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Lecture 10 Stereochemistry

Lecture 10 Stereochemistry. Duncan Wardrop November 26, 2012. Historical Importance of Stereochemisty.

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Lecture 10 Stereochemistry

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  1. Lecture 10 Stereochemistry Duncan Wardrop November 26, 2012

  2. Historical Importance of Stereochemisty Once a molecule is asymmetric, its extension proceeds also in an asymmetrical sense. This concept completely eliminates the difference between natural and artificial synthesis. The advance of science has removed the last chemical hiding place for the once so highly esteemed vis vitalis. Emil Fischer, 1894

  3. Self Test Question Which pair of molecules below are not stereoisomers? geometrical stereoisomers A B C D conformational stereoisomers • Stereoisomers: • same connectivity • different arrangement of atoms in space identical molecules enantiomeric stereoisomers

  4. Self Test Question Which structure cannot be moved in space (translation) or undergo rotation around single bonds to obtain a mirror image of the molecule (1) below? A B C D identical to 1 1 2

  5. Chirality 1 2 2 A molecule is chiral if its two mirror image forms are notsuperimposable on one another Superimposable molecules can be laid on top of one another so that all equivalent atoms (points) line up

  6. Chiral Objects chiral: non-superimposable mirror images snail mirror image

  7. Chiral Objects chiral: non-superimposible mirror images milkyway mirror image

  8. Chirality chiral: non-superimposible mirror images fingerprint mirror image

  9. Chirality chiral: non-superimposible mirror images DNA double helix mirror image

  10. Chirality chiral: non-superimposable mirror images cheir (Greek) = hand chirality is synonymous with “handedness”

  11. Molecules can Also Be Chiral chiral: non-superimposible mirror images

  12. Superimposibility to check for superimposibility, rotate mirror image 180º along a plane perpendicular to the mirror plane 180º

  13. Non-superimposible if the images are not identical, they are non superimposible; not all points (atoms) line up chiral: non-superimposible mirror images

  14. Not Everything is Chiral! achiral: not chiral; mirror images are superimposible basketball mirror image

  15. Not Everything is Chiral! achiral: not chiral; mirror images are superimposible

  16. Not Everything is Chiral! achiral: not chiral; mirror images are superimposible

  17. Symmetry Tests for Achiral Molecules Any molecule with a plane of symmetry or a center of symmetry is achiral (not chiral) A plane of symmetry bisects a molecule into two mirror image halves

  18. Symmetry Tests for Achiral Molecules Any molecule with a plane of symmetry or a center of symmetry is achiral (not chiral) plane of symmetry A plane of symmetry bisects a molecule into two mirror image halves

  19. Symmetry Tests for Achiral Molecules Any molecule with a plane of symmetry or a center of symmetry is achiral (not chiral) plane of symmetry • • A plane of symmetry bisects a molecule into two mirror image halves

  20. Symmetry Tests for Achiral Molecules Any molecule with a plane of symmetry or a center of symmetry is achiral (not chiral) plane of symmetry A plane of symmetry bisects a molecule into two mirror image halves

  21. Symmetry Tests for Achiral Molecules Any molecule with a plane of symmetry or a center of symmetry is achiral (not chiral) center of symmetry: a line drawn from center of molecule to an element extends in opposite direction to an identical element

  22. Symmetry Tests for Achiral Molecules Any molecule with a plane of symmetry or a center of symmetry is achiral (not chiral) center of symmetry: a line drawn from center of molecule to an element extends in opposite direction to an identical element

  23. Self Test Question Which molecule below is chiral? A B C D

  24. Chirality Center Chirality center: an atom bonded to ligands in a spatial arrangement which is not superimposable on its mirror image Asymmetric carbon: a carbon atom bonded to four different atoms or groups (a type of chirality center) • other terminology • commonly used: • stereocenter • stereogenic center avoid these terms; both have different meanings from chirality center

  25. Chirality Center A molecule with a single chirality center is chiral bromochlorofluoromethane is chiral

  26. Chirality Center A molecule with a single chirality center is chiral • 2-butanol is chiral • although two carbon atoms are bonded to the chirality center, they are part of different groups

  27. Chirality Center A molecule with a single chirality center is chiral • 1,2-epoxypropane: a chirality center can be part of a ring • attached to the chirality center are: • -H • -CH3 • -CH2O • -OCH2

  28. Chirality Center A molecule with a single chirality center is chiral chiral as a result of isotopic substitution

  29. 1 Chirality Center = Chiral A molecule with a single chirality center must be chiral. But a molecule with two or more chirality centers may be chiral or it may not be chiral (Sections 7.10-7.13)

