Isomer Organization. Conformational Isomers. Meso compounds. Geometric Cis/trans. Isomers

Transkripsi

1 Stereochemistry

2 Isomer Organization Isomers Stereoisomers Constitutional isomers Enantiomers Diastereomers Geometric Cis/trans Conformational Isomers Meso compounds Epimers

3 Definisi Isomer Konstitusional: Senyawa yang mempunyai jumlah atom yang sama tetapi berbeda susunannya dalam struktur (berbeda gugus fungsi atau isomer gugus fungsi). n-butana and 2-metilpropana Dimetil eter and Etanol

4 Definitions Stereoisomer: Senyawa dengan jumlah atom sama susunan atomnya sama (gugus fungsinya sama), tetapi berbeda susunan dalam ruang. cis-but-2-ena and trans-but-2-ena konformasi gauche dan anti dari butana 2 enantiomer dari bromokloroiodometana

5 Definitions Isomer Konformasional Perbedaan struktur karena rotasi ikatan tunggal karbon-karbon. Isomer konformasional dari butana Koonformasi kursi, perahu setengah perahu dari sikloheksana.

6 Definitions Isomer Geometri : Perbedaan struktur karena perbedaan posisi atom atau gugus. (atom atau gugus fungsi sama) terdapat pada ikatan rangkap C=C atau senyawa sikloalkana. But-2-ena (cis and trans) 1,2-dietilsiklopropana (cis and trans)

7 Definitions Isomer Optik : Senyawa yang berbeda susunan atom atau gugus dalam ruang pada atom C tetrahedral. (atom atau gugus fungsi sama). Enansiomer: senyawa bayangan cermin yang tidak saling menutup (Non-superimposable mirror image isomer) Diastereomer: Senyawa bukan bayangan cermin dan tidak saling menutup. (Non-superimposable, non mirror image isomer)

8 Stereokimia Beberapa benda yang tidak sama dengan bayangan cermin mereka (mereka tidak memiliki bidang simetri) - Tangan kanan berbeda dengan tangan kiri disebut Handedness. Banyak molekul organik yang memiliki bayangan cermin yang tidak sama.

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10 Enansiomer Bayangan Cermin Molekul berada dalam bentuk tiga dimensi Beberapa molekul adalah sama seperti bayangan cerminnya Beberapa molekul yang berbeda dari bayangan cerminnya merupakan stereoisomer yang disebut enantiomer

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12 Enantiomers and the Tetrahedral Carbon Enantiomers are molecules that are not the same as their mirror image They are the same if the positions of the atoms can coincide on a one-to-one basis (we test if they are superimposable, which is imaginary) This is illustrated by enantiomers of lactic acid

13 This is illustrated by enantiomers of lactic acid

14 Examples of Enantiomers Molecules that have one carbon with 4 different substituents have a nonsuperimposable mirror image enantiomer

15 Mirror-image Forms of Lactic Acid When H and OH substituents match up, COOH and CH 3 don t When COOH and CH 3 coincide, H and OH don t

16 The Reason for Handedness: Chirality Molecules that are not superimposable with their mirror images are chiral (have handedness) A plane of symmetry divides an entire molecule into two pieces that are exact mirror images A molecule with a plane of symmetry is the same as its mirror image and is said to be achiral (See Figure for examples)

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18 Chirality If an object has a plane of symmetry it is necessarily the same as its mirror image The lack of a plane of symmetry is called handedness, chirality Hands, gloves are prime examples of chiral object They have a left and a right version

19 The flask has a mirror plane, or plane of symmetry There is no mirror plane for a hand

20 Chirality Centers A point in a molecule where four different groups (or atoms) are attached to carbon is called a chirality center There are two nonsuperimposable ways that 4 different groups (or atoms) can be attached to one carbon atom If two groups are the same, then there is only one way A chiral molecule usually has at least one chirality center

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22 Chirality Centers in Chiral Molecules Groups are considered different if there is any structural variation (if the groups could not be superimposed if detached, they are different) In cyclic molecules, we compare by following in each direction in a ring

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25 Problem 1: Chirality Centers?

