wk5_lecture_2 - 10/3/2008 Draw the sawhorse projection of...

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Unformatted text preview: 10/3/2008 Draw the sawhorse projection of the following Newman projection. Draw the sawhorse projection of the following Newman projection. Cl Cl H H H H Cl C Cl H A Cl Cl Cl Cl H C C H a partial structure Cl Cl Cl H Cl H H H Cl Cl Cl H C H B Cl Cl H H H H Cl H Cl Cl C HH C H H H H H Cl Cl H H H H H Cl Cl Cl H C H B H C C Cl Cl H C HH C H Cl Cl C H Cl Cl C Cl Cl H C H H C Cl Cl H H Cl Cl C H C Cl Cl H H C H Cl Cl C Cl Cl H C H HH D HH D A What sawhorse projection is identical to the above Newman projection? 1 What sawhorse projection is identical to the above Newman projection? 2 Butane has two staggered conformations. Which is more stable? H HH C H H C H C C HH H H H H Butane has two staggered conformations. Which is more stable? H H C H H HH C C H C C HH H H H H HH C C HH C C H C A CH H H HH B CH H H HH staggered anti staggered gauche 3 staggered anti staggered gauche 4 In contrast to ethane butane has two staggered conformations. Which is more stable? H HH C H H C H C C HH H H H H In contrast to ethane butane has two staggered conformations. Which is more stable? H H C C HH H H H H HH C C H C H CH H HHH HH C C H HH C C H C CH H HHH staggered anti staggered gauche 5 staggered anti 6 1 10/3/2008 In contrast to ethane butane has two staggered conformations. Which is more stable? H HH C H H C H C C HH H H H H In contrast to ethane butane has two staggered conformations. Which is more stable? HH C C H C CH CH3 CH CH 2 CH CH 2 CH CH3 CH H HHH energy 14.6 kJ /mol 3.8 kJ /mol rotation 21 kJ /mol staggered anti staggered gauche 7 8 How many stable staggered conformations exist for butane? A. 1 CH CH3 CH CH 2 How many stable staggered conformations exist for butane? B. 2 CH CH 2 CH CH3 C. 3 D. 4 E. 5 F. 7 Although the two gauche conformations have the same energy they have different structures. energy 14.6 kJ /mol 3.8 kJ /mol rotation 21 kJ /mol energy 14.6 kJ /mol 3.8 kJ /mol rotation 21 kJ /mol 9 10 What is major factor responsible for the energy difference between the staggered and eclipsed conformers? A. Steric Effects (non bonding) B. Torsional Effects (electronic) What is major factor responsible for the energy difference between the staggered and eclipsed conformers? torsional strain 12.6 kJ /mol steric strain 14.6 kJ /mol energy energy 14.6 kJ /mol 3.8 kJ /mol rotation 14.6 kJ /mol 3.8 kJ /mol rotation 21 kJ /mol 11 12 2 10/3/2008 Which conformation, gauche or anti, would predominate in butane at room temperature? Can the Keq be calculated from an energy difference? ∆G° = -RTln Keq If ∆G° = 5.7 kJ/mole then the Keq = 10. If ∆G° = 2(5.7) or 11.4 kJ/mole then the Keq = ? A. 5 B. 10 C. 20 D. 100 A gauche B gauche anti energy anti energy 14.6 kJ /mol 3.8 kJ /mol rotation 14.6 kJ /mol 3.8 kJ /mol rotation 13 14 Can the Keq be calculated from an energy difference? ∆G° = -RTln Keq If ∆G° = 5.7 kJ/mole then the Keq = 10. If ∆G° = 2(5.7) or 11.4 kJ/mole then the Keq = 100 1 If ∆G° = 3(5.7) or 17.1 kJ/mole then the Keq = ?000 What is the Keq of the butane conformers at room temperature? ∆G° = -RTln Keq ∆G° = 3.8 kJ/mol Keq ≈ ? A. Less than 10? B. More than 10? gauche anti energy energy gauche anti 14.6 kJ /mol 3.8 kJ /mol rotation 14.6 kJ /mol 3.8 kJ /mol rotation 15 16 What is the Keq of the butane conformers at room temperature? ∆G° = -RTln Keq ∆G° = 3.8 kJ/mol [anti] [anti] Keq ≈ 4.6 ≈ [gauche] Arrange the following structures in order of increasing stability (most stable on the right). H H H C Cl C Cl H H Cl Cl HCC H H H H Cl C Cl C H H gauche anti energy A. D. B. E. C. F. 14.6 kJ /mol 3.8 kJ /mol rotation 17 18 18 3 10/3/2008 Review: All of the conformers of butane have the same connectivity and they rapidly