Vollhardt Capítulo 8 (Álcoois)
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. ethanol . 1-propanol. 28. 1,2-Ethanediol exists to a much greater extent in the gauche conformation than does 1,2- dichloroethane. Explain. Would you expect the gauche: anti conformational ratio of 2-chloroethanol to be similar to that of 1,2-dichloroethane or more like that of 1,2-ethanediol? 29. The most stable conformation of trans-1,2-cyclohexanediol is the chair in which both hydroxy groups are equatorial. (a) Draw the structure or, better yet, make a model of the compound in this conformation. (b) Reaction of this diol with the chlorosilane R3SiCl, R 5 (CH3)2CH (iso- propyl), gives the corresponding disilyl ether shown in the margin. Remarkably, this transforma- tion causes the chair to fl ip, giving a conformation where both silyl ether groups are in axial positions. Explain this observation by means of either structural drawings or models. OSiR3] ~OSiR3 C h a p t e r 8 327 30. Rank the compounds in each group in order of decreasing acidity. (a) CH3CHClCH2OH, CH3CHBrCH2OH, BrCH2CH2CH2OH (b) CH3CCl2CH2OH, CCl3CH2OH, (CH3)2CClCH2OH (c) (CH3)2CHOH, (CF3)2CHOH, (CCl3)2CHOH 31. Write an appropriate equation to show how each of the following alcohols acts as, fi rst, a base, and, second, an acid in solution. How do the base and acid strengths of each compare with those of methanol? (a) (CH3)2CHOH; (b) CH3CHFCH2OH; (c) CCl3CH2OH. 32. Given the pKa values of 22.2 for CH3O 1 H2 and 15.5 for CH3OH, calculate the pH at which (a) methanol will contain exactly equal amounts of CH3O 1 H2 and CH3O 2; (b) 50% CH3OH and 50% CH3O 1 H2 will be present; (c) 50% CH3OH and 50% CH3O 2 will be present. 33. Do you expect hyperconjugation to be important in the stabilization of alkyloxonium ions (e.g., R 1 OH2, R2 1 OH)? Explain your answer. 34. Evaluate each of the following possible alcohol syntheses as being good (the desired alcohol is the major or only product), not so good (the desired alcohol is a minor product), or worthless. (Hint: Refer to Section 7-9 if necessary.) (a) CH3CH2CH2CH2Cl CH3CH2CH2CH2OH H2O, CH3CCH3 O B (b) CH3OSO2 CH3 CH3OH HO\ufffd, H2O, \u394 (c) I HO\ufffd, H2O, \u394 OH (d) CH3CHCH2CH2CH3 I CH3CHCH2CH2CH3 H2O, \u394A OH A (e) CH3CHCH3 CH3CHCH3 CN A OH AHO\ufffd, H2O, \u394 (f ) CH3OCH3 CH3OH HO\ufffd, H2O, \u394 (g) H3C Br H2O H3C OH (h) CH3CHCH2Cl CH3 A CH3CHCH2OH CH3 AHO\ufffd, H2O, \u394 35. For every process in Problem 34 that gives the designated product in poor yield, suggest a supe- rior method if possible. 36. Give the major product(s) of each of the following reactions. Aqueous work-up steps (when necessary) have been omitted. (a) CH3CH CHCH3 H3PO4, H2O, \u394 P (Hint: See Section 8-4.) (b) CH3CCH2CH2CCH3 O B O B 1. LiAlH4, (CH3CH2)2O 2. H\ufffd, H2O (c) C O NaBH4, CH3CH2OH B H (d) LiAlH4, (CH3CH2)2O Br (e) O NaBH4, CH3CH2OH BB ; ? (CH3)2CH CH3 (f) NaBH4, CH3CH2OH H O H \u2265 ´ 37. What is the direction of the following equilibrium? (Hint: The pKa for H2 is about 38.) H\ufffd HO\ufffdH2O H2\ufffd \ufffd P r o b l e m s 328 C h a p t e r 8 H y d r o x y F u n c t i o n a l G r o u p : A l c o h o l s 38. Formulate the product of each of the following reactions. The solvent in each case is (CH3CH2)2O. (a) CH3CH O 1. LiAlD4 2. H\ufffd, H2OB (b) CH3CH O 1. LiAlH4 2. D\ufffd, D2OB (c) CH3CH2I LiAlD4 39. Write out a mechanism for every reaction depicted in Problem 38. 