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.
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.
(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.
1 The pure (S) isomer would make his customers happy. The (R) isomer has different biological properties, acting mainly as a nasal decongestant.
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.
(d)SN2 reaction; iodide is better leaving group.
(e) SN2 reaction is faster in polar aprotic solvents like acetone.
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 (c) (10 pts.) Draw one molecule that is a diastereomer of A, B, C, and Cʹ.
Q4 (50 pts.) The molecules menthol and isomenthol are depicted below.
(a) (40 pts.) Draw the two chair conformations of each compound.
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.
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.
(b) (S)-Omeprazole (“esomeprazole”, Nexium) is the newer version of the drug. Draw the structure of this molecule with the appropriate stereochemistry indicated.
(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?