interconvert by rotation about single bonds. C4H10 CH3—CH2—CH2—CH3 Homework: Using your molecular models build a six membered ring and bring it and your molecular models to class. conformers conformers conformational isomers 19 20 H C H H C H C H H H H H C C H H C C H H H H H H C C H C H H C C H H H H H H C C C H C C H H H H H H H C H C H C H H H H H cyclopropane planar 60° A cyclobutane 90° B H cyclopentane 108° CH H C cyclohexane 120° HD H H C H H H C C HCCH H H HH HH H H Which of these rings would be predicted to be the least stable? Which of these rings would be predicted to be the most stable? alkanes = CnH2n+2 cycloalkanes = CnH2n 109.5° 21 22 When these hydrocarbons are burned which one will give off the most heat per methylene group (CH2)? ΔH° predicted(from alkanes) - ΔH° observed = strain energy strain energy 116 kJ 110 kJ (27.7 kcal)/mol (26.3 kcal)/mol H H H C 42 kJ (10 kcal)/mol HC H H H C H C H C H C C H O2 CO2 + H2O + heat H H H H H H no strain energy HD H C C H C C C H O2 CO2 + H2O + heat H H H H strain energy 116 kJ 110 kJ (27.7 kcal)/mol (26.3 kcal)/mol H C H H C C H H H H H H C C H H C C H H H 42 kJ (10 kcal)/mol H H H H C C H C H H C C H O2 CO2 + H2O + heat H H H H H H H no strain energy H C C H C H C C H O2 CO2 + H2O + heat H H H H A C H H H H H H C C H B H C C H H H C C C O2 CO2 + H2O + heat O2 CO2 + H2O + heat O2 CO2 + H2O + heat H H C H H H O2 CO2 + H2O + heat H C H + 109.5° 23 H C H C H H ΔH° = 376kJ (90kcal)/mol 24 4 10/3/2008 The heats of combustion for ethylene oxide acetaldehyde are given below. Which of these compounds is more stable? What are the origins of ring strain? (A) H H C H C O H (B) H C H O C H H strain energy 116 kJ 110 kJ (27.7 kcal)/mol (26.3 kcal)/mol H C H H C C H H H H H H C C H H C C H H H 42 kJ (10 kcal)/mol H H H H C C H C H H C C H H H H H H H no strain energy H C C H C H C C H H H H H 1164 kJ/mol 1264 kJ/mol O2 C O2 H H C 60° H C H 25 109.5° H C H H H H C H C C H H H energy 2CO2 + 2H 2O 2CO H 26 What are the origins of ring strain? 1. Bond angle strain. 2. Torsional strain. strain energy 116 kJ 110 kJ (27.7 kcal)/mol (26.3 kcal)/mol H C H H C C H H H H H H C C H H C C H H H 42 kJ (10 kcal)/mol H H H H C C H C H H C C H H H H H H H no strain energy H C C H C H C C H H H H H strain energy 116 kJ 110 kJ (27.7 kcal)/mol (26.3 kcal)/mol H C H H C C H H H H H H C C H H C C H H H 42 kJ (10 kcal)/mol H H H H C C H C H H C C H H H H H H H no strain energy H C C H C H C C H H H H H C C 60° 90° 105° H C H H C H H C H H H H C H C C H H H 109.5° H H H H C C H C H H H Why does cyclobutane have about the same strain energy as cyclopropane but is much less than cyclopentane? 27 H bond angle strain 28 1. Bond angle strain. Why Why are cyclobutane and cyclopentane puckered? 2. Torsional strain. A. Bond angle strain. B. Torsional strain. strain energy 116 kJ 110 kJ (27.7 kcal)/mol (26.3 kcal)/mol H C H H C C H H H H H H C C H H C C H H H 42 kJ (10 kcal)/mol H H H H C C H C H H C C H H H H H H H no strain energy H C C H C H C C H H H H H strain energy 116 kJ 110 kJ (27.7 kcal)/mol (26.3 kcal)/mol H C H H C H C H H H H H C C H H C C H H H 42 kJ (10 kcal)/mol H H H H C C H C H H C C H H H H H H H no strain energy H C C H C H C C H H H H H C C-C bond angles planar actual C 60° 60° 90° 88° 108° 105° 120° 109.5° energy 12.6 kJ (3 kcal)/mol rotation 30 5 10/3/2008 How can we draw a cyclohexane ring? How many kinds of C-H bonds are there in cyclohexane? H H H 31 H H H H H 32 H H H H How many kinds of C-H bonds are there in cyclohexane? axial equatorial H H H H H H H H H H H 33 H 6 ...
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This note was uploaded on 09/04/2009 for the course SBU 101 taught by Professor Debag during the Spring '09 term at SUNY Stony Brook.

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