Chemistry 51 Exam #3 Name KEY November 20, 2001 This exam has nine (9) questions. Please check before beginning to make sure no questions are missing. All scratch work must be done on the attached blank pages, which will be collected. Please sign BOT cover pages.
Chemistry 51 Exam #3 Name KEY November 20, 2001 (includes honor pledge) Periodic Table of the Elements Group V V V V O Period 1. 10 1 e 1 1 2 2. 15 2 Li Be B C N O F Ne 3 4 5 6 7 8 9 10 3. 20 3 Na Mg Al Si P S Ar 11 12 Transition elements 13 14 15 16 17 18 4. 15 4 K Ca Se Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Kr 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 5. 10 5 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd n Sn Sb Te Xe 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 6. 10 6 Cs Ba La f Ta W Re Os r Pt Au g Tl Pb Bi Po At Rn 55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 7. 35 7 Fr Ra Ac 87 88 89 8. 10 Lanthanide Ce Pr Nd Pm Sm Eu Gd Tb y o Er Tm Yb Lu Series 58 59 60 61 62 63 64 65 66 67 68 69 70 71 9. 15 Actinide Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lw Series 90 91 92 93 94 95 96 97 98 99 100 101 102 103 AVCE: A picture is worth a thousand words!
-1-1. On treatment with, 2S-bromo-3R-hydroxybutane is converted into meso-2,3- dibromobutane. Provide a detailed mechanistic rationale for this transformation. (10 pts). -O O O + - - 2. S-1-deutero-1-chloro-cis-2-pentene is reacted with the acetylenic anion derived from 1- butyne to give a compound A of molecular formula C 9 13. (a) Provide the structure of A, showing all stereochemical features, and explain why A does or does not exhibit optical activity. An accurate drawing is necessary (5 pts). + S N 2 A (chiral) (b) Compound A is reduced with 2 and Pd/Pb(OAc) 2 (Lindlar's catalyst) to a di-alkene B. oes B exhibit optical activity? Explain why or why not. (5 pts). A Lindlar's catalyst B (achiral) (c) Compound A is reduced with Na/N 3 to a di-alkene C. oes C exhibit optical activity? Explain why or why not. (5 pts). A Na/N 3 C (chiral)
-2-3. Provide the most efficient synthesis possible for each of the following materials from the indicated starting compounds. Read the question carefully. There will be no credit given for preparing the correct product from incorrect starting materials, or for preparing an incorrect product from correct starting materials. (20 pts). (a) 2-Chloro-4-methylpentane from 1-propene and any inorganic reagents of your choice NBS + CuLi 2 Cu Li Li (b) 2-deuterobutane from propyne and any other reagents of your choice. 3 C 1) NaN 2 2) 3 C Na/N 3 or Lindlar Mg Mg 2 O 4. (a) raw Fischer projection formulas for all possible stereoisomers of 2,4-dibromo-3- chloropentane, and indicate enantiomeric, diastereomeric, and meso relationships. (10 pts). 1 2 3 4 (enantiomers) (meso) (meso) Compounds 1 and 2 are diastereomers of 3 and 4 (b) ow many fractions would one obtain upon distillation? Which compounds would b e found in each fraction (indicate by compound number from above). (5 pts). Three fractions: Racemic mixture of 1 and 2, and two meso compounds, 3 and 4.
-3-5. (a) Arrange the compounds below in order of decreasing reactivity toward S N 2 displacement (i.e. most reactive first). You must show structures! (5 pts). bromomethane, bromoethane, 1-bromo-3-methylbutane, 2-bromo-2-methylbutane, 2-bromo-3-methylbutane. S N 2 Reactivity 3 C C 2 C C 2 C 2 C 2 C 2 3 C C 2 3 C (b) Arrange the compounds below in order of decreasing reactivity toward E1 elimination (i.e. most reactive first). You must show structures! (5 pts). iodomethane, iodoethane, 2-iodopropane, 1-iodo-1-phenylethane, 2-iodo-2-phenylpropane E1 Reactivity C C C C 2 3 C 6. Which of the compounds indicated below are chiral? Provide a brief explanation for your answer (2-3 words plus a planar drawing will do). (10 pts). (a) cis-1,2-dihydroxycyclohexane mirror images superimposable - Achiral O O O O (b) trans-1,2-dimethylcyclohexane mirror images non-superimposable - Chiral 3 C 3 C (c) cis-1,3-dichlorocyclohexane mirror images superimposable - Achiral
(d) trans-1,4-diethylcyclohexane -4- mirror images superimposable - Achiral Et Et Et Et (e) 1,1,3-trimethylcyclohexane mirror images non-superimposable - Chiral 3 C 3 C 7. Predict the major product, including stereochemistry where applicable, for each reaction indicated below. You must draw your answer neatly in the box to receive credit. (35 pts). (a) (2R,3R)-2-omo-2,3-diphenylbutane + KO/ethanol Ph Ph (b) -C C - Na + + pure (R)-1-deutero-1-bromopropane (c) -C C - Na + + 2-chloro-2-methylbutane.
-5- (d) S-2-hydroxybutane + P 3 /ether (e) S-3-Chlorocyclopentene + methanol/ (no base) O (nearly racemic) (f) cis-1-chloro-2-tert-butylcyclohexane + KO/ethanol (g) trans-1-omo-2-methycyclohexane + KO/ethanol (racemic)
-6-8. The reaction shown below was carried out in two different solvents, methanol ( O) and acetonitrile ( CN). Which reaction occurred faster and why? llustrate by filling in the energy diagram, clearly showing any stabilizing or destabilizing effects, and the relative energies of all species involved. (10 pts). C 2 C 2 C 2 - + Na + N 3 - ------> C 2 C 2 -N 3 + Na + - O selectively solvates (and stabilizes) N 3 -, lowering its energy and resulting in a higher activation energy (red curve). CN selectively solvates Na +, thereby increasing the energy of N 3 -, and lowering the activation energy (blue curve). The reaction is therefore faster in CN. :NC- Na + :NC- N 3 - :NC- + O G Energy N 3 - O Na + N 3 O Reaction progress 9. Circle the compounds drawn below that are meso structures. (15 pts). F 3 C F 3 C 3 C 3 C