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

Disciplina:Química Orgânica II963 materiais31.941 seguidores
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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 � NaSH

(c)
O

S NaI�
CF3

O O
 (d)

H Cl
� NaN3

¥&

(e) N�CH3Cl

CH3

 (f ) KSeCN�

I

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 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�I��CH3CH2CH2Br (b)
DMSONa��CN�(CH3)2CHCH2I

(c) (CH3)2CHOHNa��OCH(CH3)2�CH3I (d) CH3OHNa��SCH2CH3�CH3CH2Br

(e)
Acetone

�CH2Cl CH3CH2SeCH2CH3 (f )
(CH3CH2)2ON(CH3)3�(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��CH3
CH2CH3

 (b) [H3C

Cl H

H Br
CH3�2 I�[

C C

(c)

[
B

0

�OCCH3

O

HO

Cl

� (d)

Š
B

0

�OCCH3

O

HO

Cl

�

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 “no
reaction” 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 “no reaction” where appropriate. (Hint: Identify the expected leaving group in each of the
substrates and evaluate its ability to undergo displacement.)

(a) K��OHCH3CH2CH2CH2Br
CH3CH2OH

� (b) K�Cl�CH3CH2I
DMF

�

(c) Li��OCH2CH3CH2Cl
CH3CH2OH

� (d) Cs�I�(CH3)2CHCH2Br CH3OH�

(e) K
��SCNCH3CH2CH2Cl

CH3CH2OH
� (f ) Li

�Cl�CH3CH2F
CH3OH

�

(g) K
�I�CH3CH2CH2OH

DMSO
� (h) Na

��SCH3CH3I
CH3OH

�

(i) Na
��OHCH3CH2OCH2CH3

H2O
�

( j)
 K

��OCCH3

O

CH3CH2I
DMSO

�
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
�

N
CH3H3C

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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 “no reaction” as your answer, if
appropriate.

(a)
CH3OHNa�Cl�CH3CH2CH2CH3� (b)

CH3OHNa��OCH3CH3CH2Cl�

(c)

AcetoneNa�I��
HH3C

H3C

Br

H
H

(d)

?
A
Cl

CH
CH3CH3CH2

G}
AcetoneNa��SCH3�

(e)

A
OH

CH3CHCH3 Na��CN� (f )
CH3CH2OH

A
CH3CHCH3

OSO2CH3
HCN�

(g)
A
OSO2CH3

CH3CHCH3 Na��CN�
CH3CH2OH

(h)
B

B G

D
SOCH2CH2CHH3C K��SCN

O

O

�
CH3OH

CH3

CH3

(i) Na
�Br�CH3CH2NH2

DMSO
� (j) Na

��NH2CH3I
NH3

�

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 “green,” 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.
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