Exam 2 - Summer 11 (Key)

Exam 2 - Summer 11 (Key) - Second Hourly Examination...

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Unformatted text preview: Second Hourly Examination Chemistry S-20 July 11, 2011 8:10 - 9:20 a.m. Name: Kev Section TF: lmportant Notes: 1. This exam consists of 8 problems on 8 pages, plus this cover sheet, and one page for scratch work at the end of the exam. 2. All answers must be written in the spaces provided. Do not wrlte anything in red ink. Answers written on the back of a page will not be graded. Anything written on the scratch page will not be graded. 3. You are allowed to use a set of molecular models for this exam. Problem Score t12 112 Total score: t12 t14 r16 t12 t12 110 / 100 Kev 1. Each of the following transformations can be carried out in three or fewer steps, Fill in the reagents required for each step. lf a step is not needed, please put an "X" next to that number. Be sure to consider the stereochemistry (if relevant) ! 1 0HCl3 , a) ct KOtS^ >+"' 2.x 3.x 1. Yts', b) W-o 2. HrO 3x c) ry" D d) .L i-a OTs ,. f cl, pgr z l(0+8,,r-, 3' HrO* 1. HeoH, 4 O("- {BI^OH 2.x 3x D .+ D )-. ! OMe OMe t12 Name: Key 2' Fill in each box with the starting material or major organic product of the indicated transformation. Be sure to provide stereochemistry if it is relevant!Any chiral starting materials or products are given as pure, single enantiomers unless otherwise specified. HorcA.zco'H Br Brz Ho,c +cozH gn b) M: & tBuoH 1. PBr3 c) ,rr"'Q)'ot 2. NaCN # OH d) -+, ?h 1. BH3, THF 2.H2O2, NaOH H (as a racemic mixture) t12 4. Consider the following molecule: Name: KeV p ?'. HsC Hsc CHs Br a) This molecule is expected to have two possible "chair" conformations. Draw clear representations of the two different conformations in the boxes below. A B 6r \4 v Clt3 b) Circle the letter of the box containing the more stable conformer you drew above. A B c) This molecule is found to react slowly with potassium ferf-butoxide to form a single product. Draw this product in the box below. You do not have to draw the product in a chair conformation. 9H. ,\ ff:#ar r KorBu BuoH I I g*' -^- l:f ,r,, !l lC HrC' CHs Hsc I I I I I I d) ln the space provided below, brief ly explain why the reaction above is so slow. Your explanation may refer to the conformational isomers you drew above. - 0nff co6"^et'.r,^ 4 l"r.* a, Ut'-\hf +4'*c-,* tr- .A,i,.*Jt^'4e & i^ h\ a,4.-cy-411*noc no!.*iowtif. t14A it eh$-afn, less r^':ll sJz,,ble ,n$.^oi +gl,^^, ttt l''r"*lfio^ s{-a*< .fr- *€.it "tX,r'^^'*-'X{ ba s,*6". l-y-.\1, s{ar^ri5#.n. c.{< accoati!#J. t14 Kev 5. The sulfonate ester below undergoes solvolysis quite readily in water to form Alcohol A. Hzo ................'.._ A a) Provide a complete curved-arrow mechanism for this transformation. K!-t$}- 5 F3-*fa il&'-J o I )tH Hro:..} b) For each of the pairs below, circle the set of conditions that would favor Sp2 mechanisms. Note that this question is NOT related to part (a) above! Substrate structure: -l-r," oR fAt,. Solvent: @* Concentration of nucleophile: water fr*]\ oR \/ Low t16 Name: Kg-V 6. Alcohols can be converted to the corresponding alkyl fluorides by treatment with lhe reagent DAST, as shown in the reaction below. This reaction proceeds analogously to treatment with phosphorus tribromide. E ./\'.oH HO //'v'F -s-NEt, tr'/ 'F --S-NEtr r/ F alkyl DAST fluoride a) Provide a complete curved-arrow mechanism for this transformation and, for each step, identify the frontier molecular orbital (donor/acceptor) interactions involved. (You only need to name the orbitals, e.g., ntoi neither "cartoon" orbital drawings nor energy-level diagrams are necessary.) ,, -oH H F>/= NE+,- 7€ Do""r: -> flo Accegtor' ,.- -9oRs.NE+. ,F o) (..J FF Dono.: h" Aa.1r+n.: d-o I d-r --\./F + *or.S /$#* Ff b) Using your knowledge of the mechanism, draw the product of the following reaction in the box provided. You must indicate stereochemistry for full credit. -+ DAST t12 Name: KE;Z 7. Provide a complete synthesis of the following desired product from the indicated starting material. You may use any other reagents you wish. You do not need to show any mechanisms. The correct answer will require three or fewer total steps. (You may ignore stereochemistry.) Starting Material: b, o#o- '*i5 or<) q^r x'y' CHzrz, aq) fo- i*:t -i"r,*-) Deslred Product t12 Kev 8. Provide a complete synthesis of the following desired product from the indicated starting material. You may use any other reagents you wish. You do not need to show any mechanisms. The correct answer will require three or fewer total steps. Starting Material: (ge ts q. is ofl Bq .4,LOMe MeoH U,,,8*^ (+ri (+/-) Desired Product t't0 ...
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This note was uploaded on 01/22/2012 for the course CHEM CHEM E-2B taught by Professor Loganmccarty during the Fall '09 term at Harvard.

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