Vollhardt  Capítulo 7 (Haloalcanos 2)
36 pág.

Vollhardt Capítulo 7 (Haloalcanos 2)


DisciplinaQuímica Orgânica II2.041 materiais37.389 seguidores
Pré-visualização12 páginas
is a good solvent for SN1 
reactions (Section 7-4).
2. We learned in Section 7-4 that different nucleophilic power has no effect on the rates of unimo-
lecular processes. The rates should be (and, experimentally, are) identical.
3. This part requires a bit more thought. According to Table 7-4 and Section 7-8, elimination by 
the E1 pathway always accompanies SN1 displacement. However, increasing the base strength 
of the nucleophile may \u201cturn on\u201d the E2 mechanism, increasing the proportion of elimination 
product. Referring to Tables 6-4 and 6-7, we see that chloride is more basic (and less nucleo-
philic) than iodide. More elimination is indeed observed with chloride than with iodide. The 
mechanism is as shown in Figure 7-7, with chloride acting as the base to remove a proton from 
the carbocation.
b. The table in the margin presents data for the reactions that take place when the chloro compound 
shown here is dissolved in acetone containing varying quantities of water and sodium azide, NaN3:
A
H3C CH
Cl
A
OH
A
N3
CH3
CH3 H3C\ufffd
H2O, NaN3,
acetone
CH CH3H3C CH
In the table, H2O% is the percentage of water by volume in the solvent, [N3
2] is the initial concentra-
tion of sodium azide, RN3% is the percentage of organic azide in the product mixture (the remainder 
is the alcohol), and krel is the relative rate constant for the reaction, derived from the rate at which 
the starting material is consumed. The initial concentration of substrate is 0.04 M in all experiments. 
Answer the following questions.
1. Describe and explain the effects of changing the percentage of H2O on the rate of the reaction 
and on the product distribution.
2. Do the same for the effects of changing [N3
2]. Additional information: The reaction rates shown 
are the same when other ions \u2014 for example, Br2 or I2 \u2014 are used instead of azide.
H2O [N3
2] RN3 krel
 % %
 10 0 M 0 1
 10 0.05 M 60 1.5
 15 0.05 M 60 7
 20 0.05 M 60 22
 50 0.05 M 60 *
 50 0.10 M 75 *
 50 0.20 M 85 *
 50 0.50 M 95 *
*Too fast to measure.
 C h a p t e r 7 277
SOLUTION
1. We begin by examining the data in the table, specifi cally rows 2 \u2013 5, which compare reactions in 
the presence of different amounts of water at constant azide concentration. The rate of substitution 
increases rapidly as the proportion of water goes up, but the ratio of the two products stays the 
same: 60% azide and 40% alcohol. These two results suggest that the only effect of increasing 
the amount of water is to make the solvent environment more polar, thereby speeding up the 
initial ionization of the substrate. Even when the proportion of water is only 10%, it is present 
in great excess and is trapping carbocations as fast as it can, relative to the rate at which azide 
ion reacts with the same intermediates (Section 7-2).
2. We note from rows 1 and 2 in the table that the reaction rate rises by about 50% when NaN3 is 
added. Without further information, we might assume that this effect is a consequence of the 
occurrence of the SN2 mechanism. Were that to be the case, however, other anions should affect 
the rate differently. However, we were told that bromide and iodide, far more powerful nucleo-
philes, affect the measured rate in exactly the same way as does azide. We can explain this obser-
vation only by assuming that displacement is entirely by the SN1 mechanism, and added ions affect 
the rate only by increasing the polarity of the solution and speeding up ionization (Section 7-4).
 In rows 5 \u2013 8 of the table, we note that increasing the amount of azide ion increases the amount 
of azide-containing product that is formed. At the higher concentrations, azide, a better nucleo-
phile than water, is better able to compete for reaction with the carbocation intermediate.
