Chemistry 51 Summer 2006 Final Exam

Transcription

1 Chemistry 51 Summer 2006 Final Exam 1. This is a two-hour, closed book exam. All answers are to be put on the question sheets. 2. Use the backsides of these pages for scratch paper. None other permitted. 3. At the end of the exam pass in all exam material. 4. There are 20 questions. Make sure you have all of them. Each question is worth five points. 5. Write your name on all pages of this exam. Do it now! What are the products or missing reactants of the following reactions? Indicate stereochemistry and make use of templates if provided. 1. O NH(CH 3 ) 2 weak acid 2 OH aq. Cl 2 acid 3. propyne C 4 H 9 Li PhCH 2 Br Na, NH 3

2 4 Br 2 CH 3 NH 2 5. O trans 2-butene 1. OsO 4 2. NaHSO 3 6. PhCO 3 H (CH 2 =CH) 2 CuLi 7. HBr Mg 1. CH 3 CHO peroxides 2. dil acid 8. Br H CH 3 base CHCl 3, KOC(CH 3 ) 3 Ph CH 3 H

3 9) Suggest a mechanism for the following reaction. H 3 C C N H + H 2 O H 3 C C O OH + NH You synthesized cyclohexene from cyclohexanol using acid to perform the dehydration.. a) Why was the acid necessary? b) The crude reaction product was purified by washing with concentrated sulfuric acid. Why was the washing with sulfuric acid done? 11. Formamide, shown below, is a completely planar molecule. O H NH 2 A) Draw a resonance structure for the molecule wherein each atom obeys the octet rule.

4 B) Utilizing the resonance structure provide an explanation of the planarity of the molecule. 12. The following compound is hydrogenated and the product mixture carefully distilled. Complete the table to characterize the product mixture and the results of the distillation. 3 H 2 catalyst 2,3,5,6,7,8-hexamethylbicyclo[2.2.2]octa-2,5,7-triene How many stereoisomers are produced? How many chiral stereoisomers? How many fractions in the distillation?

5 13) a) Complete the potential energy barrier diagram for rotation about the central bond (C2 - C3) of the 2,3-diphenylbutane stereoisomer shown below. The rotation of C2 is to be counter-clockwise as shown. The conformation shown is to be taken as zero degrees. H H 3 C Ph H 3 C H Ph Energy

6 b) Complete the template below to represent the conformation of lowest energy. H (S) H 3 C Ph c) Sketch the expected nmr spectrum for this conformation. Show splittings carefully. 14. Cis 1,2-dimethyl cyclohexane is an optically inactive molecule. Consider the three dimensional structure of the molecule and explain the lack of optical activity.

7 15. a) Rank the following for relative acid strength. Use the letters to identify the compounds. a) 4-chlorobutanoic acid b) 3-chlorobutanoic acid c) 2-chlorobutanoic acid Answer: > > b) Explain your rankings in part a) c) Rank the following for relative base strength. Use the letters to identify the compounds. A) CH 4 B) H 2 O C) NH 3 Answer: > > d) Explain your answer given in part c) 16. Limonene, a naturally occurring alkene has either structure A or B. Present an

8 argument for A or B being the correct structure. A B Provide a synthesis of the following compounds. Use only 2-propanol as the source of all carbon atoms in the target molecules. In later problems you may also use as starting material target molecules of earlier problems even if you were not able to solve the earlier problem. Do not repeat synthetic work. You may reference a compound prepared in an earlier problem by a symbol such as 14A bromo propene bromo propan-1-ol

9 16. 4-methyl pentane -1,4 - diol 17. Compound A has molecular formula C 6 H 14 O. It readily undergoes dehydration. Compound A has the following peaks in the 1 H NMR spectrum at δ 0.90 (t, 6H), 1.12 (s, 3H), 1.38 (s, 1H) and 1.48 (q, 4H). The 13 C NMR spectrum shows signals at 72.98, 33.72, 25.85, and Deduce a structure for the unknown compound.

10 18 Two mass spectra are presented below. One is 3-pentanone and the other is 2- pentanone. Present an argument for assigning the identities of each spectra.

11 19 Compound B has the molecular formula C 8 H 10 O. The 1 H NMR and IR spectra are presented below. is shown below Deduce the structure of the unkown.

12 20. The 1 H NMR spectrum of cyclohexane d 11 (cyclohexane with only one hydrogen and eleven deuterium atoms) is shown for several temperatures. The deuterium atoms are invisible in the spectrum. Offer an interpretation of the spectrum.

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14 13 C shifts