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Chemistry 241 Exam #4 April 23, 2007


1. (a) Consider the following reaction. Show the mechanism leading to the product expected from the reaction of the more reactive C=C (make the best possible cation) at LOW temperatures. In a sentence or two, why do you expect this product? (10 pts) H-Br

(b) Comment on the relative length of the indicated C-C bonds, with a brief explanation as to why the lengths are different: (5 pts)

2. Show the molecular orbitals for ethene (include phases, electrons, descriptors for levels, and HOMO/LUMO designation). Two molecules of ethene will NOT react to form a molecule of cyclobutene. Show why this result is predicted by Molecular Orbital Theory. (9 pts)

2. Draw the product from the following Diels-Alder reaction. Please be careful to indicate exo- or endo- stereochemistry, and briefly comment on why you observe the stereochemistry shown. (8 pts) heat +

3. Explain the 4n+2 Rule for aromaticity. Use it to indicate which of the following is aromatic. Be sure to show electron counts for each! What other attributes are necessary for aromaticity to occur? (8 pts)

4. Based on the following spectral information, draw the structure for ONE of the following two compounds. Show work for partial credit and be sure to indicate which one of the following two that you want me to grade! If you do not indicate, I will grade the compound 1. (20 pts) Compound 1: C4 H7BrO, strong IR absorbance at 1715 cm-1 1

H NMR: δ 2.1 (s, 3H); 2.4 (t, 2H); 4.1 (t, 2H) C NMR: δ 17.1 (q); 25.3 (t); 51.4 (t); 182.8 (s)


Compound 2: C4 H10O, medium IR absorbances at about 2950 cm-1 1

H NMR: δ 0.95 (d, 6 H); 4.22 (septet, 1 H), 4.37 (s, 3H) C NMR: δ 21.3 (q); 65.3 (d); 68.9 (q)


5. Sketch the 1H and 13C NMR of each of the following compounds. For (a), also sketch the DEPT-135 carbon spectra. O


(25 pts)






(15 pts)