40. Give the major product(s) of each of the following reactions [after work-up with aqueous acid in (d), (f ), and (h)]. (a) Mg, (CH3CH2)2OCH3(CH2)5CHCH3 Cl A (b) D2O Product of (a) (c) Br Li, (CH3CH2)2O (d) O Product of (c) BB (e) (CH3CH2)2OCH3CH2CH2Cl Mg\ufffd (f) Product of (e) CCH3 O \ufffd B (g) 2 Li Br \ufffd (CH3CH2)2O (h) CH3CCH2CH2CCH3 O 2 mol product of (g) \ufffd B O B 1 mol 41. The common practice of washing laboratory glassware with acetone can lead to unintended con- sequences. For example, a student plans to carry out the preparation of methylmagnesium iodide, CH3MgI, which he will add to benzaldehyde, C6H5CHO. What compound is he intending to synthesize after aqueous work-up? Using his freshly washed glassware, he carries out the proce- dure and fi nds that he has produced an unexpected tertiary alcohol as a product. What substance did he make? How did it form? 42. Which of the following halogenated compounds can be used successfully to prepare a Grignard reagent for alcohol synthesis by subsequent reaction with an aldehyde or ketone? Which ones cannot and why? (a) Br CH3H3C /\u2211 (b) Cl OHH /\u2211 (c) HI OCH3 /\u2211 (d) H Cl O / \u2211 (e) H Br H /\u2211 (Hint: See Problem 49 in Chapter 1.) 43. Give the major product(s) of each of the following reactions (after aqueous work-up). The solvent in each case is ethoxyethane (diethyl ether). (a) MgBr HCH O \ufffd B (b) CH3CHCH2MgCl CH3 CH3CH O \ufffd BA (c) C6H5CH2Li C6H5CH O \ufffd B (d) CH3CHCH3 MgBr \ufffd A O BB (e) H CH3CH2CHCH2CH3 C O H MgCl \ufffd A EN 44. For each reaction presented in Problem 43, write out the complete, step-by-step mechanism using curved-arrow notation. Include the aqueous acid work-up. C h a p t e r 8 329 45. Write the structures of the products of reaction of ethylmagnesium bromide, CH3CH2MgBr, with each of the following carbonyl compounds. Identify any reaction that gives more than one stereoisomeric product, and indicate whether you would expect the products to form in identical or in differing amounts. P r o b l e m s (a) O (b) H O (c) H O (d) O (e) O (f) O (g) O (h) O CH3 H / \u2211 (i) O CH3 CH3 / \u2211 ( j) OH CH3 H3C H / \u2211 /\u2211 46. Give the expected major product of each of the following reactions. PCC is the abbreviation for pyridinium chlorochromate (Section 8-6). (a) CH3CH2CH2OH Na2Cr2O7, H2SO4, H2O (b) (CH3)2CHCH2OH PCC, CH2Cl2 (c) CH2OH H Na2Cr2O7, H2SO4, H2O (d) PCC, CH2Cl2 CH2OH H (e) PCC, CH2Cl2 OH H 47. Write out a mechanism for every reaction depicted in Problem 46. 48. Give the expected major product of each of the following reaction sequences. PCC refers to pyridinium chlorochromate. (a) (CH3)2CHOH 1. CrO3, H2SO4, H2O 2. CH3CH2MgBr, (CH3CH2)2O 3. H\ufffd, H2O (b) CH3CH2CH2CH2Cl 1. \ufffdOH, H2O 2. PCC, CH2Cl2 4. H\ufffd, H2O 3. Li, (CH3CH2)2O (c) Product of (b) 1. CrO3, H2SO4, H2O 2. LiAlD4, (CH3CH2)2O 3. H\ufffd, H2O 49. Unlike Grignard and organolithium reagents, organometallic compounds of the most electropositive metals (Na, K, etc.) react rapidly with haloalkanes. As a result, attempts to convert RX into RNa or RK by reaction with the corresponding metal lead to alkanes by a reaction called Wurtz coupling. 2 RX 1 2 Na uy ROR 1 2 NaX which is the result of ROX 1 2 Na uy RONa 1 NaX followed rapidly by RONa 1 ROX uy ROR 1 NaX When it was still in use, the Wurtz coupling reaction was employed mainly for the preparation of alkanes by the coupling of two identical alkyl groups (e.g., equation 1 below). Suggest a 330 C h a p t e r 8 H y d r o x y F u n c t i o n a l G r o u p : A l c o h o l s reason why Wurtz coupling might not be a useful method for coupling two different alkyl groups (equation 2). 2 CH3CH2CH2Cl 1 2 Na uy CH3CH2CH2CH2CH2CH3 1 2 NaCl (1) CH3CH2Cl 1 CH3CH2CH2Cl 1 2 Na uy CH3CH2CH2CH2CH3 1 2 NaCl (2) 50. The reaction of two equivalents of Mg with 1,4-dibromobutane produces compound A. The reaction of A with two equivalents of CH3CHO (acetaldehyde), followed by work-up with dilute aqueous acid, produces compound B, having the formula C8H18O2. What are the structures of A and B? 51. Suggest the best synthetic route to each of the following simple alcohols, using in each case a simple alkane as your initial starting molecule. What are some disadvantages of beginning syn- theses with alkanes? (a) Methanol (b) Ethanol (c) 1-Propanol (d) 2-Propanol (e) 1-Butanol (f) 2-Butanol (g) 2-Methyl-2-propanol 52. For each alcohol in Problem 51, suggest (if possible) a synthetic route that starts with, fi rst, an aldehyde and, second, a ketone.