Chemistry 21 Exam 2 Solutions | Answer Key

University:
Amherst College
Course:
CHEM 221 | Organic Chemistry I
Academic year:

2009
Views:

214

Pages:

6
Author:

Aryan Rasmussen

Exam 2 Chem 21 Oct 21, 2009 6 /£ ? /J page 2 so 4. (15 points) In principle, the compound below can be protonated on either oxygen. But one oxygen is much more basic than the other. Draw the two conjugate acids that could be formed, and decide which protonation is the more favorable. Briefly explain your choice. Oil & ( H\n ' - 35 ’ O' I — cc*Ar*\ Ctv' Ov A I U/OW ' -f-ex j ' &V y ' '' pr (CM // ) 5 . (15 points) (a) One chair conformer of a substituted piperidine is shown below. Draw the other chair conformer. $ / "1 / ei 5 CH(CH3)2 Br •• /O place- T (V (Co~ ( b) Circle the more stable of the two. f (c) Use the data in the table at the right to estimate AG° for the ring flip above. Make sure your reasoning is clear / S */ y-c ' V ) S< yawl's y ^ SPIN' r 5 S R = Me R = Et R = iPr R - t- Bu R = CN R = vinyl R = Br R = NH2 iO -\% 0 ' wz AG° = 1.7 kcal/mol 1.8 2.2 5.0 0.2 1.7 0.5 1.4 r- frL . /** " JiKC . s^c4 ) R ) ( o> >AAV/^/A . 6 (12 points) Two isomeric cyclohexylammonium ions are shown below. © NH3 pKa = 10.3 (a) Label them cis and tram. W sk GiCst, (b) Explain briefly, in terms of structure, why one stereoisomer is more acidic than the other. }o% s U- $ jLesrc sh-sTc UK + . jfa- . /y* 4 V* ^ A-<- r. e . TS ^4us « A^eivs « - 7 (15 points) Calicene has been found experimentally to have a surprisingly large dipole moment. This can be attributed to a contribution from minor charge-separated resonance structures like A or B. (Of course, the charge can be further delocalized in each case.) Which polarization is the more important (A or B)? Briefly explain your choice. o 5! a •• f A / a, UV/T©» •ro^oa i *1 v * S } «*cA ' - Z. aT c L ^ T B ' fe ( _ coc C C 3( QX - -> av* yt / , In contrast to the corresponding cyclohexane ( cis-1-/ert-butyl-4(13 points) 8. methylcyclohexane), the lower energy chair conformation of the dioxane below is the one with the tert-butyl axial and the methyl equatorial . Explain. Ii (CH3 )3C f lAO 0) x C' Dji / j vcMU** USM ^ b* PA x ^ o ^ NH3 s *xz/uy ^ Draw all the stereoisomers of the following compound. No need 10. (15 points) conformation, just be sure that the stereochemistry is clear. Cl C Cl - CJ3//i/«vK 5 r?su-<- ~- lr\ a r C^ <7 c =C — CH3 c y* '\ to show the . 11. (25 points) Write a flow chart for the separation of the three compounds below using liquid liquid extraction. Draw structures to clearly show what compound is present at each step of the process and its protonation state. - Assume that the mixture is dissolved in diethyl ether, Et20, to begin. In addition to water, your lab is stocked with aqueous HC1, aq NaOH, and aq NaHCCb (pKa of H2CO3 is 6.3). (The sep funnel is just for decoration.) OH o2N t pKa = 4.1 pKa = 11.1 conj acid pKa = 3.2 / C02Et / conj acid pKa = 2.5 CA ^ ( ijl\ ~ jL Cy ( 00 C&'S L> 0 ^ yJJ \ ^ 0\ A \ .) Sci^ U shA * 7 )$ 1~ (L A/ c Oly *k ^ n A/o, liCO ^

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