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Optical isomerism
 

Optical isomerism

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Introduction to Optical Isomerism for people familiar with Chemsitry

Introduction to Optical Isomerism for people familiar with Chemsitry

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    Optical isomerism Optical isomerism Presentation Transcript

    • 1
      Optical Isomerism
    • Content
      History
      Enantiomers
      Reasons for molecular handedness
      Optical activity of enantiomers
      Naming Conventions
      Diastereomers
      Meso Compounds
      Resolution of Enantiomers
      Importance of Enantiomers
      2
    • Isomers
      3
    • History
      4
    • Discovery of Optical Activity
      In 1850, French Physicist Jean-BaptiseBiot observed that solutions of some organic compounds like sugar and camphor have the ability to rotate plane polarized light
      Up till then the basis of this phenomenon was not yet known
      5
    • Separation of Enantiomers
      In 1848 French Chemist Louis Pasteur separated a solution of optically inactive tartaric acid into two optically active components. 
      He observed that:
      Each of these components had identical physical properties like density,  melting point, solubility, etc
      But
      One of the components rotated plane polarized light clockwise while the other component rotated the polarized light by the same amount counter clockwise.
      6
    • Pasteur made a proposal that since the crystals of tartaric acid were mirror images of each other, their molecules were also mirror images of each other
      7
    • Additional research done by Pasteur revealed that one component of tartaric acid could be utilized for nutrition by micro-organisms but the other could not.
      Thus Pasteur concluded that  biological properties of chemical substances depend not only on the nature of the atoms comprising the molecules but also on the manner in which these atoms are arranged in space.
      8
    • The tetrahedral Carbon
      In 1874 as a student at University of Utrecht, Jacobusvan't Hoff proposed the tetrahedral carbon. His proposal was based upon evidence of isomer number:
      Conversion of CH4 into CH3R(1 isomer)
      Conversion of CH3R into CH2RR1(1 isomer)
      van't Hoff realized that the four hydrogens in CH4 had to be equivalent and a geometrical square was ruled out because it would form 2 isomers for CH2RR1. Thus he proposed the tetrahedral carbon centre.
      9
    • Explanation of Optical Isomerism
      The tetrahedral carbon not only collaborated the absence of isomers in CH2YZ, but also predicted the existence of mirror image isomers. When carbon makes four single bonds with four different groups such as CHFClBr, non-super-imposable mirror-image molecules exist
      10
    • Enantiomers
      11
    • Definition
      An enantiomer is one of two stereoisomers that are mirror images of each other and are non-superimposable.
      12
    • As seen here, the two molecules of lactic acid are mirror images of each other, but cannot superimpose.
      13
      (S)-Lactic acid
      or (+)-Lactic acid
      (R)-Lactic acid
      or (-)-Lactic acid
    • The Tetrahedral Carbon
      Enantiomers result when two compounds that are mirror-images of each other cannot superimpose
      Eg. when a tetrahedral carbon is bonded to four different substituents(CR1R2R3R4)
      For example, in lactic acid, there are four different groups: -H, -OH, -CH3, -COOH
      Can you name other examples of enantiomers formed this way?
      Alanine, Tartaric Acid, Glyceraldehyde
      14
    • Reasons for molecular handedness(Chirality)
      15
    • Chirality
      A molecule that is not superimposable to its mirror image is said to be chiral
      The most common cause of chirality in organic molecules is the presence of a carbon atom bonded with 4 different groups(eg in lactic acid), this is also known as point chirality.
      These carbon atoms are called chirality centers or chirality centres.
      Can you think of other causes of Chirality? (be creative)
      16
    • Identifying Chiral Molecules
      The obvious way: find any carbon with 4 different substituents. If there are any, then the molecule is chiral. Isotopic differences are also considered different substituents.
      Thus, -CH2-, -CH3(methyl), >C=O(carbonyl), >C=C<(alkene) and -C@C-(alkyne) groups cannot be chirality centers as they all have less than 4 substituents.
      Easier way: look for the presence of a plane of symmetry, since a symmetrical molecule is identical to its mirror image and is thus achiral
      17
    • Drawing of Enantiomers
      Fischer Projection (do not confuse with Lewis Structure)
      Natta Projection (we all know this)
      For cyclic molecules
      Haworth Projection
      Chair conformation
      18
    • 19
      1. Fischer Projection
      2. Haworth Projection
      3. Chair Conformation
      4. Natta Projections
    • Optical activity of enantiomers
      20
    • As seen from Biot’s experiment, enantiomers can rotate plane polarised light, since they are optically active
      When a beam of plane polarised light passes through a solution of a non-racemic (scalemic) mixture (ie has enantiomeric excess of one of the enantiomers), rotation of the polarisation plane occurs.
