Bertozzi_Chem3A_Midterm2_1999_Key

Bertozzi_Chem3A_Midterm2_1999_Key - Chemistry 3A - Fall...

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Unformatted text preview: Chemistry 3A - Fall 1999 Second Midterm - November 8, 1999 Professor Carolyn Bertozzi Your full signature K Qy Print your full name (Last name, First name, Middle) Please circle the section number and name of your GSI/T A. 161 Tewell, Craig 221 Jouppi, Sarah 411 Simon, Matthew 111 Evans, Jared 361 Jenkins, Stephen 421 Eissa, Hisham 121 Luchansky, Sarah 371 Kehoe, John 561 Merolle, Mauro 131 Wetzler,Modi 311 Ringgenberg, Joe 511 Brinner, Kristin 261 Wilson, Rebecca 321 Aruguete, Deborah 521 Mihardja, Shirley 211 Grunwell,Jocelyn 461 Wilson, Rebecca This exam has 17 pages; make sure that you have them all. We will only grade answers that are in the Do Not Write in this Box. designated spaces; please do your scratch work on the backs of the exam pages. Write only m answer to . ( 15 ) each problem; multiple answers will receive m credit, even if one of them is correct. -—( 20 ) 50 Note: This examination runs for a total of 90 minutes. ( ) No questions will be answered by proctors after the exam . ( 30 ) begins. Please write legibly; ambiguous or messy 30 answers will receive no credit. ( ) ( 30 ) Partial Periodic Table ( 25 ) I II III IV V VI VII VIII He 0 F Ne S Cl Ar Se Br Kr Te | Xe Total (200) C Si Ge Sn N P As Sb 5a 5b 5c 1. [15 points] For each pair of molecules below, circle the word that describes their relationship. a. H H Br H H30 H C CH3 C CH3 CH3 Br enantiomers diastereomers _ identical b. He Cl CH3 Cl enantiomers diastereomers identical 0. enantiomers diastereomers identical H3 H3 CH3CH2 Cl C CHZCH3 H30 Br H30 Br CH3 CH3 enantiomers diastereomers identical 1%? enantiomers diastereomers identical 2. [20 points] Nucleophilic substitution reactions can take place at boron atoms in a manner that is similar to substitutions at carbon. For example, compound 1 below reacts with the nucleophile H20, in cyclohexane as the solvent, to afford the corresponding substitution product 2. H H a? 3 /CH3 cyclohexane a? 3 /H + H20 ——> + CH3 CH3 CH3 H 1 2 a. The reaction follows a first-order rate law: Rate = k[1]. Using this information, show the mechanism of the reaction in a step-wise fashion using the curved- arrow notation to depict the flow of electrons. l d9 l (9 fl ./ __ #6‘ VS\ 3 /rj\ + /S\ JED" \> l H \H 6 “*3/14 b. Depict the progress of the reaction in cyclohexane on the reaction coordinate diagram on the left. Be sure to clearly indicate the relative energies of the starting materials, transition states and any intermediates (the reaction is exothermic). On the diagram on the right, depict the same reaction performed in acetone as the solvent. Be sure to show how the relative energies change with the change in solvent. Reaction in cyclohexane Reaction in acetone Reaction coordinate c. In which solvent will the reaction proceed fastest? Briefly explain your reasoning below. h Cid c('l"¢"““€- TDL f‘vFL @660qu 5R0? '1’) (7136(9‘4-2K‘nt/1L 0! Muclfirpy‘ «ewe/"9‘0 («ampq/Cfl (9 QCCMM¢' Tkl‘s . r {a is becqoje acetone 15 6v. {3043/ fail/U4, whereas. Cfidohetame 1'3” "0“P°(“V' T1“ Spit/cuf- stqgflc'zkrg Han- rectal-emf mare. HNM ‘H’lt ham: #1984 Q/‘a‘r‘rrfi 7 be can-4n. H41- #83194»qu Ltcts CQ“/¢)€ SQPq/a (135”, deohexmu 93cm” «0 5:161 ,CéquUE'CIF-“DH, palar 3. [50 points] Add the missin roducts startin