Chapter 7 - Instructor Supplemental Solutions to Problems...

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Instructor Supplemental Solutions to Problems © 2010 Roberts and Company Publishers Chapter 7 Cyclic Compounds. Stereochemistry of Reactions Solutions to In-Text Problems 7.3 Following the procedure in the solution to Problem 7.2 (in the Study Guide and Solutions Manual), we first calculate the K eq for tert -butylcyclohexane: K eq = 10 ± G °/2.30 RT = 10 –20/5.71 = 10 –3.5 = 3.1 ² 10 –4 From this we calculate, in a total concentration of mol L –1 , [ A ] = (3.1 ² 10 –4 )[ E ] ± 0.00031. There is about 0.051/0.00031 = 168 times more axial conformation of methylcyclohexane than there is axial conformation of tert - butylcyclohexane per mole. 7.6 (a) The two chair conformations of cis- 1,3-dimethylcyclohexane: 7.7 (a) A boat conformation of cis- 1,3-dimethylcyclohexane: 7.9 The more stable conformations of the two 1,4-dimethylcyclohexanes: The cis isomer has the same number of 1,4 methyl–hydrogen interactions—two—as the axial conformation of methylcyclohexane itself, and thus has a destabilizing contribution of 7.4 kJ mol –1 . There are no destabilizing interactions in the trans isomer.
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INSTRUCTOR SUPPLEMENTAL SOLUTIONS TO PROBLEMS • CHAPTER 7 2 7.10 (b) (d) 7.11 (b) The most stable conformation is the one that has the greater number of groups in the equatorial position: 7.12 (b) 1,1-Dimethylcyclohexane is achiral, and therefore cannot be optically active. (d) Cis- 1-ethyl-3-methylcyclohexane can be isolated in optically active form. 7.14 (b) The two structures differ in configuration at both asymmetric carbons. They are enantiomers. (Show that they are noncongruent mirror images.) 7.15 (b) Cyclobutane undergoes an interconversion of puckered forms analogous to the chair interconversion of cyclohexane. This interchanges axial and equatorial groups. Therefore, one conformation of trans -1,2- dimethylcyclobutane—the more stable conformation—has diequatorial substituents, and the other—the less stable conformation—has diaxial substituents. 7.16 (b) Trans- 1,2-dimethylcyclopropane is chiral.
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INSTRUCTOR SUPPLEMENTAL SOLUTIONS TO PROBLEMS • CHAPTER 7 3 7.17 (b) Bicyclo[3.2.0]heptane 7.20 (b) The model of trans- bicyclo[5.3.0]decane is easier to build. The larger is a ring, the easier it is to compress the dihedral angle of the trans bonds at the ring junction without introducing significant strain in the ring. This angle must be very close to 0 ° in order to accommodate a fused cyclopropane, that is, to bridge the ends of trans bonds with only one carbon. 7.21 (b) Although both molecules have bridgehead double bonds, the double bond in compound B is more twisted, and a model of this molecule is more difficult to build. Consequently, compound B is less stable and therefore would have the greater (more positive or less negative) heat of formation. 7.22
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This note was uploaded on 02/13/2010 for the course EGR 012 taught by Professor Witfield during the Spring '10 term at Aarhus Universitet.

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Chapter 7 - Instructor Supplemental Solutions to Problems...

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