PS_2_Answer_Key_2011

PS_2_Answer_Key_2011 - Massachusetts Institute of...

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Massachusetts Institute of Technology Organic Chemistry 5.12 Head TAs Hyung Kyoo Kwon, Yu-Pu Wang & Phil Hamzik February 23, 2011 Solutions to Problem Set #2 (1) Consider the three compounds isolated from peppermint shown below. (a) What is the isomeric relationship between A and B, A and C, and B and C? (b) Label each stereogenic center as R or S . (c) Draw the two alternate chair conformers for each compound and indicate which conformer in each pair is more stable. OH A OH B OH C ANSWER: Noticing that A, B and C are constitutionally identical isomers, they must be stereoisomers. Recall that an easy way to determine the stereoisomeric relationship between two stereoisomers is to compare each stereogenic center's configuration; if at least one but not all of the stereogenic centers differ between two molecules, they are diastereomers; if all of the stereogenic centers are different between the two, they are enantiomers ("Useful Shortcuts in Analyzing Stereochemistry," Lecture 4). For stereoisomers A, B and C, there are 3 stereogenic centers: (a) OH A OH B Inverted Inverted Same OH A OH C Inverted Inverted Inverted OH B OH C Same Same Inverted Diastereomers Enantiomers Diastereomers This material may not be reproduced or posted on an internet website without written permission of Professors Danheiser and Jamison.
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(b) Use the Cahn-Ingold-Prelog rules to assign R and S to each stereogenic center (Smith, 5.6). For stereoisomer A, the priorities of groups attached to the three stereogenic centers are shown: OH 1 2 3 H 4 OH H 1 2 3 4 OH H 1 2 3 4 R S S The configurations of all stereogenic centers can then be determined for all stereoisomers: OH A OH B OH C S S S S S R R R R (c) Here, consider the destabilization due to axial positioning of large groups (Me, OH, isopropyl) on the cyclohexane ring (Tools and Techniques #5, Lecture 5; Smith, 4.13). Though some conformations contain an isopropyl-OH gauche interaction (as shown for isomer A below), the energy of this interaction is not large enough to overcome the energetic difference between having two substituents axial and having only one substituent axial. 2 substituents axial 1 substituent axial Me H H H H OH H Me Me Gauche interaction A: FAVORED OH HO B: 2 substituents axial 1 substituent axial FAVORED OH HO 1 substituent axial 2 substituents axial C: FAVORED HO OH
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(2) This problem focuses on the trimethylcyclohexane shown below. (a) Draw the two alternate chair conformers for this compound. (b) Use equilibrium arrows of unequal length to indicate which conformer is lower energy and is favored in the interconversion of conformational isomers. (c) Draw a Newman projection for the major conformer sighting down the C(1)-C(2) and C(5)-C(4) bonds. (d)
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This note was uploaded on 02/14/2012 for the course CHEMISTRY 5.12 taught by Professor Saraho'connor during the Fall '09 term at MIT.

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PS_2_Answer_Key_2011 - Massachusetts Institute of...

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