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Isomer Presentation (


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Isomer Presentation (

  1. 1. Online practice tests, live classes, tutoring, study guides Q&A, premium content and more.
  2. 2. Isomers Isomers: different compounds with the same molecular formula Constitutional isomers: isomers with a different connectivity Stereoisomers: isomers with the same molecular formula, the same connectivity but a different orientation of their atoms in space that cannot be interconverted by rotation about a single bond 2
  3. 3. Isomerism → Constitutional Isomers and Stereoisomers Stereoisomers are isomers with the same molecular formula and same connectivity of atoms but different arrangement of atoms in space 3
  4. 4. Stereochemistry is the chemistry of molecules in three dimension 4
  5. 5. Handedness Stereochemistry of organic molecules can be understood, if we understand the meaning of handedness the fundamental reason for this is that our hands are not identical, rather they are mirror images 5
  6. 6. Chirality The mirror image Objects which are chiral have a of a chiral object is sense of “handedness” and different and will not exist in two forms. superimpose on the original object. 6
  7. 7. The reason for Handedness→ chirality • Although everything has a mirror image, mirror images may or may not be superimposable. • Some molecules are like hands. Left and right hands are mirror images, but they are not identical, or superimposable → chiral (property of handedness) 7
  8. 8. Mirror Image 8
  9. 9. Chirality and Nonchirality Mirror image: the reflection of an object in a mirror Objects that are not superposable on their mirror images are said to be chiral, that is, they show handedness Objects that are superposable on their mirror images are said to be achiral, that is, they do not show handedness. An achiral object has at least one element of symmetry 9
  10. 10. Achiral Molecules • Do these molecules contain a Plane of Symmetry (Mirror Plane)? 10
  11. 11. Chiral Molecules • The molecule labeled A and its mirror image labeled B are not superimposable. No matter how you rotate A and B, all the atoms never align. Thus, CHBrClF is a chiral molecule, and A and B are different compounds. • A and B are stereoisomers—specifically, they are enantiomers. • A carbon atom with four different groups is a tetrahedral stereogenic center. 11
  12. 12. Chiral vs. Achiral • With one stereogenic center, a molecule will always be chiral. • With two or more stereogenic centers, a molecule may or may not be chiral, e.g. Meso compound (contains a plane of symmetry or a mirror plane) 12
  13. 13. Chiral vs. Achiral Chiral: from the Greek, cheir, hand an object that is not superposable on its mirror image Achiral: an object that lacks chirality; one that lacks handedness an achiral object has at least one element of symmetry plane of symmetry: an imaginary plane passing through an object dividing it so that one half is the mirror image of the other half center of symmetry: a point so situated that identical components are located on opposite sides and equidistant from that point along the axis passing through it 13
  14. 14. Elements of Symmetry Symmetry in objects 14
  15. 15. Plane of Symmetry or Mirror plane 15
  16. 16. TWO VIEWS OF THE PLANE OF SYMMETRY plane of symmetry F F Cl Cl Br Cl Br Cl F side edge Cl Br view view
  17. 17. Plane of Symmetry Symmetry plane No symmetry plane COOH CH3 H C H H C OH COOH COOH achiral chiral 17
  18. 18. Elements of Symmetry Center of symmetry: a point so situated that identical components of the object are located equidistant on opposite sides and equidistant from the point along any axis passing through the point Br H Cl Cl H center of Br symmetry 18
  19. 19. Chiral Center The most common (but not the only) cause of chirality in organic molecules is a tetrahedral atom, most commonly carbon, bonded to four different groups A carbon with four different groups bonded to it is called a chiral center all chiral centers are stereocenters, but not all stereocenters are chiral centers. 19
  21. 21. Stereogenic Carbon Atoms Cl This is one type of …. stereocenter …. others are possible H F Br A stereogenic carbon is tetrahedral and has four different groups attached. 21
