Chapter 5                                  Stereochemistry © 2013 Pearson Education, Inc.© 2013 Pearson Education, Inc.
ISOMERS                                                                           rotation about                          ...
Achiral: Objects, molecules, that contain                      mirror elements of symmetry                           plane...
Planes of Symmetry          • A molecule that has a plane of symmetry is            achiral.© 2013 Pearson Education, Inc....
Chirality: Handedness”:             An object, molecule that lacks elements                          of symmetry.   Right-...
Enantiomers: a type of                                    stereoisomers      Compounds that are __________________mirror  ...
Stereocenters: chirality centers,                        stereogenic atom       •  Chiral or asymmetric carbon bonded to _...
Examples of Chirality Centers:                             stereocenters© 2013 Pearson Education, Inc.     Chapter 5      ...
Cis Cyclic Compounds          • Cis-1,2-dichlorocyclohexane is _______            because the molecule _______internal pla...
Trans Cyclic Compounds        • Trans-1,2-dichlorocyclopentane          _________have a plane of symmetry so the          ...
•        • Only one enantiomer of alanine is biologically active          (only the left enantiomer can be metabolized by ...
( ) and ( ) Absolute configuration; The Cahn–                 Ingold–Prelog Convention    • Assign a relative “priority” t...
In case of ties, use the next atoms along          the chain of each group as tiebreakers.      If the atoms are the same ...
• Lowest priority group        goes to the back.      • Draw an arrow from        highest to lowest priority        group....
Determine absolute                                    configuration                    CH2 Br                             ...
Assign Absolute Configuration                                                                     CH3           CH3     O ...
Draw the enantiomers of 1,3-dibromobutane and label               them as (R) and (S). (Build the models)© 2013 Pearson Ed...
OH                                      18© 2013 Pearson Education, Inc.
Properties of Enantiomers   • Same boiling point, melting point, and density,     refractive index.   • Rotate the plane o...
Optical Activity: the rotation of the angle of   polarized light                    Clockwise Counterclockwise            ...
Specific Rotation   Observed rotation depends on temperature, wavelength of    light, ____________________________________...
When one of the enantiomers of 2-butanol is placed in a      polarimeter, the observed rotation is 4.05° counterclockwise....
Biological       Discrimination© 2013 Pearson Education, Inc.   Chapter 5   23
____________ Mixtures        • Equal quantities of + and – ; Old notation (d,l); new          (±). No optical activity.   ...
Optical Purity (o.p.) sometimes called                      enantiomeric excess (e.e.)                                    ...
A commercial synthesis of naproxen, a              nonsteroidal anti-inflammatory drug (NSAID),                   gives th...
Chiral compounds that do not posses a                          chiral carbon                         A) Atropisomers    • ...
B) Allenes: Some allenes are chiral even            though they do not have a chiral carbon                         Build ...
Fischer Projections: Flat representation of                       a 3-D molecule       A _________________is at the inters...
Stereoisomers or same                                 compounds?                    CH3                  CH2CH3           ...
Stereoisomers = 2n maximum        3-chloro-2-butanol: Identify enantiomers and diasteromers                               ...
2,3-dihydroxybutandioic (tartaric acid).       The 2n rule will not apply to compounds that      have a plane of symmetry:...
Meso Compounds are achiral even though they                have chiral centers because they have                     _____...
If equilibrium exists between two chiral conformers, the                              molecule is not chiral.             ...
Cyclic molecules    • 2-methylcyclopentanol (4)    • 1,3-cyclopentanediol (3, cis is meso)                                ...
Disubstituted derivatives of cycloehexane      Methylcyclohexanols (2 Methylcyclohexanol = 4)           4-methylcyclohexan...
Disubstituted derivatives of cycloehexane     Cyclohexanediol     cis-1,2 rapid interconversion = meso         1,3-cyclohe...
•                                 Physical properties             Enantiomers have identical physical and chemical propert...
