CHE 2240 Exam 1 Solutions (1)
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CHE 2240 Spring 2020
Exam 1 Practice Answers + Explanations
Stringle
1
Which spectroscopic technique listed below involves the highest energy radiation being
absorbed by the molecules being examined?
A)
IR
B)
UV-Vis
C)
NMR
D)
MS
You should know the electromagnetic spectrum and the order.
Note that it is called mass
spectrometry and not spectroscopy, because it doesn
’t invo
lve absorption of radiation like
the other three
2
Which of the four structures below have a highest energy stretching peak?
The highest energy stretch will also be the stretch with the highest frequency (highest wavenumber).
Also,
the shortest/strongest bond.
Both the O-H peak and C-H of an alkyne show up around 3300cm-1, so they are
the highest energy stretches
3
The mass spectrum of the following molecule is expected to show a base peak with a mass-to-
charge ratio (m/z) of ____________.
A)
113
B)
128
C)
85
D)
71
E)
112
This molecule will undergo a-cleavage on either side of the C=O.
The charge will stay with the C=O part of the
fragment.
It will also preferentially do the alpha cleavage that results in the more stable (here it is 3
o
) radical.
This results in a charged fragment with 4Cs, 7Hs and an O atom for an m/z = 48 + 7 + 16 = 71
CHE 2240 Spring 2020
Exam 1 Practice Answers + Explanations
Stringle
4
Which compound would show a peak around 3300
cm
−
1
in the IR, have an M
+
at m/z = 136 and a
base peak at m/z = 105?
The peak at 3300 means alcohol (I, II and IV).
They all have a mass of 136.
They all can fragment to leave the
+ve charge on the benzylic carbon.
For I, this results in a peak at 91. For III it gives a peak at 105 (because only
the CH
2
OH leaves).
IV would just lose a methyl (CH
3
) group and have a peak at 121.
5
Which compound will most likely have a base peak at m/z = 43?
Here, it helps to figure out that 43 equals 2Cs, 3Hs and an O.
These can all do
-cleavage, but only II can lose
the propyl group as a 2
o
radical to leave the CH
3
CO
+
as the charged fragment, everything else gives larger
fragments from
-cleavage.
6
What is the key feature that all atoms with a nucleus that can be used in NMR experiments have
in common?
A)
It possesses and uneven number of protons
B)
It possesses and uneven number of neutrons
C)
The number of protons equals the number of neutrons
D)
The sum of protons plus neutrons is an uneven number
E)
It possesses more neutrons than electrons
The common nuclei used in NMR experiments, 1H, 13C, 19F, 31P all have this in common
7
In the following compound, which H atom would require the most energy to transition between
spin states?
NMR measures the energy released when nuclei drop from the higher spin state to a lower spin state.
This
energy is reported as a chemical shift.
The higher the chemical shift, the higher the energy released meaning
there was a larger gap between the spin states.
CHE 2240 Spring 2020
Exam 1 Practice Answers + Explanations
Stringle
8
Which structure would show the fewest signals it its
13
C NMR spectrum?
Look for symmetry here.
The number of signals for each compound is I
–
7, II
–
7 (no symmetry at all), III
–
6
(high symmetry here), IV
–
5 (isopropyl group gives 2, t-butyl group also just gives 2), V
–
4 (two planes of
symmetry here)
9
Which compound will have an integration pattern of 6:3:2:1:1 in its
1
H NMR spectrum?
The question should read
“
6:3:2:1:1:1
”
, then it matches compound III
Compound I is 6:3:1:1:1
Compound II is 6:3:2:1
Compound IV is 9:3:2:1:1
Compound V is 3:3:3:2:1 (the right hand methyl groups are not equivalent, one is cis to the ketone and the
other is trans
–
that makes a difference!
10
Which best describes the multiplicity of the indicated H atom in the structure below?
A)
doublet of triplets
B)
quintet
C)
doublet
D)
quartet
E)
doublet of quartets
On opposite sides of this H is a CH
3
group and a CH group.
That means it will be split into a double by the CH
and quartet by the CH
3
.