  22. 22. Stereogenic Centers • To locate a stereogenic center, examine the four groups—not the four atoms—bonded to each tetrahedral carbon atom in a molecule. • Omit from consideration all C atoms that cannot be tetrahedral stereogenic centers. These include • Methylene and methyl units, i. e. CH2 and CH3 groups respectively. • Any sp or sp2 hybridized Carbons, e.g. triple bonds, and double bonds in alkenes (C=C) and carbonyls (C=O). 22
  23. 23. Enantiomers 23
  24. 24. Enantiomers One of a pair of molecular species that are mirror images of each other and not superposable. They are mirror-image stereoisomers. 24
  25. 25. Drawing Enantiomers • To draw both enantiomers of a chiral compound such as 2-butanol, use the typical convention for depicting a tetrahedron: place two bonds in the plane, one in front of the plane on a wedge, and one behind the plane on a dash. Then, to form the first enantiomer, arbitrarily place the four groups—H, OH, CH3 and CH2CH3—on any bond to the stereogenic center. Then draw the mirror image. 25
  26. 26. Pairs of Enantiomers 26
  27. 27. Enantiomers Cl Cl rotate H F F Br Br H this molecule Cl is chiral note that the fluorine and bromine have been interchanged in the H Br enantiomer F 27
  28. 28. Enantiomers HO O O OH Lactic acid C C C C HO H H OH CH 3 H3 C 28
  29. 29. Enantiomers 3-Chlorocyclohexene Cl Cl 29
  30. 30. Enantiomers OH CH 3 CHCH 2 OH 1,2-propanediol OH OH H3 C C CH 2 OH HOH 2 C C CH 3 H H 30
  31. 31. Enantiomers A nitrogen chiral center + + N N H3 C CH2 CH3 CH3 CH2 CH3 A pai r of enanti omers 31
  32. 32. Enantiomers & Diastereomers 32
  33. 33. Enantiomers & Diastereoisomer Enantiomers: opposite configurations at all stereogenic centers. Diastereomers: Stereoisomers that are not mirror images of each other. Different configuration at some locations. 33
  34. 34. Two Stereocenters Cl Br Br Cl d H3C CH3 H3C CH3 i a H H H H s t entaiomers e r Cl Br Br Cl o m e r H3C H H CH3 s H CH3 H3C H entaiomers 34
  35. 35. Enantiomers & Diastereomers For a molecule with 1 stereocenter, 2 stereoisomers are possible For a molecule with 2 stereocenters, a maximum of 4 stereoisomers are possible For a molecule with n stereocenters, a maximum of 2n stereoisomers are possible 2n-1 pairs of enantiomers 35
  36. 36. Enantiomers & Diastereomers 2,3,4-Trihydroxybutanal two chiral centers 22 = 4 stereoisomers exist; two pairs of enantiomers CHO CHO CHO CHO H C OH HO C H H C OH HO C H H C OH HO C H HO C H H C OH CH2 OH CH2 OH CH2 OH CH2 OH A pai r of enanti omers A pai r of enanti omers (Eryt hreose) (Threose) Diastereomers: stereoisomers that are not mirror images refers to the relationship among two or more objects 36
  37. 37. Enantiomers & Diastereomers 2,3-Dihydroxybutanedioic acid (tartaric acid) two chiral centers; 2n = 4, but only three stereoisomers exist COOH COOH COOH COOH H C OH HO C H H C OH HO C H H C OH HO C H HO C H H C OH COOH COOH COOH COOH A meso compound A pair of enantiomers (plane of symmetry) Meso compound: an achiral compound possessing two or more chiral centers that also has chiral isomers 37
  38. 38. Enantiomers & Diastereomers 2-Methylcyclopentanol CH3 OH HO H3 C H H H H ci s- 2-Methyl cycl opentanol (a pai r of enanti omers) di astereomers CH3 H H H3 C H OH HO H trans- 2-Methyl cycl opentanol (a pai r of enanti omers) 38
  39. 39. Enantiomers & Diastereomers 1,2-Cyclopentanediol OH HO OH HO H H H H cis- 1,2-Cyclopentanediol (a meso compound) diastereomers OH H H HO H HO OH H trans- 1,2-Cyclopentanediol (a pair of enantiomers) 39
  40. 40. Enantiomers & Diastereomers cis-3-Methylcyclohexanol H3 C OH HO CH 3 40
  41. 41. Enantiomers & Diastereomers trans-3-Methylcyclohexanol H3 C CH 3 OH HO 41
  42. 42. Meso compounds Meso compounds are achiral by virtue of a symmetry plane, but contain a stereogenic center. plane of symmmetry mirror Cl Cl Cl Cl H3C CH3 H3C CH3 H H H H 42
  43. 43. Meso compounds Meso compound: achiral despite the presence of stereogenic centers Not optically active Superposable on its mirror image Has a plane of symmetry 43
  44. 44. The Three Stereoisomers of 2,3-dibromobutane • Because one stereoisomer of 2,3-dibromobutane is superimposable on its mirror image, there are only three stereoisomers, not four. 44
  46. 46. How Many Stereoisomers Are Possible? maximum number of stereoisomers sometimes fewer = 2n, than this number will exist where n = number of stereocenters (sterogenic carbons)
  47. 47. CH2OH OH RR RS CH3 * C CH2 CH CH CH3 SR * SS CH3 CH3 22 = 4 stereoisomers CH3 RRR RSS * RRS SRS RSR SSR * * OH SRR SSS CH3 CH 23 = 8 stereoisomers CH3
  49. 49. CONFIGURATION The three dimensional arrangement of the groups attached to an atom Stereoisomers differ in the configuration at one or more of their atoms.
