This is a long exam. Most students will not finish each problem. Carefully select your problems to work on.

Transcription

1 Chemistry 51 Exam 1, Spring 2008 =============================================== This is a closed book exam. The exam lasts 75 minutes. All answers must appear on the answer sheet. Only the answer sheet will be collected. Put your name on the answer sheet now. When the exam is over you must hand your answer sheet in promptly or a penalty may be assessed. Each question is worth 7 points. You may not leave the room during the exam. This is a long exam. Most students will not finish each problem. Carefully select your problems to work on. =============================================================== 1. Name the following compound. Use the IUPAC system and include configurational (R,S) designations. O C OH 2. You have a racemic mixture of 2-methylbutanoic acid and need to separate it into enantiomers. You have a well equipped laboratory with which to work. Provide details on a method which is likely to accomplish the separation for you. 3. The mass spectrum of propyne includes a peak at 40 daltons (atomic mass units). The peaks in a mass spectrum are interpreted as the mass of the molecular species. The peak at 40 is taken to be the mass of a propyne molecule (molecular weight of 40). The propyne is dissolved in NaND 2 / ND 3 (D is deuterium, an isotope of hydrogen) and the propyne is then recovered. The mass spectrum is taken of the recovered propyne and the peak at 40 daltons is almost absent and a new peak at 41 daltons is quite large. Give a section complete explanation for the appearance of the peak at 41 daltons

2 4. Carefully show the three dimensional structure of the substituted decalin below by completing either template A or template B as appropriate. OH substituted decalin A B 2-5) Consider the reaction of acidic dichromate ion, Cr 2 O 7 benzoic acid, acetic acid, and Cr 3+ ion. CO 2 H and 1-phenyl propane to yield Cr 2 O CH 3 CO 2 H + Cr +3 H + a) Write the balanced organic half reaction. b) Write the balanced overall reaction.

3 6) Provide a careful explanation of which compound, A or B, is the stronger acid OH OH CF 3 CF 3 A B 7. Penta-1,4-diene reacts with bromine, Br 2, at high temperature to yield 5-bromo penta- 1,3-diene. Describe the structure of the radical intermediate involved in two ways a) draw the resonance structures which contribute to the resonance hybrid b) draw the molecular orbital which holds the unpaired electron. 8. Alkenes A and B below are each reacted with HCl and an alkyl chloride is produced. Upon analysis it is found that the same alkyl chloride is produced from both A and B. Offer a mechanistic explanation and give a structure for the alkyl chloride. A B

4 9. Using the graph below construct the rotational energy curve around the central, emphasized bond. Rotate the front carbon as shown in a counter clockwise direction. H Me Me E Me t - Bu H initial, zero degree, conformation ============================================================== For the following reactions give the missing reactants or products. Show the stereochemistry. Write "NR" if there is no reaction. Put answers on the answer sheet. In some questions you are provided with templates for the answer. Use as many of the templates as needed. If additional templates are needed you should draw identical structures. If you are unsure about the notation ask the monitor. 10. propyne sia 2 BH H 2 O 2, OH -

5 11. Cl 2 All unique monochloro products (show stereochemistry) Indicate the fraction of each product in the mixture. (1:4:5) =========================================================== Provide a synthesis of the following compounds using ethyne as the sole source of carbon atoms. You may use a target molecule from an earlier question in a later question even if you were not able to synthesize it in the earlier question. Do not repeat a synthesis of an earlier compound, simply refer to it by letter. 12. but-1-yne 13. trans hex-3-ene 14. meso hexan-3,4-diol g of compound X is dissolved in ml of water. The aqueous solution is then shaken with ml of ether and the layers separated. The ether layer is evaporated. The residue of the evaporation is identified as compound X and has a mass of 6.00 g. What is the value of K D? (Be sure to put answer on answer sheet.)

6 Answer Sheet Chemistry 51, Spring 2008 All questions 7 pts Your Name Recitation Instructor: Fang ; Kunjappu ;Metlitsky Zamadar Use one one of the above templates 5. Organic half reaction: Overall reaction resonance molecular orbital

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