OChem Ch 3 Structure and Stereochemistry of Alkanes

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OChem Ch 3 Structure and Stereochemistry of Alkanes
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2013-10-28 20:30:50
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OChem Structure Stereochemistry Alkanes
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OChem Ch 3 Structure and Stereochemistry of Alkanes
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  1. common alkane names
    methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane
  2. IUPAC naming
    • 1. find longest continuous carbon chain
    • 2. number the carbons, starting closest to the first branch
    • 3. name the groups attached to the chain using the carbon number as their locator
    • 4. alphabetize substituents (ignoring the di- tri-, etc. and just focusing on the name of the substituent group, like methyl, ethyl, etc.)
    • 5. use di- tri-, etc. for multiples of same constituent

    • ex. 
    •           CH3 CH3 C2H5
    •              |         |         | 
    • CH3---CH--CH--CH--CH3

    name: 2,3-methyl-3-ethylpentane
  3. when looking for the longest chain, what if there are two possible chains with same number of carbons
    use the chain that has the most substituents
  4. when numbering the carbons, what if you have two substituents that are equidistant?
    look for the next closest group.

    ex.



    The carbon on the left is labeled 1 because it has a substituent next to it
  5. what does it mean if an alkyl group has an "n-" prefix
    n- stands for normal. It means that the alkyl group is bonded to the main carbon chain via its terminal carbon.

    ex.

    • n-propyl:
    • CH3CH2CH2-
  6. name all propyl groups
    • n-propyl:
    • CH3CH2CH2-


    • isopropyl:
    • CH3--CH--CH3
    •           |
  7. name all butyl groups
    n-butyl:

    CH3CH2CH2CH2-

    sec-butyL-

    • CH3--CH--CH2--CH3
    •             |

    • isobutyl
    •          CH3
    •               |
    • CH3---CH---CH2-

    TERT-BUTYL

    •        CH3
    •             |
    • CH3---C--CH3
    •             |
  8. what is a primary/secondary/etc. carbon?
    primary- bonded to one other carbon

    secondary - bonded to two other carbons,

    etc.
  9. how to name complex substituents?
    • •If the branch has a branch, number the carbons from the point of attachment.
    • •Name the branch off the branch using a locator number.Parentheses are used around the complex branch name

    ex.

  10. if you increase branching in alkanes, the b.p. will ______ and the mp will _______
    decrease, increase
  11. major reaction done by orgo chemists with alkanes?
    halogenation

    • CH4 +Cl2 --> CH3Cl + CH2Cl2 + CHCl3 + 
    •                    ^                                  CCl4                                
    •                     |
    •           add heat or light
    •                
  12. conformer
    structure resulting from the free rotation of a C--C single bond.
  13. ethane conformers
    • 1. Staggered
    • -dihedral angle -- 60 degrees
    • lower in energy-- stable



    • 2. eclipsed
    • 0 degrees- dihedral angle
    • least stable-- high energy

    • 3. skew
    • -anything between eclipsed and staggered
  14. propane conformers
  15. butane conformers
    • 1. totally eclipsed
    • least stable
    • dihedral angle 0 degrees

    • 2. anti conformer
    • lowerst energy
    • dihedral angle - 180 degrees

    • 3. eclipsed
    • 2nd highest in energy (2nd most unstable)
    • dihedral angle 120 degrees

    • 4. gauche, staggered conformer
    • dihedral angle - 60 degrees
    • 2nd most stable
  16. what is the conformation of higher alkanes (nonane, decane, etc.)
    • •Anti conformation is lowest in energy.
    • •“Straight chain” actually is zigzag.

  17. cycloalkane properties
    • formula:CnH2n
    • mp and bp similar to branched alkanes with same number of carbons
  18. naming cycloalkanes
    1. number carbons in ring if >1 substituent. First in alphabet gets lowest number,

    2. proceed in naming the same way you would any other hydrocarbon
  19. cycloalkane stability
    5 and 6 member rings the most stable b/c bond angles closest to 109.5 degrees. All other rings exhibit angle strain.
  20. cyclopropane
    • large ring strain due to angle compression. Very reactive, weak bonds. Torsional strain because of elclipsed hydrogens.
  21. cyclobutane
    angle strain due to compression. Torsional strain partially relieved by ring-puckering (see the bent shape of the newman projection to get an idea of what puckering looks like)

  22. cyclopentane
    puckered conformer reduces torsional strain


  23. cyclohexane
    • From most stable to least stable:
    • chair
    • twist
    • boat 
    • half chair

  24. what is a bicyclic alkane 
    two alkane rings bonded together
  25. fused rings and bridged rings
    fused rings share two adjacent carbons

    bridged rings share two nonadjacent C's




  26. name the following bridged ring:

    bicyclo[3.2.1]octane
  27. name the following fused ring:

    bicyclo[4.2.0]octane
  28. which is more stable cis or trans decalin?
    trans, b/c there is torsional strain in cis:




  29. What is the torsional strain here called?

    1,3-diaxial interactions
  30. What is the most stable conformation of trans 1-ethyl-3-methylcyclohexane?
  31. for cis bonds in cyclohexane, what is the spatial positioning of the cis bonds (think in terms of equatorial and axial)
    one equatorial, one axial
  32. How does the conformer of cyclohexane change when it has bulky functional groups?
    • •Groups like t-butyl cause a large
    • energy difference between the axial and equatorial conformer. 

    •Most stable conformer puts t-butyl equatorial regardless of other substituents.

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