CHEM 332 – Spring 2016
Name:
Problem Set 5
Discussion Section: M
T
F
Due 4/29/2016
1. Predict the major product of the following transformation.
HCl
2. Draw the major 1,2- and 1,4-addition products of the following reaction.
HBr
3. A. In the following reaction, both the 1,2- and 1,4-addition products are observed. Please
draw an arrow-pushing mechanism that rationalizes formation of both products.
+
0 °C
X
X
X2
X
X
X = Cl
50%
50%
X = Br
67%
33%
1 B. Explain why the ratios differ with bromine versus chlorine.
4. Assign the following molecules as aromatic, anti-aromatic, or non-aromatic and indicate the
number of π electrons.
Circle your answer:
Aromatic, Anti-Aromatic or Non-Aromatic
(a)
Aromatic
Anti-Aromatic
Non-Aromatic
(b)
Aromatic
Anti-Aromatic
Non-Aromatic
Aromatic
Anti-Aromatic
Non-Aromatic
(d)
Aromatic
Anti-Aromatic
Non-Aromatic
(e)
Aromatic
Anti-Aromatic
Non-Aromatic
(f)
Aromatic
Anti-Aromatic
Non-Aromatic
(c)
H
N
How many
electrons in π
system?
2 5. A. Label the hybridization of each nitrogen atom in benzimidazole.
N
N
H
B. Is benzimidazole aromatic, anti-aromatic, or non-aromatic?
C. How many π electrons are in benzimidazole?
D. Draw a 3D representation of benzimidazole clearly showing the p-orbitals of the conjugated π
system and the orbitals that contain the nitrogen lone pairs.
6. Is cyclooctatetraene aromatic? Draw a molecular orbital diagram to rationalize your answer.
3 7. Draw an arrow-pushing mechanism for the formation of both the major and minor
regioisomers of the product.
OMe
Δ
+
H
H
+
OMe
OMe O
O
H
H
major
minor
O
B. For the major regioisomer, please indicate the expected stereochemistry.
C. Why is the major regioisomer favored?
8. Draw an arrow-pushing mechanism for the following reactions.
CO2Me
N
N
N
N
CO2Me
CO2Me
Δ
+
OEt
N
N
CO2Me
4 MeO 2C
CO2Me
O
O
O
Δ
O
O
O
O
O
Δ
9. Predict the products of the following transformations and provide a reasonable arrow pushing
mechanism.
Cl
AlCl3
5 Cl
AlCl3
OH
BF 3•OEt2
60 °C
10. Provide a mechanism which is consistent with the following observation:
13 C
H 2O/HCl
13 C
benzene,
80°C
6 11. A. In an attempt to synthesize n-propylbenzene A from benzene, Dr. X performed a FriedelCrafts alkylation using n-propyl bromide in the prescence of AlBr3. Instead of obtaining the
desired product A, B was observed as the only major product. Provide a reasonable
mechanism which accounts for the formation of B.
Br
Br
AlBr3
B
observed
AlBr3
A
expected but
NOT FORMED!
B. Provide an alternative route for the synthesis of the desired product.
7