Orgo 5.7-5.9

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Orgo 5.7-5.9
2013-10-19 17:25:01
CHM 201

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  1. Sometimes, we deal with mixtures that are neither optically pure (all one enantiomer) nor racemic (equal amounts of two enantiomers). In these cases, we specify the __ of the mxiture, which is defined as what?
    • optical purity (o.p)
    • the ratio of its rotation to the rotation of a pure enantiomer
  2. How do we compute o.p?
    (observed roation)/ (rotation of pure enantiomer) x 100%
  3. The __ is a similar method for expressing the relative amounts of enantiomers in a mixture.
    enantiomeric excess (e.e)
  4. How do we compute e.e.?
    we calculate the excess of hte predominant enantiomer as a percentage of the entire mixture.
  5. For a chemically pure compound, the calculation of enantiomeric excess generally gives the same result as the calculation of optical purity, and we often do what? What would this make the formula?
    • use e.e. and o.p. interchangeably
    • o.p.=e.e.= [abs(d-l)]/(d+l) x 100%
  6. Since the two mirror-image conformations of cyclohexane interconvert and have identical energies, any sample of (for example) cis-1,2-dibromocyclohexane) must what?
    contain equal amounts of hte two mirror images.
  7. Similarly, most achiral compounds can exist in what?
    transient chiral conformations that are in equilibrium with their mirror-image conformations
  8. A molecule cannot be __ if its __ are in __ with their __. We consider such a molecule to be __.
    • optically active
    • chiral conformations
    • equilibrium
    • mirror images
    • achiral
  9. What is the general principle with conformationally mobile systems.
    • to determine whether a conformationally mobile molecule can be optically active, consider its most symmetric conformation
    • can also be said that a molecule cannot be optically active it if is in equilibrium with a structure (or conformaiton) that is achiral
  10. Inherently dchiral compounds have __.
    NO achievabel achiral conformaitons
  11. Because conformers differ only by __ about __, they are generalyl in equilibrium at room temp.
    • rotations
    • single bonds
  12. We can consider cyclohexane rings as though what? And we should consider straight-chain compounds in what?
    • they were flat
    •  (the most symmetric conformation)
    • in their most symmetric conformations (often an eclipsed conformation)
  13. Organic compounds commonly exist as rapidly interconverting __. Even ethane is __ in its __.
    When we speak of __, we intend to focus on what rather than what?
    • chiral conformations
    • chiral
    • skew conformatiosn
    • chirality
    • observable, persistent properties
    • transient conformations
  14. Most chiral organic compounds have at least __. Some compounds are chiral because what? Some compounds are chiral even though they have what? In these types of compounds, special characteristics of the molecules' shapes lend __ to the structure.
    • one asymmetric carbon atom
    • they have another asymmetric atom
    • no asymmetric atoms at all 
    • chirality
  15. Some molecules are so bulky or so highly strained that whT?
    These molecules are said to be what?
    • they cannot easily convert form one chiral conformation to the mirror-image conformation; nor can they achieve the most symmetric conformation because it has too much steric strain or ring strain
    • in a locked-in conformation
  16. When a molecule cannot pass through a conformation, it is what?
    It can exist in only one of the two __ conformations shown on the left and right. These conformations are __, and they do not __. They are __, and they can be __ and __. Each of them is __, and they have __ and __.
    • conformationally locked
    • staggered
    • nonsuperimposable mirror images
    • interconvert
    • enantiomers
    • separated
    • isolated
    • optically active
    • equal 
    • oppsoite specific rotations
  17. __ are compounds that contain the C=C=C unit, with __ meeting at a single carbon atom. The parent compound, __, has the common name __.
    • allenes
    • two C=C double bonds
    • propadiene
    • allene
  18. In allene, the central carbon is __ and __, and the two outer carbons are __ and __.
    • sp hybridized/ linear
    • sp2 hybridized/ trigonal
  19. Explain the p orbitals in an allene.
    • the central sp carbon atom must use different p orbitals to form the pi bonds with the two outer carbon atoms
    • the two unhybridized p orbitals on the sp hybrid carbon atom are perpendicular, so the two pi bonds must be perpendicular too
  20. Allene is 
    a. chiral
    b. achiral
  21. If we add substitutents to allene, what happens?
    it may become chiral