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Chapter 5

Chapter 5. Stereochemistry Chiral Molecules. Stereochemistry. Stereosiomers are molecules Same formula and connectivity Differ by arrangement in space only Molecules are non-superimposable Not identical. Achiral and Chiral Carbon.

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Chapter 5

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  1. Chapter 5 Stereochemistry Chiral Molecules

  2. Stereochemistry • Stereosiomers are molecules • Same formula and connectivity • Differ by arrangement in space only • Molecules are non-superimposable • Not identical

  3. Achiral and Chiral Carbon • Tetracarbon with 4 different atoms/groups attached  chrial center or chiral carbon

  4. Test for Chirality: Planes of Symmetry • mirror plane: an imaginary plane that bisects a molecule in such a way that two halves of the molecule are mirror images of each other • Molecules with plan of symmetry are achiral

  5. Chiral objects

  6. Chiral Molecules • Has no plane of symmetry • Have non-superimposable mirror image

  7. Chiral Molecules • Molecules that have no plane of symmetry • Molecules that have non-superimposable mirror image • Your left hand and right hand

  8. Subdivision of Isomers

  9. Stereoisomers • Enantiomers: stereoisomers whose molecules are nonsuperimposable mirror image of each other • Exact MW, BP, MP ect …except how it rotates in polarized light

  10. Stereoisomer • Diastereoisomers: stereoisomers whose molecules are not mirror image of each other

  11. Nomenclature of Enantiomers: The R,S-System • Each chiral carbon in a chiral molecule is designated a “R” (Rectus: right) or “S” (Sinitrus: left) configuration • Rules: Assign priorities to atoms directly attached to chiral carbon based on Atomic number • # 1 = highest atomic # • # 4 = lowest atomic #

  12. Nomenclature of Enantiomers: The R,S-System • If there’s a tie (e.g two carbons) move to the next carbon atom until tie is broken • If there’s a double bond = 2 single bonds • With the lowest priority group (#4) point away from you, rotate 14 • Clockwise  R configuration • Counterclockwise  S configuration

  13. Nomenclature of Enantiomers: The R,S-System • Vinyl group, is of higher priority than the isopropyl group

  14. Examples • Assign (R) or (S) designation to each of the following compounds

  15. Molecules with Multiples Chiral Centers • The maximum number of stereiosomers can be predicted • 2n • n = number of chirality centers

  16. Naming compounds with multiple chirality center • Followed same rules for naming R,S • Designate position of chiral carbon

  17. Molecules with multiple chirality center

  18. Examples • Draw alll possible stereoisomers and provide an appropriate name for each stereoisomer

  19. Fisher Projection Formulas • Show three dimension structure • Compound with several chiral centers • Used carelessly, these projection formulas can easily lead to incorrect conclusion

  20. Examples • Draw all stereoisomers of

  21. Meso compounds • A structure with two chirality centers does not always have four possible stereoisomers. • achiral occurred within chiral molecules when carbon is inverted • Look for internal plane of symmetry

  22. Properties of Enantiomers • Recall: the molecules of enatiomers are not superposable one on the other • Enantiomers have identical M.P, B.P • Different physical properties and direction which they rotate plane-polarized light • Same amount in rotation but opposition direction • Different reaction rate when interact with another chiral moleucle

  23. Plane-Polarized light • When regular light beam is passed through a polarizer, all of the light waves, except those whose electromagnetic fields ossicllate in a single direction, are filter out •  plane-polarized light •  optically active

  24. Plane-Polarized light

  25. Polarimeter • A substance that is rotated in a clockwise rotation • α (measured of degree) is positive (+) • Dextrorotatory • A substance that is rotated in a counterclockwise rotation • α is negative (-) • levorotatory

  26. Specific Rotation

  27. Specific Rotation • Depends on the temperature and the wavelength of light that is employed

  28. Specific Rotation • The direction of rotation of plane-polarized light is often incorporated into the names of optically active compounds • No obvious correlation exists between the (R) and (S) configurations of enantiomers and the direction ([(+) or (-)] in which they rotate plane-polarized light

  29. Racemic Mixtures • An equimolar mixture of two enantiomers is called a racemic mixture (racemate, racemic form) • No net rotation of plane-polarized light • 50:50 mixture • Resulted from chemical reaction of achiral molecule

  30. Enantiomeric excess • A sample of an optically active substance that consists of a single enantiomers is said to be enantiomerically pure or to have an enantiomeric excess of 100%

  31. Resolution: Separating Enantiomers • A common way of separation uses the conversion into diastereomers, that are not mirror images of each other • Recrystallization or chromatography

  32. Resolution: Separating Enantiomers

  33. Stereoisomerism of cyclic compounds • Is trans-1,2-dimethylcyclopentane superposable on its mirror image • Is cis-1,2-dimethylcyclopentane superposable on its mirror image? • Is cis-1,2-dimethylcyclopentane a chiral molecule? • Would cis-1,2-dimethylcylcopentane show optical activity? • What is the stereoisomeric relationship between 1 and 3

  34. Cyclohexane Derivative

  35. Compounds with Chirality Centers Other than Carbons

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