STEREOISOMERS ARRANGEMENTS IN 3D- SPACE

Transcription

1 STEREOISOMERS ARRANGEMENTS IN 3D- SPACE 1

2 Isomers 2

3 Physiological of Stereoisomers can have very different physiological 3

4 Oranges and Lemons found in oranges found in lemons 4

5 Chirality Handedness : Right glove doesn t fit the let hand. Mirror- image object is different from the original object. 5

6 Achiral Objects that can be superposed are achiral. 6

7 Stereoisomers Enan)omers: Nonsuperimposable mirror images, different molecules with different 7

8 Chiral Molecules Chiral molecules have an asymmetric center. An asymmetric center is an atom that is a[ached to different groups. 8

9 Chiral Carbons Carbons with four different groups a[ached are chiral. It s mirror image will be a different compound (enan@omer). 9

10 are nonsuperimposable mirror images. 10

11 The two isomers are called are different compounds: they can be separated. have the same physical and chemical 11

12 Achiral Compounds Take this mirror image and try to superimpose it on the one to the le4 matching all the atoms. Everything will match. 12

13 Planes of Symmetry A molecule that has a plane of symmetry is achiral. 13

14 Chiral and Achiral Molecules Chiral compounds have nonsuperimposable mirror images. Achiral compounds have superimposable mirror images (they are molecules). 14

15 Asymmetric Center versus Stereocenter Asymmetric center: an atom a[ached to four different groups. Stereocenter: an atom at which the interchange of two groups produces a stereoisomer. 15

16 (R) and (S) Nomenclature Different molecules must have different names. Usually only one will be biologically around the chiral carbon is specified with (R) and (S). 16

17 Rule 1: Cahn Ingold Prelog Rules Sequence Rules (IUPAC) Look at the four atoms directly a[ached to the chirality center, and rank them according to atomic number. With the lowest priority group away, look at remaining 3 groups in a plane Clockwise is designated R (from La@n word for right ) Counterclockwise is designated S (from La@n word for let )

18 Sequence Rules Rule 2: If a decision cannot be reached by ranking the first atoms in the subs@tuents, look at the second, third, or fourth atoms un@l difference is found 18

19 Sequence Rules Rule 3: bonded atoms are equivalent to the same number of single- bonded atoms

20 Assign (R) or (S) Working in 3- D, rotate the molecule so that the lowest priority group is in back. Draw an arrow from highest to lowest priority group. Clockwise = (R), Counterclockwise = (S) 20

21 Assign Atomic number: F > N > C > H Once priori@es have been assigned, the lowest priority group (#4) should be moved to the back if necessary. 21

22 Assign Draw an arrow from Group 1 to Group 2 to Group 3 and back to Group 1. Ignore Group 4. Clockwise = (R) and Counterclockwise = (S) 22

23 Example When to put the lowest priority group in the back, keep one group in place and rotate the other three. 23

24 Example 24

25 Example Draw the enantiomers of 1,3-dibromobutane and label them as (R) and (S). (Making a model is particularly helpful for this type of problem.) Solu@on The third carbon atom in 1,3-dibromobutane is asymmetric. The bromine atom receives first priority, the ( CH 2 CH 2 Br) group second priority, the methyl group third, and the hydrogen fourth. The following mirror images are drawn with the hydrogen atom back, ready to assign (R) or (S) as shown. 25

26 of Same boiling point, point, and density. Same index. Rotate the plane of polarized light in the same magnitude, but in opposite Different with other chiral molecules: site of enzymes is for a specific enan@omer. Taste buds and scent receptors are also chiral. Enan@omers may have different smells. 26

27 rotate the plane of polarized light in opposite but same number of degrees. 27

28 Polarimeter Not related to (R) and (S) 28

29 Specific Observed depends on the length of the cell and as well as the strength of temperature, and wavelength of light. [α] = α (observed) c l Where α (observed) is the rota@on observed in the polarimeter, c is concentra@on in g/ml and l is length of sample cell in decimeters. 29

30 Example When one of the enantiomers of 2-butanol is placed in a polarimeter, the observed rotation is 4.05 counterclockwise. The solution was made by diluting 6 g of 2-butanol to a total of 40 ml, and the solution was placed into a 200-mm polarimeter tube for the measurement. Determine the specific rotation for this enantiomer of 2-butanol. Solu@on Since it is levorotatory, this must be ( )-2-butanol The concentration is 6 g per 40 ml = 0.15 g/ml, and the path length is 200 mm = 2 dm. The specific rotation is [α] D = (0.15)(2) =

31 Biological 31

32 Racemic Mixtures Equal of d- and l- (d,l) or (±) No The mixture may have different boiling point (b. p.) and point (m. p.) from the 32

33 Racemic Products If reagents combine to form a chiral molecule, a racemic mixture is formed. 33

34 Purity purity (o.p.) is called excess (e.e.). One is present in greater amounts. o.p. = observed rota@on rota@on of pure enan@omer X