New Reactions
1. Bimolecular Substitution \u2014 SN2 (Sections 6-2 through 6-9, 7-5)
Primary and secondary substrates only
}0
GC CI
Direct backside displacement with 100% inversion of configuration
H3C CH3
CH2CH3 CH2CH3
I\ufffd\ufffd
H H
O NuO }0
GNu\ufffdð
2. Unimolecular Substitution \u2014 SN1 (Sections 7-1 through 7-5)
Secondary and tertiary substrates only
A
OCH3
CH3
A
CH3
Through carbocation: Chiral systems are racemized
CBr
A
OCH3
CH3
A
CH3
CNuOCH3
\ufffdBr\ufffd
Nu\ufffdð
CH3
CH3
C\ufffd
G
D
3. Unimolecular Elimination \u2014 E1 (Section 7-6)
Secondary and tertiary substrates only
A
OCH3
CH3
A
CH3
Through carbocation
CCl OCH3
\ufffdCl\ufffd
B\ufffdð
CH2
CH3H3C
C
B
GD
BH\ufffd
CH3
CH3
C\ufffd
G
D
4. Bimolecular Elimination \u2014 E2 (Section 7-7)
\ufffd \ufffdBH I\ufffdPCH3CH2CH2I CH3CH CH2
Simultaneous elimination of leaving group and neighboring proton
B\ufffdð
Important Concepts
1. Secondary haloalkanes undergo slow and tertiary haloalkanes undergo fast unimolecular substitution 
in polar media. When the solvent serves as the nucleophile, the process is called solvolysis.
2. The slowest, or rate-determining, step in unimolecular substitution is dissociation of the C \u2013 X 
bond to form a carbocation intermediate. Added strong nucleophile changes the product but not 
the reaction rate.
I m p o r t a n t C o n c e p t s
278 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
3. Carbocations are stabilized by hyperconjugation: Tertiary are the most stable, followed by sec-
ondary. Primary and methyl cations are too unstable to form in solution.
4. Racemization often results when unimolecular substitution takes place at a chiral carbon.
5. Unimolecular elimination to form an alkene accompanies substitution in secondary and tertiary 
systems.
6. High concentrations of strong base may bring about bimolecular elimination. Expulsion of the 
 leaving group accompanies removal of a hydrogen from the neighboring carbon by the base. 
The stereo chemistry indicates an anti conformational arrangement of the hydrogen and the 
leaving group.
7. Substitution is favored by unhindered substrates and small, less basic nucleophiles.
8. Elimination is favored by hindered substrates and bulky, more basic nucleophiles.
Problems
25. What is the major substitution product of each of the following solvolysis reactions?
H3C
H3C
Br
H
CH3OH
/\u2211
/\u2211
(a) 
A
A
CH3
CH3
CH3CBr
CH3CH2OH
 (b) 
A
Br
(CH3)2CCH2CH3
CF3CH2OH
 (c) 
CH2CH3
CH3OH
Cl
(d) 
A
A
O
B
C
Br
CH3
CH3
HCOH
O
 (e) 
A
A
CH3
CH3CCl
CH3
D2O
 (f) 
A
A
CH3
CH3CCl
CH3
H
OD
26. For each reaction presented in Problem 25, write out the complete, step-by-step mechanism 
using curved-arrow notation. Be sure to show each individual step of each mechanism 
 separately, and show the complete structures of the products of that step before going on to 
the next.
27. Write the two major substitution products of the reaction shown in the margin. (a) Write a 
mechanism to explain the formation of each of them. (b) Monitoring the reaction mixture reveals 
that an isomer of the starting material is generated as an intermediate. Draw its structure and 
explain how it is formed.
28. Give the two major substitution products of the following reaction.
CH3CH2OH
H3C
OSO2CH3
C6H5
C6H5H3C
H
29. How would each reaction in Problem 25 be affected by the addition of each of the following 
substances to the solvolysis mixture?
(a) H2O (b) KI
(c) NaN3 (d) CH3CH2OCH2CH3 (Hint: Low polarity.)
30. Rank the following carbocations in decreasing order of stability.
\ufffd
\ufffd
CH3CH3H CH2\ufffdH
 C h a p t e r 7 279
31. Rank the compounds in each of the following groups in order of decreasing rate of solvolysis in 
aqueous acetone.
(a) 
A
CH3
CH3CHCH2CH2Cl
A
A
CH3
Cl
CH3CHCHCH3
A
A
CH3
Cl
CH3CCH2CH3
(b) 
B
OCCH3
O
OHCl
(c) 
H3C ClClBr
32. Give the products of the following substitution reactions. Indicate whether they arise through the 
SN1 or the SN2 process. Formulate the detailed mechanisms of their generation.
 CH3CH2OH
(a) (CH3)2CHOSO2CF3 uuuy (b) 
CH3 Excess CH3SH, CH3OH
Br
 (C6H5)3P, DMSO NaI, acetone
(c) CH3CH2CH2CH2Br uuuuuy (d) CH3CH2CHClCH2CH3 uuvuy
33. Give the product of each of the following substitution reactions.