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Vollhardt Capítulo 7 (Haloalcanos 2)

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an unusually poor nucleophile, even compared with ordinary alcohols such
as ethanol. Suggest an explanation for the poor nucleophilicity of the solvent. (b) What can
you say about the relative rates of the two steps (rate1 and rate2), and how do they compare
to those in the usual SN1 reaction mechanism? (c) How might increasing carbocation stability
and decreasing solvent nucleophilicity affect the relative magnitudes of rate1 and rate2 in an
SN1 process? (d) Write the complete mechanism for the reaction above.

53. Match each of the following transformations to the correct reaction profi le shown here, and
draw the structures of the species present at all points on the energy curves marked by capital
letters.

CH3

CH3

C BrRBr �
A

A
O

 C h a p t e r 7 283

A

B

E

Reaction coordinate

(i)

C

D

E

Reaction coordinate

(ii)

J

I

M

L N

K

E

Reaction coordinate

(iv)

E

F

G

H

E

Reaction coordinate

(iii)

(a) (CH3)3CCl 1 (C6H5)3P uy (b) (CH3)2CHI 1 KBr uy
(c) (CH3)3CBr 1 HOCH2CH3 uy (d) CH3CH2Br 1 NaOCH2CH3 uy

54. Formulate the structure of the most likely product of the following reaction of 4-chloro-4-methyl-
1-pentanol in neutral polar solution.

(CH3)2CCH2CH2CH2OH HCl C6H12O�
Cl
A

 In strongly basic solution, the starting material again converts into a molecule with the molecular
formula C6H12O, but with a completely different structure. What is it? Explain the difference
between the two results.

55. The following reaction can proceed through both E1 and E2 mechanisms.

C6H5CH2CCl C6H5CH C6H5CH2C CH2C(CH3)2 �
CH3

CH3

A
CH3
A

A
P P

NaOCH3, CH3OH

 The E1 rate constant kE1 5 1.4 3 10
24 s21 and the E2 rate constant kE2 5 1.9 3 10

24 L mol21 s21;
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 NaOCH3? (c) At what concentra-
tion of base does exactly 50% of the starting material react by an E1 route and 50% by an E2
pathway?

56. 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).

OCH3O
C

Li�I�
CH3I�

EN O
�Li�O

CEN

pyridine

 Suggest several experiments that would allow you to determine the likely mechanism of this
process.

P r o b l e m s

N
Pyridine

284 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

57. Ethers containing the 1,1-dimethylethyl (tert-butyl) group are readily cleaved with
dilute, strong acid, as shown in the example below.

A

A
O C

CF3CO2H, H2O
O O

CH3

CH3 OH �

CH3 H3C

H3C
CH2C

G

G
P

 Suggest a plausible mechanism for this process. What role might the strong acid play?

58. 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 acid of the
nucleophile is given in parentheses.

(a) N3
2 (4.6) (b) H2N

2 (35) (c) NH3 (9.5)
(d) HSe2 (3.7) (e) F2 (3.2) (f) C6H5O

2 (9.9)
(g) PH3 (212) (h) NH2OH (6.0) (i) NCS

2 (20.7)

59. Cortisone is an important steroidal anti-infl ammatory agent. Cortisone can be synthesized effi -
ciently from the alkene shown here.

Alkene Cortisone

H3C

A P

K

N
H3C

≥
H

O

O

O
OHC

%

%

HOCH2
/∑

H3C

HO}

%

H3C
% ∞ O

OO

O

 Of the following three chlorinated compounds, two give reasonable yields of the alkene
shown above by E2 elimination with base, but one does not. Which one does not work well,
and why? What does it give during attempted E2 elimination? (Hint: Consider the geometry
of each system.)

A CB

H3C

HO}

%

%

≥
Cl

H3C ∞

O

OO

O

[
H3C

HO}

%

%

≥
H

Cl H3C ∞

O

OO

O

≈
H3C

HO}

%

%

≥
H

Cl
H3C ∞

O

OO

O

60. The chemistry of derivatives of trans-decalin is of interest because this ring system
is part of the structure of steroids. Make models of the brominated systems (i and ii) to help you
answer the following questions.

