09-29 - 9/29/2009 The C-C-C bond angles in cyclopropane are...

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Unformatted text preview: 9/29/2009 The C-C-C bond angles in cyclopropane are 60. H C C H H C H C C H H H C C H H C H C C H H H H H H H H H H H C C H H C H H H H H H H C C cyclohexane cyclopropane cyclohexane incorrect 0% A. Very sure this is true thi C. Maybe this is true D. Maybe this is false correct correct 100% 80% 60% 60% 80% 100% 20% 40% 40% 20% 0% B. B. Somewhat sure this is true E. Somewhat sure this is false F. Very sure this is false The C-C-C bond angles in cyclohexane are 120. H C C H H C H C C H H What are the C-C-C bond angles in cyclohexane? H C C H H C H C C H H H C H H C H H C H H H H H H H H H H C C H H C H H H H H H H C C cyclopropane cyclohexane cyclopropane cyclohexane A. Very sure this is true thi B. B. Somewhat sure this is true C. Maybe this is true D. Maybe this is false E. Somewhat sure this is false F. Very sure this is false 3 109.5° 4 What are the shapes of these cycloalkanes? H H H H H C C H H C C H H H H H H H C C H C H H C C H H H H H H H C C H C H C C H H Which of these cycloalkanes is most stable? H H H H H C C H H C C H H H H H H H C C H C H H C C H H H H H H H C C H C H C C H H H C H H C H C H H H H H H H C C H C H H H H C C cyclopropane cyclobutane cyclopentane cyclohexane A B C D planar actual 60° 60° 90° 88° 108° 105° 120° 109.5° 5 1 9/29/2009 Which of these cycloalkanes is most stable? H H H H H C C H H C C H O2 CO2 + H2O + heat H H H H H H C C H C H H C C H O2 CO2 + H2O + heat H H H H H H C C H C H C C H O2 CO2 + H2O + heat H The heats of combustion for ethylene oxide acetaldehyde are given below. Which of these compounds is more stable? H H H H C H H C H C H C (A) H H C H C O H (B) H C H O C H H O2 CO2 + H2O + heat 1164 kJ/mol 1264 kJ/mol O2 Hº/CH2 (kJ/mole) = O2 697 686 664 ) = 659 kJ/mole 659 Hº/CH2 in unstrained alkanes ( strain energy 116 (kJ/mole) = 116 energy 110 110 42 no strain energy 2CO 2CO2 + 2H 2O 8 What are the origins of ring strain? What are the origins of ring 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 H H C 60° H H C 109.5° H C H H H H C H C C H H H H H 9 H C C H H C H H H C H H C C H H H H H bond angle strain, electrons are further from the nuclei 10 1. 1. Bond angle strain. 2. Torsional strain. 1. 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° 109.5° H H H H C C H C H H energy Why does cyclobutane have about the same strain energy as cyclopropane but is much less than cyclopentane? H 12.6 kJ (3 kcal)/mol 11 rotation 2 9/29/2009 Why are cyclobutane and cyclopentane puckered? A. Bond angle strain. B. Torsional strain. How can we draw a cyclohexane ring? 42 kJ (10 kcal)/mol H H C H C C H 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 no strain energy H H C C H C H C C H H H H H H H H C C H C C-C bond angles 60° 88° 105° 109.5° 13 14 How many chemically different kinds of C-H bonds are there in cyclohexane? A. 1 B. 2 C. 3 D. 4 E. 6 F. 12 How many methylcyclohexanes are there? axial methyl group H H H H H H H H H H H H H H H H H H H H equatorial methyl group H H A. 1 H H 15 B. 2 C. 3 D. 4 E. 6 F. 12 16 Which of the following methyl cyclohexanes is most stable? axial methyl group How many methylcyclohexanes are there? * * A B equatorial methyl group * 17 18 3 9/29/2009 Why is the axial conformation less stable than the equatorial conformation? Why is the