H C C C C H C H C C H H. Cl H CH 3 O. Br CH 3 OH H 3 C. H Br H

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1 CEMISTRY MITERM # 3 answer key June 17, 2005 Statistics: Average: 69 pts (69%); ighest: 96 pts (96%); Lowest: 46 pts (46%) Number of students perfming at above average: 16 (53%) 1. (10 pts) Mark as true (T) false () the following statements. o not explain! () Enantiomers are superimposable mirr images; () iastereomers are non-superimposable mirr images; () A molecule with me than one chirality center has me than one enantiomer; () Meso fms are achiral but optically active; () Chiral compounds have only one internal mirr plane of symmetry; () Both S N 2 S N 1 reactions occur with a complete inversion of configuration; (T) Methyl substrates react fastest in S N 2 reactions; (T) Tertiary substrates react fastest in S N 1 E1 reactions; () Internal alkynes are me acidic than terminal alkynes (T) ydration of terminal alkynes obeys the Markovnikov rule; 2. Circle ALL that apply: A. (3 pts) Enantiomers: a. Are mirr images ; b. ave the same absolute configuration; c. Rotate the plane of polarized light in opposite directions; d. Are chiral molecules ; B. (3 pts) S N 2 reactions: a. Are stepwise; b. ccur with inversion of configuration; c. ccur fastest in polar aprotic solvents ; d. Require strong nucleophiles ; C. (3 pts) E2 becomes me preferred than S N 2 if: a. Stronger nucleophile/base is used; b. Bulkier nucleophile/base is used; c. Me sterically hindered substrate is used; d. The temperature is reduced;. (3 pts) ydroxyl groups can be converted into better leaving groups by: a. eprotonation; b. Protonation; c. Conversion to tosylates ; d. Use of polar aprotic solvents; 3. (7 pts) Identify the chiral molecules among the structures below: C C C C C C C C 3 C 3 C 3 C C C 3 C

2 4. (4 pts) Provide structures f the following compounds: 2,2-dimethyl-3-hexyne 1-buten-3-yne 1-penten-4-yn-3-ol phenylacetylene 5. (4 pts) Consider the following pairs of structures. Identify the relationship between the two components of each pair (same, constitutional isomers, enantiomers, diastereomers). C 3 C 3 identical C 3 C 3 identical identical diastereomers 6. (4 pts) Consider the ischer projection A of a particular stereoisomer of 3-amino-2-hydroxybutanoic acid: 2 N C A Among the ischer projections given below, select those that represent the same stereoisomer as A. C 3 C 2 N C 3 C C 3 C N 2 N 2 2 N C 3 C C 3 7. (4 pts) Write an acceptable 3-dimensional structural presentation (i.e. a ischer projection, bold--dashed wedge, etc., properly unambiguously showing stereochemistry) f each of the following compounds. A. (2S, 3S)-2,3-dichlopentane; C 3 C 2 C 3 B. meso-1,3-dichlocyclopentane;

3 8. (4 pts) Indicate the stronger nucleophile in each pair. o not explain! I S 9. (4 pts) Indicate the better leaving group in each pair. o not explain! NR 2 NR 3 R R I C (4 pts) In each of the following cases, indicate which compound will react faster in S N 2 conditions. o not explain! I 11. (8 pts) Write complete chemical equations ( predict the products) f the following reactions: A. 2-pentyne 2 /Lindlar catalyst; B. Propyne 2 S 4 /gs 4 ; C. 3-heptyne Na/liq. N 3 ; 2 Lindlar Pd 2 S 4 gs 4 Na liq. N 3. Acetylene NaN 2 /N 3, followed by propyl bromide; C C 1. NaN 2 /liq. N (8 pts) Write the product ( products) you would expect to obtain in each of the following reactions. In case of a mixture, indicate which product you expect to be the maj component.

4 NaN 3 M N 3 NaI acetone I NaC 3 C 3 (R)-2-bromobutane NaS MS C 3 S C 2 C (4 pts) In each of the following reactions, identify the prochiral atom functional group label it as enantiotopic diastereotopic. prochiral MCPBA + C 2 2 (R)-4-bromocyclohexene prochiral 2 2 stereoisomers, but not mirr images, therefe diastereomers. The prochiral group is the C=C bond it is diastereotopic + stereoisomers nonsuperimposable mirr images, therefe enantiomers. The prochiral group is the C=C bond it is enentiotopic 14. (3 pts) The S N 2 substitution reaction below starts with the (S)-tosylate yields a nitrile product that also has (S) stereochemistry. ffer a brief rationalization. Ts Ts NaCN M NaCN M? NC Inversion ALWAYS occurs in S N 2 reactions. owever, in this particular case the oder of priity of groups at the chiral center changes upon substitution - the outgoing group (the leaving group) is #1, while the incoming group (from the nucleophile) is #2. Thus, even though the inversion occurs, the designation stays the same, (S). 15. (4 pts) The synthetic transfmations shown below are unlikely to occur as written. Tell what is wrong with each one of them predict the true product. A. K + - C(C 3 ) 3 (C 3 ) 3 C C(C 3 ) 3 Bulky strong nucleophile/base. Will not give a product of substitution, but elimination (i.e. E2 rather than S N 2) Actual product: + +

5 B. + Na N 2 + Na N 2 N 2 + Na Na + N 3 As written the reaction implies a substitution (S N 2) of by an amide anion. Two problems: 1) is a very po leaving group 2) Alcohols are stronger acids than ammonia, so an acid-base reaction will occur instead. 16. (3 pts) Suppose your task is to prepare isopropyl methyl ether. Two possible reactions are given below. Which reaction should give a better yield of the desired product? iefly explain your choice. C 3 + I + I isopropyl methyl ether + C 3 I + I second reaction is better, since methyl substrates are best f S N 2 reactions. The first process will yield a large percentage of elimination product 17. (4 pts) etermine the crect starting material required to prepare meso-2,3-dibromobutane, if 2 /C 4 is used as a reagent. Solution: The product is a vicinal dibromide, those are generated by addition of bromine to alkenes. This, combined with the fact that the reaction of alkene with bromine is an anti-addition process, leads to the conclusion that the required starting material is trans-2- butene, as illustrated in the scheme below. 3 C C 3 trans-2-butene + 3 C C C C 3 3 C C 3 meso-2,3-dibromobutane 18. (4 pts) Suggest a plausible synthetic sequence f the following transfmation (Me than one step required!!). (R)-2-heptanol??? N 3 Ts pyridine Ts NaN 3 M N (5 pts) ow would you carry out the transfmation shown below? Me than one step is required.???

6 2 C NaN 2 /N NaN 2 /liq. N 3 2. C 3 I 2 Lindlar Pd 20. (3 pts) BNUS PRBLEM (In der to receive credit f this problem, it has to be solved entirely!!). In the reaction below, 2 is added to fumarate in the presence of the enzyme fumarase. Is the enzyme catalyzing syn anti addition of 2? Suppt your answer by appropriate structures. C C C C C + 2 fumarase fumarate C Syn-addition: C C C C C C Anti-addition: C C C C C C Therefe the enzyme catalyzes the anti-addition of 2