CEM 351 2nd EXAM/Version A Friday, October 17, :50 2:40 p.m. Room 138, Chemistry

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1 Name (print) Signature Student # Section Number CEM 351 2nd EXAM/Version A Friday, October 17, :50 2:40 p.m. Room 138, Chemistry Ann Swerkey Grade? 1.(20 2.(20. 3.(20 4.(20 5.(20 6.(20 TOTAL 100 Score Note: Answer any 5 questions for a total score of 100 pts. Be sure to look over all the questions first before beginning the exam, and indicate which five questions are to be graded by checking the corresponding box. Recitation Day and Time Instructors: Room 1. T 9:10-10:00 a.m. Yu Zhang M 10:20-11:10 a.m. Yu Zhang 218A 3. T 10:20-11:10 a.m. Tulika Datta Th 10:20-11:10 a.m. Tulika Datta T 10:20-11:10 a.m. Yu Zhang T 11:30-12:20 p.m. Tulika Datta 218B 7. Th 11:30-12:20 Tulika Datta M 4:10-5:00 p.m. Yu Zhang 136

2 1. (20 pts) Consider the following eight representations of brominated hydrocarbons: C 3 C 2 C(C 3 ) 2 A 1-omo-2,2-dimethylpropane B (E)-(C 3 ) 2 CC=CC 2 C D 2-omo-4-methyl-1-pentene E 3-omo-3-methylcyclopentene F C 3 3 C G In this collection, identify a) (4 pts) The best substrate for S N 2 reaction with azide ion (N 3, a strong nucleophile): D. b) (4 pts) The best substrate for E1/S N 1 reactions in 2 O/EtO: F. c) (6 pts) A chiral substrate that undergoes neither substitutions nor eliminations C. For your chosen compound, explain why it is so inert. Be sure to consider each reaction mode (S N 1, S N 2, E1, E2) carefully. Compound C is a bridgehead bromide in a bicyclo[2.2.1]framework. S N 2 fails because no nucleophile can approach from the (caged in) back (and it s a tertiary bromide anyway); S N 1 and E1 fail because the bridgehead carbon can t achieve the planar sp 2 geometry required for cation formation; and E2 fails because it would make a twisted alkene whose p orbitals, held almost perpendicular, can have little or no π-bonding (in violation of edt s rule ). d) (2 pts) Two representations of the same compound: E and G. e) (4 pts) Use the box below to draw the product of your chosen S N 2 reaction in (a): N 3 2

3 2. (20 pts) Strong base treatment of 2-bromocyclopentanol can form a bicyclic ether, commonly known as cyclopentene oxide, via an intramolecular S N 2 reaction as shown. The base first deprotonates the O to form an alkoxide, which then displaces the bromide as shown. Base: O O 2-bromocyclopentanol O cyclopentene oxide (a) (6 pts) Would cis- or trans-2-bromocyclopentanol be better for this reaction? Please circle your answer and draw the compound in the box provided Cis Trans O (b) (2 pts) If the C-O carbon has the (S) configuration, what is the configuration at the C- carbon in your answer to (a)? Please circle your answer. (R) (S) (c) (4 pts) Provide a name and formula for the hydrocarbon (shown) you would have if the O atom in cyclopentene oxide were replaced by a C 2 fragment. Name: Bicyclo[3.1.0]hexane Formula: C 6 10 (d) (4 pts) Cyclopentene oxide (an epoxide) can undergo S N 2 attack by an external nucleophile to displace O, essentially the reverse of the process in (a). Draw the product(s), including stereochemistry, that you would expect from attack of C 3 S. O SC 3 3 CS O (e) (4 pts) Would you expect cyclopentene oxide to show any optical rotation? ow about your product mixture in (d)? Explain briefly. Optical Rotation? Cyclopentene oxide: Yes No Meso (note the internal mirror plane) Product(s) in (d): Yes No Racemic 50:50 mix of enantiomers 3

4 3. (20 pts) Quick / answers (4 pts each): (a) If a molecule has a stereogenic center it must be chiral. (b) It is possible to have a trans double bond in a cyclic hydrocarbon. (c) Nucleophiles don t have to be negatively charged. (d) Diastereomers usually have opposite optical rotations. (e) There are two types of carbon sites in chair cyclohexane. Note: it should have said carbon atoms in this question. Some took it as bond sites on C, so credit was given for either answer. 4. (20 pts) Quick chemical answers (4 pts each): (a) Circle the better nucleophile for S N 2 reactions: 2 O N 3 (b) Circle the better leaving group: F 2 O (c) Circle the better solvent for E1/S N 1 reactions: O Tetrahydrofuran (d) Circle the higher energy isomer: Mixture: 80% C 3 C 2 O 20% 2 O (e) Circle the more stable C 6 13 carbocation: 4

5 5. (20 pts) (a) (12 pts) Provide full IUPAC names to represent the following structures. Be sure to pay attention to stereochemistry. (i) (E,3R)-1,3-dichloro-4-methyl-1-pentene (ii) (3R,6S)-1,3,4,6-tetramethyl-1,4-cyclohexadiene (iii) (1R,3S)-1-t-Butyl-3-methylcyclohexane (b) (2 pts) Which of the compounds in (a) is achiral? Please circle your answer (i) (ii) (iii) (c) (6 pts) The compound in (i) has three other stereoisomers; draw them below: 5

6 6. (20 pts) In your environmentally conscientious search for cleaner-burning fuels, you consider the following S N 2 synthesis of the gasoline additive MTBE (methyl t-butyl ether): (C 3 ) 3 C-O K + + C 3 -I (C 3 ) 3 C-O-C 3 + K + + I (run in (C 3 ) 3 CO solvent) (a) (4 pts) Identify the nucleophilic reactant in this process and circle it (C 3 ) 3 C-O K + C 3 -I (b) (4 pts) Draw curved arrows to show the flow of electrons in the reaction (redrawn below). (C 3 ) 3 C-O K + + C 3 -I (C 3 ) 3 C-O-C 3 + K + + I Igor, your new lab technician, attempts to repeat the reaction but confuses the reagents and puts the following reaction together: C 3 -O K + + (C 3 ) 3 C-I? (attempted in TF solvent) At room temperature, not much happens, so Igor attempts to push the reaction by heating it. Instead of getting his MTBE, he isolates a strong-smelling gas, which upon analysis proves to have the formula C 4 8. (c) (6 pts) Why didn't Igor's experiment work to make MTBE via the S N 2 reaction? Explain. The S N 2 reaction doesn t work on a tertiary halide like t-butyl iodide because approach from the back side of the C-I bond is blocked by the three methyl groups. So elimination takes over when the reaction is pushed. (d) (6 pts) What was the C 4 8 hydrocarbon that he did make instead (please draw it), and what kind of reaction was responsible? Igor made isobutylene (2-methylpropene) via an E2 reaction. E1 would not work in the polar aprotic TF solvent; separation of the ions is too difficult. isobutylene 6