  50. 50. CONFIGURATION → R,S convention 1 2 clockwise 2 1 counter clockwise C C 4 4 3 3 view with substituent of lowest priority in back R (rectus) S (sinister)
  51. 51. Rules for Labeling Stereogenic Centers with R or S • Since enantiomers are two different compounds, they need to be distinguished by name. This is done by adding the prefix R or S to the IUPAC name of the enantiomer. • Naming enantiomers with the prefixes R or S is called the Cahn-Ingold-Prelog system. • To designate enantiomers as R or S, priorities must be assigned to each group bonded to the stereogenic center, in order of decreasing atomic number. The atom of highest atomic number gets the highest priority (1). 51
  52. 52. Priority Rules for Naming Enantiomers (R or S) • If two atoms on a stereogenic center are the same, assign priority based on the atomic number of the atoms bonded to these atoms. One atom of higher priority determines the higher priority. 52
  53. 53. Priority of Isotopes on a Stereogenic Center • If two isotopes are bonded to the stereogenic center, assign priorities in order of decreasing mass number. Thus, in comparing the three isotopes of hydrogen, the order of priorities is: 53
  54. 54. Priority Rules for Multiple Bonds in (R or S) Labeling • To assign a priority to an atom that is part of a multiple bond, treat a multiply bonded atom as an equivalent number of singly bonded atoms. For example, the C of a C=O is considered to be bonded to two O atoms. • Other common multiple bonds are drawn below: 54
  55. 55. Examples Assigning Priorities 55
  56. 56. Cahn-Ingold-Prelog System for Naming Enantiomers R or S 56
  57. 57. R or S Enantiomers 57
  58. 58. Positioning the Molecule for R/S Assignment 58
  59. 59. R-enantiomer (Clockwise Rotation) S-enantiomer (Counterclockwise Rotation) 59
  60. 60. Manipulation of Chiral Molecules 60
  61. 61. The molecule is rotated to put the lowest priority group back If the groups descend in priority (a,b then c) in clockwise direction the enantiomer is R If the groups descend in priority in counterclockwise direction the enantiomer is S 61
  62. 62. R,S Convention Priority rules (Cahn, Ingold, Prelog) Each atom bonded to the stereocenter is assigned a priority, based on atomic number. The higher the atomic number, the higher the priority 1 6 7 8 16 17 35 53 H CH3 NH2 OH SH Cl Br I Increasing Priority 62
  63. 63. R,S Convention If priority cannot be assigned on the basis of the atoms bonded to the stereocenter, look to the next set of atoms. Priority is assigned at the first point of difference. 1 6 7 8 CH2 H CH2 CH3 CH2 NH2 CH2 OH Increasing Priority 63
  64. 64. R,S Convention Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of similar atoms by single bonds C C i s treated as -CH=CH2 -CH-CH2 O O C i s treated as -CH C O H C C i s treated as C CH C C H C C 64
  65. 65. Priorities 1. -OH H HO COOH 2. -COOH C 3. -CH3 CH3 4. -H (R)-(-)-lactic acid H HOOC OH C CH3 (S)-(+)-lactic acid 65
  66. 66. Bromochlorofluoroiodomethane 1 1 I I 4 4 F C C 2 F Cl Br Br Cl 3 3 2 R S Enantiomers 66
  67. 67. R and S Assignments in Compounds with Two or More Stereogenic Centers. • When a compound has more than one stereogenic center, the R and S configuration must be assigned to each of them. One stereoisomer of 2,3-dibromopentane The complete name is (2S,3R)-2,3-dibromopentane 67
  68. 68. Stereoisomerism of Cyclic Compounds 1,4-dimethylcyclohexane Neither the cis not trans isomers is optically active Each has a plane of symmetry 68