CH3

CH3
Br
0

%

≥

i

CH3

CH3Br

%

≥
≥

ii

(a) One of the molecules undergoes E2 reaction with NaOCH2CH3 in CH3CH2OH considerably
faster than does the other. Which molecule is which? Explain. (b) The following deuterated
analogs of systems i and ii react with base to give the products shown.

 C h a p t e r 7 285P r o b l e m s

CH3

CH3
Br

D
H

i-deuterated

D0

%

≥

NaOCH2CH3, CH3CH2OH

CH3

CH3

(All D retained)
%

≥#≥

CH3

CH3Br
ii-deuterated

H

%

≥

NaOCH2CH3, CH3CH2OH

CH3

CH3

(All D lost)
%

≥
≥

D
H
#≥

 Specify whether anti or syn eliminations have taken place. Draw the conformations that the mol-
ecules must adopt for elimination to occur. Does your answer to (b) help you in solving (a)?

Team Problem
61. Consider the general substitution-elimination reactions of the bromoalkanes.

 Nu/base
R O Br uuuy R O Nu 1 alkene

 How do the reaction mechanisms and product formation differ when the structure of the sub-
strate and reaction conditions change? To begin to unravel the nuances of bimolecular and
unimolecular substitution and elimination reactions, focus on the treatment of bromoalkanes A
through D under conditions (a) through (e). Divide the problem evenly among yourselves so
that each of you tackles the questions of reaction mechanism(s) and qualitative distribution of
product(s), if any. Reconvene to discuss your conclusions and come to a consensus. When you
are explaining a reaction mechanism to the rest of the team, use curved arrows to show the
fl ow of electrons. Label the stereochemistry of starting materials and products as R or S, as
appropriate.

Preprofessional Problems
62. Which of the following haloalkanes will undergo hydrolysis most rapidly?

(a) (CH3)3CF (b) (CH3)3CCl (c) (CH3)3CBr (d) (CH3)3CI

63. The reaction

(CH3)3CCl
CH3O�

H3C

H3C
CH2C

G

D
P

 is an example of which of the following processes?

(a) E1 (b) E2 (c) SN1 (d) SN2

Br
A

Br

D

B

0

´

Br

D

C

´

≥

 D

Br

(a) NaN3, DMF (b) LDA, DMF (c) NaOH, DMF (d) CH3CO�Na�, CH3COH

O
B

O
B

 (e) CH3OH

286 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

64. In this transformation,

H2O, acetoneA CH3CH2C(CH3)2
OH
A

 what is the best structure for A?

(a) BrCH2CH2CH(CH3)2 (b) CH3CH2CBr
CH3

CH3

A

A

(c) CH3CH2CH

CH3

CH2Br

A

A
 (d) CH3CHCH(CH3)2

Br
A

65. Which of the following isomeric carbocations is the most stable?

(a)

CH2�

 (b)

� CH3

(c)
�

CH3
 (d)

�

CH3

66. Which reaction intermediate is involved in the following reaction?

2-Methylbutane 2-bromo-3-methylbutane
(not the major product)

Br2, hv

(a) A secondary radical (b) A tertiary radical
(c) A secondary carbocation (d) A tertiary carbocation

	Chapter 7: FURTHER REACTIONS OF HALOALKANES
	7-1: Solvolysis of Tertiary and Secondary Haloalkanes
	7-2: Unimolecular Nucleophilic Substitution
	7-3: Stereochemical Consequences of SN1 Reactions
	7-4: Effects of Solvent, Leaving Group, and Nucleophileon Unimolecular Substitution
	7-5: Effect of the Alkyl Group on the SN1 Reaction: Carbocation Stability
	Chemical Highlight 7-1: Unusually Stereoselective SN1 Displacement in Anticancer Drug Synthesis
	7-6: Unimolecular Elimination: E1
	7-7: Bimolecular Elimination: E2
	7-8: Competition Between Substitution and Elimination: Structure Determines Function
	7-9: Summary of Reactivity of Haloalkanesnull
	Chapter Integration Problemsnull
	New Reactions
	Important Concepts
	Problems