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Unformatted text preview: CHM 2210 Section 4830 Fall 2007 Exam 2A Name_______________________________ UFID________________________________ Note: for the entirety of this exam, when drawing resonance structures, rearrangements, and/or mechanisms, arrows must be drawn to show the movement of electrons. 1. This question concerns the light-initiated monochlorination of the molecule shown below.
1 2 a. (4 pts) Draw the mechanism for the initiation step. initiation step Cl Cl 2Cl b. (6 pts) In the propagation step, possible sites for abstraction of a hydrogen are indicated 1 and 2. Draw all major resonance structures resulting from hydrogen abstraction at site 1. resonance structures c. (2 pts no partial credit (NPC)) On the energy diagram below, label in the box which graph depicts hydrogen abstraction through site 1 (write 1) and which depicts hydrogen abstraction through site 2 (write 2). d. (2 pts NPC) Call the products resulting from attack of the chlorine radical at site 2 P2 and the product resulting from attack of the chlorine radical at site 1 P1. Raising the temperature will cause the ratio of P1:P2 to (circle one). --tend towards 75:25 (P1:P2) --tend towards 50:50 --tend towards 25:75 2 --tend towards complete conversion to P2 --tend towards complete conversion to P1 --not affect the ratio whatsoever 1 2.a. (2 pts NPC) From the options below, circle the choice which best describes what is occurring in the diagram at left. hyperconjugation hydrogen bonding dipole-dipole interactions resonance inductive effect London forces b. (4 pts, 2 pts each, NPC) Below, 3 alkylbromides are shown undergoing conversion to their respective carbocation. Circle the least stable carbocation. Circle the starting material that is most likely to undergo an SN1 reaction. Br H3 C CH 3 CH 2 CH3 H 3C CH3 Br H 3C CH 3 H 3C Br H 3C H 3C H 3C CH 3 3.a. (4 pts; 2 pts each, NPC) From the options below, circle the condition(s) under which the general SN2 reaction would be favorable: Having leaving group(s) that are weak bases Having leaving group(s) that are strong bases Using a nonpolar solvent Using a polar solvent b. (6 pts) The reaction below proceeds through an SN2 mechanism. Draw the three-dimensional structure of the starting material, and draw the Fischer projection of the product of the SN2 reaction.
Br H H OCH3 three-dimensional structure H 3C H Br H 3CO F H F H Fischer projection OEt H or H H F OEt EtO or CH3 H OCH3 H F CH 3 OCH 3 F CH3 CH3 CH 2O - K+ (1 equivalent) SN2 CH3 OCH3 4. (20 pts) Label the stereocenter(s) (if any) S or R on the molecule below (2 pts). Assume the reaction below proceeds through an SN1 mechanism. Draw a three-dimensional picture of the intermediate (3 pts; -1 if structure not planar). Also, draw any major resonance structure(s) that may be present (3 pts). Then, draw the major product(s) of the above reaction, being sure to clearly detail stereochemistry in the product(s) (8 pts). Finally, indicate whether or not optical activity is expected in the reactant and product(s) (circle yes or no) (2 pts each).
Br H2O SN 1 (R)
three-dimensional intermediate structure intermediate resonance structure(s) product(s) OH HO HO Is optical activity present in the reactant? Yes / No Is optical activity present in the product? Yes / No 5. (12 pts) Assume the reaction below proceeds through an SN2 mechanism. a. Draw the mechanism (showing any intermediates or transition states) and be sure to clearly detail stereochemistry in the product(s) (1 pt for correct arrows; 3 pts for intermediate; 4 pts for product). b. Indicate whether or not optical activity is expected in the reactant and product(s) (circle yes or no) (2 pts each). OCH3 Br F H CH 3OF H SN2 F OCH3 H F OCH3 Br F H OCH 3 OCH3 F H Is optical activity present in the reactant? Yes / No Is optical activity present in the product? Yes / No 6. (20 pts) Assume the molecule below is drawn in its most stable chair conformation. The reaction shown proceeds through an SN1 mechanism. a. Label the stereocenter(s) (if any) S or R (1 pt each, 4 pts total). b. Draw the intermediate (5 pts; you had to show the geometry around the carbocation as planar). c. Then draw the intermediate resulting from the most favorable hydride shift (5 pts). d. Finally, draw all the products resulting from the two intermediate you've drawn (6 pts). (Note: the answers are shown in the two most common forms they were received in; either was completely acceptable.)
Br Br CH 3CH 2OH SN1 CH3 (1R,2R,4R,5R) CH3 H intermediate H H CH 3 CH3 intermediates from hydride shift H H CH3 CH 3 OCH 2CH3 products CH 3 HH CH 3 H H OCH2CH3 CH 3 CH 3 CH3 H H OCH 2CH 3 CH3 H H CH3 CH 3 OCH 2CH3 BONUS (6 pts) Normally, one would expect racemization from the above reaction, yet one product is vastly preferred over the other. Why? Circle the product resulting from this preferred approach. Although normally a carbocation intermediate can be approached from above or below by the nucleophile, in this case the axial methyl sterically hinders approach from below. As a result, the nucleophile preferentially adds from above to result in the products circled. 7. (18 pts). In the boxes provided, draw the best product(s) for the reaction shown above each box. (6 pts each; loss of chirality in the first and third mechanism must be noted either by writing `racemic', drawing both enantiomers, or as shown below, drawn without wedges or dashes. By drawing only one enantiomer, you imply that the product is chiral. This resulted in a loss of 2 pts.)
Br SN1 CH 3OH OCH 3 H 3CO or H 3CO OCH 3 or , , OCH 3 SN2 Br Br + K
+ O O K + DMF O O or O O H3C CH 2CH 3 OH + H3C CH 2 CH 3 Br SN1 H3C CH 2CH 3 O H3 CH 2C BONUS (4 pts, no partial credit) The Fischer projection of a hypothetical molecule with arbitrary substituents A, B, C and D is given on the left with the stereochemistry indicated below. Based on this, indicate the stereochemistry of the other three molecules in the boxes below each molecule.
A D C R B A C S D B D A S C B C B S D A ...
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This test prep was uploaded on 04/09/2008 for the course CHM 2210 taught by Professor Reynolds during the Spring '01 term at University of Florida.
- Spring '01