  30. Self Test Question How many chirality centers in a molecule of codeine? A. 7 B. 6 C. 5 D. 4 E. 3

  31. Cahn–Ingold–Prelog (CIP) Priority Rules R-S Notation for Chirality Centers

  32. CIP R/S Notation Step One: Locate chirality center chirality center

  33. CIP R/S Notation Step Two: Orient molecule so that lowest ranked (CIP priority) atom or group points away from you chirality center

  34. CIP R/S Notation Step Three: Number 3 highest priority groups in order of increasing priority (CIP sequence rules) 3 chirality center 2 1

  35. CIP R/S Notation Step Four: Determine rotation direction of groups in decreasing priority 3 chirality center 2 1 clockwise (rectus) = R counterclockwise (sinister) = S (R)-1-chloroethanol

  36. Self Test Question Which molecule has a chirality center with an absolute configuration of R? A B C D 2 3 clockwise = R 1

  37. Enantiomers molecules which are non-superimposible mirror images are called enantiomers; enantiomer describes a relationship between two molecules (enantiomer: opposite handedness) (S)-2-butanol (R)-2-butanol enantiomers have opposite configurations (R/S) at every chirality center

  38. Enantiomers molecules which are non-superimposible mirror images are called enantiomers; enantiomer describes a relationship between two molecules (enantiomer: opposite handedness) (S)-2-chloro-2-butanol (R)-2-chloro-2-butanol enantiomers have opposite configurations (R/S) at every chirality center

  39. Isomers isomers Do the molecules have the same connectivity? constitutional (structural) no yes stereoisomers Can the molecules be interconverted by rotation around single bonds? yes no conformational configurational

  40. Stereoisomers configurational stereoisomers Is the isomerism at a doulbe bond? yes no geometrical optical Are the compounds non-superimposible mirror images? no yes enantiomers diastereomers

  41. Self Test Question Which pair of molecules are enantiomers? A B C D Remember: if a molecule has a plane or center of symmetry, it is not chiral and therefore has no enantiomers.

  42. Self Test Question Which molecule is (R)-1-hexen-3-ol? A B C D E 1 clockwise = R 3 2

  43. Self Test Question The three asymmetric carbon atoms in penicillin G are labeled a-c. List the R/S configuration of each in order of a,b,c. A. R,R,S B. S,S,R C. R,S,R D. S,R,S E. S,R,R a b c

  44. Self Test Question The three asymmetric carbon atoms in penicillin G are labeled a-c. List the R/S configuration of each in order of a,b,c. A. R,R,S B. S,S,R C. R,S,R D. S,R,S E. S,R,R a N(CCH) S(SC) C(OON)

  45. Self Test Question The three asymmetric carbon atoms in penicillin G are labeled a-c. List the R/S configuration of each in order of a,b,c. A. R,R,S B. S,S,R C. R,S,R D. S,R,S E. S,R,R b

  46. Self Test Question The three asymmetric carbon atoms in penicillin G are labeled a-c. List the R/S configuration of each in order of a,b,c. A. R,R,S B. S,S,R C. R,S,R D. S,R,S E. S,R,R C(SCC) c C(OOO)

  47. Self Test Question The three asymmetric carbon atoms in penicillin G are labeled a-c. List the R/S configuration of each in order of a,b,c. A. R,R,S B. S,S,R C. R,S,R D. S,R,S E. S,R,R a b c

  48. Self Test Question List the R/S configuration of each asymmetric carbon in the enantiomer of penicillin G in the order a,b,c. Remember: enantiomers have opposite configurations at every chirality center. A. R,R,S B. S,S,R C. R,S,R D. S,R,S E. S,R,R (R) b b a a (R) c c (S)

  49. Stereoisomers with >1 Chirality Center (2S,3R)-3-chloro-2-hexanol or (2S,3R)-3-chlorohexan-2-ol • Number longest chain so that highest priority group has lowest locant. • List locants of asymmetric carbons in increasing order within parentheses at the beginning of name. • List stereodescriptor (R/S) after each asymmetric locant. • R & S are italicized when typed • a comma separates each steredescriptor (no spaces) • stereodescriptors are not considered when alphabetizing subgroups • asymmetric locants are not written if their exclusion is unambiguous ( ) - , chiral locant R/S chiral locant R/S parent class suffix

  50. Drawing Fisher Projections Step One: Draw (imagine) totally eclipsed conformation

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