twist boat conformation less stable than the equatorial or axial conformation? A. bond angle strain twist boat B. torsional strain C. steric strain (van der Waals) A. bond angle strain twist boat B. torsional strain C. steric strain (van der Waals) >21 kJ (5 kcal)/mol energy energy G° energy energy G° >21 kJ (5 kcal)/mol 7.28 kJ (1.74 kcal)/mol axial equatorial 19 7.28 kJ (1.74 kcal)/mol axial equatorial 20 1,3-diaxial 1,3-diaxial interaction Why is the twist boat conformation less stable than the equatorial or axial conformation? What is the approximate axial/equatorial Keq at RT? A. bond angle strain B. torsional strain twist boat C. steric strain (van der Waals) A. A. 0.031 B. B. 0.16 C. 1.8 D. D. 19 E. 230 F. F. 21000 when Keq = G° = 5.7 KJ/mol then Keq = 101 19 = 107.28/5.7 = 101.28 >21 kJ (5 kcal)/mol energy energy G° energy energy G° 7.28 kJ (1.74 kcal)/mol axial equatorial 21 7.28 kJ (1.74 kcal)/mol axial equatorial 22 How much of the equatorial conformation of methylcyclohexane would be present at room temperature? G° = -RTln Keq How many different 1,2-dimethylcyclohexane structures exist in the chair conformation? H CH CH CH CH3 3 H A. 1 B. 2 C. 4 D. 5 E. 7 F. 8 5% 95% 23 24 4 9/29/2009 How many different 1,2-dimethylcyclohexane structures exist in the chair conformation? H CH CH CH CH3 3 H Which of these represent different conformations? H CH CH3 A. 1 B. 2 C. 4 D. 5 E. 7 F. 8 H CH CH3 H CH CH3 H axial equatorial CH CH CH CH3 3 equatorial H H CH CH3 equatorial CH CH3 H H CH CH3 CH CH3 CH CH3 H H CH CH3 H axial H CH CH3 CH CH3 H axial H axial B. + C. equatorial CH CH3 A. 25 + + 26 Are Are and or and interconvertable by a conformational change? Do these different configurations have different names? different conformations H CH CH CH CH3 3 H ? CH CH3 H CH CH3 H H H CH CH CH CH3 3 H CH CH3 H CH CH3 H3C CH CH3 H H CH CH3 H H CH CH3 H3C H H CH CH3 CH CH3 ? CH CH3 CH CH3 H H CH CH3 H H CH CH3 CH CH3 H H different configurations 27 trans-1,2-dimethylcyclohexane cis-1,2-dimethylcyclohexane 28 Which of the following isomers is the most stable? How can the stabilities of these isomers be estimated? H CH CH CH CH3 3 H CH CH3 H H CH CH3 CH CH3 CH CH3 H H H CH CH3 H CH CH3 H H CH CH CH CH3 3 CH CH3 H H CH CH3 CH CH3 CH CH3 H H H H H H H CH CH3 H CH CH3 A B C D a gauche butane interaction CH CH3 H3C 29 H H H H 30 5 9/29/2009 How can the stabilities of these isomers be estimated? H CH CH CH CH3 3 H H H CH CH3 H H H CH CH3 H H H H CH CH3 CH CH3 H How many gauche butane interactions are present in trans diaxial-1,2-dimethylcyclohexane? H CH CH CH CH3 3 H H CH3 H H H H CH3 H H H H CH CH3 CH CH3 H H H CH CH3 H CH CH3 H H H CH CH3 H CH CH3 3 interaction 3 interactions 1 interaction H 4 interactions 3 interaction 3 interactions A=0 a gauche butane interaction one diaxial interaction B=1 C=2 D=3 E=4 a gauche butane interaction one diaxial interaction 31 32 What is the order of stability of these different isomers? H CH CH CH CH3 3 CH CH3 > H H CH CH3 = CH CH3 CH CH3 H H CH CH3 > H H CH CH3 1 interaction H 4 interactions 3 interactions 3 interactions increasing stability a gauche butane interaction one diaxial interaction 33 6 ...
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This note was uploaded on 12/25/2009 for the course CHE 320 taught by Professor Ff during the Spring '09 term at SUNY Stony Brook.

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