chem3A-Fall05-mt2-Vollhardt-soln

chem3A-Fall05-mt2-Vollhardt-soln - New: . Mg”? WHY—i...

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Unformatted text preview: New: . Mg”? WHY—i EXAMINATION 2 Chemistry 3A Name: [Print first name beffi second! Use capital letters!} Peter Vollhardt November 8, 2005 r E Please check the name of your TA and corresponding section number. Complete the remaining information if applicable. Section # TA's Name Section # TA’s Name Making up an | Grade , from Professor . (Please indicate the semester during which you took previous Chem 3A: ). Piease write the answer you wish to be graded in the spaces provided. Do scratch work on the back of the pages. This test should have 14 numbered pages. Check to make sure that you have receEved a complete exam. A good piece of advice: read carefully over the questions (at least twice); make sure that you understand exactly what is being asked; avoid sloppy structures or phrases. it is better to be pedantic in accuracy! Good Luck! DO NOT WRITE lN THiS SPACE l. __ (60) It. (50) Ill. (40) IV. (40) V. (30) VI. (30) Total: W (250) 1 M! m 2 I. [60 Points] Add the missing starting materiais, reagents, or products (aqueous work-up is assumed where necessary). Don't forget stereochemistry! a. S I + S(CH3)2 —* Br 1 equtvalent b. CH3 0 ,OSCHS H + ":CEN : W CHa Pure enantiomer OCH3 CH3 + Na" 'OC(CH3)3 —-m“'--'——>— LiAlH4 —> Circle one: if” Racemic Pure enantiomer W . . Achlral J mnmmmAfimMmmwmwwm OH i CH3 H I-j OH 0 “CH CH u 0 2 3 CHSS/ CH3 H o ' 'Ahélcohol NaOH, CH30H CH30H2Li H. [50 Points] The following reactions proceed (predominantly) by 8N2, 8N1, E2, or E1 pathways, respectively. Give the major product (one only) in each case and answer the questions by circling the most applicable statement. 3- H30 OH H‘”, CHSOH, 100°C -H20 § r0 ‘r'QJE-iwfw ’@Q #6 : 4 f7 Mechanism: 8N2 8N1 E2 At lower temperatures one of the foliowing ratios will increase: 3N2 / 8N1 E2 / E1 8N2 / E2 Na” '00 H3, —---M--——-———> CI Mechanism: 8N2 8N1 @ E1 Changing the alkoxide to CHSS" K+ causes gflg of the following ratios to increase: E2 1 E1 sN2'r/i;\) 5N1 / E1 E2 I 3N2 rm” Li+ 'N[CH(CH3)2]2 iLDAl. THF Mechanism: 8N2 an E1 Changing the reagent from LDA to ammonia, NH3, causes M of the following ratios to increase: 2‘ SN2/ E2} E2 / E1 8N2 I an rearrangement / 8N2 mevwfiflfiflfi d. /SLCH3 CH38H, CH30H (solvent) CH3 A secondary cycloalkyi methyl sutfide Mechanism: 8N2 8N1 E2 Et Changing the solvent to hexane will have gag of the following effems: «W’WTSW-w—w I @decreaQ 1:56 ' 4m: m-W’W’m" 8N2 / 8N1 increases E1/ 8N1 increases E2 1 E1 increases NaOH (1 equiv.). CHscHQOH (solvent) lntramolecular Williamson synthesis 'W' OH Mechanism: 8N2 8N1 E2 E1 0 u Changing the solvent to DMF [CH3CN(CH3)2] causes one of the following effects: MM’F’M E1 ISN1 increases "” rate increases rate decreases Wm. mxmmnfl""' 8N2 / 8N1 increases 'wnnuwnflNnkA/‘mnaa 8 ill. [40 Points] Explain the foiiowing observations by a detaiied mechanism (i.e., write a scheme with structures, use arrow-pushing, etc). Do not add any reagents! This is not a synthesis! a. H13in + W] H20. 80°C “*H———>- Watch stereochemistry! IV. [40 Points] a. Provide a viable synthesis of the foElowing compound from any starting materials containing four carbons or less. Work backwards! oer-12w3 10 Provide a viable conversion of starting material to product. You may use any additional organic or inorganic compounds in your scheme. ‘11 V. [30 Points] A researcher carried out the radical bromination of 2-methylpropane, (CH3)3CH, and isolated the expected (CH3)3CBr, Up. 82.4 gC. However, careful distillation revealed another compound, b.p. 150 9C, with the 1H NMR spectrum shown below. mwwawmwmm raxmfi-aw-M'AM-rmmmumm¢mwx 12 b. Assign the spectrum by labeling the hydrogens giving rise to the absorption centered at 5 = 1.90 ppm with the letter “A” and those at 5 = 3.87 ppm with "B" in the drawing in the box above. c. Give one reason for your assignment in b., in one sentence. Note: There are several valid answers, just pick one. If you give more, your question will not be graded. - we XrWW; t Vl. [80 Points} Place an X mark in the box preceding the most accurate statement. Only % answer is allowed. a. Nucleophilicity in CHSOH increases down the periodic table, because E the nucleophilic atoms get heavier E! the basicity of the nucleophile increases polarizability increases :1 solvation is impeded by protic solvents 13 b. in 1H NMR, the chemicai shift decreases with increasing strength of the external magnet increases with deshielding by electron withdrawing groups on the attached carbon increases with increasing strength of the external magnet DEED decreases with deshielding by electron withdrawing groups on the attached carbon c. The rate of cyclization of bromoalcohols to cyclic ethers decreases in the order (the numbers stand for the resulting ring size) a 3- 4' 5- a m 4' s: a: s [:1 s, e, 4, s d. In the reaction shown, the proportion of product A can be increased by OH OH 6 (CH3)SCBr + Nal —3> (magi: (CH3)3COCH3 A 3' (D 3. 3 (Ct adding excess Nal E changing the leaving group to methanesuifonate |:| entropy e. Carbocation stability increases with alkyl substitution nucleophilicity hydride shifts EDD leaving group ability V0111] ardt Teaches 3A * The End * 14 ...
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This note was uploaded on 10/25/2009 for the course CHEM 3A taught by Professor Fretchet during the Spring '08 term at University of California, Berkeley.

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chem3A-Fall05-mt2-Vollhardt-soln - New: . Mg”? WHY—i...

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