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Estereoq
 

Estereoq

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    Estereoq Estereoq Presentation Transcript

    • ESTEREOQUÍMICA
    • enantiomers
      diastereomers
      Isomers
      constitutional
      isomers
      stereoisomers
    • Molecular Chirality: Enantiomers
    • Chirality
      A molecule is chiral if its two mirror image forms are not superposable upon one another.
      A molecule is achiral if its two mirror image forms are superposable.
    • Bromochlorofluoromethane is chiral
      Cl
      It cannot be superimposed point for point on its mirror image.
      Br
      H
      F
    • Bromochlorofluoromethane is chiral
      Cl
      Cl
      Br
      Br
      H
      H
      F
      F
      To show nonsuperimposability, rotate this model 180° around a vertical axis.
    • Bromochlorofluoromethane is chiral
      Cl
      Br
      Cl
      Br
      H
      H
      F
      F
    • Another look
    • Enantiomers
      nonsuperimposable mirror images are called enantiomers
      and
      are enantiomers with respect to each other
    • Chlorodifluoromethaneis achiral
    • Chlorodifluoromethaneis achiral
      The two structures are mirror images, but are not enantiomers, because they can be superimposed on each other.
    • The Chirality Center
    • w
      x
      y
      C
      z
      The Chirality Center
      a carbon atom with fourdifferent groups attached to it
      also called:
      chiral centerasymmetric centerstereocenter
      stereogenic center
    • H
      F
      Cl
      C
      Br
      Chirality and chirality centers
      A molecule with a single chirality center is chiral.
      Bromochlorofluoromethane is an example.
    • H
      CH3
      CH2CH3
      C
      OH
      Chirality and chirality centers
      A molecule with a single chirality center is chiral.
      2-Butanol is another example.
    • CH3
      CH2CH2CH2CH3
      CH3CH2CH2
      C
      CH2CH3
      Examples of molecules with 1 chirality center
      a chiral alkane
    • OH
      Examples of molecules with 1 chirality center
      Linalool, a naturally occurring chiral alcohol
    • H2C
      CHCH3
      O
      Examples of molecules with 1 chirality center
      1,2-Epoxypropane: a chirality center
      can be part of a ring
      attached to the chirality center are:
      —H
      —CH3
      —OCH2
      —CH2O
    • CH3
      H
      C
      CH2
      CH3
      Examples of molecules with 1 chirality center
      Limonene: a chirality center can be part of a ring
      attached to thechirality center are:
      —H
      —CH2CH2
      —CH2CH=
      —C=
    • H
      CH3
      D
      C
      T
      Examples of molecules with 1 chirality center
      Chiral as a result of isotopic substitution
    • A molecule with a single chirality centermust be chiral.
      But, a molecule with two or more chirality centers may be chiral or it may not.
    • Properties of Chiral Molecules:Optical Activity
    • Optical Activity
      A substance is optically active if it rotates the plane of polarized light.
      In order for a substance to exhibit opticalactivity, it must be chiral and one enantiomer must be present in excess of the other.
    • Light
      has wave properties
      periodic increase and decrease in amplitude of wave
    • Light
      optical activity is usually measured using light having a wavelength of 589 nm
      this is the wavelength of the yellow light from a sodium lamp and is called the D line of sodium
    • Polarized light
      ordinary (nonpolarized) light consists of many beams vibrating in different planes
      plane-polarized light consists of only those beams that vibrate in the same plane
    • Nicol prism
      Polarization of light

    • Rotation of plane-polarized light
    • AbsoluteandRelative Configuration
    • Configuration
      Relative configuration compares the arrangement of atoms in space of one compound with those of another.
      Absolute configuration is the precise arrangement of atoms in space.