  69. 69. 1,3-dimethylcyclohexane The trans and cis compounds each have two stereogenic centers The cis compound has a plane of symmetry and is meso The trans compound exists as a pair of enantiomers 69
  70. 70. Properties of Stereoisomers 70
  71. 71. Properties of Stereoisomers Enantiomers have identical physical and chemical properties in achiral environments Diastereomers are different compounds and have different physical and chemical properties meso tartaric acid, for example, has different physical and chemical properties from its enantiomers (see Table 3.1) 71
  72. 72. Plane-Polarized Light Ordinary light: light vibrating in all planes perpendicular to its direction of propagation Plane-polarized light: light vibrating only in parallel planes Optically active: refers to a compound that rotates the plane of plane-polarized light 72
  73. 73. Plane-Polarized Light plane-polarized light is the vector sum of left and right circularly polarized light circularly polarized light reacts one way with an R chiral center, and the opposite way with its enantiomer the result of interaction of plane-polarized light with a chiral compound is rotation of the plane of polarization 73
  74. 74. Plane-Polarized Light Polarimeter: a device for measuring the extent of rotation of plane-polarized light 74
  75. 75. Optical Activity observed rotation: the number of degrees, α, through which a compound rotates the plane of polarized light dextrorotatory (+): refers to a compound that rotates the plane of polarized light to the right levorotatory (-): refers to a compound that rotates of the plane of polarized light to the left specific rotation: observed rotation when a pure sample is placed in a tube 1.0 dm in length and concentration in g/mL (density); for a solution, concentration is expressed in g/ 100 mL COOH COOH C H H C H3 C OH CH3 HO (S)-(+)-Lacti c aci d (R)-(-)-L actati c aci d 21 21 [ α] D = +2.6° [ α] D = -2.6° 75
  76. 76. Optical Purity Optical purity: a way of describing the composition of a mixture of enantiomers [α ]sam p l e Percent opti cal puri ty = x 100 [α ]p u re en an ti o mer Enantiomeric excess: the difference between the percentage of two enantiomers in a mixture [R] - [S] Enan ti omeri c excess (ee) = x 100 = %R - %S [R] + [S] optical purity is numerically equal to enantiomeric excess, but is experimentally determined 76
  77. 77. Resolution Racemic mixture: an equimolar mixture of two enantiomers because a racemic mixture contains equal numbers of dextrorotatory and levorotatory molecules, its specific rotation is zero Resolution: the separation of a racemic mixture into its enantiomers 77
  78. 78. Racemates • An equal amount of two enantiomers is called a racemate or a racemic mixture. A racemic mixture is optically inactive. Because two enantiomers rotate plane-polarized light to an equal extent but in opposite directions, the rotations cancel, and no rotation is observed. 78
  79. 79. Specific Rotation • Specific rotation is a standardized physical constant for the amount that a chiral compound rotates plane- polarized light. Specific rotation is denoted by the symbol [α] and defined using a specific sample tube length (l, in dm), concentration (c in g/mL), temperature (25 0C) and wavelength (589 nm). 79
  80. 80. Discovery of Enantiomers “There is no doubt that in dextro - + tartaric acid there COO Na exists an H C OH assymetric arrangement HO C H having a nonsuperimposible - + COO Na image.” 80
