1. Molecules with more than One Chiral Carbon1

2. 2,3-Dihydroxybutanoic acid
CH3CHCHCOH HO OH 3 2
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 6

3. 2,3-Dihydroxybutanoic acid
CH3CHCHCOH HO OH 3 2
4 Combinations = 4 Stereoisomers
Carbon-2 R R S S
Carbon-3 R S R S 6

4. 2,3-Dihydroxybutanoic acid
CH3CHCHCOH HO OH 3 2
4 Combinations = 4 Stereoisomers
What is the relationship between these stereoisomers?
Carbon-2 R R S S
Carbon-3 R S R S 6

5. 2,3-Dihydroxybutanoic acid
CH3CHCHCOH HO OH 3 2
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

6. R S R S R S HO CO2H CH3 H OH CO2H CH3 H HO OH [] = -9.5° [] = +9.5°
enantiomers
CO2H H
CH3 HO R S
CO2H
CH3 H OH R S
enantiomers
[] = +17.8°
[] = -17.8° 12

7. 2,3-Dihydroxybutanoic acid
CH3CHCHCOH HO OH 3 2
but not all relationships are enantiomeric
stereoisomers that are not enantiomers are diastereomers
Carbon-2 R R S S
Carbon-3 R S R S 6

8. constitutional isomers stereoisomers enantiomers diastereomers8

9. R S R S R S HO CO2H CH3 H OH CO2H CH3 H HO OH [] = -9.5° [] = +9.5°
enantiomers
diastereomers
CO2H H
CH3 HO R S
CO2H
CH3 H OH R S
enantiomers
[] = +17.8°
[] = -17.8° 12

10. Figure Number:
Title: Table 5.1  Physical Properties of the Stereoisomers of Tartaric Acid
Caption:
Notes:

11. Fischer Projections
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
CO2H
CH3 13

12. Fischer projections
transform molecule to eclipsed conformation in order to construct Fischer projection 13

13. Fischer projections
CO2H
CH3 OH H 13

14. easiest to apply using Fischer projections
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 6

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

16. Threo
when carbon chain is vertical, same (or analogous) substituents on opposite sides of Fischer projection
CO2H
CH3 OH H HO OH
CO2H
CH3 H HO
+17.8°
–17.8° 14

17. Figure Number: UN-A
Title: Erythro and Threo Diastereomers
Caption: Erythro enantiomers are diastereomers of threo enantiomers.
Notes: Diastereomers are stereoisomers which are not mirror images of one another. Erythro enantiomers are stereoisomers with two adjacent chirality centers, which have two similar groups on the same side of the carbon chain, and threo enantiomers have the two similar groups on opposite sides of the carbon chain.

18. Perspective formula
Fischer projection

19. Figure Number: UN
Title: 3-Bromo-2-butanol Isomers
Caption: Named perspective formulas and Fischer projections of the isomers of 3-bromo-2-butanol.
Notes: The first two structures on the left are enantiomers of each other. The last two structures on the right are also enantiomers. Each of the structures in the left pair is a diastereomer of each of the structures in the right pair.

20. Achiral Molecules with Two Chiral Centers
It is possible for a molecule to have chiral carbons yet be achiral. 10

21. Symmetry Tests for Chirality
Any molecule with a plane of symmetry or a center of symmetry must be achiral. 19

22. Plane of symmetry
A plane of symmetry bisects a molecule into two mirror image halves. Chlorodifluoromethane has a plane of symmetry. 21

23. Plane of symmetry
A plane of symmetry bisects a molecule into two mirror image halves. Chlorodifluoromethane has a plane of symmetry. 21

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

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

26. Center of symmetry
A point in the center of the molecule 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. 21

27. Center of symmetry
A point in the center of the molecule 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. 21

28. 2,3-Butanediol CH3CHCHCH3 HO OH
Consider a molecule with two equivalently substituted chiral carbons such as 2,3-butanediol. 6

29. Three stereoisomers of 2,3-butanediol
2R,3R
2S,3S
2R,3S
chiral
chiral
achiral 6

30. Three stereoisomers of 2,3-butanediolOH
CH3 HO
CH3 OH H HO H
CH3 OH
2R,3R
2S,3S
2R,3S
chiral
chiral
achiral 6

31. Three stereoisomers of 2,3-butanediol
these two are enantiomers
2R,3R
2S,3S
chiral
chiral 6

32. Three stereoisomers of 2,3-butanediolOH
CH3 HO
CH3 OH H HO
these two are enantiomers
2R,3R
2S,3S
chiral
chiral 6

33. Three stereoisomers of 2,3-butanediol
the third structure is superposable on its
mirror image
2R,3S
achiral 6

34. Three stereoisomers of 2,3-butanediol
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
2R,3S
achiral 6

35. Three stereoisomers of 2,3-butanediol
CH3 H
CH3 OH
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 HO H HO H
CH3
2R,3S
achiral 6

36. 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 6

37. Three stereoisomers of 2,3-butanediolHO
CH3 H
CH3 OH
A line drawn the center of the Fischer projection of a meso form bisects it into two mirror- image halves.
2R,3S
achiral 6

38. S R R Cyclic compounds chiral meso
There are three stereoisomers of 1,2-dichloro- cyclopropane; the achiral (meso) cis isomer and two enantiomers of the trans isomer. 22

39. Chirality: di-substituted cyclopentanes and cylcohexanes

40. 1,2-Disubstituted Cyclopentanes

41. 1,3-Disubstituted Cylcopentanes

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

44. Cyclohexane Stereochemistry Cis isomers

45. 1,2-disubstituted-cis-cyclohexane Stereochemistry

46. Cyclohexane Stereochemistry Trans isomers.
Point

47. Molecules with Multiple chiral carbons10

48. 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 24

49. 4 chiral carbons 16 stereoisomers Example Hexaldose sugar O
HOCH2CH—CH—CH—CHCH OH
4 chiral carbons
16 stereoisomers 6

50. Cholic acid HO OH H H3C CH2CH2CO2H CH3 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 26

51. 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 24

52. How many stereoisomers?
3-Penten-2-ol R E E S HO H H OH Z R Z S HO H H OH 27