Vollhardt  Capítulo 7 (Haloalcanos 2)
36 pág.

Vollhardt Capítulo 7 (Haloalcanos 2)


DisciplinaQuímica Orgânica II1.861 materiais36.788 seguidores
Pré-visualização12 páginas
Which of these transformations 
should proceed faster in a polar, aprotic solvent (such as acetone or DMSO) than in a polar, protic 
solvent (such as water or CH3OH)? Explain your answer on the basis of the mechanism that you 
expect to be operating in each case.
(a) CH3CH2CH2Br 1 Na
1 2CN uy (b) (CH3)2CHCH2I 1 Na
1 N3
2 uy
(c) (CH3)3CBr 1 HSCH2CH3 uy (d) (CH3)2CHOSO2CH3 1 HOCH(CH3)2 uy
34. Propose a synthesis of (R)-CH3CHN3CH2CH3, starting from (R)-2-chlorobutane.
35. Two substitution reactions of (S)-2-bromobutane are shown here. Show their stereochemical 
outcomes.
(S)-CH3CH2CHBrCH3
B
HCOH
O
(S)-CH3CH2CHBrCH3
B
HCO\ufffdNa\ufffd, DMSO
O
36. Propose a stereocontrolled synthesis of cis-1-acetoxy-3-methylcyclopentane (margin), starting 
from trans-1-chloro-3-methylcyclopentane.
37. The two seemingly similar reactions shown below differ in their outcomes.
 NaOH, CH3CH2OH
CH3CH2CH2CH2Br uuuuuuy CH3CH2CH2CH2OH
 NaSH, CH3CH2OH
CH3CH2CH2CH2Br uuuuuuy CH3CH2CH2CH2SH
 The fi rst proceeds in high yield. The yield of the product in the second, however, is diminished 
by the formation of (CH3CH2CH2CH2)2S in substantial quantities. Discuss the formation of this 
by-product mechanistically, and explain why it occurs in the second case but not in the fi rst.
38. Write all possible E1 products of each reaction in Problem 25.
39. Formulate the complete step-by-step mechanisms for all the E1 processes that you identifi ed in 
Problem 38.
CH3
B
%
(
OCCH3
O
cis-1-Acetoxy-
3-methylcyclopentane
P r o b l e m s
280 C h a p t e r 7 F u r t h e r R e a c t i o n s o f H a l o a l k a n e s
40. Write the products of the following elimination reactions. Specify the predominant mechanism 
(E1 or E2) and formulate it in detail.
(a) 
KOC(CH3)3,
(CH3)3COH
CH2Cl
H
 (b) 
A
KBr,
acetone
F
CH3CHCH2CH3
(c) Br
CH2CH3
H3C
H
H2O
 (d) NaNH2, liquid NH3
I
 NaNH2, NH3 KOC(CH3)3, (CH3)3 COH
(a) (CH3CH2)3CBr uvuuuy (b) CH3CH2CH2CH2Cl uuuuuuuvuy
(c) 
A
CH
Br
Excess KOH, CH3CH2OH
 (d) 
NaOCH3, CH3OH
Cl
CH3
41. From the list of reagents (a) \u2013 (f ) below, choose all those that are most likely to give primarily 
(i) SN2 reaction with primary RX; (ii) E2 reaction with primary RX; (iii) SN2 reaction with sec-
ondary RX; (iv) E2 reaction with secondary RX.
(a) NaSCH3 in CH3OH (b) (CH3)2CHOLi in (CH3)2CHOH
(c) NaNH2 in liquid NH3 (d) KCN in DMSO
(e) 
A
N
H
A
N
in
Li
 (f ) 
B
CH3CH2CH2CONa in DMF
O
42. Predict the major product(s) that should form from reaction between 1-bromobutane and each 
of the following substances. By which reaction mechanism is each formed \u2014 SN1, SN2, E1, or 
E2? If it appears that a reaction will either not take place or be exceedingly slow, write \u201cno 
reaction.\u201d Assume that each reagent is present in large excess. The solvent for each reaction 
is given.
(a) KCl in DMF (b) KI in DMF (c) KCl in CH3NO2
(d) NH3 in CH3CH2OH (e) NaOCH2CH3 in CH3CH2OH (f ) CH3CH2OH
(g) KOC(CH3)3 in (CH3)3COH (h) (CH3)3P in CH3OH (i) CH3CO2H
43. Predict the major product(s) and mechanism(s) for reaction between 2-bromobutane (sec-butyl 
bromide) and each of the reagents in Problem 42.
44. Predict the major product(s) and mechanism(s) for reaction between 2-bromo-2-methylpropane 
(tert-butyl bromide) and each of the reagents in Problem 42.
45. Three reactions of 2-chloro-2-methylpropane are shown here. (a) Write the major product of each 
transformation. (b) Compare the rates of the three reactions. Assume identical solution polarities 
and reactant concentrations. Explain mechanistically.
 (CH3)3CCl
H2S, CH3OH
 