    • Configuration
      Relative configuration compares the arrangement of atoms in space of one compound with those of another.until the 1950s, all configurations were relative
      Absolute configuration is the precise arrangement of atoms in space. we can now determine the absolute configuration of almost any compound
    • CH3CHCH2CH3
      CH3CHCH
      CH2
      OH
      OH
      Relative configuration
      Pd
      [] + 33.2°
      [] + 13.5°
      No bonds are made or broken at the chirality centerin this experiment. Therefore, when (+)-3-buten-2-ol and (+)-2-butanol have the same sign of rotation, the arrangement of atoms in space is analogous. The twohave the same relative configuration.
    • H
      HO
      H
      HO
      OH
      OH
      H
      H
      Two possibilities
      H2, Pd
      H2, Pd
      But in the absence of additional information, we can't tell which structure corresponds to(+)-3-buten-2-ol, and which one to (–)-3-buten-2-ol.
    • H
      HO
      H
      HO
      OH
      OH
      H
      H
      Two possibilities
      H2, Pd
      H2, Pd
      Nor can we tell which structure corresponds to(+)-2-butanol, and which one to (–)-2-butanol.
    • H
      HO
      H
      HO
      OH
      OH
      H
      H
      Absolute configurations
      H2, Pd
      [] +33.2°
      [] +13.5°
      H2, Pd
      [] –13.5°
      [] –33.2°
    • CH3CH2CHCH2Br
      CH3CH2CHCH2OH
      CH3
      CH3
      Relative configuration
      HBr
      [] -5.8°
      [] + 4.0°
      Not all compounds that have the same relativeconfiguration have the same sign of rotation. No bondsare made or broken at the chirality center in thereaction shown, so the relative positions of the atoms are the same. Yet the sign of rotation changes.
    • The Cahn Ingold PrelogR-S Notational System
    • Two requirements for a systemfor specifying absolute configuration
      1. need rules for ranking substituents at chirality center in order of decreasing precedence
      2. need convention for orienting molecule so that order of appearance of substituents can be compared with rank
      The system that is used was devised by R. S. Cahn, Sir Christopher Ingold, and V. Prelog.
    • The Cahn-Ingold-Prelog Rules
      1. Rank the substituents at the chirality center according to same rules used in E-Z notation.
      2. Orient the molecule so that lowest-ranked substituent points away from you.
    • 1
      1
      4
      3
      3
      4
      2
      2
      Example
      Order of decreasing rank:4 > 3 > 2 > 1
    • The Cahn-Ingold-Prelog Rules
      1. Rank the substituents at the chirality center according to same rules used in E-Z notation.
      2. Orient the molecule so that lowest-ranked substituent points away from you.
      3. If the order of decreasing precedence traces a clockwise path, the absolute configuration is R. If the path is counterclockwise, the configuration is S.
    • 1
      1
      4
      3
      3
      4
      2
      2
      counterclockwise
      clockwise
      R
      S
      Example
      Order of decreasing rank:43 2
    • H
      H
      CH2CH3
      CH3CH2
      C
      HO
      C
      OH
      CH3
      H3C
      (S)-2-Butanol
      (R)-2-Butanol
      Enantiomers of 2-butanol
    • Very important! Two different compounds with the same sign of rotation need not have the same configuration.
      Verify this statement by doing Problem. All four compounds have positive rotations. What are their configurations according to the Cahn-Ingold-Prelog rules?
    • H
      H3C
      R
      H
      H
      Chirality center in a ring
      —CH2C=C > —CH2CH2 > —CH3 > —H
    • Fischer Projections
      Purpose of Fischer projections is to show configuration at chirality center without necessity of drawing wedges and dashes or using models.
    • Rules for Fischer projections
      H
      Cl
      Br
      F
      Arrange the molecule so that horizontal bonds at chirality center point toward you and vertical bonds point away from you.
    • Rules for Fischer projections
      H
      Br
      Cl
      F
      Projection of molecule on page is a cross. When represented this way it is understood that horizontal bonds project outward, vertical bonds are back.
    • Rules for Fischer projections
      H
      Br
      Cl
      F
      Projection of molecule on page is a cross. When represented this way it is understood that horizontal bonds project outward, vertical bonds are back.
