CHEMISTRY 332 FALL 08 EXAM II October 22-23, 2008

Transcription

1 First Three Letters of Last Name NAME Network ID CHEMISTRY 332 FALL 08 EXAM II October 22-23, 2008 The following materials are permissible during the exam: molecular model kits, course notes (printed, electronic, or hand written), textbook, calculator, Internet browser. You are not permitted to receive assistance, in any way, from another person during the exam. Receiving aid on an exam from any person, by any means of communication, is considered academic misconduct and can result in a failing grade for that exam. Answers must be submitted via ACE Organic during the scheduled examination period. The paper copy of your exam will be collected, but not graded. Your goal is to answer every question correctly, in as few attempts as possible. The exam will be graded out of 200 points, but there are 203 points possible. The total number of residual points is 89.

2 QUESTION 1 [3 pts. full credit; 0 pts. residual credit] The carbon skeleton of acepentalene (C 10 H 6 ) is shown below. Assume all 10 carbon atoms to be sp 2 hybridized. Build a model of acepentalene in SHMO. Adjust the electron count to completely fill all of the pi MO levels that are below the energy of the carbon 2p atomic orbital. This electron configuration gives acepentalene "aromatic" properties. How many electrons are in the pi system of acepentalene when it exhibits aromatic character (check one box only)? QUESTION 2 [8 pts. full credit; 4 pts. residual credit] The skeleton of nitrogen-containing acepentalene A is shown below. Build a model of A in SHMO, again assuming that each atom is sp 2 hybridized. Follow the instructions in QUESTION 1 to adjust the pi electron count of A such that it will have aromatic properties. Based on the electron count that gives this molecule aromatic character, draw a resonance contributor of A. 1

3 QUESTION 3 [8 pts. full credit; 4 pts. residual credit] The same pi electron count was required to give aromatic character to acepentalene (QUESTION 1) and A (QUESTION 2). Assume that this same number of pi electrons is required for any heteroacepentalene. The skeleton of oxygencontaining acepentalene B is shown below, each atom being in the sp 2 hybridization state. Draw a resonance contributor of B that is expected to show aromatic character (SHMO will not help you for this problem because it is not programmed for this type of oxygen atom). 2

4 QUESTION 4 [16 pts. full credit; 8 pts. residual credit] A recent publication describes efforts directed towards the synthesis of oxygen-containing acepentalene B (J. Am. Chem. Soc. 2008, 130, ). The first steps of the synthesis are outlined below. Add molecular iodine (I 2 ) to box 1. Draw intermediates in boxes 2 and 3 and curved arrows for each step to show the electron flow consistent with the frontier orbital interactions or elementary mechanism step as indicated. QUESTION 5 [10 pts. full credit; 5 pts. residual credit] The next step of the reaction sequence involves an [E2] elimination using the strong organic base, DBU. Draw both the elimination product and the conjugate acid of DBU in box 6. Provide curved arrows to show the electron flow for this elimination. Hint: One nitrogen of DBU is more basic than the other. 3

5 QUESTION 6 [10 pts. full credit; 5 pts. residual credit] Paste the structure of the elimination product from box 6, QUESTION 5 into box 7. When the structure in box 7 is subjected to acidic conditions, the oxonium ion in box 9 is isolated. Provide the structure of the carbocation intermediate in box 8 and show curved arrow notation for both steps consistent with the indicated elementary steps. QUESTION 7 [3 pts. full credit; 0 pts. residual credit] How is the hydrogen labeled (a) related to the hydrogen labeled (b)? 4

6 QUESTION 8 [3 pts. full credit; 0 pts. residual credit] The 13 C NMR spectrum of the indicated oxonium ion was recorded; how many different types of carbon atoms were observed? QUESTION 9 [3 pts. full credit; 0 pts. residual credit] Consider the carbocation intermediate in box 8 of QUESTION 6. How is the space above the carbocation related to the space below it? QUESTION 10 [6 pts. full credit; 0 pts. residual credit] When the oxonium ion was subjected to an aqueous solution of potassium cyanide, the indicated nitrile was obtained as a single diastereomeric product. Which elementary step was involved in this transformation? 5

7 QUESTION 11 [16 pts. full credit; 10 pts. residual credit] Draw the missing intermediates and use curved arrows to show the electron flow, consistent with the instructions provided. The second half of the mechanism is in QUESTION 12, in case you need to work the problem backwards. (Ref: J. Org. Chem. 2008; ASAP Article; DOI: /jo ) 6

8 QUESTION 12 [16 pts. full credit; 10 pts. residual credit] Draw the missing intermediates and use curved arrows to show the electron flow, consistent with the instructions provided. The first half of the mechanism is in QUESTION 11. Paste the intermediate from Box 13 of the previous question into Box 13 here. 7

9 QUESTION 13 [16 pts. full credit; 8 pts. residual credit] Draw the missing intermediates and use curved arrows to show the electron flow, consistent with the instructions provided. The second half of the mechanism is in QUESTION 14, in case you need to work the problem backwards. (Ref: J. Org. Chem. 2008, 73, 7498) 8

10 QUESTION 14 [16 pts. full credit; 8 pts. residual credit] Draw the missing intermediates and use curved arrows to show the electron flow, consistent with the instructions provided. The first half of the mechanism is in QUESTION 13. 9

11 QUESTION 15 [8 pts. full credit; 0 pts. residual credit] The overall transformation for the mechanism written in QUESTIONS 13 and 14 is shown below. Check all of the statements that are true about this reaction, its mechanism, or the intermediates involved. QUESTION 16 [6 pts. full credit; 3 pts. residual credit] Draw the best resonance contributor for the conjugate base of oxonium ion J. 10

12 QUESTION 17 [8 pts. full credit; 0 pts. residual credit] Build a model of oxonium ion J in SHMO. Which two atoms are most likely to be attacked by a nucleophile (check two boxes)? QUESTION 18 [6 pts. full credit; 3 pts. residual credit] Draw the best resonance contributor for a tautomer of T in which the -OH oxygen becomes an O1 oxygen, and the N2 nitrogen becomes an N3 nitrogen. 11

13 QUESTION 19 [8 pts. full credit; 0 pts. residual credit] Use SHMO to build a model of J, the conjugate base of J, compound T, and the tautomer of T. According to the energy levels calculated by SHMO, rank these from most reactive electrophile to least reactive electrophile (most reactive =1; least reactive = 4). 12

14 QUESTION 20 [16 pts. full credit; 10 pts. residual credit] Draw the missing intermediates and use curved arrows to show the electron flow, consistent with the instructions provided. The second half of the mechanism is in QUESTION 21, in case you need to work the problem backwards. (Ref: Tetrahedron Lett. 2008, 49, 4446) QUESTION 21 [12 pts. full credit; 8 pts. residual credit] Draw the missing intermediates and use curved arrows to show the electron flow, consistent with the instructions provided. 13

15 QUESTION 22 [6 pts. full credit; 3 pts. residual credit] Draw the most stable tautomer of the product from QUESTION