B
(CH3)3CCl
CH3CO\ufffdK\ufffd, CH3OH
O
 (CH3)3CCl
CH3O\ufffdK\ufffd, CH3OH
46. Give the major product(s) of the following reactions. Indicate which of the following mechanism(s) 
is in operation: SN1, SN2, E1, or E2. If no reaction takes place, write \u201cno reaction.\u201d
 C h a p t e r 7 281
(e) 
NaOCH2CH3, CH3CH2OH(CH3)2CHCH2CH2CH2Br (f) 
C
Br
CH2CH3
CH2CH2CH3H3C
A
NaI, nitromethane}& H
(g) 
KOH,
CH3CH2OHOH
H 
(h) CH2CH2CH2BrCl
Excess
KCN,
CH3OH
(i) 
(R)-CH3CH2CHCH3
CH3OSO2
NaSH, CH3CH2OH
A
 
( j) CH3OH
CH2CH3I
(k) 
A
Br
KOH, CH3CH2OH(CH3)3CCHCH3 (l) 
B
CH3CH2Cl
CH3COH
O
P r o b l e m s
47. Fill in the blanks in the following table with the major product(s) of the reaction of each haloalkane 
with the reagents shown.
48. Indicate the major mechanism(s) (simply specify SN2, SN1, E2, or E1) required for the formation 
of each product that you wrote in Problem 47.
49. For each of the following reactions, indicate whether the reaction would work well, poorly, or 
not at all. Formulate alternative products, if appropriate.
 Reagent
Haloalkane H2O NaSeCH3 NaOCH3 KOC(CH3)3
CH3Cl 
CH3CH2CH2Cl 
(CH3)2CHCl 
(CH3)3CCl 
(a) CH3CH2CHCH3
Br
A
CH3CH2CHCH3
OH
A
NaOH, acetone
(b) CH3CHCH2Cl
H3C
A
CH3CHCH2OCH3
ACH3OH
H3C
 (c) 
HCN, CH3OH
H Cl H CN
(d) H3C
CH3
CH3 CH2CH2CH2CH2OH
CH3SO2O
C
O
A
A
O O
Nitromethane
H3C
(e) 
H3C H3CCH2I
NaSCH3, CH3OH
CH2SCH3
 (f ) CH3CH2CH2Br uuuuuy CH3CH2CH2N3
(g) (CH3)3CCl uuuuuuy (CH3)3CI (h) (CH3CH2)2O uuuy (CH3CH2)2O
1
CH3 1 I
2
(i) CH3I uuuy CH3OCH3 ( j) (CH3CH2)3COCH3 uuuuuy (CH3CH2)3CBr
NaN3, CH3OHNaN3, CH3OH
NaI, nitromethaneNaI, nitromethane CH3ICH3I
CH3OHCH3OH NaBr, CH3OHNaBr, CH3OH
282 C h a p t e r 7 F u r t h e r R e a c t i o n s o f H a l o a l k a n e s
(k) CH3CHCH2CH2Cl CH3CHCH CH2
CH3
A
CH3
A
P
NaOCH2CH3, CH3CH2OH
(l) CH3CH2CH2CH2Cl CH3CH2CH CH2P
NaOCH2CH3, CH3CH2OH
50. Propose 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 reagents and conditions that 
will maximize the yield of the desired material (compare Problem 53 in Chapter 6).
(a) CH3CH2CHICH3, from butane (b) CH3CH2CH2CH2I, from butane
(c) (CH3)3COCH3, from methane (d) Cyclohexene, from cyclohexane
and 2-methylpropane
(e) Cyclohexanol, from cyclohexane (f) 
S
S
, from 1,3-dibromopropane
51. [(1-Bromo-1-methyl)ethyl]benzene, shown in the margin, undergoes solvolysis in 
a unimolecular, strictly fi rst-order process. The reaction rate for [RBr] 5 0.1 M RBr in 9:1 
acetone : water is measured to be 2 3 1024 mol L21s21. (a) Calculate the rate constant k from 
these data. What is the product of this reaction? (b) In the presence of 0.1 M LiCl, the rate is 
found to increase to 4 3 1024 mol L21s21, although the reaction still remains strictly fi rst order. 
Calculate the new rate constant kLiCl and suggest an explanation. (c) When 0.1 M LiBr is pres-
ent instead of LiCl, the measured rate drops to 1.6 3 1024 mol L21s21. Explain this observation, 
and write the appropriate chemical equations to describe the reactions.
52. In this chapter we have encountered many examples of SN1 solvolysis reactions, all of which 
proceed according to the following scheme:
Rate1 \ufffd k1[RX] Rate2 \ufffd k2[R\ufffd][Nu ]R X X\ufffd R\ufffd R OH2\ufffd
\ufffd
\ufffd
\ufffd
OH2ð\u161
 Loss of a proton gives the fi nal product. Although there is considerable evidence for the inter-
mediacy of a carbocation, it is not directly observed normally because its combination with a 
nucleophile is so rapid. Recently, examples of SN1 solvolyses have been found that give rise to 
very unusual observations. One example is
C
H
CH3O OCH3
Cl
CF3CH2OHA C
H
CH3O
CH2CF3
OCH3
O
A
A
 Upon mixing the colorless substrate and solvent, a reddish-orange color is observed immediately, 
signaling the formation of an intermediate carbocation. This color fades over a period of about 
a minute, and analysis of the solution reveals the presence of the fi nal product in 100% yield. 
(a) There are two reasons for the buildup of a detectable concentration of carbocation in this 
case. One is that the carbocation derived from dissociation of this particular substrate is unusu-
ally stable (for reasons we will explore in Chapter 22). The other is that the solvent (2,2,2-
trifl uoroethanol) is