1. Stereochemistry Chiral Molecules
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