272_quiz3solutions - a possible hydrogen bond between the...

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Chain Chem 272 Quiz 3 1. Calculate the barrier to rotation for the designated bond. Show your work! 2. (a) Draw the anti and gauche conformations for ethylene glycol (HOCH 2 CH 2 OH). (b) Ethylene glycol is unusual in that the gauche conformation is more stable than the anti conformation. Offer an explanation. Name: H 3 C C(CH 3 ) 3 CH 3 H 3 C CH 3 H H H H 3 C H 3 C CH 3 H H H OH H H OH H H staggered no costly interactions eclipsed 3 CH 3 /H eclipsing interactions 3 x 1.4 kcal/mol = 4.2 kcal/mol H HO H OH H H HO OH H H H H H OH HO H H H staggered, anti conformer staggered, gauche conformer Though the gauche conformer picks up a small amount of strain due to the gauche interaction between the hydroxyl groups, this is offset by two factors - one of them is
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Unformatted text preview: a possible hydrogen bond between the hydroxyl groups. Recall, a hydrogen bond is worth ~1 kcal/mol, which is apparently enough to offset the gauche interaction. We can estimate the gauche interaction is worth slightly less than 0.88 kcal/mol (methyl-methyl gauche = 0.88 kcal/mol; hydroxyl groups are smaller than methyl groups, but a C–O bond is shorter than a C–C bond). possible hydrogen bond Answer Key...
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This note was uploaded on 03/07/2011 for the course CHEM 272 taught by Professor Jarrett,j during the Spring '08 term at University of Hawaii, Manoa.

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