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3: Chapter 3 Conformations and Cycloalkanes

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    • 3.1: Conformations of Ethane
      Conformational isomerism involves rotation about sigma bonds, and does not involve any differences in the connectivity or geometry of bonding. Two or more structures that are categorized as conformational isomers, or conformers, are really just two of the exact same molecule that differ only in terms of the angle about one or more sigma bonds.
    • 3.2: Conformations of Other Alkanes
      Ethane has only two conformers of note - staggered and eclipsed.  Alkanes that are more complex than ethane, like propane and butane have a greater variety in possible conformers and their relative energies.
    • 3.3: Cis-Trans Isomerism in Cycloalkanes
      Stereoisomers are molecules that have the same molecular formula, the same atom connectivity, but they differ in the relative spatial orientation of the atoms.  Di-substituted cycloalkanes are one class of molecules that exhibit stereoisomerism.  Di-substituted cycloalkane stereoisomers are designated by the nomenclature prefixes cis (Latin, meaning on this side) and trans (Latin, meaning across).
    • 3.4: Stability of Cycloalkanes - Ring Strain
      Small cycloalkanes, like cyclopropane, are dramatically less stable than larger cycloalkanes due to ring strain.  Ring strain is caused by increased torsional strain, steric strain, and angle strain, in the small, nearly planar ring of cyclopropane.  Larger rings like cyclohexane, have much lower ring straing because they adopt non-planar conformations.
    • 3.5: Conformations of Cycloalkanes
      Overall ring strain decreases in cycloalkane rings that are large enough to allow the carbon-carbon bonds to rotate away from planar structures.  For this reason, cyclopentane is significantly more stable, than cyclopropane and cyclobutane.
    • 3.6: Conformations of Cyclohexane
      Rings larger than cyclopentane would have angle strain if they were planar. However, this strain, together with the eclipsing strain inherent in a planar structure, can be relieved by puckering the ring. Cyclohexane is a good example of a carbocyclic system that virtually eliminates eclipsing and angle strain by adopting non-planar conformations.
    • 3.7: Axial and Equatorial Bonds in Cyclohexane
      The hydrogens of cyclohexane exist in two distinct locations - axial and equatorial.  The two chair conformations of cyclohexane rapidly interconverts through a process called ring flip.
    • 3.8: Conformations of Monosubstituted Cyclohexanes
      Mono-substituted cyclohexane prefers the ring flip conformer in which the substituent is equatorial.  1,3-diaxial interactions occur when the substituent is axial, instead of equatorial. The larger the substituent, the more pronounced the preference.
    • 3.9: Conformations of Disubstituted Cyclohexanes
      The most stable configurational isomer of a disubstituted cyclohexane will be the isomer that has the most stable conformational isomer.
    • 3.10: Conformations of Polycyclic Molecules
      Polycyclic molecules are common and important in nature. Biologically important polycyclic molecules are found in cholesterol, sex hormones, birth control pills, cortisone, and anabolic steroids