chapter 4 lect 2 finish cycloalkanes l.
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Chapter 4 Lect. 2 Finish Cycloalkanes. Hydrogens on Cyclohexane Two types of H on cyclohexane chair conformations 6 C-H bonds perpendicular to the ring axis = Equatorial 6 C-H bonds parallel to the ring axis = Axial Drawing chair conformations 3 sets of 2 parallel lines give the ring

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chapter 4 lect 2 finish cycloalkanes
Chapter 4 Lect. 2 Finish Cycloalkanes
  • Hydrogens on Cyclohexane
    • Two types of H on cyclohexane chair conformations
      • 6 C-H bonds perpendicular to the ring axis = Equatorial
      • 6 C-H bonds parallel to the ring axis = Axial
    • Drawing chair conformations
      • 3 sets of 2 parallel lines give the ring
      • Axial bonds straight up and down on alternating carbons
      • Equatorial bonds are parallel to one C-C bond away
slide2
Ring Flipping
      • Ea for interconverting between chair conformations = 10.8 kcal/mol
      • Happens about 100,000 times per second for cyclohexane
      • Ring inverts to give the other conformation
      • Every equatorial H becomes axial, every axial H become equatorial
  • Substituted Cyclohexanes
    • Equatorial vs. Axial Positions: Methylcyclohexane
      • Two possibilities for where CH3 group can go
      • Axial position has methyl group close to 2 axial H’s
        • 1,3-diaxial interaction
        • Like gauche butane
      • Equatorial position has methyl group out away from other H’s: like anti-
      • DGo = -1.7 kcal/mol going axial to equatorial methyl: 95% equatorial
      • Energy difference increases as the substituent gets larger: 99.99% equatorial for (1,1-dimethylethyl) substituent (tertiary butyl)
slide3
Multiple Substituents

1) Predict which substituent “wins” equatorial position based on size

slide4
Same substituent on one carbon: no change when ring flips
      • Same substituent cis on two different carbons: no change with ring flip
      • Same substituent trans on two different carbons: diaxial vs. diequatorial
  • Polycyclic Alkanes
    • Fused rings: when 2 or more cycloalkanes share the same 2 adjacent carbons
      • Decalin = 2 fused cyclohexanes
        • Fused Bicyclic
        • 2 shared carbons are called ring fusion carbons
slide5
Cis and trans isomers of decalin have different properties
  • Bridged Bicyclics: 2 rings share any 2 carbons
    • Norbornane
    • Shared carbons are called Bridgehead Carbons

C. Cool structures from Fused/Bridged Cycloalkanes

Tetrahedrane

(C4H4)

Cubane

C8H8

slide6
Natural Product Cycloalkanes
    • Natural Products = organic compounds produced by plants or animals
      • Classification by
        • Structure
        • Physiological activity
        • Taxonomy
        • Biochemical origin
      • Drugs, dyes, raw materials, etc…
      • Organic Chemists try to synthesize from simple molecules
        • Expensive
        • Rare
    • Taxol
      • Terpene (biochemical origin classifications)
      • Comes from Isoprene
      • Powerful anticancer drug
    • Steroids
      • Hormones that regulate body biochemistry are often steroids
      • Birth control pills contain estrogen and progesterone