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CHE 2240 Spring 2020
Exam 1 Practice Answers + Explanations
Stringle
11
Which one structure shown below possesses an H atom whose signal will be split into a doublet
of doublets?
Here you are looking for an H atom that has two different Hs on either side of it.
This should lead initially to A,
D and E.
If you start looking at what is different about these three structures is that two (A and D) have high
symmetry.
If we look at A, the H meta to the two methyl groups has a single H on either side of it, but both of
those Hs are equivalent, so this H would give a triplet.
Looking at D, the Hs meta to the Cl are equivalent and
do not split one another, so these signals would just be doublets.
This leaves E with no symmetry and the H
meta to the CH
3
and Cl so the H on either side will split it, each splitting it into a doublet leading to a doublet
of doublets
12
Which one compound does not contain any diastereotopic H atoms?
You need to think of these as 3-D structures to get the idea of what makes Hs diastereotopic.
But you can also
look for molecules with a chiral center that has a CH
2
group beside it (B, C, and D).
If it is a ring, you don
’
t
need a chiral center, just a group that would become a chiral center if an adjacent H was replaced (E).
This
leaves structure A with no chiral center and no ring.
13
Which one of the four compounds shown below would display 2 signals in its DEPT-90 spectrum
and 3 positive signals in its DEPT-135 spectrum?
DEPT-90 shows CH groups.
Only C contains two CH groups.
Positive signals in DEPT-135 are CH and CH
3
, and C
also has a single methyl group to give 3 positive signals.
CHE 2240 Spring 2020
Exam 1 Practice Answers + Explanations
Stringle
14
Which compound will show 9 signals in its
13
C NMR?
Again, lack of symmetry is in play here
–
A and E.
A has 9, 6 from the unsymmetrical ring and 3 from the
ketone group.
E also has 6 from the ring, 3 from the ketone group but has one extra signal from the CH
3
group.
15
Which compound below would absorb the highest wavelength radiation in the UV-Vis region?
The higher the wavelength, the lower the energy.
The more conjugated a compound, the lower the energy
gap between
and
* orbitals so the least amount of energy needed for excitation (this is what happens
when the radiation is absorbed).
C is the most conjugated (all 4
bonds are conjugated)
16
Which of the following structures are possible products of the following reaction?
A)
1 and 3
B)
2 and 4
C)
1, 2 and 5
D)
2, 3 and 4
E)
all of them
This is allylic bromination.
This leads to 2.
1 and 3 have lost carbons, which is not what happens in allylic
bromination.
5 has the Br on a vinylic carbon, not allylic, this is not a possible product.
4 is a possible product
because of resonance in the intermediate radical formed when an H is abstracted from the right hand CH
3
group.
CHE 2240 Spring 2020
Exam 1 Practice Answers + Explanations
Stringle
17
What is(are) the major product(s) of the following reaction when carried out at low
temperatures?
A)
2
B)
3
C)
4
D)
1 and 4
E)
3 and 5
First, this reaction involves Cl
−
attacking an allylic carbocation.
4 and 5 do not have the Cl in an allylic position,
these are not possible products. 3 would be formed through attack at a 3
o
allylic carbocation, so this would be
the product formed through the lowest energy pathway
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CHE 2240 Spring 2020
Exam 1 Practice Answers + Explanations
Stringle
18
Which one structure below represents the major product of the following Diels-Alder reaction?
You may want to re-draw the diene.
Note that both double bonds are trans, so when you redraw the diene,
they still must be trans:
These structures have been drawn with an endo approach of the dienophile.
I added the Hs to the structure
just the emphasize that they will end up in the up position, which means all the other groups will be down
(dashed). This would lead to structure B
CHE 2240 Spring 2020
Exam 1 Practice Answers + Explanations
Stringle
19
Which of the following set of structures (
A
–
E
) represent the reactants which would be used to
produce the Diels-Alder adduct shown below?
1
2
A)
B)
C)
D)
E)
When there is a bridge on the cyclohexene ring, it means the diene was in a ring structure.
This ring involved
the 4 carbons of the diene plus the one carbon of the bridge.
This gives us B and D.
The ketone groups are
trans on the product, which means they were trans on the dienophile.
This means the answer is B.
