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Ch7 problems - 276 Chapter FURTHER REACTIflNSOl HALOMKANES...

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Unformatted text preview: 276 Chapter] FURTHER REACTIflNSOl HALOMKANES 2. Unimolecular Substitution—SN] (Sections 7-1 through 7-5) _g._t,-'__Write the . . W 'te Secondary and tertiary substrates only ignitih 1 €ng CH3 (EH3 .' intermedi / :N ‘ . CH3—(IJBr TB}? CH3WC+\ —~u—> CHg—CIJNu I 5.35. Give the t CH3 CH3 CH3 | Through carbocation: Chiral systems are raoemized 3. Unimolecular Elimination-13] (Section 7-6) H Secondary and tertiary substrates only (EH3 /CH3 a EH2 gag-ling wou] CH3—CC1 u—_> (:Hg—c+ '—> c + BH I fish-swing I 7C! \CH3 H3C/ \CH3 I list-$0 CH2 fté] NaNg Through carbocation aghast that f 4. Bimolecular Elimination—E2 (Section 7-7) :3— CH3CH2CH21 _'> CH3CH=CH2 + BH + If Simultaneous elimination of leaving group and neighboring proton Important Concepts , I 1. Secondary haloalkanes undergo slow and tertiary haloal— 5. Uuimolecular elimination to form an alkene accompanist?!i kanes undergo fast unimolecular substitution in polar me— substitution in secondary and tertiary systems. dia. When the solvent serves as the nucleophile, the process the c: . . 6. High concentrations of strong base may bring about I 13 called solvolysrs. . . . . . molecular elimination. Expulsmn of the leavmg group an 2. The slowest, or rate-detennining, step in unimolecular sub— companies removal of a hydrogen from the neighbo " ' stitution is dissociation of the C—X bond to form a carbo- carbon by the base. The stereochemistry indicates an a”; _ cation intermediate. Added strong nucleophile changes the conformational arrangement of the hydrogen and the leéfi product but not the reaction rate. ing group. I 3. Carbocations are stabilized by hyperconjugation: Tertiary 7. Substitution is favored by unhindered substrates and smflifi are the most stable, followed by secondary. Primary and less basic nucleophiles. methyl cations are too unstable to form in solution. 8. Elimination is favored by hindered substrates and bull: 4. Racemization often results when unimolecular substitution more basic nucleophiles. takes place at a chiral carbon. _. [ems ’ 22. hat is the major substitution product of each of the following solvolysis reactions? CH3 Br CH3CH2 (:1 i cmcnzon I CF3CH10H (a) CHg(l3Br —> (b) (CH3)2CCH2CH3 —~—> (6) CH3 Br 0 CH3 CH3 . l HtIiOH I 520 I 9;; (d) GHQ-[.13 _s (e) CH3CIIC1 —-> (f) CH3C!‘C1 - | CH3 CH3 CH3 @or each reaction presented in Problem 22, write out the complete, step~by-step .. I mechanism using curved arrow notation. Be sure to show each individual step of 8391" mechanism separately and show the complete structures of the products of that steP- before going on to the next. _: finite-111$ two major substitution products of the reaction shown in the margin. ' 1.: a mechanism to explain the formation of each of them. (b) Monitoring the L mixture reveals that an isomer of the starting material is generated as an 'iediate. Draw its structure and explain how it is formed. '. . fiwmfl two major substitution products of the following reaction. OSOZCH3 H3C C5H5 CH3CH20H _'—) H3C C5H5 H . sjj malt-i each reaction in Problem 22 be affected by the addition of each of the : . Mining substances to the solvolysis mixture? a. mo (b) KI Na”: (d) CH3CH20CH2CH3 (Hint: Low polarity.) . finit' Illefollowing carbocations in decreasing order of stability. H CH3 CH; CH3 ‘IL + flint the compounds in each of the following groups in order of decreasing rate of F: mligoiysis in aqueous acetone. (‘le (EH3 (EH3 {a} CHECHCH3CH2C1 CH3CH$HCH3 CH3CCH2CH3 neigli C1 C1 cates . (H) d I m Eh“! 