chem3A-sp11-mt2-Bertozzi-soln

chem3A-sp11-mt2-Bertozzi-soln - Chemistry 3A Spring 2011...

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Chemistry 3A – Spring 2011 Second Midterm – April 1, 2011 Professor Carolyn Bertozzi Page 1 of 20 Your full signature__ Key ________________________________________________ Print your full name_____________________________________________________ (Last Name, First Name, Middle) Your SID______________________________________________________________ GSI’s name or section number____________________________________________ This exam has 20 pages; make sure that you have them all . We will only grade answers that are in the designated spaces; please do your scratch work on the last page or the backs of pages. Write only one answer to each problem; multiple answers will receive no credit, even if one of them is correct. Note: This examination runs for a total of 120 minutes. Please write legibly; ambiguous or illegible answers will receive no credit . Partial Periodic Table I II III IV V VI VII VIII H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Ga Ge As Se Br Kr Rb Sr In Sn Sb Te I Xe Do Not Write in this Box: 1.________( 16 ) 2.________( 8 ) 3.________( 18 ) 4.________( 9 ) 5.________( 12 ) 6.________( 12 ) 7.________( 8 ) 8.________( 12 ) 9.________( 5 ) Total______( 100 )
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1. [16 points] For each pair of reactions, circle the one that will proceed fastest and provide a brief explanation for your choice in the space below the pair . Your explanation must include the reaction type (i.e., S N 2, S N 1, E1 or E2). a. Explanation : Neopentyl electrophiles are more sterically encumbered than secondary electrophiles and are slower in S N 2 reactions. b. Explanation: The carbocation character in the transition state of the S N 1 reaction is stabilized by resonance, lowering the activation energy, and resulting in a faster reaction rate.
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c. Explanation: Iodide ion is a better leaving group than a bromide ion and consequently the electrophile with the better leaving group will react faster in the E2 reaction. d. Explanation: The strength of the base affects the rate of E2 reaction, and basicity decreases going down a column of the periodic table.
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