposite faces of the double bond—one from the top face and one from the bottom face. i f Br --i:-! \ 7 H H r a l l l ./ l i' | Br H Br 1;: ‘ Cyclopentene trans-1,2-Dibromo- ‘ cis-1,2-Dibromo- cyclapentane cyclopentane HmI-r- i-.-;r::5rm=) . (NOTformedl' fl An explanation tor the observed anti stereochemistry of addition was sug- gested in 1937 by George Kimball and Irving Roberts, who proposed that the Thomson Click Organic Process to view an animation of the bromonium ion intermediate and nvnrhlrf fnrmnfinn in H15: ullu Pluuuvl xvuuuuuu IAI uuu reaction. George Andrew 0|al1 [19277) was born in Budapest Hungary, and received a doctorate in 1949 at the Technical University of Budaoest. During the Hungarian revolution in i956, he immigrated In Canada and ruined the Dow [Zhen‘iicai Company. Atrer moving to the United States, he was pro- fessor oi chemistry at Case Western Reserve University HESS—i977) and then at the Uni- versity of Southern California (1977— l. He received the i394 Nobel Prize in chemistry forhis wort: on carbocations, 7,2 AdditionU‘lHalogenstDAlkenes 217 reaction intermediate is not a carbocation but is instead a bromonium ion, RzBr+, formed by additiOn of Br+ to the alkene. (Similarly, a n‘lzloronium ion con— tains a positively charged divalent chlorine, R3C1+.) l'he bromonium ion is formed in a single Step by interaction of the alltene with Brz and simultaneous loss of Br". J ,zfir: - i, " l :Br “ft /‘\ I , (C C\ —v ,_7C*C‘\_ + .l' An alkene A bromonium ion How does the formation of a bromonium ion account for the observed anti stereochemistry of addition to cyclopentene? it a bromonium ion is fOrmed as an intermediate, we can imagine that the large bromine atom might "shield" one side ofthe molecule Reaction with Br' i0n in the second step could then occur only trom the opposite, unshielded side to give trans product. Top side open to attack Bottom side shielded from attack trans-3,2-Dibromo- cyclopentane Bromonium ion intermediate Cyclopentene The bromonium ion postulate, made more than 75 years ago to explain the stereochemistry of halogen addition to alkenes, is a remarkabJe example of deductive logic in chemistry. Arguing from experimental results, chemists were able to make a hypothesis about the intimate mechanistic details of alltene electrophilic reactions. Subsequently, strong evidence supporting the mecha nism came from the work of George Olah, who prepared and studied stable 218 CHAPTER 7 Alkenes: Reactions and Synthesis Problem 7.3 Problem 7.4 7.3 Thomson Click Organic interactive to use a web—based palette to predict products of the addition of hypohalous acid to alkenes‘ solutions of cyclic bromonium ions in liquid 502‘ There’s no question that bromonium ions exist. 1 H3C‘ K4?“ :er: .-'| -- , H i-‘ Fe -‘ \ 3LT: 3C7\Cic“ f. _. (Cicx '/ \~CH3 Liqiiid 802 H30 1/ \cCHa r T CH3 H \ r.1 Bromonium io lctthn in :n \aluu u -e Icni v29 3 nlannl UIUlI I Alkene halogenation reactions occur in nature just as they do in the labora- tory but are limited primarily to marine organisms, which live in a halide-rich environment. The reactions are carried out by enzymes called Imioperoxidzisa, which use H202 to oxidize Br‘ or Ci‘ ions to a biological equivalent of 81"“ or (31+. Electrophiiic addition to the double bond of a substrate molecule then yields a bromonium or chloronium ion intermediate just as in the laboratory, and reaction with another halide ion completes the process For example, the following tetrahalide, isolated from the red alga Plocmiiium r'artilagiizezmz, is thought to arise from fi-ocimene by twofold additiOn of BrCl through the cor— responding bromonium ions. r// \ \ / 1‘ “Br'” \ M e _‘|_ - Er/YK/Y Q or at 2i" fl-Ocimene What product would you expect to obtain from addition 01" C12 to 1,2-dimethyl— cyclohexene? Show the StGl'COt hemistry of the product. Addition of HCl to 1,2-diinethylcyciohexene yields a mixture of two products. Show the stereochemistry of each, and explain why a mixture is formed. Addition of Hypohaluus Acids to Aikenes: Halohydrin Formation Yet another example 01’ an electrophilic addition is the reaction of alkenes with the hypohalous acids l—loiCl or HO—Br to