Vollhardt Capítulo 6 (Haloalcanos)
DisciplinaQuímica Orgânica II2.041 materiais • 37.388 seguidores
4/18/09 7:54:10 AM user-s172 /Users/user-s172/Desktop/Tempwork/Don'tDelete_Job/FREE036:Vollhardt/FREE036-/Users/user-s172/Desktop/Tempwork/Don'tDelete_Job/FREE036:V 246 C h a p t e r 6 P r o p e r t i e s a n d R e a c t i o n s o f H a l o a l k a n e s 4. The kinetics of the reaction of nucleophiles with primary (and most secondary) haloalkanes are second order, indicative of a bimolecular mechanism. This process is called bimolecular nucleophilic substitution (SN2 reaction). It is a concerted reaction, one in which bonds are simultaneously broken and formed. Curved arrows are typically used to depict the fl ow of elec- trons as the reaction proceeds. 5. The SN2 reaction is stereospecifi c and proceeds by backside displacement, thereby producing inversion of confi guration at the reacting center. 6. An orbital description of the SN2 transition state includes an sp 2-hybridized carbon center, partial bond making between the nucleophile and the electrophilic carbon, and simultaneous partial bond breaking between that carbon and the leaving group. Both the nucleophile and the leaving group bear partial charges. 7. Leaving-group ability, a measure of the ease of displacement, is roughly proportional to the strength of the conjugate acid. Especially good leaving groups are weak bases such as chloride, bromide, iodide, and the sulfonates. 8. Nucleophilicity increases (a) with negative charge, (b) for elements farther to the left and down the periodic table, and (c) in polar aprotic solvents. 9. Polar aprotic solvents accelerate SN2 reactions because the nucleophiles are well separated from their counterions but are not tightly solvated. 10. Branching at the reacting carbon or at the carbon next to it in the substrate leads to steric hin- drance in the SN2 transition state and decreases the rate of bimolecular substitution. Problems 31. Name the following molecules according to the IUPAC system. (a) CH3CH2Cl (b) BrCH2CH2Br (c) CH3CH2CHCH2F CH2CH3 (d) (CH3)3CCH2I (e) CCl3 (f) CHBr3 32. Draw structures for each of the following molecules: (a) 3-ethyl-2-iodopentane; (b) 3-bromo-1,1-dichlorobutane; (c) cis-1-(bromomethyl)-2-(2-chloroethyl)cyclobutane; (d) (trichloromethyl)cyclopropane; (e) 1,2,3-trichloro-2-methylpropane. 33. Draw and name all possible structural isomers having the formula C3H6BrCl. 34. Draw and name all structurally isomeric compounds having the formula C5H11Br. 35. For each structural isomer in Problems 33 and 34, identify all stereocenters and give the total number of stereoisomers that can exist for the structure. 36. For each reaction in Table 6-3, identify the nucleophile, its nucleophilic atom (draw its Lewis structure fi rst), the electrophilic atom in the substrate, and the leaving group. 37. A second Lewis structure can be drawn for one of the nucleophiles in Problem 36. (a) Identify it and draw its alternate structure (which is simply a second resonance form). (b) Is there a second nucleophilic atom in the nucleophile? If so, rewrite the reaction of Problem 36, using the new nucleophilic atom, and write a correct Lewis structure for the product. 38. For each reaction shown here, identify the nucleophile, its nucleophilic atom, the electrophilic atom in the substrate molecule, and the leaving group. Write the organic product of the reaction. (a) CH3I 1 NaNH2 S (b) Br \ufffd NaSH (c) O S NaI\ufffd CF3 O O (d) H Cl \ufffd NaN3 ¥& (e) N\ufffdCH3Cl CH3 (f ) KSeCN\ufffd I 3157T_ch06_215-250.indd Page 246 4/23/09 4:24:51 PM user-s1723157T_ch06_215-250.indd Page 246 4/23/09 4:24:51 PM user-s172 /Users/user-s172/Desktop/FREE036-06/Users/user-s172/Desktop C h a p t e r 6 247 39. For each reaction presented in Problem 38, write out the mechanism using the curved-arrow notation. 