1. 8/2/2018
Stereoisomers
Dr Seemal Jelani Chem-261

2. All molecules have a mirror image – but for most molecules it is the same molecule.
8/2/2018
Dr Seemal Jelani Chem-261

3. For some molecules the mirror image is a different molecule (the mirror image is non- superimposable).
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Dr Seemal Jelani Chem-261

4. 8/2/2018
Dr Seemal Jelani Chem-261

5. Left and right hands are an example of non-superimposable mirror images.
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Dr Seemal Jelani Chem-261

6. Such molecules are said to be chiral or optically active.
This usually happens when a molecule contains a C atom with four different groups attached (chiral / asymmetric C).
Such molecules are said to be chiral or optically active.
8/2/2018
Dr Seemal Jelani Chem-261

7. The optical isomers are called enantiomers.
These are distinguished by +/-, D/L or more correctly R/S.
A 50/50 mixture of the two enantiomers is called a racemic mixture or a racemate.
8/2/2018
Dr Seemal Jelani Chem-261

8. 1-chloro-3-methylpentane
TASK Which of the following molecules are optically active?
8/2/2018
propan-2-ol
2-chlorobutane
1-chlorobutane
3-methylhexane
butanone
2-methylbutanoic acid
butan-2-ol
1-chloro-3-methylpentane
Dr Seemal Jelani Chem-261

9. propan-2-ol
8/2/2018
NOT OPTICALLY ACTIVE
Dr Seemal Jelani Chem-261

10. 2-chlorobutane
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OPTICALLY ACTIVE

11. 1-chlorobutane
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NOT OPTICALLY ACTIVE
Dr Seemal Jelani Chem-261

12. 3-methylhexane
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OPTICALLY ACTIVE

13. butanone
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NOT OPTICALLY ACTIVE
Dr Seemal Jelani Chem-261

14. propan-2-ol
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NOT OPTICALLY ACTIVE
Dr Seemal Jelani Chem-261

15. OPTICALLY ACTIVE 2-methylbutanoic acid 8/2/2018
Dr Seemal Jelani Chem-261
OPTICALLY ACTIVE

16. butan-2-ol
8/2/2018
Dr Seemal Jelani Chem-261
OPTICALLY ACTIVE

17. OPTICALLY ACTIVE 1-chloro-3- methylpen tane 8/2/2018
Dr Seemal Jelani Chem-261
OPTICALLY ACTIVE

18. Molecules that are optical isomers are called enantiomers.
8/2/2018
Enantiomers have identical chemical and physical properties, except:
Their effect on plane polarised light
Their reaction with other chiral molecules
Dr Seemal Jelani Chem-261

19. 2-Chlorobutane
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20. 3-Chlorocyclohexene
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Dr Seemal Jelani Chem-261

21. Formula to calculate stereoisomers
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For a molecule with 1 chiral center, 2x1 = 2 stereoisomers are possible
For a molecule with 2 chiral centers, a maximum of 2x2 = 4 stereoisomers are possible
For a molecule with n chiral centers, a maximum of 2n stereoisomers are possible
Dr Seemal Jelani Chem-261

22. Stereoisomers with tow chiral carbon atoms
8/2/2018
Molecules with two similar chiral center gives
Enantiomers
Diastereomers and Meso compounds
Dr Seemal Jelani Chem-261

23. 2,3,4-Trihydroxybutanal R S S R two chiral centers
8/2/2018
two chiral centers
22 = 4 stereoisomers exist; two pairs of enantiomers
R (a) S (b) R (c) S (d)
Dr Seemal Jelani Chem-261
Maximum stereoisomers= 22 =4

24. Maximum stereoisomers= 22 =4
8/2/2018
Maximum stereoisomers= 22 =4
(a ) and (b) are non superposable mirror images of each other therefore they are a “pair of enantiomers”
Dr Seemal Jelani Chem-261

25. 8/2/2018
2,3,4-Trihydroxybutanol exists in 2 pairs of enantiomer (a,b) and (c,d)
(a) = (2R,3R) erythrose
(b) = (2S,3S) erythrose
(c) = (2R,3S) threose
(d) = (2S,3R) threose
Dr Seemal Jelani Chem-261

