CHEMISTRY MIDTERM # 3 March 31, The total number of points in this midterm is 100. The total exam time is 120 min (2 h). Good luck!

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1 Name: CEMISTRY 1-01 MIDTERM # March 1, 2009 The total number of points in this midterm is 100. The total exam time is 120 min (2 h). Good luck! 1. (11 pts) Mark as true (T) false (F) the following statements. Do not explain! Enantiomers are superimposable mirr images; If a compound has an enentiomer, it must be chiral; Every chiral compound has a diastereomer; Diastereomers are non-superimposable mirr images; A molecule with me than one chirality center has me than one enantiomer; Molecules with me than one chirality center are always chiral; S N 1 is a first-der substitution because it requires only one molecule of the substrate; Both S N 2 S N 1 reactions occur with a complete inversion of configuration; Methyl substrates react fastest in S N 2 reactions; Tertiary substrates react fastest in S N 1, E1 E2 reactions; The nucleophile is a Lewis base; 2. Circle ALL that apply: A. ( pts) S N 2 reactions: a. Are stereospecific; b. ccur with inversion of configuration; c. ccur fastest in polar aprotic solvents; d. Require strong nucleophiles; B. ( 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; C. ( 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; D. ( pts) ydroxyl groups can be converted into better leaving groups by: a. Deprotonation; b. Protonation; c. Conversion to tosylates; d. Use of polar aprotic solvents;. (6 pts) Assign the absolute configuration to each chirality center in the following molecules. C P C C ( C C) 2 N 4. (8 pts) Identify the chiral molecules among the structures below: C C C NC CN C C C C C C C C C C C

2 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 C F C F C 6. (4 pts) Among the Fischer projections below, select those that crespond to (2R,S)--bromo-4-penten-2-ol. C C C C C 2 C C C C 2 C 2 C C 2 7. (6 pts) Write an acceptable -dimensional structural presentation (i.e. a Fischer projection, bold--dashed wedge, etc., properly unambiguously showing stereochemistry!!) f each of the following compounds. A. (1S, S)-1,-dibromocyclohexane; C. meso-1,-dichlocyclopentane; B. (R)--methyl--hexanol; 8. (2 pts) Draw the structure of a meso compound with molecular fmula C (4 pts) Indicate the stronger nucleophile in each pair. Do not explain! I F N P S 10. (4 pts) Indicate the better leaving group in each pair. Do not explain! 2 I R SR C 11. (4 pts) In each of the following cases, indicate which compound will react faster in S N 2 conditions. Do not explain!

3 I 12. (10 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. C C 2 Na C C 2 NaI acetone K (C ) C C C 2 5 NaCN DMF NaC C (R)-2-bromobutane NaN DMS 1. ( pts) Treatment of 1-propanol with sodium bromide (Na) fails to produce 1-bromopropane, but the conversion readily occurs if 1-propanol is treated with. Provide a brief explanation f these observations. Na 14. (2 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 (S)-isomer NaCN DMF? (S)-isomer 15. (4 pts) In each of the following reactions, show clearly if a mixture of enantiomers diastereomers is fmed. MCPBA C 2 2

4 (2 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 + I + I isopropyl methyl ether + C I + I 17. (7 pts) Choose the alkyl halide(s) from the following list of C 6 1 isomers that meet each criterion below: (1) 1-bromohexane (2) -omo--methylpentane () 1-omo-2,2-dimethylbutane (4) -omo-2-methylpentane (5) 2-omo--methylpentane A. The compound(s) that can exist as enantiomers; B. The compound(s) that can exist as diastereomers; C. The compound that gives the fastest S N 2 reaction with NaC in methanol; D. The compound(s) that give an E2 but no S N 2 reaction with NaC in methanol; E. The compound that gives the fastest S N 1 reaction; 18. (4 pts) Suggest a plausible synthetic sequence f the following transfmation (Me than one reaction required!!). (R)-2-heptanol??? N (pure enantiomer!) 19. ( pts) F the species below, complete the Lewis structure draw two me valid resonance structures. early show, using the curved arrow notation, the shift of electron pairs. Rank the resonance structures. N N N azide anion

5 20. (2 pts) BNUS PRBLEM (In der to receive credit f this problem, it has to be solved entirely!!). Ethers are compounds of the type R R. They are rather inert in both S N 1 S N 2 reaction conditions, except f their reaction with hydroiodic acid, I. Ethers are cleaved (broken into two fragments) by I, as shown f methyl ether below. Write a detailed mechanism f the reaction. C C + I C I + C