Diastereomers: Cis-trans Isomerism on                         Double Bonds        • These stereoisomers are not mirror    ...
Diastereomers: Cis-trans                       Isomerism on Rings          • Cis-trans isomers are not mirror images, so  ...
Comparing Structures© 2013 Pearson Education, Inc.      Chapter 5      41
Resolution (separation) of                         enantiomers   • In 1848, Louis Pasteur     noticed that a salt of     r...
Chemical Resolution of                             Enantiomers                React the _______________with a ______      ...
Resolution of (R)- and (S)-2-butanol© 2013 Pearson Education, Inc.    Chapter 5              44
Chromatographic                   ____________of Enantiomers© 2013 Pearson Education, Inc.   Chapter 5      45
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  • Copyright © 2006 Pearson Prentice Hall, Inc.
  • Copyright © 2006 Pearson Prentice Hall, Inc.
  • Ch506/25

    1. 1. Chapter 5 Stereochemistry © 2013 Pearson Education, Inc.© 2013 Pearson Education, Inc.
    2. 2. ISOMERS rotation about Compounds with the single bonds same molecular formula Conformations same different connectivity connectivity rotation restricted Stereoisomers Constitutional Conformational Isomers Isomers stereocenters but no chiral centers with chiral centers Atropisomers Cis,Trans m ore than (E,Z) Isomers one chiral center one chiral center (can be called achiral chiral diastereomers) mirror Enantiomers Meso not mirror images images Compounds Diastereomers Enantiomers 2© 2013 Pearson Education, Inc.
    3. 3. Achiral: Objects, molecules, that contain mirror elements of symmetry plane HO OH© 2013 Pearson Education, Inc. Chapter 5 3
    4. 4. Planes of Symmetry • A molecule that has a plane of symmetry is achiral.© 2013 Pearson Education, Inc. Chapter 5 4
    5. 5. Chirality: Handedness”: An object, molecule that lacks elements of symmetry. Right-hand glove does not fit the left hand, it is chiral, its mirror image is different from the original© 2013 Pearson Education, Inc. Chapter 5 5
    6. 6. Enantiomers: a type of stereoisomers Compounds that are __________________mirror images.© 2013 Pearson Education, Inc. Chapter 5 6
    7. 7. Stereocenters: chirality centers, stereogenic atom • Chiral or asymmetric carbon bonded to ______ different groups is the most common type of a chirality center • Its mirror image will be _________compound (enantiomer). The double-bonded carbon atoms in cis-trans isomers are the most common types of stereocenters.© 2013 Pearson Education, Inc. Chapter 5 7
    8. 8. Examples of Chirality Centers: stereocenters© 2013 Pearson Education, Inc. Chapter 5 8
    9. 9. Cis Cyclic Compounds • Cis-1,2-dichlorocyclohexane is _______ because the molecule _______internal plane of symmetry. Both structures above are _______________© 2013 Pearson Education, Inc. Chapter 5 9
    10. 10. Trans Cyclic Compounds • Trans-1,2-dichlorocyclopentane _________have a plane of symmetry so the images are ______________and the molecules above are _______________© 2013 Pearson Education, Inc. Chapter 5 10
    11. 11. • • Only one enantiomer of alanine is biologically active (only the left enantiomer can be metabolized by the enzyme) • Need a way to distinguish between them.© 2013 Pearson Education, Inc. Chapter 5 11
    12. 12. ( ) and ( ) Absolute configuration; The Cahn– Ingold–Prelog Convention • Assign a relative “priority” to each group bonded to the asymmetric carbon according to Z I Br Cl S F O N 13 C C 12 H 2 H 1© 2013 Pearson Education, Inc. Chapter 5 12
    13. 13. In case of ties, use the next atoms along the chain of each group as tiebreakers. If the atoms are the same triple bonds takes priority over double and double over single© 2013 Pearson Education, Inc. Chapter 5 13
    14. 14. • Lowest priority group goes to the back. • Draw an arrow from highest to lowest priority group. • Clockwise = (R), Counterclockwise = (S)© 2013 Pearson Education, Inc. Chapter 5 14
    15. 15. Determine absolute configuration CH2 Br HS H2CH2 C CH(CH3)2 CN C C CH2 CH3 IH2CH2 C Cl C ClH2 C CF 3 H2 NH2 C CH2 OH H© 2013 Pearson Education, Inc. Chapter 5 15
    16. 16. Assign Absolute Configuration CH3 CH3 O O OH OH OH H H OH C C C C CH2 CH2 H H OH OH CH 3 CH3 CH3 CH3 CH3 CH3 C H H Cl l C C CH2 CH2 CH3 CH3 16© 2013 Pearson Education, Inc.
