Vollhardt  Capítulo 6 (Haloalcanos)
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Vollhardt Capítulo 6 (Haloalcanos)

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(a) How would you characterize BMIM as a solvent? Polar or nonpolar?
Protic or aprotic? (b) How would changing the solvent from ethanol to BMIM affect the rate of
the nucleophilic substitution reaction between sodium cyanide and 1-chloropentane?

51. (2S,3S)-3-Hydroxyleucine is an amino acid (Chapter 26) that is a key component in the structures
of many “depsipeptide” antibiotics, such as sanjoinine (margin). (a) Find the part of the sanjoinine
molecule that is derived structurally from (2S,3S)-3-hydroxyleucine. (b) Although many depsi-
peptide antibiotics occur in nature, the quantities available are too small to be useful pharmaceu-
tically; thus these molecules must be synthesized. (2S,3S)-3-Hydroxyleucine, which is also not
available in quantity from nature, must be synthesized as well. Possible starting materials are the
four diastereomers of 2-bromo-3-hydroxy-4-methylpentanoic acid (margin). Draw structural for-
mulas for each of these diastereomers and identify which of the four should be the best starting
material for a preparation of (2S,3S)-3-hydroxyleucine.

52. Iodoalkanes are readily prepared from the corresponding chloro compounds by SN2
reaction with sodium iodide in acetone. This particular procedure is especially useful because the
inorganic by-product, sodium chloride, is insoluble in acetone; its precipitation drives the equi-
librium in the desired direction. Thus, it is not necessary to use excess NaI, and the process goes
to completion in a very short time. Because of its great convenience, this method is named after
its developer (the Finkelstein reaction). In an attempt to synthesize optically pure (R)-2-iodoheptane,
a student prepared a solution of (S)-2-chloroheptane in acetone. In order to ensure success,
he added excess sodium iodide and allowed the mixture to stir over the weekend. His yield of
2-iodoheptane was high, but, to his dismay, he found that his product was racemic. Explain.

53. Using the information in Chapters 3 and 6, propose the best possible synthesis of
each of the following compounds with propane as your organic starting material and any other
reagents needed. [Hint: On the basis of the information in Section 3-7, you should not expect to
fi nd very good answers for (a), (c), and (e). One general approach is best, however.]

(a) 1-Chloropropane (b) 2-Chloropropane (c) 1-Bromopropane
(d) 2-Bromopropane (e) 1-Iodopropane (f) 2-Iodopropane

PF6�N�
CH3CH2CH2CH2 CH3

N

1-Butyl-3-methylimidazolium
(BMIM) hexafluorophosphate

OOH

OH
NH2

(2S,3S)-3-Hydroxyleucine

0

´

O
O

O

HO

NH

NH

O
NH

(CH3)2N
Sanjoinine

&

*

(

OOH

2-Bromo-3-hydroxy-4-methyl-
pentanoic acid

OH
Br

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 C h a p t e r 6 249

54. Propose two syntheses of trans-1-methyl-2-(methylthio)cyclohexane (shown in the margin),
beginning with the starting compound (a) cis-1-chloro-2-methylcyclohexane; (b) trans-1-chloro-2-
methylcyclohexane.

55. In each pair of molecules that follows, indicate the member of the pair that would be better suited
in its indicated function for an SN2 reaction.

P r o b l e m s

Š

[

CH3

SCH3

(a) Nucleophile: NH3, PH3 (b) Substrate: Br, Br

(c) Solvent:
C

H N(CH3)2,
B
O

C

H NH2

B
O

 (d) Leaving group: CH3OH, CH3SH

56. Rank each of the following sets of molecules in order of increasing SN2 reactivity.

(a) CH3CH2Br, CH3Br, (CH3)2CHBr
(b) (CH3)2CHCH2CH2Cl, (CH3)2CHCH2Cl, (CH3)2CHCl

(c) CH3CH2Cl, CH3CH2I, Cl

(d)

(CH3CH2)2CHCH2Br, CH3CH2CH2CHBr, (CH3)2CHCH2Br
A
CH3

57. Predict the effect of the changes given below on the rate of the reaction

CH3Cl 1
2OCH3 ––¡

CH3OH
CH3OCH3 1 Cl

2.

