wk6_lecture_1 - 10/7/2008 H H HO HO 1 2 H How can we draw a...

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Unformatted text preview: 10/7/2008 H H HO HO 1 2 H How can we draw a cyclohexane ring? 3 4 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 5 B. 2 C. 3 D. 4 E. 6 F. 12 7 1 10/7/2008 Which of the following methyl cyclohexanes is most stable? axial methyl group How many methylcyclohexanes are there? * * A B equatorial methyl group * 8 9 Why is the axial conformation less stable than the equatorial conformation? A. bond angle strain twist boat B. torsional strain C. steric strain (van der Waals) H H H CH CH HH H 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) >21 kJ (5 kcal)/mol energy ΔG° energy ΔG° >21 kJ (5 kcal)/mol 7.28 kJ (1.74 kcal)/mol axial equatorial 11 7.28 kJ (1.74 kcal)/mol axial equatorial 12 What is the approximate axial/equatorial Keq at RT? A. A. ≈ 0.031 B. ≈ 0.16 C. ≈ 1.8 D. ≈ 19 E. ≈ 230 F. ≈ 21000 How How much of the equatorial conformation of methylcyclohexane would be present at room temperature? ∆G° = -RTln Keq when ∆G° = 5.7 KJ/mol then Keq = 101 Keq = = 107.28/5.7 = 101.28 ≈ 19 energy ΔG° energy 7.28 kJ (1.74 kcal)/mol axial equatorial 13 5% 95% 17 2 10/7/2008 How many 1,2-dimethylcyclohexane structures exist in the chair conformation? H CH CH CH CH3 3 H How many 1,2-dimethylcyclohexane structures exist in the chair conformation? H CH CH CH CH3 3 H A. 1 B. 2 C. 3 D. 4 E. 5 F. 6 A. 1 B. 2 C. 3 D. 4 E. 5 F. 6 H CH CH3 H CH CH3 CH CH3 H H CH CH3 CH CH3 CH CH3 H H 18 19 Which of these represent different conformations? H CH CH3 Are Are and or and interconvertable by a conformational change? equatorial CH CH CH CH3 3 equatorial H different conformations axial H CH CH CH CH3 3 H H CH CH3 ? CH CH3 H CH CH3 H H equatorial CH CH3 H axial H CH CH3 CH CH3 H axial H CH CH3 H H CH CH3 axial B. E. + + C. F. + + equatorial CH CH3 ? CH CH3 CH CH3 H H A. D. + + different configurations 20 21 Do these different configurations have different names? What is the order of stability of these different isomers? H CH CH CH CH3 3 H H CH CH3 H CH CH3 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 H H H CH CH3 H CH CH3 increasing stability CH CH3 CH CH3 H H trans-1,2-dimethylcyclohexane cis-1,2-dimethylcyclohexane 22 23 3 10/7/2008 What is the order of stability of these different isomers? What is the order of stability of these different isomers? H CH CH CH CH3 3 H CH CH3 H H H H CH CH3 H H H H CH CH3 CH CH3 H H CH CH CH CH3 3 H CH CH3 H H CH CH3 CH CH3 CH CH3 H H H H H H H CH CH3 H CH CH3 H H H H CH CH3 H CH CH3 1 interaction 4 interactions 3 interaction 3 interactions increasing stability a gauche butane interaction increasing stability a gauche butane interaction ≈ one diaxial interaction CH CH3 H3C H H H H 24 ≈ 25 What is the order of stability of these different isomers? How can alkanes and cycloalkanes be prepared? catalytic hydrogenation 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 C C HH catalyst catalyst (Pd/C) (Pd/C) H C H C increasing stability a gauche butane interaction ≈ one diaxial interaction ≈ 26 29 How can alkanes and cycloalkanes be prepared? How can alkanes and cycloalkanes be prepared? catalytic hydrogenation catalytic hydrogenation HH C C HH catalyst catalyst (Pd/C) (Pd/C) H C H C C C HH catalyst catalyst (Pd/C) (Pd/C) H C H C syn or cis addition of H2. 30 31 4 10/7/2008 How can alkanes and cycloalkanes be prepared? 13C Nuclear Magnetic Resonance Spectroscopy? catalytic hydrogenation C C HH catalyst catalyst (Pd/C) (Pd/C) H C H C Nuclear Magnetic Resonance Spectroscopy Instrument 13 13C pure pure chemical compound C5H10O * H H H H H H H * H catalyst catalyst (Pd/C) (Pd/C) * 32 X 33 Give the structure of C5H10O. Give the structure of C5H10O. Each Each chemically different carbon atom in a structure results in one absorption The The position of a carbon atom absorption depends upon its chemical environment pure chemical compound C5H10O no double bonds or rings = C5H12O double bonds or rings? * Each Each chemically different carbon atom in a structure results in one absorption The The position of a carbon atom absorption depends upon its chemical environment O H O pure chemical compound C5H10O O O * A B O C D * E * * X 34 * X 36 5 ...
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