26 Solution:

27 Optical Activity Light restricted to pass through a plane is plane-polarized Plane-polarized light that passes through solutions of achiral compounds remains in that plane Solutions of chiral compounds rotate plane-polarized light and the molecules are said to be optically active Phenomenon discovered by Biot in the early 19 th century

28 Optical Activity Light passes through a plane polarizer Plane polarized light is rotated in solutions of optically active compounds Measured with polarimeter Rotation, in degrees, is [ ] Clockwise rotation is called dextrorotatory (d) atau (+) Anti-clockwise is levorotatory (l) atau (-)

29 Measurement of Optical Rotation A polarimeter measures the rotation of plane-polarized that has passed through a solution The source passes through a polarizer, and then is detected at a second polarizer The angle between the entrance and exit planes is the optical rotation.

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31 A Simple Polarimeter Measures extent of rotation of plane polarized light Operator lines up polarizing analyzer and measures angle between incoming and outgoing light

32 Specific Rotation To have a basis for comparison, define specific rotation, [ ] D for an optically active compound [ ] D = observed rotation/(pathlength x concentration) = /(l x C) = degrees/(dm x g/ml) Specific rotation is that observed for 1 g/ml in solution in cell with a 10 cm path using light from sodium metal vapor (589 nanometers)

33 Specific Rotation and Molecules Characteristic property of a compound that is optically active the compound must be chiral The specific rotation of the enantiomer is equal in magnitude but opposite in sign (or direction).

34 Pasteur s Discovery of Enantiomers (1849) Louis Pasteur discovered that sodium ammonium salts of tartaric acid crystallize into right handed and left handed forms The optical rotations of equal concentrations of these forms have opposite optical rotations The solutions contain mirror image isomers, called enantiomers and they crystallized in distinctly different shapes such an event is rare

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36 Relative 3-Dimensionl Structure The original method was a correlation system, classifying related molecules into families based on carbohydrates Correlate to D- and L- glyceraldehyde D-erythrose is the mirror image of L- erythrose This does not apply in general

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38 Sequence Rules for Specification of Configuration A general method applies to the configuration at each chirality center (instead of to the the whole molecule) The configuration is specified by the relative positions of all the groups with respect to each other at the chirality center The groups are ranked in an established priority sequence (the same as the one used to determine E or Z) and compared. The relationship of the groups in priority order in space determines the label applied to the configuration, according to a rule

39 Sequence Rules (IUPAC) Assign each group priority according to the Cahn-Ingold-Prelog scheme With the lowest priority group pointing away, look at remaining 3 groups in a plane Clockwise is designated R (from Latin for right ) Counterclockwise is designated S (from Latin word for left )

40 R-Configuration at Chirality Center Lowest priority group is pointed away and direction of higher 3 is clockwise, or right turn

41 Examples of Applying Sequence Rules If lowest priority is back, clockwise is R and counterclockwise is S R = Rectus S = Sinister

42 Practice Problem 2

43 Problem 8: Assign R or S

44 Problem 9 : R or S?

45 Solution:

46 Problem : Same structure or Enantiomers?

47 6 Diastereomers Molecules with more than one chirality center have mirror image stereoisomers that are enantiomers In addition they can have stereoisomeric forms that are not mirror images, called diastereomers See Figure R,3R 2R,3S 2S,3S 2S,3R

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51 Problem 10: Assign configurations

52 Problem 11: R or S?

53 Problem 12: Assign R or S

54 Tartaric acid: Enantiomers What are they?

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56 7. Meso Compounds Tartaric acid has two chirality centers and two diastereomeric forms One form is chiral and the other is achiral, but both have two chirality centers An achiral compound with chirality centers is called a meso compound it has a plane of symmetry

57 Problem 13: Meso?

58 Problem 14: R or S?

59 Molecules with More Than Two Chirality Centers Molecules can have very many chirality centers Each center has two possible permanent arrangements (R or S), generating two possible stereoisomers The number of possible stereoisomers with n chirality centers is 2 n

60 Problem 15: Chirality centers?

61 Solution:

62 Problem 16 : R or S?

63 Solution:

64 9. Physical Properties of Stereoisomers Enantiomeric molecules differ in the direction in which they rotate plane polarized but their other common physical properties are the same Diastereomers have a complete set of different common physical properties

65 10.Racemic Mixtures and Their Resolution A 50:50 mixture of two chiral compounds that are mirror images does not rotate light called a racemic mixture (named for racemic acid that was the double salt of (+) and (-) tartaric acid The pure compounds need to be separated or resolved from the mixture (called a racemate)

66 10.Racemic Mixtures and Their Resolution To separate components of a racemate (reversibly) we make a derivative of each with a chiral substance that is free of its enantiomer (resolving agent) This gives diastereomers that are separated by their differing solubility The resolving agent is then removed

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