TAMU Chemistry Question

Name: ____KEY_________________University of Pittsburgh
Organic Chemistry 1 • Fall 2022
Instructor: Dr. Yiming Wang
Midterm Exam #2
October 19, 2022, 6:00 PM – 7:15 PM E.S.T.
Q1 (50 pts): ______
Q2 (50 pts): ______
Q3 (50 pts): ______
Q4 (50 pts): ______
EC (15 pts): ______
________________________
Exam Score (200 pts total): ______
Instructions: Please turn off all electronic devices before the exam. During the exam, you are
permitted: a molecular model kit and two 8½ in. × 11 in. sheets of handwritten notes. (Front and
back; no typed or electronic notes!) Stencils for drawing conformations are not permitted. Put
your name on every page in case your exam packet is disassembled during the grading
process. The use of blue or black ink (rather than pencil) is strongly encouraged. Please let me
or the TAs know if you need additional sheets of scrap paper.
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Q1 (50 pts., 10 pts. each) Provide a short response and/or a brief explanation for each of the
questions below. Use no more than three (3) sentences and one (1) drawing for an explanation.
An excessively long response may be penalized even if it is correct.
(a) Using dash-wedge notation, draw all stereoisomers of the molecule shown below. Show
each stereoisomer only once. Give the total number of stereoisomers in the box provided.
2 points for every non-repeated stereoisomer shown. 4 points for correct number of
stereoisomers.
(b) Assign (R) or (S) for all stereocenters in the two structures shown below. State the
relationship between the structures. Are they different depictions of the same molecule,
enantiomers, or diastereomers?
1.5 points each for (R)/(S) assignment. 4 points for correct relationship.
(c) For the reaction shown below, circle the solvent (MeOH, CHCl3, DMF) that results in the
fastest reaction rate. Briefly explain your choice.
5 points for correct choice, 5 points for explanation.
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(d) An enterprising, self-employed chemist (who isn’t nearly as good as Walter White)
prepared a batch of N-methyl-1-phenylpropan-2-ammonium chloride with [α]D25 = +8.
The Merck Index indicates that [α]D25 = +20 for a pure sample of the (S) isomer.1 What is
the percent enantiomeric excess (% e.e.) of the product?
The enantiomer excess is 8/20, or, as a percentage, 40% e.e.
7 points if division is shown, but not converted to a percentage.
(e) Predict the product(s) of the following substitution reaction. Be sure to show all
stereoisomers formed. (Note: When predicting the product, show only the stable organic
product(s) formed at the end of the reaction. Do not show any reactive intermediates
involved in the mechanism.)
5 points if only one diastereomer is shown. No credit for showing carbocation or an
incorrect Lewis structure. 5 point deduction for showing products that are still protonated.
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The pure (S) isomer would make his customers happy. The (R) isomer has different biological properties,
acting mainly as a nasal decongestant.
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Q2 (50 pts., 10 pts. each) For each of the subproblems below, choose the faster reaction by
circling it. For each pair of reactions you are comparing, assume that all other reaction conditions
that are not explicitly shown (e.g., reagent concentration, temperature, etc.) are the same for both
reactions. You do not have to explain your choice.
(a)
Beta branching slows down SN2 reactions.
(b)
SN2 reaction; heavier element in same column is more nucleophilic.
(c)
Secondary (benzylic) carbocation forms more readily in SN1 reaction.
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(d)
SN2 reaction; iodide is better leaving group.
(e)
SN2 reaction is faster in polar aprotic solvents like acetone.
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Q3 (50 pts.) Answer the following questions about structures A − C, shown below. For this
question, you may assume that we are operating at temperatures at which chair flips occur
rapidly.
(a) (30 pts.) For each of A, B, and C, state whether the molecule is chiral or achiral.
A: achiral
B: chiral
C: chiral
10 points for each correct statement.
(b) (10 pts.) State the relationship between B and C. Are they different depictions of the
same molecule, enantiomers, or diastereomers? (Think carefully before you answer!
Hint: First consider how C and Cʹ are related to each other.)
This is a tricky question. First note that C and Cʹ are the same molecule. For C, Cʹ (as
well as B), the dash/wedge bonds between carbon and bromine are superfluous – there is
no stereochemistry there! Now note that B and Cʹ are chiral molecules and that they are
minor images of each other. Thus, B and C (which is the same as Cʹ) are enantiomers.
(c) (10 pts.) Draw one molecule that is a diastereomer of A, B, C, and Cʹ.
(You could show the structure with chlorines down and bromine up; it’s the same
molecule.)
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Q4 (50 pts.) The molecules menthol and isomenthol are depicted below.
(a) (40 pts.) Draw the two chair conformations of each compound. Circle the more stable
chair conformation for each compound. (Potentially useful A values: Me (1.70), iPr
(2.20), OH (0.94).)
8 points for each correct chair. 4 points for each correct conformer circled.
(b) (10 pts.) Based on the conformations available to these two molecules, which compound
is more stable, menthol or isomenthol? Give a one sentence justification.
Menthol is more stable, since there is a conformation in which all substituents are
equatorially disposed.
5 points for correct choice, 5 points for correct explanation.
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Extra credit (15 pts., 5 pts. each).
(a) Racemic omeprazole (Prilosec) is a proton pump inhibitor used to treat heartburn. Circle
the stereocenter in this molecule.
Sulfoxides have stereogenic lone pairs that are configurationally stable.
(b) (S)-Omeprazole (“esomeprazole”, Nexium) is the newer version of the drug. Draw the
structure of this molecule with the appropriate stereochemistry indicated.
I have explicitly showed the lone pair here. As long as your depiction is unambiguous,
the lone pair does not need to be shown.
(c) (Continuation of Q1d): In a 10 g sample of the product he made, how many grams each
of the (R) and (S) isomers are present?
The sample would contain 3 grams of (R) and 7 grams of (S).
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Use this sheet for additional responses to the exam questions if you run out of space. Be sure to
label your additional responses with the question number.
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