0(ch3 c1 OH :s and “IE-Hi sandbfi__" Br Cl H3 C1 ”@0 G é 1r ‘ _-=' - ctions? ISL :3? Gm the products of the following substitution reactions. Indicate whether they arise ""- -"ll?|ltfiiEI1 the SNl or the 8N2 process. Formulate the detailed mechanisms of their generation. Cl - | Cflao'fi'l . , CH CH on C 3 E cnjsu, CHJOH 7 "—41- :3133} it.“Hngonos02(11=3 #9 (1}) “La Br OH I (C H ) P DMSO N81 . . 1 , acetone 0 .3? ciCH3GH2CHZCH213r £1—> (d) CH3CH2CHC1CH2CH3 _> a“ We the product 0f each of the following substitution reactions. Which of these trans- ' _fhmnfltions should proceed faster in a polar, aprotic solvent (such as acetone or DMSO) 'I an in a p _ . polar, protic solvent (such as water or CH30H)? Explain your answer on the of each ‘blfig-EE of the mechanism that you expect to be operating in each case. it step It} CH3CH2CHZBr + Na+ ’CN __> (h) (CH3)2CHCH21 + Nat+ N3_ —~> “it! tCHnfiBr + HSCHZCHg —+ (a) (canzcnosozcng + HOCH(CH3)2 —_> 277 278 Chaplen FURTHER REACTIONS 0i HALUALKANES 31. Propose a synthesis of (R)-CH3CHNgCH2CH3, starting from (R)-2—chlorobutane. @ Two substitution reactions of (SD-Z-bromobutane are shown here. Show their stereo- chemical outcomes. 0 ll HCOH (S)—CH3CH2CHBrCH3 ——~——> O HgO’Nafl, nMso (S)-CH3CH2CHBrCH3 —> 33. The two seemingly similar reactions shown below differ in their outcomes. N CH, CH CH OH CHgCHZCHZCHzBr % CH3CH2CH2CH20H N SH, CH CH OH CH3CI-I2CH2CH2Br ;—2_> CH3CH2CH2CHZSH The first proceeds in high yield. The yield of the product in the second, however, is diminished by the formation of (CH3CH2CH2CH2)ZS in substantial quantities. Discuss the formation of this by-product mechanistically, and explain why it occurs in the second case but not in the first. .Write all possible E1 products of each reaction in Problem 22. 35. Write the products of the following elimination reactions. Specify the predominant mechanism (E1 or E2) and formulate it in detail. (a) (CH3CH2)3CBr —m—>NaNH2‘ “13 (b) CH3CH2CH2CH2C1 ———-KOC‘CH’)3‘ (CH3)3C°H, Cl Excess KOH, CH3CH20H NaOCHh CHIOH ——~.—> ——a Br @Predict the major product(s) that should form from reaction between 1-brornobutane and each of the following substances. By which reaction mechanism is each formedeNl, ' 8N2, E1, or E2? If it appears that a reaction will either not take place or be exceedingly slow, write “no reaction.” Assume that each reagent is present in large excess. The sol- vent for each reaction is given. (a) KCl in DMF (13) KI in DMF (c) KCl in CH3N0: (d) NH3 in CH3CH20H (e) NaOCHZCHg in CHSCH'ZOH (f) CH3CH20H (g) KOC(CH3)3 in (CH3)3COH (h) (CH3)3P in CH30H (i) CH3C02H 37. Predict the major product(s) and mechanism(s) for reaction between 2-brornobutane (sec-butyl bromide) and each of the reagents in Problem 36. @Predict the major product(s) and mechanism(s) for reaction between 2-brorno—2-methyl- propane (tert-butyl bromide) and each of the reagents in Problem 36. 39. Three reactions of 2-chloro—2—methylpropane are shown here. (3) Write the major product of each transformation. (b) Compare the rates of the three reactions. Assume identical solution polarities and reactmit concentrations. Explain mechanistically. H25, CHJOH (CH3)3CC1 —-—> 0 CHagO'Kfl CH30H (CH3)3CC1 ——> CEO—K”, CHSOH (CH3)3CC1 ———~—.——> thlems 279 1 hiya-fl ”‘:'”W 11]; major product(s) of the following reactions. Indicate which of the following " ' 'ihmtiflml-‘il is in operation: 8N1, SNZ, E1, or E2. If no reaction takes place, write ‘16. tereolc _ . Illa ”HUN-3“” u CHZCI KOC(CH3>3. l|2 fifgne .15]. g (CHs)aC0H (b) CH3CHCH2CH3 —> :r. H CHQCHa I _ H20 um Hap Bf —’ (d) NaNH2,liquidNH3 ——> H . Br ‘ I c _ N OCH1CH3, CH CH10H /’ \ ' {a} rcngygcmnzcnzcnznr Ma 3 (f) H3C CIH CCH2CH2CH3 Nfl‘m‘mmmm . 