35 Excess excess tells us how much of an excess of one is in a mixture. 35

36 Calculate % Composi@on The specific rota@on of (S)- 2- iodobutane is Determine the % composi@on of a mixture of (R)- and (S)- 2- iodobutane if the specific rota@on of the Mixture is Sign is from the enan6omer in excess: levorotatory o.p. = X 100 = 20% l = 60% d = 40% 36

37 R and S Versus (+) and ( ) Some R enan@omers are (+) and some are ( ). Some S enan@omers are (+) and some are ( ). 37

38 If One Is (+), the Other Is ( ) 38

39 Chirality of Conformers If equilibrium exists between two chiral conformers, the molecule is not chiral. Judge chirality by looking at the most symmetrical conformer. Cyclohexane can be considered to be planar, on average. 39

40 Chirality of Isomers The two chair of cis- 1,2- dibromocyclohexane are nonsuperimposable, but the interconversion is fast and the molecules are in equilibrium. Any sample would be racemic and, as such, 40

41 Nonmobile Conformers The planar of the biphenyl is too sterically crowded. The compound has no around the central C C bond and thus it is conforma@onally locked. The staggered conforma@ons are chiral: They are nonsuperimposable mirror images. 41

42 Allenes Some allenes are chiral even though they do not have a chiral carbon. Central carbon is sp hybridized. To be chiral, the groups at the end carbons must have different groups. 42

43 2,3- Pentadiene Is Chiral 43

44 Fischer Flat of a 3- D molecule. A chiral carbon is at the intersec@on of horizontal and ver@cal lines. Horizontal lines are forward, out- of- plane. Ver@cal lines are behind the plane. 44

45 Fischer 45

46 Fischer Rules Carbon chain is on the line. Highest oxidized carbon is at top. of 180 in plane doesn t change molecule. Do not rotate 90! 46

47 180 A of 180 is allowed because it will not change the 47

48 90 A 90 will change the of the horizontal and groups. Do not rotate a Fischer projec@on

49 Fischer Mirror Images Fisher are easy to draw and make it easier to find and internal mirror planes when the molecule has 2 or more chiral centers. H C l C H 3 C l H C H 3 49

50 Fischer (R) and (S) Lowest priority (usually H) comes forward, so assignment rules are backwards! Clockwise is (S) and counterclockwise is (R). Example: C H 3 (S) H C l C l H (S) C H 3 50

51 Compounds with Two Asymmetric Centers Diastereomers 1 and 2 are enan@omers. 3 and 4 are enan@omers. Diastereomers are stereoisomers that are not enan@omers. 1 and 3 are diastereomers. 1 and 4 are diastereomers. 2 and 3 are diastereomers. 2 and 4 are diastereomers. Diastereomers have different physical and chemical proper@es. 51

52 Formulas of the Four Stereoisomers 52

53 Diastereomers Molecules with two or more chiral carbons. Stereoisomers that are not mirror images. 53

54 A Meso Compound Has a Superimposable Mirror Image Meso compounds are op@cally inac@ve even though they have asymmetric centers. 54

55 Two Asymmetric Centers: Three Stereoisomers (a meso compound and a pair of enan@omers) 55

56 Number of Stereoisomers The 2 n rule will not apply to compounds that may have a plane of symmetry. 2,3- dibromobutane has only 3 stereoisomers: (±) enan6omer and the meso diastereomer. 56

57 A Meso Compound A compound with two asymmetric centers that have the same four groups bonded to each asymmetric center will have three stereoisomers: a meso compound and a pair of enan@omers. 57

58 Meso Compounds Meso compounds have a plane of symmetry. If one image was rotated 180, then it could be superimposed on the other image. Meso compounds are achiral even though they have chiral centers. 58

59 Two Asymmetric Centers, Four Stereoisomers The cis stereoisomers are a pair of enan@omers. The trans stereoisomers are a pair of enan@omers. 59

60 an Asymmetric Center An asymmetric center is a[ached to four different groups. two asymmetric centers, four stereoisomers 60

61 No Asymmetric Centers There are only two stereoisomers: cis and trans. 61

62 No Asymmetric Centers There are only two stereoisomers: cis and trans. 62

63 A Meso Compound For cyclic compounds with the same bonded to two asymmetric centers, cis = a meso compound and trans = a pair of enan@omers. 63

64 Two or More Chiral Carbons When compounds have two or more chiral centers they have diastereomers, or meso isomers. have opposite at each corresponding chiral carbon. Diastereomers have some matching, some opposite Meso compounds have internal mirror planes. Maximum number of isomers is 2 n, where n = the number of chiral carbons. 64

65 Comparing Structures 65

66 of Diastereomers Diastereomers have different physical so they can be easily separated. differ only in with other chiral molecules and the in which polarized light is rotated. are difficult to separate. Convert into diastereomers to be able to separate them. 66

67 of React the racemic mixture with a pure chiral compound, such as tartaric acid, to form diastereomers, then separate them. 67

68 Chromatographic of 68

69 by hand by chromatography 69

70 Physical of Stereoisomers 70

71 Nitrogen and Phosphorus Can Be Asymmetric Centers 71