yield 1,2-halo alcohols called haiohydrins. Halohydrin formation doesn’t take place by direct reaction of an alkene with HOBr or HOCl, however, Rather, the addition is done indirectly by reaction of the alkenc with either Br; 01' Cl; in the presence of water. X \ / t / C:C IC—C\ + X / \ \‘ An alkene A halohydrin Figure 7.1 MECHANISM: Mechanism of bromohydrin for mation by reaction of an alkene with Brz in the presence of water. Water acts as a nucleophile to react with the intermediate biomonium ion. 7.3 Addition of Hypohalous Acids to Alkenes: Halohydrin Formation 21 We Saw in the previous section that when Brz reacts with an alkene, th- cyclic hromonium ion intermediate reacts with the only nucleophile preseni Br’ ion. it the reaction is carried 0th in the presence oll an additional nuclec phile, however, the intermediate brornonium ion can be intercepted by th added nucleophile and diverted to a dit'ferent product. In the presence of water for instance, water competes with Br‘ ion as nucleophile and reacts with th bromonium ion intermediate to yield a bromalzydrin. The net effect is additior of l-lOeBt to the alltene by the pathway shown in Figure 7.]. H / CH3 [3! I Br 02c / H3C H 0 Reaction of the allrene with Brz yields a bromonium ion 0 intermediate, as previously discussed. ,———| :IB\r:;_') —C.\ + Br" ,r \‘CH3 H l H 3 x . . “2' ill 9 Water acts as a nucleophile, usmg ‘- alone pair of electrons to open the bromonium ion ring and form a 6| bond to carbon. Since oxygen donates i'ts electrons in this step, it _ now has the positive Charge. Bk [CH3 l/H ,C—C H l \ ‘ H3C :‘ ‘ ’ l' _-,,-\ i I -‘, I. L 9 Loss of a proton (H+l from oxygen then gives H30+ and the 9 neutral bromohydrin addition V Product. Bi 9:3 ,C—(f’ + l' H 1 \ H3C ‘ 3—Bromo-2-butanol [a bromohydrin) ln practice, few silken-es are soluble in water, and bromohydrin formation is often carried out in a solvent such as aqueous dimethyl sultoxide, CF1350CF13 (DMSO), using a reagent called N-brornosuccinimide (NBS) as a source of BIZ. N83 is a stable, easily handled compound that slowly decom- poses in water to yield Brz at a controlled rate. Bromine itself can also be used [20 CHAPTER 7 Alkenes: Reactions and Synthesis Problem 7.5 Problem 16 7.4 in the addition reaction, but it is more dangerous and more difficult to han- dle than NBS. j H N—Bi H l mil-ifiii \ / / /C§c/H O A/C\C/Br l _l_ l-l;i',CH3500H3lDlVISOl l ,/ \, V H V ” ” Styrene Z-Bromo-l-phenylethanol (70%] Note that the aromatic ring in the preceding example does not react with Brz under the conditions used, even though it appears to contain three carbon—carbon double bonds. As we’ll see in Chapter 15, aromatic rings are a good deal more stable than might be expected. What product would you expect from the reaction of cyclopentene with NBS and water? Shaw the stereochemistry. When an unsymmetrical alkene such as propene is treated with N-bromosuccin— imicle in aqueous dimethyl sullioxide, the major product has the bromine atom bonded to the less highly substituted carbon atom. is this h'lal‘kOVnikOV or non- MarkovnikOV orientation? Explain. OH Brz, H20 I CH3CH=CH2 —s CH3CHCHZBr Addition of Water to Alkenes: Oxymercuration Water adds to aikenes to yield alcohols, a process called Indiana”. The reaction takes place on treatment of the aikene with water and a strong acid catalyst (HA) by a mechanism similar to that of HX addition. Thus, protonation of an alkene double bond yields a carbocation intermediate, which reacts with water to yield a protonated alcohol product (ROI-13+). Loss of H+ from this protonated alcohol gives the neutral alcohol and regenerates the acid catalyst (Figure 7.2). Acid—catalyzed alkene hydration is particularly suited to largerscale indus trial procedures, and approximately 300,000 tons of ethanol are manufactured each year in the United States by hydration of ethylene. The reaction is of little value in the typical laboratory, however, because it requires high temperatures—— 250 “C in the case of ethylenekand strongly acidic conditions. H H \C_C/ IrixiV-i‘Z-alril'y‘s‘i CH H OH / " \ + H20 250=c 3C 2 H H Ethanol Ethylene ...
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