2 **——> 3 :r, is _ .- Excess mm.- OH K0“ KCNa . .. . . .. CH3CH20H CH30H me :th _ H W—> (h) c1 cnzcnzcnznr —-> CH3C 2 I ant (3 OH H: . NaSH,CH3CH2 H . —~——> .‘flli $302 (31-13 ——.O—> (J) {)s'. .- 3 ' - {R}-CH3CH2CHCH3 _ . Br (1|) {’Oh' _- I K0H,CH,CH20H CHICOH fill? (CH3)3CCHCH3 ——-*~—> (l) CH3CH2C1 —“~—> giltlhx: blanks in the following table with the major product(s) of the reaction of an _- Each haloelkane with the reagents shown. -5 ' - :di Reagent Le . . M Hfllfl'fllkflflfi H20 NaSeCHs NaOCH; KOC(CH3)3 :Hzfla; '_ —' ’03 Elite: EH3CHZCH2C1 {CiifliCl-ICI ' tfifizlgCCI ___w_ ———— m— —--— 1__- I—_—_—————_—__—— I -. ___I:|iL'otI.iE the major mechanism(s) (simply specify 3N2, 8N1, E2, or E1) required for the ".ml'n'Ialiutl of each product that you wrote in Problem 41. fifnr each of the following reactions, indicate whether the reaction would work well, ““33 fimfl'fi or not at all. Formulate alternative products, if appropriate. NaOH, acetone in) CH3CH2C|HCH3 ~—‘) CH3CH2$HCH3 Br OH H Cl H CN H30 H3C -- i CH3OH i HCN CH30H ii i!” CH3CHCH2C1 ——> cngcncnzocn3 (c) ——’—> *- || 280 Chapter] FURTHER REACTIONS OF HALOALKANES CH3 i Nitromethane (d) CH3-—(F3--CH2CHZCH2CH20H —-——> H3C O CH38020 H3O CH3 CHZI CH3 CstCHg I N N , CH 01-! (e) w (r) CH3CH2CHzBr L;_> CH3CH2CH2N3 I . | I ‘ CH I + (g) (CH3)3CCI —~—->N“ ’ “meme (CH3)3CI (h) (CH3CH2)20 ——>3 (CH3CH2)20CH3 +1- , CH 0H NaB , CH 0H first: (1) CH31 ._3_, CH30CH3 (j) (CH3CH2)3COCH3 A (CHgCH2}3CBr Liifil I CH3 CH3 I “i"- | NaOCH2CH3,CH3CH20H l I if. Eli-“9i (k) CH3CHCH2CH2C1 —~—~—; CH3CHCH=CH2 I as =1 H . NaOCH2CH3, CH3CH10H ”y M (I) CH3CH2CH2CH2C1 “a CH3CH2CH=CH2 l - ' ||| ( .ropose syntheses of the following molecules from the indicated starting materials. Make use of any other reagents or solvents that you need. In some cases, there may be no alternative but to employ a reaction that results in a mixture of products. If so, use | I reagents and conditions that will maximize the yield of the desired material (compare I Problem 47 in Chapter 6). E ' (a) CH3CH2CHICH3, from butane (b) CHBCHZCHZCHQI, from butane | ' (c) (CH3)3COCH3, from methane (d) Cyclohexene, from cyclohexane and 2-methylpropane S _ | (e) Cyclohexanol, from cyclohexane (f) CS3, from 1,3~d1bromopropane . ' CH3 45. (“NEW [(l-Bromo-l-methyl)ethy1]benzene, shown in the margin, undergoes I (ii—Br solvolysis in a unimolecular, strictly first-order process. The reaction rate for [RBI] = 0.1 M RBr in 9: 1 acetone : water is measured to be 2 X 10—4 mol L”ls—1. (:1) Calculate; H CH3 the rate constant k from these data. What is the product of this reaction? (b) In the pres-'1 ence of 0.1 M LiCl, the rate is found to increase to 4 X 10—4 mol L—ls'l, although the II reaction still remains strictly first order. Calculate the new rate constant kLiCI and suggests an explanation. (c) When 0.1 M LiBr is present instead of LiCl, the measured rate draw: to 1.6 X 10'4 mo] L'lsfil. Explain this observation, and write the appropriate chemical equations to describe the reactions. I 46. n this chapter we have encountered many examples of SNl solvolysis reactions, all 01‘ fl which proceed according to the following scheme: RBI :éiH2 l=klRX 2:k2R+N: ‘ R1} —e——>m { 1 x- + R+ ———>ram [ ""1 RfiéHg Loss of a proton gives the final product. Although there is considerable evidence for flf-f I intermediacy of a carbocation, it is not directly observed normally because its combinilt I tion with a nucleophile is so rapid. Recently, examples of SN} solvolyses have been found that give rise to very unusual observations. One example is (EHQCFE. $1 (I) I' "What: F1 3 _ i ..:__. CH30 c OCH3 34% CH3O c 0cm 1' Marnie H H l Within I.