CHEM 237-Davis
Organic Chemistry
Examination 4
May 6, 2009
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(Last,
__________________________
First)
Initial of Last Name
Instructions
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• Print the initial of your last name in the box above
• Please sign the Honor Code on the following page
• DO NOT OPEN THIS EXAM UNTIL INSTRUCTED
• Answer questions in the spaces provided
PAGE
1
CHEM 237-Davis, Examination 4
YOUR NAME
__________________________
(Last, First)
University Honor Code Acknowledgment
I have neither given nor received assistance in taking this examination.
Signature
Points
Problem 1
(10)
Problem 2
(10)
Problem 3
_ (10)
Problem 4
(20)
Problem 5
(10)
Problem 6
(20)
Problem 7
(20)
Problem 8
(10)
Problem 9
_ (15)
Total
(125)
EXAM BEGINS ON NEXT PAGE 3
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2
1. a) Use structures and concise wording to explain why the energy barrier for rotation
about the C(2)-C(3) bond in 1,3-butadiene 1 is much higher than the energy required to
rotate about the C(2)-C(3) bond in 1-butene 2. (5 pts)
2
4
2
1
4
1
3
3
1
2
b) Circle the best answer (5 pts). The shortest C-C single bond in compound 3 is:
a) C1-C6
b) C2-C3
c) C4-C5
d) C5-C9
e) C6-C7
8
7
6
1
9
5
10
2
4
3
3
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3
2. Bromination of cyclopentadiene gives 1 as the major product. However, two other
compounds, 2 and 3, are formed in small amounts. Provide a step-wise mechanism that
explains formation of the minor products, compounds 2 and 3. Explain why one gets both
the trans isomer 2 and the cis isomer 3. (10 pts)
Br
+ Br-Br
+
Br
Br
Br
Br
+
Br
1
Major Product
2
Minor Product
3
Minor Product
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4
3 . One might predict that reaction of 1 and 2-methyl-1-chloro-butane 2 in the presence of
a Lewis acid catalyst, AlCl3, should give compound 3. But, compound 3 was a minor
product. Instead, compound 4 is the major product of this Friedel-Crafts alkylation.
OCH3
OCH3
CH3
+
H3C
1
OCH3
AlCl3 (cat)
+
Cl
2
CH3
CH3
H3C
CH3
CH3
3
4
Major Product
Minor Product
(10 pts) Provide a detailed arrow pushing mechanism for the following step.
CH3
H3C-H2C
H3C
H
H3C-H2C
H
+ H
+
H
+
H3C
H
Cation A
O
1
CH3
O
CH3
Cation B
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5
4. (20 pts) Chloride anion can be a nucleophile or a base. Shown below are 2 possible
processes for reaction of Cl- with a typical carbocation. Process A (addition) involves the
Cl- acting as a nucleophile and process B (elimination) involves the Cl- acting as a base.
a) Use the bond dissociation energies provided below to determine ΔHo values for the
addition reaction and for the elimination reaction. Enter your calculated ΔHo values into
the empty boxes. (10 pts)
Addition
R
H
+
R
H
H
_
+
Cl
Cl
H
ΔHo=
kcal/mol
ΔHo=
kcal/mol
Elimination
R
R
H
+
_
H
+
H
Cl
+
H
Cl
Table of Bond Dissociation Energies
Bond
ΔHo (kcal/mol)
C-Cl
85
C-C Π (pi)
60
C-H
90
C-Cl
85
H-Cl
100
b) Using the ΔHo values that you calculated in Part 5a circle the best answer (5 pts):
1. The addition reaction is thermodynamically favored over the elimination reaction
2. The elimination reaction is thermodynamically favored over the addition reaction
3. Both reactions are equally favorable
4. Both reactions are endothermic as written
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6
c) Reaction of Cl- anion with the cyclodienyl carbocation B only proceeds via the
elimination pathway. No addition product is observed.
CH3
H 3C-H 2C
H 3C
H
+
O
Cl-
H
CH3
H 3C-H 2C
H 3C
H
H
+
+
O
CH3
O
CH3
+
ClO
CH3
4
Elimination to give compound 4 is the only observed process.
Addition is never observed
Circle the answer that best explains the above information (5 pts):
1. The addition of Cl- to carbocation B is disfavored because of steric hindrance.
2. The elimination reaction is favored because Cl- is a strong base.
3. The elimination reaction is favored because the reaction product 4 is aromatic.
4. The addition reaction is disfavored because the depicted reaction is endothermic.
5. (10 pts) The following questions concern the relative stability of carbocations.
a) Circle the least stable carbocation. (5 pts)
CH3
H3C
H3C
H
+
O
CH3
H3C
H3C
H
H
+
H2C
CH3
H
H
H
H
+
CH3
CH3
H3C
H3C
CH3
H3C
H3C
+
C
H
O
H
CH3
b) Circle the least stable carbocation. (5 pts)
CH3
H3C
H3C
H
H
+
O
CH3
CH3
H3C
H
CH3
H
CH3
H3C
H3C
H3C
H3C
H
+
+
O
CH3
CH3
H
+
H
O
O
CH3
H
H
+
H Cl
CH3
H 3C-H 2C
H 3C
CH3
H 3C-H 2C
H 3C
H
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7
CH3
H
Cl
CH3
6. (20 pts) Friedel-Crafts acylation of furan gives 2-acetyl-furan 1, rather than 3-acetylfuran 2, as the major product. Use resonance structures to explain this specific
regiochemistry. Make sure to compare and contrast!
O
O
O
O
+
AlCl3
O
+
Cl
1
O
2
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8
7. (20 pts) Circle the more stable cation in each pair.
OH
OH
a)
+
+
+
+
b)
O
c)
H
N+
H
H
d)
N
+
+
H
e)
NO2
NH2
+
H
H
Br
H
Br
+
+
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9
8. (10 pts) The following energy diagram represents the electrophilic bromination of
benzene. Draw the structure of intermediate A in the box. Identify the rate determining
step in the reaction by circling the best statement below.
Br
+
Br2
FeBr3
+
H-Br
.
Energy
Structure of A
A
+ Br2
Br
+ H-Br
Reaction Progress
Circle the best answer. The rate-determining step is
a) Formation of HBr from Intermediate A
b) Formation of Bromobenzxene from Intermediate A
c) Formation of Intermediate A from benzene and bromine
d) Fridel-Crafts alklyaltion of benzene
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10
9. (15 pts) Draw the structure of the major product expected for the following 3 reactions.
OCH3
+
O
AlCl3
Cl
H3CS
O
H3C
NO2 +
O
Cl
O
O
AlCl3
AlCl3
+
Cl
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11
For Scratch work-nothing on this page will be graded
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12