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Chemistry
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CHEM 241-Davis
Exam 1-Thurs Sep. 23, 2021
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I promise to be honest in taking this exam
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Discussion #
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YOUR NAME ______________ ______________ First Name Last Name
Problem 1: Structure _____ (10) Problem 2: Alkyne Chemistry _____ (30) Problem 3: Conjugated Dienes _____ (35) Problem 4: Diels-Alder _____ (25) Extra Credit: Haiku Poem _____ (3) Total
_____
(100)
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4
4
Blank
5
5
Q1. Structure and Bonding from Chem 231 Assessment (10 pts).
Guanidine
1
is
protonated to give guanidinum cation
2
. Guanidinium cation
2
is planar and all of
its 3 C-N distances are identical (1.33 angstroms).
i)
Write the hybridization of the indicated atoms in guanidinium
2
. (4 pts).
ii) Using the framework,
draw and label
a) orbitals with non-bonding
electrons and b) orbitals that make up the
π
bond for
2
. (6 pts)
N
N
N
H
H
H
H
H
H
+
N
N
N
H
H
H
H
H
+
H
+
N
N
N
H
H
H
H
H
H
+
1
2
120
o
1.33 A
1.33 A
C
N
H
2
N
H
2
N
H
H
+
6
6
Q2. Alkyne Chemistry (30 pts).
Hydroboration can be done on alkynes. The initial
product is enol isomer
1
. Under acidic conditions enol
1
isomerizes to aldehyde
2
.
a) Complete the stepwise mechanism for acid-catalyzed isomerization of enol
1
to
aldehyde
2
by completing the diagram below. Show all electron pairs, formal
charges, both resonance forms of the reaction intermediate and all arrows
indicating electron flow. (15 pts)
b) Using these bond dissociation energies, calculate
Δ
H
o
for isomerization of enol
1
to aldehyde
2
(5 pts)
C-C
π
60 kcal/mol
C-O
π
95 kcal/mol
C-H
σ
100 kcal/mol
C-O
σ
85 kcal/mol
C-C
σ
85 kcal/mol
O-H
σ
110 kcal/mol
Δ
H
o
= _________________ (kcal/mol)
CH
3
1) BH
3
2) H
2
O
2
CH
3
Enol 1
H
HO
H
Aldehyde 2
H
3
O
+
(cat)
Alkyne
H
CH
3
O
H
H
H
CH
3
H
HO
H
O
H
H
H
Carbocation Resonance Form
Another Resonance Form
O
H
H
H
Enol 1
Reaction Intermediate
Aldehyde 2
CH
3
O
H
H
H
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7
7
c) Complete the reaction coordinate-energy diagram, showing relative positions
of enol
1
and aldehyde isomers
2
and the resonance-stabilized intermediate that
exists between
1
and
2
. Assume that the first step, protonation of enol
1
is the
rate-determing step in the isomerization reaction. Label all transition states as
TS
≠
,
Δ
G
≠
(E
act
) for each step, and
Δ
G
o
for the isomerization of
1
to
2
(10 pts)
Free Energy
Reaction Coordinate
CH
3
H
HO
H
Enol 1
Aldehyde 2
8
8
Q3. Conjugated Dienes (35 points).
Addition of 1 equivalent of HBr to diene
1
gives a mixture of 4 different products,
2-5
, when carried out at 20
o
C for 1 hour.
a) (8 pts) When the reaction is done at -20
o
C for 15 min, compound
3
is formed
first, making it the kinetic product. Below, complete the drawings to show the
carbocation intermediates that give compounds
2-5
.
b) (10 pts)
Mechanism.
Use the structures provided below to complete drawing
the stepwise mechanism for formation of compound
3
from diene
1
and HBr.
Show all electron flow and formal charges and make sure to indicate the proper
arrows (resonance or equilibrium) between various species.
c) (4 pts) When the same reaction is done at 50
o
C for 24 hours, compound
2
is
the predominant product. In 5 words or less, justify this result.
Explanation (5 words or less)
__________________________________________________
CH
3
CH
3
CH
3
CH
3
Carbocation 2
Carbocation 3
Carbocation 4
Carbocation 5
CH
3
CH
3
CH
3
Br
1
3
CH
3
CH
3
HBr
THF
CH
3
CH
3
CH
3
CH
3
Br
Br
Br
Br
1
2
3
4
5
9
9
d) (8 pts) Compound
4
exists as 2
diastereomers,
cis
4
and
trans
4
:
Complete the drawings to indicate the lowest energy conformation for both
cis
4
and
trans
4
. Only draw the substituents (-CH
3
, -Br and –H) at the indicated bonds
in
cis
4
and
trans
4
. Assume that a –CH
3
group is larger than a –Br atom.
e) (5 pts) Circle the best answer for the following statements
Cis
4
is lower in energy (more stable) than is trans
4
True
False
Cis
4
can react with base to regenerate diene
1
True
False
Trans
4
can react with base to regenerate diene
1
True
False
Diene
1
is more reactive in Diels-Alder reaction than diene
6
True
False
Diene
1
is more reactive in Diels-Alder reaction than diene
7
True
False
1
2
1
2
cis
4
trans
4
CH
3
1
OCH
3
7
CH
3
1
H
3
C
H
3
C
6
CH
3
cis
4
trans
4
CH
3
Br
CH
3
Br
1
2
1
2
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10
10
Q4.
Diels-Alder Chemistry (25 pts).
Shown below are 2 possible Diels-Alder
approaches to make compound
1
. Method B is much better than method A.
a) Draw a mechanism for formation of
1
from diene
4
and dienophile
5. (3 pts)
b) One reason Method B is superior to Method A is because diene
4
is
conformationally restricted, whereas diene
2
is conformationally mobile. Draw
the two conformations for diene
2
. Then circle the conformation that is favored at
equilibrium (6 pts)
Reactive Conformation 2
Unreactive Conformation 2
O
4
5
+
O
1
O
O
O
2
3
4
5
1
Method A
+
+
O
1
Method B
11
11
c) Another reason that method B is best is because
5
is a better dienophile than is
compound
3.
Complete drawing resonance forms to clearly show why the C=C
double bond in
5
is more electron deficient than the C=C double bond in
3
.
Indicate all charges and electron pairs in each resonance form (8 pts).
d
) Compound
1
can exist as 2 isomers, either cis-
1
or trans-
1
. (4 pts)
Circle True or False to the following statements:
i) Diels-Alder reaction of
4
and
5
gives only cis-
1
True
False
ii) Trans-
1
is thermodynamically more stable than cis-
1
True
False
iii) Compounds cis-
1
and trans-
1
are enantiomers
True
False
iv) Compounds cis-
1
and trans-
1
are diastereomers
True
False
O
O
3
5
O
cis-1
O
trans-1
12
12
e) Below are shown 4 transitions states for the Diels-Alder
reaction of
1
and
2
. Circle either Endo or Exo to indicate
whether the illustration is depicting an endo or exo
transition state. (4 pts)
Bonus (3 pts):
“
A traditional Japanese Haiku is a three-line poem with seventeen
syllables, written in a 5/7/5 syllable count. Often focusing on images from
Nature, Haiku emphasizes simplicity, intensity, and directness of expression.”
Write a Haiku poem about either Alkynes or the Diels-Alder Reaction
Graded this way: 3 points---“I wish I’d thought of that”
1 point--- “Yeah, that’s 5-7-5”
O
O
O
H
H
H
O
H
Endo Exo
Endo Exo
Endo Exo
Endo Exo
O
2
1
+
H
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