‘ mixing the colorless substrate and solvent, a reddish-orange color is observed hitch“- signaling the formation of an intermediate carbocation. This color fades . period Of about a minute, and analysis of the solution reveals the presence of the I‘m“ _ppodufli in 100% yield. (3) There are two reasons for the build-up of a detectable Irwntm‘infi of carbocation in this case. One is that the carbocation derived from Witttifiil of this particular substrate is unusually stable (for reasons we will explore CHzCHfl' l 'l] ELM-Ill" 22). The other is that the solvent (2,2,2—trifluoroethanol) is an unusually '-..i‘ Rhapmle, even compared with ordinary alcohols such as ethanol. Suggest an lhn for the poor nucleophilicity of the solvent. (b) What can you say about the 3H3 +1"? 'firoct'fi? (d) Write the complete mechanism for the reaction above. ‘ h mil-teach of the following transformations to the correct reaction profile shown here, Iiitltuw the structures of the species present at all points on the energy curves marked :fifigapiml letters. Reaction coordinate %—> Reaction coordinate ——> (ii i} (iv) i rate-I'd? . : chemié‘tm E ".I 3118, all Eo‘fl'fi Reaction coordinate —eh—) Reaction coordinate ——> ‘12 . __' with} (GHQSCCI + (cfinsnr —_—> (b) (CH3)2CHI + KBr —--> nce forifil - iii (CH3)3CBr + HOCHZCI-Ia ——> (d) CH3CH2Br + NaOCHZCHg ——> : combi . b mutate the structure of the most likely product of the following reaction of 4-chloro- e een '-"d"'m'$th5'1~1-pentanol in neutral polar solution. C] | (CH3)2CCH2CH2CH20H _—> HCl + cfiuuo _ ;_.l“_ Stmngly basic solution, the starting material again converts into a molecule with I'hfl molecular formula C6H120, but with a completely different structure. What is it? "Ell-”plain the difference between the two results. Problems 281 282 CllapiEI] FURTHER REACTIONS 0F HALOAlKAllES \ IN/ Pyridine 49. The following reaction can proceed through both E1 and E2 mechanisms. CH3 CH3 1 NaOCHg, CH30H | C5H5CH2C|C1 _“_""'“"""—)‘ C6H5CH=C(CH3)2 + C5H5CH2C=CH2 CH3 The El rate constant k5] = 1.4 X 10’4 s‘1 and the E2 rate constant kEz = 1.9 X 10'4 L mol—1 s_1; 0.02 M haloalkane. (a) What is the predominant elimination mechanism " -’\ with 0.5 M NaOCH3? (b) What is the predominant elimination mechanism with 2.0 M L NaOCHg? (c) At what concentration of base does exactly 50% of the starting material I '- react by an El route and 50% by an E2 pathway? I 50. The compound below is an example of a methyl ester. Methyl esters react with lithium __ — iodide to give lithium carboxylate salts. The solvent in this example is pyridine (margin).-_ II I I Him ' 'fiits'n': 0% /0CH3 0% /0‘Li+ penal i C C Li‘I' fl + CH3I pyridine Suggest several experiments that would allow you to determine the likely mechanism of ‘ this process. 51. Ethers containing the 1,1-dimethylethyl (tert~butyl) group are readily _ _ _ cleaved with dilute, strong acid, as shown in the example below. jfi-‘fiflfl mastitis CH3 H3c meantime l CF3COZH, H20 \ OmCl—CHg, —-> OH + /C=CH2 all I Suggest a plausible mechanism for this process. What role might the strong acid play? . | ._,_a H . 