L5a - Rank these conformations in order of increasing...

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1 1 Most stable A B C 2% 94% 4% 1. A 2. B Rank these conformations in order of increasing stability (i.e., least stable to most stable) ©Dr. Kay Sandberg CH 3 H CH 3 Br H H CH 3 H CH 3 H H Br CH 3 H CH 3 H Br H AB C Click the letter of the first one in your list. Lecture notes & HW may be posted later than 8 pm. 2 g a u c h e ©Dr. Kay Sandberg H H Br CH 3 CH 3 H CH 3 H CH 3 Br H H CH 3 H CH 3 H H Br CH 3 H CH 3 H Br H C anti gauche gauche Br HH CH 3 CH 3 H H Br H CH 3 CH 3 H anti gauche most stable most stable??? A & B have same stability 3 ©Dr. Kay Sandberg Br CH 3 CH 3 H H Br H CH 3 CH 3 H H H Br CH 3 CH 3 H CH 3 H CH 3 Br H H CH 3 H CH 3 H H Br CH 3 H CH 3 H Br H ABC anti gauche van der Waals strain van der Waals strain X 2 same conformation 4 ©Dr. Kay Sandberg Last lecture This lecture Conformations of alkanes •eclipsed, anti, gauche •energy difference of conformations •description of steric or strain energy Conformations of cycloalkanes Configurations of cycloalkanes 5 Potential Energy, kJ/mol 0 60 120 300 360 CH 3 H H H H CH 3 CH 3 H H CH 3 H H CH 3 H H H H 3 C H CH 3 H H H H CH 3 CH 3 H H CH 3 H H CH 3 H H H CH 3 H CH 3 H H H H H 3 C ©Kay Sandberg 3 14 25 9 van der Waals & torsional strain torsional strain (eclipsed bonds) no torsional strain (no eclipsed bonds) no VDW strain 6 ©Dr. Kay Sandberg ac b de eclipsed anti no strain!!!!
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2 7 Potential Energy, kJ/mol 60 120 300 360 CH 3 H H H H CH 3 CH 3 H H CH 3 H H CH 3 H H H H 3 C H CH 3 H H H H CH 3 CH 3 H H CH 3 H H CH 3 H H H CH 3 H CH 3 H H H H H 3 C ©Kay Sandberg 3 14 25 9 van der Waals & torsional strain vdw strain (atoms too close) gauche torsional strain (eclipsed bonds) no strain no torsional strain (no eclipsed bonds) 8 Shapes of cycloalkanes Ideal bond angle for alkanes (sp 3 )? 109.5 o H Bond angle in cyclopropane? Section 3.5 60 o ©Kay Sandberg “wanna be” angle = 109.5 o “hafta be” angle = 60 o 9 Shapes of cycloalkanes Ideal bond angle for alkanes (sp 3 )? 109.5 o H Bond angle in cyclopropane? 60 o Angle strain: destabilization due to deviation from ideal angle values. Planar & bent bonds ©Kay Sandberg Section 3.5 10 ©Kay Sandberg bowed bonds C C HH H H straight bonds Section 3.5 11 ©Kay Sandberg turn 90 o a b c de 1) torsional eclipsed bonds 2) angle bending deviation from normal bond angles High energy results from: All bonds separated by 1 bond are forced to be eclipsed!! Section 3.5 Total of 9 bonds 4 bonds missing? f g h i a d f h i 12 ©Dr. Kay Sandberg Strain Energy Types 1) van der Waals 2) torsional Atoms too close eclipsed bonds Strain E = Steric E 3) angle bending deviation from normal bond angles 4) bond stretching deviation from normal bond lengths
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3 13 Shapes of cycloalkanes Ideal bond angle for alkanes?
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This note was uploaded on 01/25/2009 for the course CH 221 taught by Professor Melander during the Spring '07 term at N.C. State.

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L5a - Rank these conformations in order of increasing...

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