26. Both erythrose and threose belong to class carbohydrates
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Both erythrose and threose belong to class carbohydrates
Erythrose is found in erythrocytes (RBC)
Dr Seemal Jelani Chem-261

27. Determine the relationship
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Find the relationship between the following
(a) and (c)
(a) and (d)
(b) and (c)
(b) and (d)
They are non-superposable, but are not mirror images so they are Diastereoisomers.
Dr Seemal Jelani Chem-261

28. 2-Methylcyclopentanol
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DIASTEREOISOMER
Dr Seemal Jelani Chem-261
2-Methylcyclopentanol

29. Stereoisomers that are not mirror images
Diastereomers:
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Stereoisomers that are not mirror images
Refers to the relationship among two or more objects
Dr Seemal Jelani Chem-261

30. 8/2/2018
Dr Seemal Jelani Chem-261

31. 1,2-Cyclopentanediol O H H O O H H O H H H H cis- 1,2-Cyclopentanediol
8/2/2018 O H H O O H H O H H H H
cis-
1,2-Cyclopentanediol
(a meso compound)
diastereomers
Dr Seemal Jelani Chem-261 O H H H H O H H O O H H
trans-
1,2-Cyclopentanediol
(a pair of enantiomers)

32. Practice Four stereoisomers of 1,2,3- butanetriol
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Four stereoisomers of 1,2,3- butanetriol
Write IUPAC names and show the R and S configuration of each stereocenter
Identify enantiomers
Identify diasteromers
Dr Seemal Jelani Chem-261

33. 8/2/2018
Dr Seemal Jelani Chem-261

34. MESO COMPOUNDS 2,3-Dihydroxybutanedioic acid (tartaric acid) 8/2/2018
Dr Seemal Jelani Chem-261
2,3-Dihydroxybutanedioic acid (tartaric acid)

35. Multiple sterocenters
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Multiple sterocenters
Special symmetry leads to reduce number of stereoisomers than expected by the 2n Rule
Example: 2,3- Dihydroxybutanedioic acid
Commonly called Tartaric acid
Dr Seemal Jelani Chem-261

36. TA is a colorless, crystalline compound.
8/2/2018
TA is a colorless, crystalline compound.
During the fermentation of grape juice, potassium bitartrate (one carboxyl group is present as a potassium salt, -COO-K+), deposits as a crust on the sides of wine casks
Dr Seemal Jelani Chem-261

37. 8/2/2018
This is collected and purified and sold commercially as a cream of tartar
Dr Seemal Jelani Chem-261

38. (c) (d) (a) (b)
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Dr Seemal Jelani Chem-261

39. Carbon 2 and 3 are stereocenters
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Carbon 2 and 3 are stereocenters
2n 2x2=4 stereoisomers or stereorepresentations
(a) and (b) are
Non superposable mirror images
Therefore (a) and (b) are
A pair of Enantiomer
Dr Seemal Jelani Chem-261

40. (c) and (d) are mirror images but they are superposable How?
8/2/2018
(c) and (d) are mirror images but they are superposable
How?
Imagine that you first rotate (d) by in the plane of the paper, cut and place it on (c)
In doing so we found that (d) is superposable on (c) therefore we
Dr Seemal Jelani Chem-261

41. 8/2/2018
Say that (c) and (d) are not different molecules, they are the same molecules just oriented differently Because (c) and its mirror image are superposable so (c) is achiral
Dr Seemal Jelani Chem-261

42. Another way to determine that (c) is Achiral
8/2/2018
We can see that it has plane of symmetry that bisect the molecule in such a way that the top half is the reflection of the bottom half
Thus even though ( c) has two sterocenters it is achiral
Dr Seemal Jelani Chem-261

43. conclusion
8/2/2018
The stereoisomers of tartaric acid are represented ( c) or (d) is called
MESO COMPOUND
So meso compound is an achiral compound that contains two or more stereocenters
Dr Seemal Jelani Chem-261

44. 2n = 4, but only three stereoisomers exist
8/2/2018
Two chiral centers
2n = 4, but only three stereoisomers exist
Dr Seemal Jelani Chem-261