    17. 17. Draw the enantiomers of 1,3-dibromobutane and label them as (R) and (S). (Build the models)© 2013 Pearson Education, Inc. Chapter 5 17
    18. 18. OH 18© 2013 Pearson Education, Inc.
    19. 19. Properties of Enantiomers • Same boiling point, melting point, and density, refractive index. • Rotate the plane of polarized light in the same magnitude, but in opposite directions. • Different interaction with other chiral molecules: – Active site of enzymes is selective for a specific enantiomer. – Taste buds and scent receptors are also chiral. Enantiomers may have different smells.© 2013 Pearson Education, Inc. Chapter 5 19
    20. 20. Optical Activity: the rotation of the angle of polarized light Clockwise Counterclockwise Dextorotatory (+) Levorotatory (-) Not related to (R) and (S)© 2013 Pearson Education, Inc. Chapter 5 20
    21. 21. Specific Rotation Observed rotation depends on temperature, wavelength of light, _________________________________________ Normalize: [α] = α (observed) c•l α(observed) is the rotation observed in the polarimeter c is concentration in g/mL, l is length of sample cell in decimeters.© 2013 Pearson Education, Inc. Chapter 5 21
    22. 22. 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.© 2013 Pearson Education, Inc. Chapter 5 22
    23. 23. Biological Discrimination© 2013 Pearson Education, Inc. Chapter 5 23
    24. 24. ____________ Mixtures • Equal quantities of + and – ; Old notation (d,l); new (±). No optical activity. • Can be formed from reactions of achiral compounds© 2013 Pearson Education, Inc. Chapter 5 24
    25. 25. Optical Purity (o.p.) sometimes called enantiomeric excess (e.e.) observed rotation o.p. = X 100 rotation of pure enantiomer The specific rotation of (S)-2-iodobutane is +15.90°. Determine the % composition of a mixture of (R)- and (S)-2-iodobutane if the specific rotation of the mixture is -3.18°.© 2013 Pearson Education, Inc. Chapter 5 25
    26. 26. A commercial synthesis of naproxen, a nonsteroidal anti-inflammatory drug (NSAID), gives the S enantiomer in 97% ee Calculate the percentages of the R and S enantiomers in this mixture. CH3 COOH H3 CO (S)-Naproxen 26© 2013 Pearson Education, Inc.
    27. 27. Chiral compounds that do not posses a chiral carbon A) Atropisomers • The planar conformation of the biphenyl derivative is too sterically crowded. The compound has no rotation around the central C—C bond and thus it is conformationally locked. • The staggered conformations are ____________: They are non-superimposable mirror images.© 2013 Pearson Education, Inc. Chapter 5 27
    28. 28. B) Allenes: Some allenes are chiral even though they do not have a chiral carbon Build the model© 2013 Pearson Education, Inc. Chapter 5 28
    29. 29. Fischer Projections: Flat representation of a 3-D molecule A _________________is at the intersection of horizontal and vertical lines. _____________ lines are forward, towards you. ___________ lines are behind the plane. Away from you© 2013 Pearson Education, Inc. Chapter 5 29
    30. 30. Stereoisomers or same compounds? CH3 CH2CH3 H H Cl CH3 CH2CH3 Cl CH2CH3 H CH2CH3 CH3 Cl CH3 Cl CH3 H Cl Cl H H CH2CH3 CH3 CH2CH3 30© 2013 Pearson Education, Inc.
