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Monosubstituted Cyclohexanes

Monosubstituted Cyclohexanes. Substituents can occupy an axial or equatorial position Equatorial is preferred by larger groups. Why?. 7.3 Monosubstituted Cyclohexanes. Conformational Analysis.

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Monosubstituted Cyclohexanes

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  1. Monosubstituted Cyclohexanes • Substituents can occupy an axial or equatorial position • Equatorial is preferred by larger groups. Why? 7.3 Monosubstituted Cyclohexanes. Conformational Analysis

  2. 1,3-Diaxial Interactions: van der Waals repulsions between an axial substituents on a cycloalkane ring 1,3-Diaxial Interactions

  3. Problems • What is the energy cost of a 1,3-diaxial strain in methylcyclohexane? • What about bromocyclohexane? • Which of the two molecules above will have a higher percentage of its molecules in the equatorial conformation?

  4. More on Ring Flip • Note that the “up” substituent remains up and the “down” substituent remains down after chair interconversion

  5. Disubstituted Cyclohexanes1-chloro-2-methylcyclohexane • Trans: the two groups have an up-down relationship • Cis: the two groups have a down-down (or up-up) relationship

  6. Conformational Analysis • For disubstituted derivatives, the larger group will preferentially occupy the equatorial position 7.4 Disubstituted Cyclohexanes

  7. Problem • Which subsituent will most likely occupy the equatorial position for a greater amount of time in cis-1-chloro-2-methylcyclohexane? • Prove this with calculation.

  8. Polycyclic Molecules • Polycyclicmolecules: compounds with two or more rings fused together • Spirocyclic: two rings that have only a single common atom • Bicyclic: two rings that share two or more common atoms

  9. 7.6 Bicyclic and Polycyclic Compounds

  10. Classification and Nomenclature • Fused and bridged bicyclic compounds 7.6 Bicyclic and Polycyclic Compounds

  11. Naming Bicyclic Systems • Indicate the number of rings using the prefix “bicyclo-” • Indicate the bridge lengths • Number of atoms connecting one bridgehead atom to another (excluding the bridgehead atoms) • Separate by full periods and place in square brackets. • Cited in decreasing order of size (e.g. [3.2.1]) • the name of the hydrocarbon indicating the total number of skeletal atoms

  12. Problems • Name the following polycyclic molecules: • If you were just given the IUPAC names for a couple of bicyclic molecules, how would you tell the difference between a fused and a bridged variety? Norbornane Decalin

  13. Cis and Trans Ring Fusion: Decalin 7.6 Bicyclic and Polycyclic Compounds

  14. Each ring in cis-decalin can undergo ring flip • Ring fusion causes trans-decalin to be conformationally locked

  15. Ring Fusion with Small Rings • Bicyclic compounds with small rings are restricted to cis ring fusion • Trans fusion would incur too much ring strain 7.6 Bicyclic and Polycyclic Compounds

  16. Organic compounds with 20 carbon, tetracyclic core Variations in substituents dictate biological activity Steroids 7.6 Bicyclic and Polycyclic Compounds

  17. Many consist of all trans-fused rings No conformational change Many have methyl groups at C-10 and C-13 Steroids 7.6 Bicyclic and Polycyclic Compounds

  18. Stereochemistry of Cycloalkene ReactionsAddition Reactions • Syn-addition: • Anti-addition: 7.9 Stereochemistry of Chemical Reactions

  19. Stereochemistry of Bromine Addition • Addition of bromine to an alkene is a highly stereoselective reaction • Exclusively anti-addition • Gives all trans products 7.9 Stereochemistry of Chemical Reactions

  20. Stereochemistry of Halohydrin Formation • Exclusively anti-addition • Gives all trans products

  21. Stereochemistry of Hydroboration-Oxidation • Hydroboration is a stereospecific syn-addition 7.9 Stereochemistry of Chemical Reactions

  22. Stereochemistry of Hydroboration-Oxidation • The oxidation of organoboranes is a stereospecific substitution reaction 7.9 Stereochemistry of Chemical Reactions

  23. Stereochemistry of Hydroboration-Oxidation • The two steps of hydroboration-oxidation result in net syn-addition of H-OH to the alkene • Note that the trans designation of the name has nothing to do with the way H-OH added 7.9 Stereochemistry of Chemical Reactions

  24. Stereochemistry of Hydrogenation • Catalytic hydrogenation is a stereospecific syn-addition 7.9 Stereochemistry of Chemical Reactions

  25. Problem • Draw the products for the following reactions

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