40. A solution containing 0.1 M CH3Cl and 0.1 M KSCN in DMF reacts to give CH3SCN and KCl with an initial rate of 2 3 1028 mol L21 s21. (a) What is the rate constant for this reaction? (b) Calculate the initial reaction rate for each of the following sets of reactant concentrations: (i) [CH3Cl] 5 0.2 M, [KSCN] 5 0.1 M; (ii) [CH3Cl] 5 0.2 M, [KSCN] 5 0.3 M; (iii) [CH3Cl] 5 0.4 M, [KSCN] 5 0.4 M. 41. Write the product of each of the following bimolecular substitutions. The solvent is indicated above the reaction arrow. P r o b l e m s (a) AcetoneNa\ufffdI\ufffd\ufffdCH3CH2CH2Br (b) DMSONa\ufffd\ufffdCN\ufffd(CH3)2CHCH2I (c) (CH3)2CHOHNa\ufffd\ufffdOCH(CH3)2\ufffdCH3I (d) CH3OHNa\ufffd\ufffdSCH2CH3\ufffdCH3CH2Br (e) Acetone \ufffdCH2Cl CH3CH2SeCH2CH3 (f ) (CH3CH2)2ON(CH3)3\ufffd(CH3)2CHOSO2CH3 42. Determine the RyS designations for both starting materials and products in the following SN2 reactions. Which of the products are optically active? (a) H Cl Br\ufffd\ufffdCH3 CH2CH3 (b) [H3C Cl H H Br CH3\ufffd2 I\ufffd[ C C (c) [ B 0 \ufffdOCCH3 O HO Cl \ufffd (d) \u160 B 0 \ufffdOCCH3 O HO Cl \ufffd 43. For each reaction presented in Problems 41 and 42, write out the mechanism using curved-arrow notation. 44. List the product(s) of the reaction of 1-bromopropane with each of the following reagents. Write \u201cno reaction\u201d where appropriate. (Hint: Carefully evaluate the nucleophilic potential of each reagent.) (a) H2O (b) H2SO4 (c) KOH (d) CsI (e) NaCN (f ) HCl (g) (CH3)2S (h) NH3 (i ) Cl2 ( j) KF 45. Formulate the potential product of each of the following reactions. As you did in Problem 44, write \u201cno reaction\u201d where appropriate. (Hint: Identify the expected leaving group in each of the substrates and evaluate its ability to undergo displacement.) (a) K\ufffd\ufffdOHCH3CH2CH2CH2Br CH3CH2OH \ufffd (b) K\ufffdCl\ufffdCH3CH2I DMF \ufffd (c) Li\ufffd\ufffdOCH2CH3CH2Cl CH3CH2OH \ufffd (d) Cs\ufffdI\ufffd(CH3)2CHCH2Br CH3OH\ufffd (e) K \ufffd\ufffdSCNCH3CH2CH2Cl CH3CH2OH \ufffd (f ) Li \ufffdCl\ufffdCH3CH2F CH3OH \ufffd (g) K \ufffdI\ufffdCH3CH2CH2OH DMSO \ufffd (h) Na \ufffd\ufffdSCH3CH3I CH3OH \ufffd (i) Na \ufffd\ufffdOHCH3CH2OCH2CH3 H2O \ufffd ( j) K \ufffd\ufffdOCCH3 O CH3CH2I DMSO \ufffd B 46. Show how each of the following transformations might be achieved. (a) A (R)-CH3CHCH2CH3 OSO2CH3 A N3 (S)-CH3CHCH2CH3 (b) H Br H CH3 CH3O CH3 H CN H CH3 CH3O CH3 (c) ! 0 % `Br H H 0 % SCH3 H H (d) A N CH3 N \ufffd N CH3H3C 3157T_ch06_215-250.indd Page 247 4/18/09 7:54:36 AM user-s1723157T_ch06_215-250.indd Page 247 4/18/09 7:54:36 AM user-s172 /Users/user-s172/Desktop/Tempwork/Don'tDelete_Job/FREE036:Vollhardt/FREE036-/Users/user-s172/Desktop/Tempwork/Don'tDelete_Job/FREE036:V 248 C h a p t e r 6 P r o p e r t i e s a n d R e a c t i o n s o f H a l o a l k a n e s 47. Rank the members of each of the following groups of species in the order of basicity, nucleophi- licity, and leaving-group ability. Briefl y explain your answers. (a) H2O, HO 2, CH3CO2 2; (b) Br2, Cl2, F2, I2; (c) 2NH2, NH3, 2PH2; (d) 2OCN, 2SCN; (e) F2, HO2, 2SCH3; (f) H2O, H2S, NH3. 48. Write the product(s) of each of the following reactions. Write \u201cno reaction\u201d as your answer, if appropriate. (a) CH3OHNa\ufffdCl\ufffdCH3CH2CH2CH3\ufffd (b) CH3OHNa\ufffd\ufffdOCH3CH3CH2Cl\ufffd (c) AcetoneNa\ufffdI\ufffd\ufffd HH3C H3C Br H H (d) ? A Cl CH CH3CH3CH2 G} AcetoneNa\ufffd\ufffdSCH3\ufffd (e) A OH CH3CHCH3 Na\ufffd\ufffdCN\ufffd (f ) CH3CH2OH A CH3CHCH3 OSO2CH3 HCN\ufffd (g) A OSO2CH3 CH3CHCH3 Na\ufffd\ufffdCN\ufffd CH3CH2OH (h) B B G D SOCH2CH2CHH3C K\ufffd\ufffdSCN O O \ufffd CH3OH CH3 CH3 (i) Na \ufffdBr\ufffdCH3CH2NH2 DMSO \ufffd (j) Na \ufffd\ufffdNH2CH3I NH3 \ufffd 49. For each reaction presented in Problem 48 that actually proceeds to a product, write out the mechanism using the curved-arrow notation. 50. The substance 1-butyl-3-methylimidazolium (BMIM) hexafl uorophosphate (margin) is a liquid at room temperature, even though it is a salt composed of positive and negative ions. BMIM and other ionic liquids constitute a new class of solvents for organic reactions, because they are capable of dissolving both organic and inorganic substances. More important, they are relatively benign environmentally, or \u201cgreen,\u201d because they can be easily separated from reaction products and reused virtually indefi nitely. Therefore they do not constitute a waste-disposal problem, unlike conventional solvents.