Chemistry 3719 Chapter 3.txt

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marvlis
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132703
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Chemistry 3719 Chapter 3.txt
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2012-02-03 20:40:34
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3719
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O Chem
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  1. Why is an anti conformation typically more stable than gauche in an acyclic alkane?
    Large susbtituents are further apart in anti conformations which minimizes tortional strain
  2. What are the torsion angles typically associated with eclipsed, anti, and gauche conformations?
    • 0 degrees for eclipsed
    • 180 degrees for anti
    • 60 degrees for gauche
  3. Which eclipsed conformation should be more strained, C1-C2 in 1,2-diiodoethane or C1-C2 in 1,2-difluoroethane?
    The eclipsed conformation in 1,2-diiodoethane should be more strained since I is much bigger than F
  4. Gauche and anti conformations are both what type of overall conformation?
    Both are staggered conformations
  5. Why is cyclobutane not planar (i.e. flat)?
    Being flat would result in significant eclipsing interactions, puckering reduces this
  6. How many overall conformations are available for cyclohexane?
    An infinite number of conformations are possible
  7. Why are boat conformations typically less stable than chairs?
    So-called "flag-pole" interactions destabilize boat conformations
  8. Why are half-chair conformations less stable than chair conformations?
    A half-chair conformation will have significant eclipsing interactions around the ring
  9. Match the compounds with the appropriate ring-flip equilibrium constant:
    t-butylcyclohexane, fluorocyclohexane, isopropylcyclohexane:

    K
    = 32.2, K >9999, K = 1.5
    K = 32.2, isopropylcyclohexane

    K >9999, t-butylcyclohexane

    K = 1.5, fluorocyclohexane
  10. Which is more stable, cis-1,2-dimethylcyclopropane or trans-1,2-dimethylcyclopropane?
    The trans isomer is more stable since it avoids 1,2-torsional strain between the methyl groups
  11. Which is more stable, cis-1,2-dibromocyclohexane or trans-1,2-dibromocyclohexane?
    The trans isomer will be more stable as it can exist in a chair conformation that has no 1,3-diaxial interactions
  12. Which releases more heat upon combustion, cis-1,2-dimethylcyclopropane or trans-1,2-dimethylcyclopropane?
    The cis isomer is less stable so it will release more energy
  13. Why is the ring-flip equilibrium constant for t-butylcyclohexane so much larger than for methylcyclohexane?
    The t-butyl group is very much larger than methyl so the axial possibility is greatly disfavoured
  14. How many carbons are present in bicyclobutane?
    4

  15. What is wrong with the name bicyclo[0.1.4]heptane?
    The numbers are usually arranged in descending order
  16. Why do substituents larger than H prefer to be equatorial in cyclohexanes?
    To avoid destabilizing 1,3-diaxial interactions
  17. What would the equilibrium constant be for the ring-flip process in cyclohexane itself?
    K = 1
  18. Which should be more stable, cis-1,4-dibromocyclohexane or trans-1,4-dibromocyclohexane?
    The trans isomer since both large substituents will be equatorial in the preferred chair conformation
  19. Why does cyclopropane generate more heat per CH2 group than cyclohexane?
    The strain energy within cyclopropane accounts for the extra energy released
  20. Is the energy difference between reactants and products directly or inversely proportional to the equilibrium constant for that reaction?
    Directly proportional (deltaG = -RTlnK)

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