  81. 81. Tartaric Acid OH OH OH OH HOOC H H COOH H COOH HOOC H (+)-tartaric acid (-)-tartaric acid OH OH meso ALSO FOUND (as a minor component) HOOC COOH H H [α]D = 0 more about this meso -tartaric acid compound later 81
  82. 82. Diastereoisomer Stereoisomers that are not mirror images of H COOH NH2 COOH each other. C H 2N C H Different C C H OH H OH configuration at CH3 CH3 some locations. 82
  83. 83. Diastereomers Threonine: 2 pairs H COOH NH2 COOH H 2N H of enantiomers C C C C H OH HO H CH3 H 3C 2R,3R 2S,3S 2R,3S & 2S,3R 2R, 3R 2S, 3S 2S,3S 2R,3R 2R,3S & 2S,3R COOH COOH 2R,3S 2S,3R 2R,3R & 2S,3S H C NH2 H 2N C H 2S,3R 2R,3S 2R,3R & 2S,3S C C HO H H OH H 3C CH3 2R, 3S 2S, 3R 83
  84. 84. Enantiomers & Diastereomers For tartaric acid, the three possible stereoisomers are one meso compound and a pair of enantiomers. Meso compound: an achiral compound possessing two or more stereocenters. 84
  85. 85. Symmetry Plane 2R, 3S and 2S, 3R COOH COOH HO H are identical H C OH C Molecule has a plane C C HO H H OH COOH of symmetry COOH 2R, 3R 2S, 3S perpendicular to C-C COOH COOH and is therefore H C OH HO C H achira C C H OH HO H COOH COOH 2R, 3S 2S, 3R 85
  86. 86. Symmetry Plane 2R, 3S and 2S, 3R COOH COOH are identical H C OH HO C H Molecule has a plane HO C H H C OH of symmetry COOH COOH perpendicular to C-C 2R, 3R 2S, 3S and is therefore COOH Mirror COOH achira H C OH HO H image is C One meso H C OH HO identical C H compound and a COOH COOH pair of enantiomers 2R, 3S 2S, 3R 86
  87. 87. 2-Bromo-3-chlorobutane mirror Cl Br Br Cl S R S R CH3 CH3 CH3 CH3 H H H H enantiomers 1 diastereomers Cl Br Br Cl S S R R CH3 H H CH3 H CH3 CH3 H enantiomers 2 87
  88. 88. 2,3-Dichlorobutane Cl Cl Cl Cl S R mirror image CH3 CH3 CH3 is identical CH3 H H H H meso diastereomers Cl Cl Cl Cl S S R R CH3 H H CH3 H CH3 CH3 H enantiomers 88
  89. 89. Tartaric Acid (-) - tartaric acid (+) - tartaric acid [α]D = -12.0o [α]D = +12.0o mp 168 - 170o mp 168 - 170o solubility of 1 g solubility of 1 g 0.75 mL H2O 0.75 mL H2O 1.7 mL methanol 1.7 mL methanol 250 mL ether 250 mL ether insoluble CHCl3 insoluble CHCl3 d = 1.758 g/mL d = 1.758 g/mL meso - tartaric acid [α ]D = 0 o solubility of 1 g mp 140o 0.94 mL H2O d = 1.666 g/mL insoluble CHCl3 89
  90. 90. Fischer Projections 90
  91. 91. CH 3 H OH Fischer Projections CH 2 CH 3 Fischer projection: a two-dimensional representation showing the configuration of a stereocenter horizontal lines represent bonds projecting forward vertical lines represent bonds projecting to the rear the only atom in the plane of the paper is the stereocenter 91
  92. 92. Fischer Projections COOH COOH C H OH H OH CH3 CH3 How? (R)-lactic acid 92
  93. 93. Fischer Projections COOH COOH H C H OH OH CH3 CH3 93
  94. 94. Fischer Projections COOH COOH H OH H OH CH3 CH3 94
  95. 95. Fischer Projections 1. Orient the stereocenter so that bonds projecting away from you are vertical and bonds projecting toward you are horizontal 2. Flatten it to two dimensions OH CH3 CH 3 C (1) H C OH (2) H OH H CH 3 CH3 CH 2 CH 2 CH 3 CH 2 CH 3 (S)-2-Butanol (S)-2-Butanol (3-D formula) (Fischer projection) 95
  96. 96. Assigning R,S Configuration Lowest priority group goes to the top. View rest of projection. A curved arrow from highest to lowest priority groups. Clockwise - R (rectus) Counterclockwise - S (sinister) 96
  97. 97. Assigning R,S Configuration 4 H 2 3 H 3C COOH OH 1 s-lactic acid 97