(a) Change substrate from CH3Cl to CH3I; (b) change nucleophile from CH3O
2 to CH3S

2;
(c) change substrate from CH3Cl to (CH3)2CHCl; (d) change solvent from CH3OH to (CH3)2SO.

58. The following table presents rate data for the reactions of CH3I with three different nucleophiles
in two different solvents. What is the signifi cance of these results regarding relative reactivity of
nucleophiles under different conditions?

Nucleophile krel, CH3OH krel, DMF

Cl2 1 1.2 3 106

Br2 20 6 3 105

NCSe2 4000 6 3 105

59. Explain the outcome of the following transformations mechanistically.

(a) �
CH3CH2OHHSCH2CH2Br NaOH S

(b) DMF�BrCH2CH2CH2CH2CH2Br NaOH
OExcess

(c) CH3CH2OH�BrCH2CH2CH2CH2CH2Br NH3
N
H

Excess

60. SN2 reactions of halocyclopropane and halocyclobutane substrates are very much
slower than those of analogous acyclic secondary haloalkanes. Suggest an explanation for this
fi nding. (Hint: Consider the effect of bond-angle strain on the energy of the transition state; see
Figure 6-4.)

61. Nucleophilic attack on halocyclohexanes is also somewhat retarded compared with that on acyclic
secondary haloalkanes, even though in this case bond-angle strain is not an important factor.
Explain. (Hint: Make a model, and refer to Chapter 4 and Section 6-9.)

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250 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

Team Problem
62. Compounds A through H are isomeric bromoalkanes with the molecular formula C5H11Br. With

your team, draw all eight constitutional isomers. Indicate any stereocenter(s), but do not label
it (them) as R or S until you have completed your analysis. Using the data below, assign structures
to A through H. Divide the problem into equal parts to share the effort of fi nding a solution.
Reconvene and discuss your analysis. At this point, you should indicate the stereochemistry with
wedged and hashed lines as appropriate.

• Treatment of compounds A through G with NaCN in DMF followed second-order kinetics and
showed the following relative rates:

A > B . C . D > E . F .. G

• Compound H does not undergo the SN2 reaction under the preceding conditions.
• Compounds C, D, and F were found to be optically active, each having S absolute confi guration

at the stereocenter. Substitution reactions of D and F with NaCN in DMF proceeded with inver-
sion of confi guration, while treatment of C in the same way proceeded with retention of con-
fi guration.

Preprofessional Problems
63. The SN2 reaction mechanism best applies to

(a) cyclopropane and H2 (b) 1-chlorobutane and aqueous NaOH
(c) KOH and NaOH (d) ethane and H2O

64. The reaction CH3Cl 1 OH
2 ¡ CH3OH 1 Cl2 is fi rst order in both chloromethane and hy droxide.

Given the rate constant k 5 3.5 3 1023 mol L21 s21, what is the observed rate at the following
concentrations?

[CH3Cl] 5 0.50 mol L
21 [OH2] 5 0.015 mol L21

(a) 2.6 3 1025 mol L21 s21 (b) 2.6 3 1026 mol L21 s21 (c) 2.6 3 1023 mol L21 s21

(d) 1.75 3 1023 mol L21 s21 (e) 1.75 3 1025 mol L21 s21

65. Which ion is the strongest nucleophile in aqueous solution?

(a) F2 (b) Cl2 (c) Br2 (d) I2 (e) all of these are equally strong

66. Only one of the following processes will occur measurably at room temperature. Which one?

(a) F ClOð ðš� š� (b) Oq ðð ICH3N C
�

(c) Oq ðð ICH3N N (d) CH2 CH2OP Pk
k

Ok
k

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	Chapter 6: PROPERTIES AND REACTIONS OF HALOALKANES
	6-1: Physical Properties of Haloalkanes
	Chemical Highlight 6-1: Halogenated Steroids as Anti-Inflammatory and Anti-Asthmatic Agents
	6-2: Nucleophilic Substitution
	6-3: Reaction Mechanisms Involving Polar Functional
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