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Unformatted text preview: Circle one: I wish to have my exam . | wish to pick up put in the rack. my exam. Printed Name . (Please print clearly) Signature . CHEMISTRY 252 Exam || Februaty 28, 2007 100 Points 6:30-8:30 PM This exam has 8 grobiems on pages 2 through 8. RULES 1. The use of a calculator and model kits are not permitted. 2. This exam is closed book and closed note. No aids other than writing implements are permitted. 3. Answer the questions in the spaces provided on this exam. 4. If you wish to ask a question about procedures or about a problem on the exam, raise your hand. TOTAL: I100 1. Predict the major product or products (be inclusive) that you would expect to be formed in five of the following seven reactions. If you feel that no reaction will occur, then answer no reaction. Be sure to answer only five problems. If you answer more than five, then only your first five wlll be graded. (15 points! 3 pts. each) 2 equiv. OH (a) MeMgBr 1) FCC, CHQCI2 (b) ()2 2) H2N-NHPh 1) excess MeOH HCI 0 2) H2, Pd on Carbon (0) H 3) HCI, excess water S 1) nBuLi 2) CH2=CHCH28r (d) H S 3) W", H20, HgClg 1) BH3 2) FCC, CHzClz 3) Pth-CHE O 1) NaBH4, MeOH CE! (0 H 2) H’r workup Br 1) Li : H (g) 2JH9(0AC)2 H20, H2504 2. {III Assign the structure of the unknown compound that gives rise to the proton NMR and carbon NMFl illustrated below. Please pay attention to the notes written on the spectra. All of the NMR signals for the molecule are shown and are “referenced” to tetramethylsilane. The multiplicities for the off resonance 13'0 NMFl spectrum are given in parenthesis (s=singlet, d=doub|et, t=triplet, q=quartet). The IR spectrum for the molecule shows strong, sharp stretches at 1700 cm'1 and 1674 cm". By high resolution mass spectroscopy, the molecular formula for the compound was found to be CaHtaBrO. in addition, the molecule was found to have an optical rotation [01020 = -28.2° (0:0.04 gme, CHEClg) (Hint: The molecule is chr'rall). At the end of the problem, be sure that you clearly indicate what structure you want graded. Please note that we will give you partial credit for assigning fragments of the molecule correctly, but only for fragments that are incorporated into your final structure. There is extra room at the top of the next page if you need it.--(15 points) lfl-I “I ll" lZI 1N _ 3| 6| III!“ Zl nnl 3. Suggest a sequence of steps that would alfow for completion of the foilowing transformation. Make sure that you show your retrosynthetic analysis. (10 points) OH 9 OH 6 ' mm 4. :1. Recently, a graduate student in the department made the two molecules illustrated below. The two molecules represent an approximately 70:30 ratio of double bond stereoisomers. Protons Hc—He do not feel the effect of the double bond isomers and appear in the same place for both isomers. With that in mind, label the protons (Hat, Hbt, Hac, Hbc, Hc, Hd, and He) on the portion of the proton NMR shown below the structures. Please ignore any fine coupling. For example, the signal at 5.9 should be treated as a doublet. (7 points) Hac‘“ ' .‘Hl Tia I“ ' H P W W [H 5.4 5.2 6.0 5.8 5.5 I.‘ 5.2 5.0 I |_.._I n_'__..2 |_'__f I_,_|_'._I_._....._.|.._'_| I IJ! 0.11 h“ I.” ‘ O. n O. n I.“ II. I. b. Draw a splitting tree that explains the coupling pattern observed for the signal at 4.3 ppm. (3 points) c. Does the major product in the mixture have a cis or a trans double bond? Why? (5 points) 5. a. Write a “curved-arrow” mechanism for the reaction illustrated below. (5 points) 0 TSOH OBu H (toluene sulfonic acid) OBu BuOH H 3A molecular sieves b. Three angstrom molecular sieves are often used to scavenge water from a reaction. Why was this done for the reaction above? (5 points) 6. The presence of an aromatic ring in a molecule often causes some protons to go downfield and others to go upfield. Let’s see if we can explain why. :1. Use a drawing to illustrate what is meant by anisotropy. For your drawing, use benzene as your example of an aromatic ring. (5 points) b. Using your drawing, why do the protons on an aromatic ring give rise to NMR signals that are all the way downfield (high chemical shift) in the 7-8 ppm region. (5 points) c. Upfield (lower frequency/chemical shift) shifts are usually observed in protons that are not directly bound to the aromatic ring but rather located close to the aromatic ring due to a variety of molecular constraints. Why might the signal for a proton located close to an aromatic ring be upfield from where you would normally expect to find it? (5 points) 7. a. Write a “curved-arrow” mechanism for the following epimerizati on reaction. (5 points) 0 K+B+CH3 O s” CH3 H CH3 “ CH3 b. Knowing that the pKa of a proton a to a ketone is approximately 20 and that the pKa of t- BuOI-I is approximately 19, explain why this reaction occurs readily at room temperature. (5 points) 8. Explain the following (keep your answers short): (2 points each) 3. Why are n to 71:* transitions forbidden? 1). Why are aldehydes more reactive than ketones? c. Why do alcohol oxidations need water to form acids? (1. Why are amines most often not chiral even when they have four different groups (three different substituents and a lone pair) surrounding the nitrogen? e. Why are amines weaker acids than alcohols? ...
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