Name: ANSWER KEY - Bama.Ua.Edu - The University of Alabama

Name: ANSWER KEY (print) CWID:
Chemistry 231, Fall 2011
Dr. Shaughnessy
Exam #2-Version 1
October 11 th , 2011
Honor Pledge: I promise or affirm that I will not at any time be involved with cheating,
plagiarism, fabrication, or misrepresentation while enrolled as a student at The University of
Alabama. I have read the Academic Honor Code, which explains disciplinary procedures that
will result from the aforementioned. I understand that violation of this code will result in
penalties as severe as indefinite suspension from the University.
_____________________________(signature)
You will have 1 hour and 20 minutes to complete this exam. When
time is called please stop all work and turn in your exam.
Show all work. Partial credit will be given where appropriate.
This exam has 9 pages. Make sure you have all 9 pages, and that they
are correctly copied, before starting the exam. The last page has useful
information for you.
You may use molecular models to help you answer questions on this
exam.
You will not need a calculator or any other electronic device to complete
this exam.
Please put all cell phones (turn off or to silent), ipods, calculators, or
other electronic devices away in a book bag.
Your book and all notes should be placed out of sight of yourself and
your neighbors before the exam begins.

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Problem Score
1 (12)
2 (20)
3 (22)
4 (18)
5 (16)
6 (16)
Total (100)
There are a total of 104 points, but the exam
is worth 100 points.
2

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1. For each problem below, circle the best answer. In each case, there is only one correct
answer (12 points)
a. Which substituent would have the highest priority in the Cahn-Ingold-Prelog rules? The
point of attachment to the stereocenter is the arrow.
F
I
b. Which molecule below is not chiral?
O
B O
F
CH 3
c. Which carbocation would be the most stable?
d. Which molecule below would not have an index of hydrogen deficiency of 3?
O
Br
e. Which molecule below is a meso compound?
Br
Br
H 3C
C
f. Which molecule below would be least likely to act as a Lewis acid in a reaction?
H Cl
F F O
Br OH
H
B
H H
N
Mg 2+
OH
Br F CH HO OH
3
CH3 HO 2C
CH 3
Br
H 3C CH 3
Br
SnH 3
CO 2H
3

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2. For each reaction below, provide the missing starting material(s) or product(s). Provide the
starting material that would give the provided product as the only product. Draw the major
product or products expected. You may ignore stereochemistry in drawing the starting
materials or products. (20 points)
a.
b.
c.
d.
O
N
O
OH
+
+
HBr
S
+ HCl
H 2O
H 2SO 4 (cat)
Two products formed
O
N
O
O
Br
OH
Cl
Cl
H
S
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3. Stereocenter configurations and nomenclature. (22 points)
a. Provide the missing name for compound below. Be sure to indicate any stereochemical
configurations in your name.
(4Z,6S)-2,6-dimethyl-3,5-dipropyl-2,4-octadiene
b. For each molecule below, provide the R/S or E/Z configurations for each chiral center or
stereogenic alkene.
O
O
F
F
Circle the highest priority
substituents on each alkene
carbon and assign as E or Z
Z
Dextromethorphan-cough suppressent in
Robutussin, Nyquil, Vicks, Dimetapp, etc
H 3CO
N
H
CH 3
3
2
2
H
1
4
3
S
Label the substituents on each circled
chiral center in order of priority and then
assign the R or S configuration
b. Label each pair of molecule below as identical, enantiomers, or diastereomers.
H 2N H
H NH2
Cl
Cl
Diastereomers
S
4
1
HO
H
Br
HO H
Br
Enantiomers
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4. Compare the acidity of the two amines shown below. (18 points)
a. Draw the key resonance structures that help to stabilize the negative charge in the
conjugate bases below.
i.
ii.
H
O
H
O
NH 2
NH 2
C
O
NH
H
-H +
-H +
H
H
O
NH
b. Which compound would you predict to be more acidic? In 2 sentences, or less, support
your answer.
NH
The molecule in part i is more acidic. It has one additional resonance structure than ii,
where the negative charge can be moved onto the oxygen.
NH
C
O
There is no way to move the negative
charge onto the aldehyde oxygen
O
NH
H
C
O
H
H
H
O
O
NH
NH
NH
NH
C
O
C
O
H
H
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5. For each reaction below, circle the side of the reaction that would be favored. You have not
been provided pKa values, so use your knowledge of how the structure of an acid affects its
relative acidity. Provide a short (one sentence) explanation for the direction of the
equilibrium. (16 points)
a.
b.
c.
d.
H 3CS
O
OH
A
O
O
+ +
Cl
H
O
3CS
O
B is a stronger acid than A because Cl is a stronger inductive electron-withdrawing
group than SCH 3.
F + HCl HF + Cl
HCl is a stronger acid than HF because Cl is larger than F (lower on periodic table).
O
O
NH
O
+ H N +
O
N is more electronegative than C, so the N-H bond is more acidic.
H C N + C C H
C N + H C C H
HCN is a stronger acid than acetylene because of the higher inductive
withdrawing effect of N vs. C.
O
O
O
O
H
H
Cl
O
B
7
OH

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6. Shown below is the synthesis of methyl tert-butyl ether (MTBE). MTBE was used as a
gasoline additive, but its use was discontinued due to problems with it contaminating
groundwater. Provide a detailed mechanism for this reaction that shows each intermediate.
Use arrows to show electron motion and indicate any formal charges. Note that the reactivity
of methanol (CH3OH) is similar to that of water. (16 points)
+
+ HOCH 3
H
O CH3 H
(from HOCH 3
and H 2SO 4)
H
O CH3
H
H 2SO 4
(cat)
+ HOCH 3
OCH 3
methyl tert-butyl ether (MTBE)
H
HOCH 3
OCH 3
H
+ O CH 3
H
H
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