    31. 31. Stereoisomers = 2n maximum 3-chloro-2-butanol: Identify enantiomers and diasteromers 31© 2013 Pearson Education, Inc.
    32. 32. 2,3-dihydroxybutandioic (tartaric acid). The 2n rule will not apply to compounds that have a plane of symmetry: a meso compound Assign absolute configuration to each stereocenter 32© 2013 Pearson Education, Inc.
    33. 33. Meso Compounds are achiral even though they have chiral centers because they have _____________________© 2013 Pearson Education, Inc. Chapter 5 33
    34. 34. 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. The two chair conformations of cis-1,2-dibromocyclohexane are _____________________, but the interconversion is fast and the molecules are in equilibrium. Any sample would be ________________ and, as such, ___________________.© 2013 Pearson Education, Inc. Chapter 5 34
    35. 35. Cyclic molecules • 2-methylcyclopentanol (4) • 1,3-cyclopentanediol (3, cis is meso) 35© 2013 Pearson Education, Inc.
    36. 36. Disubstituted derivatives of cycloehexane Methylcyclohexanols (2 Methylcyclohexanol = 4) 4-methylcyclohexanol (2) 3-methyl cyclohexanol (4) 36© 2013 Pearson Education, Inc.
    37. 37. Disubstituted derivatives of cycloehexane Cyclohexanediol cis-1,2 rapid interconversion = meso 1,3-cyclohexanediol (3) 37© 2013 Pearson Education, Inc.
    38. 38. • Physical properties Enantiomers have identical physical and chemical properties in achiral environments (except one) • Diastereomers are different compounds and have different physical and chemical properties. COOH COOH COOH H C OH HO C H H C OH HO C H H C OH H C OH COOH COOH COOH (R,R)-Tartaric acid (S,S)-Tartaric acid Meso tartaric acid specific rotation +12.7 -12.7 0 melting point (¡ C) 171-174 171-174 146-148 density at 20¡C (g/cm 3 ) 1.7598 1.7598 1.660 solubility in water at 20¡C (g/100 mL) 139 139 125 pK1 (25¡ C) 2.98 2.98 3.23 pK2 (25¡C) 4.34 4.34 4.82 38© 2013 Pearson Education, Inc.
    39. 39. Diastereomers: Cis-trans Isomerism on Double Bonds • These stereoisomers are not mirror images of each other, so they are not ___________. They are diastereomers.© 2013 Pearson Education, Inc. Chapter 5 39
    40. 40. Diastereomers: Cis-trans Isomerism on Rings • Cis-trans isomers are not mirror images, so these are diastereomers.© 2013 Pearson Education, Inc. Chapter 5 40
    41. 41. Comparing Structures© 2013 Pearson Education, Inc. Chapter 5 41
    42. 42. Resolution (separation) of enantiomers • In 1848, Louis Pasteur noticed that a salt of racemic (±)-tartaric acid crystallizes into mirror- image crystals. • Using a microscope and a pair of tweezers, he physically separated the enantiomeric crystals. • Pasteur had accomplished the first artificial resolution of enantiomers.© 2013 Pearson Education, Inc. Chapter 5 42
    43. 43. Chemical Resolution of Enantiomers React the _______________with a ______ chiral compound, such as tartaric acid, to form _________________, then separate them.© 2013 Pearson Education, Inc. Chapter 5 43
    44. 44. Resolution of (R)- and (S)-2-butanol© 2013 Pearson Education, Inc. Chapter 5 44
    45. 45. Chromatographic ____________of Enantiomers© 2013 Pearson Education, Inc. Chapter 5 45
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