  98. 98. Rules of Motion  Can rotate 180°, but not 90° because 90° disobeys the Fischer projection. Same groups go in and out of plane COOH COOH CH3 CH3 H OH 180 HO H =H OH HO H = CH3 CH3 COOH COOH 98
  99. 99. Rules of Motion  Can rotate 180°, but not 90° because 90° disobeys the Fischer projection. Different groups go in and out of plane This generates an enantiomeric structure COOH COOH H H H OH 90 H 3C COOH =H OH H 3C COOH = CH3 CH3 OH OH (R)-lactic acid (S)-lactic acid 99
  100. 100. Rules of Motion  One group can be held steady and the others rotated. COOH COOH H OH same as HO CH3 CH3 H 100
  101. 101. Rules of Motion To determine if two Fischer projections represent the same enantiomer carry out allowed motions. H C 2H 5 OH H 3C C 2H 5 HO H H CH3 OH CH3 C 2H 5 A B C 101
  102. 102. H C 2H 5 OH H 3C C 2H 5 HO H H CH3 OH CH3 C 2H 5 Rules of Motion A B C By performing two allowed movements on B, we are able to generate projection A. Therefore, they are identical. CH2CH3 HO H CH3 CH3CH2 HO H H CH2CH3 CH3 CH2CH3 CH3 CH3 HO B A 102
  103. 103. H C 2H 5 OH H 3C C 2H 5 HO H H CH3 OH CH3 C 2H 5 Rules of Motion A B C Perform one of the two allowed motions to place the group with lowest priority at the top of the Fischer projection. OH CH2CH3 H H CH 3 180 H OH CH3 H 3C CH2CH 90 CH2CH3 OH OH 103 C not A
  104. 104. Priorities HOOC 1. NH2 CH3 H 2. COOH H 2N H HOOC NH2 CH3 CH3 3. CH3 4. H H H HOOC HOOC NH2 HOOC NH2 CH3 CH3 H 2N H CH3 S - stereochemistry 104
  105. 105. 1-Bromo-2-chlorocyclohexane Br Cl Cl Br cis enantiomers diastereomers Br Br trans Cl Cl enantiomers 105
  106. 106. 1-Bromo-2-chlorocyclopropane Br R Cl Cl R Br S S cis enantiomers diastereomers Br R Br R S S trans Cl Cl enantiomers 106
  107. 107. 1,2-Dibromocyclopropane mirror image identical Br Br Br Br cis meso diastereomers Br Br trans Br Br enantiomers 107
  108. 108. Biological Significance of Stereoisomers Structure causes Properties Stereochemistry Biological effects Example •Pasteur’s plant mold metabolized (+)-tartaric acid but not (-)-tartaric acid 108
  109. 109. Biological Significance of Stereoisomers Thalidomide O •Marketed in 50 countries 1956-1962 Sedative for “hysterical” pregnant women N O Antiemetic to combat morning sickness N •Caused thousands of birth defects O O H Teratogen: causes fetal abnormalities One stereocenter •Sold as racemic mixture: 1:1 mixture of enantiomers R enantiomer = antiemetic (not teratogenic) S enantiomer = teratogenic (not antiemetic) •Single-enantiomer drug not useful: quickly racemizes in body 109
  110. 110. Biological Significance of Stereoisomers Another Biological Effect: Odor O O enantiomers H H (R)-(-)-carvone (S)-(+)-carvone smells like spearmint smells like caraway Mirror image molecules do not have “mirror image effects” 110
  111. 111. Biological Significance of Stereoisomers Of Hands, Gloves, and Biology Why do stereoisomers have different biological properties? •Many biological effects involve interaction with a cavity in enzyme or receptor •Good fit to cavity (i.e., strong binding) triggers enzyme or receptor R H OH •Enzymes and receptors are proteins; built from amino acids: H2N O •Most amino acids are chiral, so protein cavity is also chiral •Metaphor: Stereoisomer = left hand or right hand Protein hole = left glove or right glove Left hand fits left glove but not right glove Left hand triggers “left protein” but not “right protein” •(R)-carvone triggers spearmint smell receptor but not caraway smell receptor 111
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