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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

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Chapter 4 Lect. 2 Finish Cycloalkanes

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  1. 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

  2. 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)

  3. Multiple Substituents 1) Predict which substituent “wins” equatorial position based on size

  4. 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

  5. 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

  6. 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

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