- NH2 or NH 3 H 2 O or CH 3 COO - BF 3 or F - I Br. or Br

Transcription

1 CEMSTRY MDTERM # 3 answer key November 09, 2004 Statistics: Average: 71 pts (71%); ighest: 90 pts (90%); Lowest: 49 pts (49%) Number of students perfming at above average: 22 (51%) 1. (8 pts) Mark as true (T) false (F) the following statements. Do not explain! (F) omination of alkenes occurs via syn addition; (F) ydrogenation of alkenes occurs via anti addition; (F) Chiral molecules have only one internal mirr plane of symmetry; (F) Enantiomers are superimposable mirr images; (F) Diastereomers are non-superimposable mirr images; (F) A molecule without a chiral center can never be chiral; (T) S N 2 reactions are stereospecific; (F) S N 2 reactions are accelerated by polar protic solvents; 2. Circle ALL that apply: A. (2 pts) The following reactions of alkenes give the Markovnikov product: a. ydrogenation; b. Free-radical addition of ; c. ydrobation-xidation; d. Epoxidation; B. (2 pts) The following reactions are stereospecific: a. omination of alkenes ; b. S N 2 reactions; c. ydrobation-xidation of alkenes ; d. Acid-catalyzed hydration of alkenes; C. (2 pts) The following reactions are used to introduce a hydroxyl group in a molecule: a. Acid-catalyzed hydartion; b. zonolysis followed by reductive hydrolysis; c. Reaction of halogen with an alkene in water; d. ydrobromination of alkenes; D. (2 pts) Circle the impossible combinations: a. A chiral molecule without a chirality center; b. An achiral molecule with one chirality center; c. An achiral molecule with two chirality centers; d. A chiral molecule with a superimposable mirr image; E. (2 pts) Circle the crect statements about the S N 2 mechanism: a. Concerted; b. Reactions are fastest with tertiary substrates; c. as a bimolecular rate-determining step; d. s accelerated by polar protic solvents; 3. (4 pts) n each of the following pairs, indicate the substrate with higher reactivity in S N 2 reactions. Do not explain! 4. (6 pts) F each of the following pairs, indicate the stronger nucleophile. Do not explain! - N2 N 3 2 C 3 C - BF 3 F - (C 3 ) 3 P (C 3 ) 3 N - - C 3 S C 3

2 5. (8 pts) Draw an acceptable three-dimensional representation (i.e. Fischer projection bald- dashed-wedge structure) f each of the following molecules: A. (2R,3S)-2,3-dibromohexane. C 3 C 2 C 2 C 3 B. meso-3,4-hexanediol. C 2 C 3 C 2 C 3 C. (R)-1,1,2-trimethylcyclohexane; 3 C C 3 C 3 D. (1R,2R)-1,2-dibromocyclohexane. 6. (14 pts) F each of the following pairs, indicate the relationship between the compounds (i.e. identical, structural isomers, enantiomers, diastereomers). Do not explain! C 3 F F C 3 identical enantiomers 2 C C 3 3 C C 2 F F enantiomers identical C 3 C 3 3 C C 3 diastereomers identical C C C C C C identical C 3 C 3 7. (4 pts) When 4-penten-1-ol is treated with aqueous 2, a cyclic bromo ether is fmed, rather than the expected bromohydrin. Propose a detailed mechanism f this conversion

3 8. (4 pts) The reaction of addition of to 3-bromocyclohexene leads to the generation of a single product out of four possible ones. ffer an explanation, using appropriate structural drawings equations. Explanation: only product not fmed not fmed not fmed attack from opposite side exclusive protonation at one of the double-bonded carbons, since it leads to the fmation of the me stable bromonium cation. t can be attacked by the nucleophile ( - ) only from the back side, leading to the fmation of a single product: trans-1,2-dibromocyclohexane 9. (4 pts) The two Fischer projections below crespond to two stereoisomers of 2,3-dibromobutane. Circle the structure that is fmed by addition of 2 to trans-2-butene. ffer brief structural rationalization f your answer. C 3 C 3 (2R,3S)-2,3-dibromobutane C 3 3 C (2R,3R)-2,3-dibromobutane 3 C C 3 2 t is the meso-fm anti-addition 3 C C 3 3 C C 3 meso-fm (2R,3S)-2,3-dibromobutane 10. (12 pts) Fill in the blanks in the following reaction scheme. ndicate proper regiochemistry / stereochemistry wherever applicable: N 3 NaN 3 DMS RR 2 2 C 4 2 MCPBA C Zn, 2

4 11. (14 pts) Predict the principal ganic product in each of the following reactions. f a mixture is fmed, indicate the expected maj product. ndicate explicitly stereochemistry/regiochemistry, wherever applicable. 1. B 3.TF 2. - / Na acetone + KC(C 3 ) 3 t-butanol + NaC 3 C 3 C 3 C 2 C 2 C 3 + LiCN DMF NC C 3 C 2 C 2 C 3 N 3 + NaN 3 DMS 1,2-dibromoethane + Na + - SC 2 C 2 S - Na + S S 12. (4 pts) The following reactions all have serious drawbacks to them. ndicate the potential problem with each one. C 3 1. B3.TF 2. - / 2 2 C 3 ydrobation-oxidation results in syn-addition of water to a C = C bond. Therefe the C 3 -groups can never be cis (C 3 ) 2 S RR zonolysis is not going to be selective with respect to one of the C = C bonds in the starting material. They will both be cleaved. As written, the reaction implies an anti-markovnikov addition of. owever, only can add to a C=C bond in an anti-markovnikov fashion in the presence of RR. The rest of X still follow the Markovnikov rule 13. (4 pts) utline a plausible synthetic sequence f the following transfmation:? CN KC(C 3 ) 3 t-butanol NaCN DMF RR

5 14. (4 pts) Which of the following alcohols CANNT be made selectively by hydrobation oxidation. iefly explain why. C 3 C 3 Because the starting material, 2-pentene, will give a mixture of alcohols Because hydrobation -oxidation is stereospecific. t is a syn-addition the should be cis to each other in the product alcohol Because hydrobation -oxidation is regiospecific yields only the anti-markovnikov alcohol 15. (4 pts) BNUS PRBLEM (n der to receive credit f this problem, it has to be solved entirely!!). n treatment with, a racemic mixture of (2R,3R)-3-bromo -2-butanol its enantiomer is converted to (+/-)-2,3-dibromobutane. n the same conditions, the racemic mixture of (2S,3R)-3-bromo -2-butanol its enantiomer is converted to meso-2,3-dibromobutane. A. Provide the Fischer projections f the starting materials products. C 3 C 3 C 3 C 3 C 3 C 3 C 3 C 3 (2R,3R) (2S,3S) (+/-)-2,3-dibromobutane C 3 C 3 C 3 C 3 C 3 C 3 C 3 C 3 (2S,3R) (2R,3S) meso-2,3-dibromobutane B. Do the reactions occur with inversion retention of configuration at the chiral center? RETENTN