Stereoisomers

1. Stereoisomers Dr Seemal Jelani Chem-261

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

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

4. Dr Seemal Jelani Chem-261

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

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

8. TASKWhich of the following molecules are optically active? • 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 NOT OPTICALLY ACTIVE Dr Seemal Jelani Chem-261

10. 2-chlorobutane Dr Seemal Jelani Chem-261 OPTICALLY ACTIVE

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

12. 3-methylhexane Dr Seemal Jelani Chem-261 OPTICALLY ACTIVE

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

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

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

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

17. 1-chloro-3-methylpentane Dr Seemal Jelani Chem-261 OPTICALLY ACTIVE

18. Molecules that are optical isomers are called enantiomers. • 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 Dr Seemal Jelani Chem-261

20. 3-Chlorocyclohexene Dr Seemal Jelani Chem-261

21. Formula to calculate stereoisomers • 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 • 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 • 22 = 4 stereoisomers exist; two pairs of enantiomers R (a) S (b) R (c) S (d) Dr Seemal Jelani Chem-261

24. 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. 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 • Erythrose is found in erythrocytes (RBC) Dr Seemal Jelani Chem-261

27. Determine the relationship • 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. DIASTEREOISOMER Dr Seemal Jelani Chem-261 2-Methylcyclopentanol

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

30. Dr Seemal Jelani Chem-261

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

32. Practice • 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. Dr Seemal Jelani Chem-261

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

35. 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. • 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. This is collected and purified and sold commercially as a cream of tartar Dr Seemal Jelani Chem-261

38. (c) (d) (a) (b) 1800 Dr Seemal Jelani Chem-261

39. 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? Imagine that you first rotate (d) by 1800 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. 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 • 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 • 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. Two chiral centers • 2n = 4, but only three stereoisomers exist Dr Seemal Jelani Chem-261