materials or rea ents. Be sure to clearly indicate stereochemistry where relevant. The workup step is assumed where necessary. DO NOT include workup steps in any of the given boxes. a. b. 1 r 59 OH N3 2- U01 C. 0 1. NaBH4 2. PBr3 3. LDA (j —-—> d. 1- rcc m 0H Ch~‘<:::7f‘3r 1equw.NaSH _> % 1.NaBr 9’4% 3 arbo = ____> M 0 H2804, A OH ——> I 1. GD (9 ' Ma 0 C ( MUMS H pcK*QM¢ Hllh. H30 1. CH3MgBr 2. NaNH2 3. CHaBr —-——> 4. [30 points] For each pair of reactions below, mark the box next to one that will proceed fastest and also circle the letter corres ondin to the statement that best explains your choice. No credit will be given for a correct answer to the first part of the question with an incorrect reason in the second part. a. wig—CH3 Nal, acetone ..\\\I R O —_> + H3 0 ii O—fi—CHa Nal, acetone _,‘\\1 R El 0 ——> + NaO—fi—C H3 0 A. 8N2 reactions are faster in polar solvents. CB) B-Branching retards the 8N2 reaction. C. Carbocations are stabilized by increased substitution. W—fi/Y Cl Poor solvation of an anion increases base strength. B. 3° electrophiles do not undergo 8N2 reactions. C. Hindered nucleophiles are less reactive than unhindered nucleophiles. 10 0. Br OCH3 CH30H r CH3 CH3OH D —» + A. Methanol is a weak nucleophile. B. 8N2 reactions proceed faster at less substituted carbon atoms. @ 3° carbocations are more stable than 2° carbocations. d. CH3CHZCI CHsCHzCl OS ———> SCHZCH3 A. More polarizable nucleophiles are more reactive. Stronger bases are better nucleophiles within a row of the periodic table. C. Resonance delocalization stabilizes carbocations. 11 NaSH, CHSOH E] \O‘Br ..\\\SH —_> NaSeH, CH30H «O‘Br .mSeH —._> A. The nucleophile is not involved in the rate determining step. B. Stronger bases are better nucleophiles. © Stronger bases hydrogen bond more strongly with protic solvents. f. 8 H20: A R o—fi—CF3 ——> + HO—fi—CF3 O O 8 H20: A R E] +o—E—0013 —> + HO—E—CCIQ, The bond to the leaving group is cleaved in the rate-determining step. B. E2 reactions follow second-order kinetics. C. Inductive effects destabilize carbocations. 12 [30 points] Provide a detailed step-wise mechanism for the following transformations using the curved-arrow notation (i.e., "electron-pushing"). Keep in mind that each individual step should contain no more than three arrows. These are NOT synthetic problems - do not add any new reagents to your mechanism. a. o'S '. —> / N + NaI 3 optically active racemic 19 + .( ,Sl 'Seg adfiml p Kuflufil Mil/ma/fhh ngj M,“ ,l W a h 9'03 13 6. 15 [30 points] Provide a viable synthetic route for the following target molecule using organic reagents of two carbon atoms or less and any inorganic reagents of your choice. Show your synthesis in the forward direction and in step-wise fashion. You do not need to show workup steps - they are assumed. (It may help to work backwards on scratch paper before writing your answer in the fonNard direction below.) K )1 CH3 mgr- L P 8 r3 6 f H M w“? l “l 0 H o/\ ELM” ‘9 H’KH “a” 7. [25 points] Provide a viable s nthetic route from the startin material to the roduct usin 16 any reagents of your choice. Again, show your synthesis in step-wise fashion in the forward direction. H H3 & ""“Wpr & - - - ' ' - - - _ _ __-> 2. .CHgHflSf i a! 0 b Cusflsbrk 0H 3» é M? 5'? ...
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Bertozzi_Chem3A_Midterm2_1999_Key - Chemistry 3A - Fall...

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