    • Physical Properties of Enantiomers
    • Physical properties of enantiomers
      Same: melting point, boiling point, density, etc
      Different: properties that depend on shape of molecule (biological-physiological properties) can be different
    • Odor
      CH3
      CH3
      O
      O
      H3C
      H3C
      CH2
      CH2
      (–)-Carvonespearmint oil
      (+)-Carvonecaraway seed oil
    • H
      H3C
      CH2CH(CH3)2
      C
      C
      HO
      O
      Chiral drugs
      Ibuprofen is chiral, but normally sold asa racemic mixture. The S enantiomer is the one responsible for its analgesic and antiinflammatory properties.
    • Chiral MoleculeswithTwo Chirality Centers
      How many stereoisomers when a particular molecule contains two chirality centers?
    • O
      CH3CHCHCOH
      HO
      OH
      2,3-Dihydroxybutanoic acid
      2
      3
      What are all the possible R and S combinations of the two chirality centers in this molecule?
      Carbon-2 R R S S
      Carbon-3 R S R S
    • O
      CH3CHCHCOH
      HO
      OH
      2,3-Dihydroxybutanoic acid
      2
      3
      4 Combinations = 4 Stereoisomers
      Carbon-2 R R S S
      Carbon-3 R S R S
    • O
      CH3CHCHCOH
      HO
      OH
      2,3-Dihydroxybutanoic acid
      2
      3
      4 Combinations = 4 Stereoisomers
      What is the relationship between these stereoisomers?
      Carbon-2 R R S S
      Carbon-3 R S R S
    • O
      CH3CHCHCOH
      HO
      OH
      2,3-Dihydroxybutanoic acid
      2
      3
      enantiomers: 2R,3R and 2S,3S
      2R,3S and 2S,3R
      Carbon-2 R R S S
      Carbon-3 R S R S
    • CO2H
      CO2H
      [] = -9.5°
      [] = +9.5°
      R
      S
      HO
      OH
      H
      H
      enantiomers
      OH
      HO
      H
      H
      R
      S
      CH3
      CH3
      CO2H
      CO2H
      S
      R
      OH
      HO
      H
      H
      enantiomers
      OH
      H
      HO
      H
      R
      S
      [] = -17.8°
      [] = +17.8°
      CH3
      CH3
    • O
      CH3CHCHCOH
      HO
      OH
      2,3-Dihydroxybutanoic acid
      2
      3
      but not all relationships are enantiomeric
      stereoisomers that are not enantiomers are diastereomers
      Carbon-2 R R S S
      Carbon-3 R S R S
    • Isomers
      constitutional
      isomers
      stereoisomers
      enantiomers
      diastereomers
    • CO2H
      CO2H
      R
      S
      HO
      OH
      H
      H
      OH
      HO
      H
      H
      R
      S
      CH3
      CH3
      diastereomers
      CO2H
      CO2H
      S
      R
      OH
      HO
      H
      H
      OH
      H
      HO
      H
      R
      S
      CH3
      CH3
      [] = -9.5°
      [] = +9.5°
      enantiomers
      enantiomers
      [] = -17.8°
      [] = +17.8°
    • Fischer Projections
      recall for Fischer projection: horizontal bonds point toward you; vertical bonds point away
      staggered conformation does not have correct orientation of bonds for Fischer projection
      CO2H
      CH3
    • Fischer projections
      transform molecule to eclipsed conformation in order to construct Fischer projection
    • CO2H
      OH
      H
      H
      OH
      CH3
      Fischer projections
    • Two chirality centers in a ring
      S
      R
      S
      R
      trans-1-Bromo-1-chlorocyclopropane
      nonsuperposable mirror images; enantiomers
    • Two chirality centers in a ring
      S
      S
      R
      R
      cis-1-Bromo-1-chlorocyclopropane
      nonsuperposable mirror images; enantiomers
    • Two chirality centers in a ring
      S
      S
      R
      R
      cis-1-Bromo-1-chloro-cyclopropane
      trans-1-Bromo-1-chloro-cyclopropane
      stereoisomers that are notenantiomers; diastereomers