20
Which diene would react most rapidly in a Diels-Alder reaction?
Dienophiles are electrophiles and react faster with electron-withdrawing groups.
It therefore makes sense
that the dienes, which are the nucleophiles, will react faster with electron-groups.
It was also mentioned that
dienes in small rings are already in the s-cis and will react faster as a result. This leads to E as the diene that
will react the fastest and the methoxy group is a resonance donating group.
CHE 2240 Spring 2020
Exam 1 Practice Answers + Explanations
Stringle
21
Which represents the correct number of
MOs and number of
electrons in the structure
below?
MOs
electrons
A)
8
4
B)
16
10
C)
8
8
D)
9
10
E)
18
5
Ignore the methyl group on the N atom.
Count all the atoms that are in the two rings (9).
Each of these atoms
has a p-orbitals and the number of
MOs = the number of p-orbitals.
There are 8
electrons in the 4
bonds
and the N atom with no
bond contributes its lone pair (2 electrons) for a total of 10
electrons
22
Which one ion shown below is not aromatic?
Here, you just count up the number of p electrons and see if it agrees with the 4n + 2 rule.
A
= 10, n = 2;
B
= 6,
n = 1;
C
= 6, n = 1;
D
= 4, n = 0.5;
E
= 6, n = 1.
D is the answer because it has 4n electrons instead of 4n + 2.
23
What is the name of the following compound?
A)
m
-amino-
o
-chlorotoluene
B)
1-chloro-2-methyl-4-aminobenzene
C)
4-chloro-3-methylaniline
D)
1-amino-4-chloro-3-methylbenzene
E)
p
-chloro-
m
-methylanisole
It is an aniline, not toluene, due to amino groups being higher priority than alkyl groups.
When it is aniline,
the ring carbon with the NH
2
is C1.
This means the other groups are on C3 and C4.
Putting it all together gives
answer C
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CHE 2240 Spring 2020
Exam 1 Practice Answers + Explanations
Stringle
24
Which compound shown below would have the name
m
-methoxyacetophenone?
Acetophenone is a phenyl group with a ketone group (hence the phenone part of the name), and one of the 8
benzene derivatives whose structure you need to memorize.
B and E are aldehdyes.
C has a ketone, but the
ring is a cyclohexane not a phenyl ring.
A and D are both methoxyacetophenone, but D has the methoxy meta
(m) to the ketone
25
Use the following electrophilic addition of HBr to the diene shown below to answer the
following questions
A)
Draw the two carbocations formed in the first step of this reaction.
B)
For each carbocation, draw its resonance contributor
C)
Identify which carbocation is lower in energy, and explain briefly
D)
Draw the two-step mechanism showing the formation of the major product
for this reaction when run under kinetic control
26
Use the structure shown below to answer the following questions:
A)
Draw an energy level diagram showing the
MOs of the compound shown above.
B)
On this diagram, clearly identify which are bonding MOs and which are anti-bonding MOs.
C)
Fill in the diagram with the correct number of
electrons.
D)
Is this compound aromatic?
Explain.
CHE 2240 Spring 2020
Exam 1 Practice Answers + Explanations
Stringle
27
Use the following information regarding an unknown compound, compound
X
, to answer the
following questions and ultimately determine its structure
13
C NMR data: 16, 22, 31, 42, 125, 128, 130, and 150
ppm
1
H NMR data:
chemical shift
integration
multiplicity
chemical shift
integration
multiplicity
0.8
3
triplet
2.6
1
multiplet
1.2
3
doublet
7.1
3
multiplet
1.6
2
multiplet
7.2
2
multiplet
IR Spectrum of compound
X
CHE 2240 Spring 2020
Exam 1 Practice Answers + Explanations
Stringle
Mass Spectrum of compound
X
(the M
+
peak occurs at m/z = 134)
A)
What are the
two
key pieces of information about the structure of compound
X
that can
be obtained from analyzing the IR?
B)
Making note of your answer above as well as the information from the mass spectrum as
well as the
13
C and
1
H NMR spectra, what is the formula of compound
X
?
Show your work and briefly explain your reasoning!
C)
Draw the structure of compound
X
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Exam 1 Practice Answers + Explanations
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