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Molecules with more than One Chiral Carbon
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  1. Molecules with more than OneChiral Carbon

  2. O CH3CHCHCOH HO OH 2,3-Dihydroxybutanoic acid 2 3 What are all the possible R and S combinations of the two chiral carbons in this molecule? Carbon-2 R R S S Carbon-3 R S R S

  3. O CH3CHCHCOH HO OH 2,3-Dihydroxybutanoic acid 2 3 4 Combinations = 4 Stereoisomers Carbon-2 R R S S Carbon-3 R S R S

  4. O CH3CHCHCOH HO OH 2,3-Dihydroxybutanoic acid 2 3 4 Combinations = 4 Stereoisomers What is the relationship between these stereoisomers? Carbon-2 R R S S Carbon-3 R S R S

  5. O CH3CHCHCOH HO OH 2,3-Dihydroxybutanoic acid 2 3 enantiomers: 2R,3R and 2S,3S 2R,3S and 2S,3R They have the same physical properties. Carbon-2 R R S S Carbon-3 R S R S

  6. CO2H CO2H R S HO OH H H OH HO H H R S CH3 CH3 CO2H CO2H S R OH HO H H OH H HO H R S CH3 CH3 [] = -9.5° [] = +9.5° enantiomers enantiomers [] = +17.8° [] = -17.8°

  7. O CH3CHCHCOH HO OH 2,3-Dihydroxybutanoic acid 2 3 stereoisomers that are not enantiomers are diastereomers but not all relationships are enantiomeric Carbon-2 R R S S Carbon-3 R S R S

  8. Isomers constitutional isomers stereoisomers enantiomers diastereomers

  9. CO2H CO2H R S HO OH H H OH HO H H R S CH3 CH3 CO2H CO2H S R OH HO H H OH H HO H R S CH3 CH3 [] = -9.5° [] = +9.5° enantiomers diastereomers enantiomers [] = +17.8° [] = -17.8°

  10. Fischer Projections CO2H recall for Fischer projection: horizontal bonds point toward you; vertical bonds point away staggered conformation does not have correct orientation of bonds for Fischer projection CH3

  11. Fischer projections transform molecule to eclipsed conformation in order to construct Fischer projection

  12. CO2H OH H H OH CH3 Fischer projections

  13. Erythro and Threo stereochemical prefixes used to specify relative configuration in molecules with two chiral carbons easiest to apply using Fischer projections orientation: vertical carbon chain

  14. CO2H OH H H OH CH3 Erythro when carbon chain is vertical, same (or analogous) substituents on same side of Fischer projection CO2H H HO HO H CH3 –9.5° +9.5°

  15. CO2H CO2H OH H H HO HO H OH H CH3 CH3 Threo when carbon chain is vertical, same (or analogous) substituents on opposite sides of Fischer projection +17.8° –17.8°

  16. Perspective formula Fischer projection

  17. Achiral MoleculeswithTwo Chiral Centers It is possible for a molecule to have chiral carbons yet be achiral.

  18. Any molecule with a plane of symmetryor a center of symmetry must be achiral. Symmetry Tests for Chirality

  19. Plane of symmetry A plane of symmetry bisects a molecule into two mirror image halves. Chlorodifluoromethane has a plane of symmetry.

  20. Plane of symmetry A plane of symmetry bisects a molecule into two mirror image halves. Chlorodifluoromethane has a plane of symmetry.

  21. Plane of symmetry A plane of symmetry bisects a molecule into two mirror image halves.1-Bromo-1-chloro-2-fluoroethene has a planeof symmetry.

  22. Plane of symmetry A plane of symmetry bisects a molecule into two mirror image halves.1-Bromo-1-chloro-2-fluoroethene has a planeof symmetry.

  23. Center of symmetry A point in the center of themolecule is a center of symmetry if a line drawn from it to some element, when extended an equal distance in the opposite direction, encounters an identical element.

  24. Center of symmetry A point in the center of themolecule is a center of symmetry if a line drawn from it to any element, when extended an equal distance in the opposite direction, encounters an identical element.

  25. CH3CHCHCH3 HO OH 2,3-Butanediol 3 2 Consider a molecule with two equivalently substituted chiral carbons such as 2,3-butanediol.

  26. Three stereoisomers of 2,3-butanediol 2R,3R 2S,3S 2R,3S chiral chiral achiral

  27. CH3 CH3 CH3 H HO OH OH H H HO OH H OH H H CH3 CH3 CH3 Three stereoisomers of 2,3-butanediol 2R,3R 2S,3S 2R,3S chiral chiral achiral

  28. Three stereoisomers of 2,3-butanediol these two areenantiomers 2R,3R 2S,3S chiral chiral

  29. CH3 CH3 H HO OH H HO OH H H CH3 CH3 Three stereoisomers of 2,3-butanediol these two areenantiomers 2R,3R 2S,3S chiral chiral

  30. Three stereoisomers of 2,3-butanediol the third structure is superposable on its mirror image 2R,3S achiral

  31. Three stereoisomers of 2,3-butanediol therefore, this structure and its mirror imageare the same it is called a meso form a meso form is an achiral molecule that has chiral carbons 2R,3S achiral

  32. CH3 OH H OH H CH3 Three stereoisomers of 2,3-butanediol CH3 therefore, this structure and its mirror image are the same it is called a meso form a meso form is an achiral molecule that has chiral carbons H HO H HO CH3 2R,3S achiral

  33. Three stereoisomers of 2,3-butanediol meso forms have a plane of symmetry and/or a center of symmetry plane of symmetry is most common case top half of molecule is mirror image of bottom half 2R,3S achiral

  34. CH3 CH3 H HO OH H OH H HO H CH3 CH3 Three stereoisomers of 2,3-butanediol A line drawnthe center ofthe Fischer projection of ameso formbisects it intotwo mirror-image halves. 2R,3S achiral

  35. chiral R R Cyclic compounds meso S R There are three stereoisomers of 1,2-dichloro-cyclopropane; the achiral (meso) cis isomer and two enantiomers of the trans isomer.

  36. Chirality:di-substituted cyclopentanes andcylcohexanes

  37. 1,2-Disubstituted Cyclopentanes

  38. 1,3-Disubstituted Cylcopentanes

  39. plane of symmetry plane of symmetry As long as any one conformer of a compound has a plane of symmetry, the compound will be achiral

  40. Cyclohexane StereochemistryCis isomers

  41. 1,2-disubstituted-cis-cyclohexaneStereochemistry

  42. Cyclohexane StereochemistryTrans isomers . Point

  43. MoleculeswithMultiple chiral carbons

  44. How many stereoisomers? maximum number of stereoisomers = 2n where n = number of structural units capable of stereochemical variation structural units include chiral carbons and cis and/or trans double bonds number is reduced to less than 2n if meso forms are possible

  45. O HOCH2CH—CH—CH—CHCH OH OH OH OH Hexaldose sugar Example 4 chiral carbons 16 stereoisomers

  46. CH3 HO H CH3 CH2CH2CO2H H H3C H H OH HO H Cholic acid 11 chiral carbons 211 = 2048 stereoisomers one is "natural" cholic acid a second is the enantiomer of natural cholic acid 2046 are diastereomers of cholic acid