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For cis-trans isomers to exist

For cis-trans isomers to exist. (stereogenic atoms). stereocenter: atom at which interchange of two groups produces a stereoisomer. Designating the Configuration of Cis-Trans Isomers. configuration: the three-dimensional arrangement of groups about a stereocenter. Z

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For cis-trans isomers to exist

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  1. For cis-trans isomers to exist (stereogenic atoms) stereocenter: atom at which interchange of two groups produces a stereoisomer

  2. Designating the Configuration of Cis-Trans Isomers configuration: the three-dimensional arrangement of groups about a stereocenter Z (zusammen – together) E (entgegen – opposite)

  3. Cahn-Ingold-Prelog Sequence Rules Rule 1: Of the two atoms attached to one C of the C=C, the one with the higher atomic number has the higher priority. C has higher priority than H Cl has higher priority than F (Z)-1-chloro-1-fluoro-1-propene

  4. Rule 2: If the two atoms attached to the carbon are the same, compare the atoms attached to them in order of decreasing priority. The decision is made at the first point of difference. C bonded to O and 2 H’s C bonded to 3 H’s C bonded to 2 C’s and one H C bonded to C and 2 H’s

  5. Rule 3: Double bonds and triple bonds in the groups attached to the C=C are treated as though they are constructed from two or three single bonds, respectively.

  6. This is the Z-stereoisomer.

  7. Fig. 6-5, p. 188

  8. Conformations: the various shapes that a molecule can assume by rotations about single bonds Newman projections less stable more stable by 2.9 kcal/mol torsional strain: the destabilization caused by eclipsed bonds

  9. Fig. 6-7, p. 191

  10. Fig. 6-8a, p. 195

  11. Table 6-1, p. 194

  12. Cyclobutane less angle strain than cyclopropane lots of torsional strain (if planar) lowest energy conformation is nonplanar angle strain + torsional strain = 26.0 kcal/mol Cyclobutane rings are less common.

  13. equatorial hydrogens axial hydrogens

  14. Methylcyclohexane axial methyl is destabilized by 1.7 kcal/mol (axial strain energy) K = 18 5% 95% CH3 is axial CH3 is equatorial

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