Chem 11 Oa-Section 2. Exam #2. 1 October 2004

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1 " Chem 11 Oa-Section 2 Exam #2 1 October 2004 Name: K~ Instructions Please read each question carefully and answer it completely and clearly. Do the problems in the order that is easiest for you. Point value is indicated with each question. There are a total of 100 points possible on the exam. Read through the entire exam before beginning. There should be 5 pages including this page and a score tabulation page. You should use clear, carefully drawn structures and pictorial representations for all molecules. Use precise and graphic arrow notation. Show 3-dimensional and resonance forms where necessary. Do not overlook the stereochemistry. Compose your answers so that the logic of your analysis is unmistakable.

Read through the entire exam before beginning. There should be 5 pages including this page and a score tabulation page.

2 1. Circle the correct IUPAC name corresponding to each of the following compounds. (3 pts each! 9 pts total) Q ( R)-2-fluoro-5, 6-dimethyl-5-heptene ( R)-6- fluoro-2, 3-d imethyl-2-heptene ~;;~ or ( 5)-2, 3-d imethyl-2-hepten-6-o1 2. a) Calculate the index of hydrogen deficiency (ID) for a molecule with the formula C6sOCI2, (5 pts) ID-= i ((2nt2)- (8..2C-1 )) n:.(p b) In a few words, what does the ID of this molecule tell you about its structure? Draw one possible isomer. This is not an essay question. (4 pts) II-ID: 2. -=> 2. units of UhSa tur-a..nct) (Tr bondslri~) possibl.q isoyy'le..ys: Q 1:1 O, I c..\ -=~ C.I O ( \'Y)o,ny oth;4-fs ~ o..re. t>os'sl b\q.) Ct C..I 3. The following molecule is one of the 10 fragments that were used by Professor Kishi's research group at arvard University to assemble Palytoxin. Label each chiral center as either R or $. (Kishi, V. et. al. J. Am. Chem. Soc. 1989, 111, 7525) (9 pts) 4. A recent article contained the following scheme. While the mechanism of this transformation is beyond the scope of this course, good conformational drawings are not. Each of the conformational drawings below has at least one error. The errors are not in the substituent labels. Clearly indicate three errors in these drawings for full credit. (Fodor, G. B. Tetrahedron 1999, 55, 7391) (6 pts) 'k 0.-\ \ 0,. ( e. ::'9 dk"o,u.)y\ o..~ O-r\0/ ~ ~ or ~ n()1r ~c: r\\ \Y\ 0., ~d~

a) Calculate the index of hydrogen deficiency (ID) for a molecule with the formula C6sOCI2, (5 pts) ID-= i ((2nt2)- (8..2C-1 )) n:.

3 5. a) Five compounds of the molecular formula CS1202 are depicted below. Choose compound(s) from those shown that fit each description. Place the letter corresponding to the compound(s) in the appropriate box. (3 pts each/ 15 pts total) O..r O O O O ~ O h b X;Z XA ~ \\\ A B D I-I I-I ( E all meso compounds a pair of enantiomers all structures that are achiral a pair of diastereomers two structures that describe the same molecule b) In a few words, comment on the optical rotation of a 1 :1 mixture of compounds A and B. What can you say about the optical rotation of a similar 1 :1 mixture of compounds D and E? (6 pts) " \ D: E => r"c) e.ornm2rt\ Co...Y\ be mcade j dl<is-\e.ye.o mer~ do no1 e'1(.h\bit opt\c~ o..c.-\iv\fus fuq,,1 o.re re\a..-\ed. 6. a) Draw two chair conformations of trans-1-ethyl-4-methylcyclohexane that are related to each other by a ring flip. Circle the (10 pts) <;:3 ""'- \- \ \ C.1-!2.C.."" 3 y C3 " W ~ ~~ ~nfby~n b) Is trans-1-ethyl-4-methylcyclohexane chiral? Mark the appropriate box. (3 pts) D Chiral

$.r O O O O ~ O h b X;Z XA ~ \\\ A B D I-I I-I ( E all meso compounds a pair of enantiomers all structures that are achiral a pair of diastereomers two structures that describe the same molecule b) In$

4 7. a) The 3-dimensional structure of 2,3-dimethylbutane is given below. This question requires you to draw three Newman projections. Draw one Newman projection to represent the conformer as shown with respect the indicated bond. Draw two additional Newman projections representing the lowest and highest energy conformations of 2,3-dimethylbutane. (3 pts each/ 9 pts total) ""(- C3 -- ~...,.. C3 3C --- C ~ " highest en~gy b) Do a "back of the envelope" calculation and predict the Ere, for the lowest energy conformer you just drew. Don't forget units. What interactions are responsible for the Ere,? (6 pts) Erel :. <2> \<.31 mot 8. In class, we discussed A values as a means of gauging the size of a substituent located on a monosubstituted cyclohexane ring. The A values for the ethyl, isopropyl (-C(C3)2). and tertbutyl (-C(C3)3) groups are 1.8 kcal/mol, 2.2 kcal/mol, and 4.8 kcal/mol, respectively. Progression through this series adds one additional methyl group per substituent. One might expect that given the incremental nature of this series, the difference in A value would be consistent as one went from the ethyl to isopropyl groups (difference = 0.4 kcal/mol) and then again from the isopropyl to tert-butyl substituents. Upon analysis, however, the difference in A value between an isopropyl and tert-butyl substituent seems disproportionately large (2.6 kcal/mol). Using a few words and several structures, explain the large difference in A value between the isopropyl and tert-butyl substituents. (12 pts) at a-t.,' I~prop-\\ CO-n adopt 0. ecviorrnatioh,u.)hlc.-h pl0...c.ls 0. intl.) ~ C:lj\$.X Of +'v'u r(~. -tgrj- butyl ty\us+ pta(!i 0, LClr8er C3 'jroup tntb 9. Explain the concept of chirality in a way that my mom (an accountant in the Midwest with no fu~. real chemistry background) could understand. Use 1-2 sentences and appropriate images in your posltidn answer. (6 pts) ~.your na-nd s O-re. ch\w. l\1~ o.rq. w1\ '(\1) r.\\"{)q~~ \::J~ L(}X\ r\0\ be.~frx'~~d on t)'\e. ~~Y. ~y -\t1$m~ c'on1 ~\&1 ) ) r ~

Draw two additional Newman projections representing the lowest and highest energy conformations of 2,3-dimethylbutane. (3 pts each/ 9 pts total) ""(- C3 -- ~...,.. C3 3C --- C3 -.

CONFORMATIONAL ANALYSIS PRACTICE EXERCISES. 1) Draw a Newman projection of the most stable conformation of 2-methylpropane.

CONFORMATIONAL ANALYSIS PRACTICE EXERCISES 1) Draw a Newman projection of the most stable conformation of 2-methylpropane. 2) The structures below are: C 3 C 3 C 3 C 3 A) not isomers. B) conformational