52. Give the mechanism and major product for the reaction of a secondary haloalkane in a " polar aprotic solvent with the following nucleophiles. The pKa value of the conjugate $131., acid of the nucleophile is given in parentheses. ' '- (a) N3’ (4.6) (b) H2N_ (35) (c) NH3 (9.5) (d) HSe‘ (3.7) (e) F‘ (3.2) (f) C6H5O’ (9.9) I (g) PH3 (—12) (h) NHZOH (6.0) (i) NCS‘ (—0.7) II ”E, I __.,,_.I J 53. Cortisone is an important steroidal anti—inflammatory agent. Cortisone can be synthe- H sized efficiently from the alkene shown here. /O—CH2 . “a V CH2 5‘ tit-TEE: \ CH20H JEFF ._ ._ OmC—O ‘ . \ - . CH3 H0" Alkene Cortisone Of the following three chlorinated compounds, two give reasonable yields of the alkcne shown above by E2 elimination with base, but one does not. Which one (1063 (“id-I" work well, and why? What does it give during attempted E2 elimination? (Hint: C011; sider the geometry of each system.) 283 ing matefitjti H0" A B c I ' if The chemistry of derivatives of tmns-decalin is of interest because this Stem is part of the structure of steroids. Make models of the brominatEd systems flbfllia ii] to help you answer the following questions. CH3 CH3 I ; mechanism, I II: Br CH3 i (EH3 i Br I .. n readily fit}.¢|n: of the molecules undergoes E2 reaction with NaQCH2CH3 in CH3CH20H consid- '*‘"_,}.r fits‘ter than does the other. Which molecule is which? Explain. (b) The following I Iterated analogs of systems i and ii react with base to give the products shown. \ . - C=C / Hg- CH3 CH3 I NaOCH2CHa. CH3CH20H (All D retained) ._' l- : ._ ———) - acid plant's 'fi—i‘ F D : talkanei- . -H Br CH3 CH3 conjugitfttt' i'diflmmfl CH3 CH3 NaOCHzCHa. CHsCH20H (A11 D lost) ___) )6 sy‘nthfl- l éHa CEH3 Br ii-deuterated ;.'.3_hccify whether anti or syn eliminations have taken place. Draw the conformations that Itfimolaeulw must adopt for elimination to occur. Does your answer to (b) help you in {waiting (a)? Problem fitidcr the general substitution-elimination reactions of the bromoalkanes. N lb R—Br fl RHNn + alkene HM do the reaction mechanisms and product formation differ when the structure of the _ {Whats and reaction conditions change? To begin to unravel the nuances of bimolecu- TIE-find unimolecular substitution and elimination reactions, focus on the treatment of _ _'Il"ifllfimlkanes A through D under conditions (a) through (e). Divide the problem evenly .‘ilfiifing'yourselves so that each of you tackles the questions of reaction mechanism(s) fiflflflflaliltttivfl distribution of product(s), if any. Reconvene to discuss your conclusions 'J‘i'd come to a consensus. When you are explaining a reaction mechanism to the rest of is of the _ one does-3:1";t Flint: Cori—i | | _._‘__ 284 Chaptell FURTHER REACTIONS OF HALGMKAHES the team, use curved arrows to show the flow of electrons. Label the stereochemistry of starting materials and products as R or S, as appropriate. Ar N >5 ER' 3 mime“ Strait? MB A 1' C D 0 0 H H (a) NaNg, DMF (b) LDA,DMF (c) NaOH,DMF (d) CH3CO‘Na+,CH3COH (e) (111303: Preprofessional Problems 56. Which of the following haloalkanes will undergo hydrolysis most rapidly? - _ (a) (CH3)3CF (b) (CH3)3CC1 (C) (CH3)3CBI (d) (CH3)3CI 5'7. The reaction H3C CH30' \ (CH3)3CC1 —~—> /C=CH2 H3C is an example of which of the following processes? (a) E1 (1)) E2 (c) SNl (d) SN2 58. In this transformation, H10, acetone A —~——) CH3CH2CI(CH3)2 0H what is the best structure for A? r13 (a) BrCHZCHZCH(CH3)2 (b) CH3CH2CBr CH3 ‘EH3 (c) CH3CH2(F3H (d) CHgCIHCH(CH3)2 CHzBr Br 59. Which of the following isomeric carbocations is the most stable? ' CH; + CH: (a) 0/ (b) gen3 CH3 (c) + (d) 0/ + 60. Which reaction intermediate is involved in the following reaction? B h 2-methy1butane —1—v-> 2-bromo~3-1nethylbutane (not the major product) (a) A secondary radical (b) A tertiary radical (c) A secondary carbocation (d) A tertiary carbocation ...
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