      21
    • Polarimeter
      22
    • Optical Activity
      Observed rotation
      The number of degrees, , through which a compound rotates the plane of polarized light.
      Dextrorotatory (+)
      Acompound that rotates the plane of polarized light to the right.
      Levorotatory (-)
      Refers to a compound that rotates the plane of polarized light to the left.
      23
    • Specific Rotation
      Specific rotation refers to the observed rotation for a sample in a tube 1.0 dm in length and at a concentration of 1.0 g/mL.
      The degree of rotation also depends on the wavelength of the light (the yellow sodium D2 line near 589 nm is commonly used for measurements)
      24
    • Calculating Specific Rotation (liquids)
      The specific rotation of a compound is given by:
      Where:
      [a] is the specific rotation
      T is the temperature
      l is the wavelength of light used
      l is the path length of sample (in decimetres)
      d is the density of the sample (in g/cm3 for pure liquids)
      The sign of the rotation (+ or -) is always given
      25
    • Calculating Specific Rotation (liquids)
      The specific rotation of a compound is given by:
      Where:
      [a] is the specific rotation
      T is the temperature
      l is the wavelength of light used
      l is the path length of sample (in decimetres)
      c is the concentration of the sample (in g/cm3)
      The sign of the rotation (+ or -) is always given
      26
    • Optical Purity
      Optical purity: A way of describing the composition of a mixture of enantiomers.
      Enantiomeric excess: The difference between the percentage of two enantiomers in a mixture.
      optical purity is numerically equal to enantiomeric excess, but is experimentally determined.
      27
    • Naming Conventions
      28
    • Types of naming
      R,S designation
      +, - designation (explained just now)
      d,l Nomenclature
      29
    • R-, S- Naming Convention
      This convention labels each chiralcenterR or S according to a system by which its substituents are each assigned a priority based on atomic number.
      The priority of substituents are assigned based on Cahn–Ingold–Prelog priority rules
      30
    • Cahn–Ingold–Prelog priority rules
      Assignment of Priority
      Compare the atomic number of the atoms directly attached to the chirality center. The group with the atom of higher atomic number receives higher priority. (eg –Cl > –OH > –NH2 > –CH3)
      If different isotopes of the same element is attached to the chiral center, the group with the higher mass number recieves higher priority (2H > 1H)
      31
    • If double or triple bonded groups are encountered as substituents, they are treated as an equivalent set of single-bonded atoms.For example, C2H5–  <  CH2=CH–  <  HC≡C– 
      32
      • If there is a tie, consider the atoms at distance 2 from the chiralitycenter—as a list is made for each group of the atoms bonded to the one directly attached to the chiralitycenter. Each list is arranged in order of decreasing atomic number. Then the lists are compared atom by atom; at the earliest difference, the group containing the atom of higher atomic number receives higher priority. (eg. –COOH > –CH3)
      33
    • If there is still a tie, each atom in each of the two lists is replaced with a sub-list of the other atoms bonded to it (at distance 3 from the chiralitycenter), the sub-lists are arranged in decreasing order of atomic number, and the entire structure is again compared atom by atom. This process is repeated, each time with atoms one bond farther from the chiralitycenter, until the tie is broken.
      34
    • Identifying the Configuration
      After having ranked the four groups attached to the stereo-center, orientate the molecule such that the lowest ranking group points directly back
      Look at the 3 substituents left facing you: If the priority of the remaining three substituents decreases in clockwise direction, it is labeledR (for Rectus, Latin for right), if it decreases in counterclockwise direction, it is S (for Sinister, Latin for left).
      35
    • d,l Nomenclature
      The d/l system names isomers after the spatial configuration of its atoms, by relating the molecule to glyceraldehyde. Glyceraldehyde is chiral itself, and its two isomers are labeled d and l
      This nomenclature is still used in certain organic compounds like saccharides and amino acids
      36
    • 37
    • Identifying the Configuration
      For Monosaccharides:
      The absolute configuration of all monosaccharides is denoted by the configuration at the chiralitycenter furthest from the anomeric centre (the carbonyl carbon in the open chain representation) 
      If, in the Fischer projection, that centre has the hydroxyl group on the right, it is a d-sugar; if on the left, it is an l-sugar. By convention, the "D" and "L" symbols are written in small capitals.
      38
    • Non amino-acids such as Lactic, Ascorbic, Tartaric Acid also follow the same rules as Saccharides.
      39
    • Identifying the Configurations
      For Amino Acids
      CORN Rule:
      The groups: COOH, R, NH2 and H (where R is a variant carbon chain)are arranged around the chiralcenter carbon atom. Starting with the hydrogen atom away from the viewer, if these groups are arranged clockwise around the carbon atom, then it is the d-form. If counter-clockwise, it is the l-form.
      40
    • Relation to other naming conventions
      The d/l labeling is unrelated to (+)/(−); it does not indicate which enantiomer is dextrorotatory and which is levorotatory.
      Rather, it says that the compound's stereochemistry is related to that of the dextrorotatory or levorotatory enantiomer of glyceraldehyde—the dextrorotatory isomer of glyceraldehyde is, in fact, the d- isomer.
      41
    • Diastereomers
      42
    • 2n Rule
      As a general rule the maximum number of isomers for a compound is 2n, with n being the number of chirality centers.
      *As seen in the next slide
      43
    • 44
    • Definition
      Diastereomers refer to enantiomers that are not mirror images of each other
      They can be chiral or achiral
      Eg D-glucose and D-galactose
      45
    • Stereochemistry in Sugars
      Aldose
      An aldose is a monosaccharide that contains only one aldehyde group per molecule. The chemical formula takes the form Cn(H2O)n.
      The maximum number of chirality centers for any aldose is 2n-2
      Since the “head”(-CH=O) and “tail”(-CH2OH) carbons cannot be chiral centers
      46
    • Examples (Aldotetrose)
      Aldotetroses are Aldoses with 4 carbons
      As such, the maximum number of enantiomers of aldotetroses is 4 (24-2)
      The 4 possible enantiomers of Aldotetroses
      47
    • Other examples of diastereomers
      48
    • Reasons
      As we can see, although Erythrose and Threose are stereoisomers, they are not enantiomers, since their molecules are not mirror-images of each other. They are thus called diastereomers
      49
      Erythrose
      Threose
      They are not mirror images of each other
    • This is because stereoisomers are only enantiomers because all their chirality centers have opposite configurations, but if not all chiralitycentres have opposite configurations, then they are diastereomers.
      50
      Example of d(+)-Glucose and D(+)-Galactose
    • Meso Compounds
      51
    • Definition
      A meso-compound is a non-optically active member of a set of stereoisomers.
      It does not give a (+) or (-) reading on a polarimeter
      It is non-optically active as it is achiral
      It is achiral since it has a plane of symmetry
      52
    • Examples
      Tartaric acid has 3 stereoisomers
      d-(2S,3S)-(−)-tartaric acid
      l-(2R,3R)-(+)-tartaric acid
      (2R,3S)-mesotartaric acid
      53
      d-(2S,3S)-(−)-tartaric acid
      l-(2R,3R)-(+)-tartaric acid
      (2R,3S)-mesotartaric acid
    • Mesotartaric Acid
      Let’s assume the mirror image of mesotartaric acid:
      Flip the molecule:
      And you find that you get the same molecule
      54
      Flip this molecule 180 degrees
    • Meso Compounds
      Thus, we can say that mesotartaric acid is not an enantiomer, as it is superimposible with its mirror image.
      This is due to the fact that there is a plane of symmetry in the molecule
      55
      There is a plane of symmetry
      Rotate this bond 180 degrees
    • Resolution of Enantiomers
      56
    • Resolution of Enantiomers
      In the lab, if we make chiral compounds from achiral starting materials, we are bound to get a racemic mixture.
      The way we separate the mixture is known as resolution
      Resolution is important as most of the time enantiomerically pure compounds are required
      57
    • Crystallisation
      One way of resolution would be crystallisation (like what Pasteur did), it’s use is limited to solid compounds (eg Sodium Ammonium Tartrate)
      It does not work for liquid compounds that do not crystallise under ordinary conditions
      58
    • The most common way of resolution would be to use an acid-base reaction between a racemic mixture of chiral carboxylic acid and an amine base to yield an ammonium salt, which can be crystallised.
      59
      +
    • Resolution of enantiomers
      Another way would be using a chemical reaction to produce a diastereomer.
      This can be done in various ways, by esterification or by forming diastereomeric salt (Adduct).
      60
    • Resolution of enantiomers (Esterification)
      One way of resolution of enantiomers would be using an ester
      61
      TsOHor H2SO4
      +
      +
      Chiral but enantiomerically pure
      Diastereomers
      Chiral but Racemic
    • Diastereomers have different physical properties (solubility, boiling point etc), and should be quite easy to separate by distillation/crystallisation/chromatography
      When a pure enantiomer (diastereomer) is separated, the original intended products can be obtained
      62
      Enantiomerically pure product
      NaOH, H2O
      +
    • Resolution of enantiomers (Forming a diastereomeric salt)
      When a racemic mixture of a carboxylic acid reacts with a single enantiomer of a chiral base, diastereomers are obtained.
      Diastereomers can then be separated just like previously
      63
    • Acidification of the two diastereomeric salts resolved the original racemic mixture.
      64
      Racemic Mixture
      Enantiomerically pure compound
    • Importance of Enantiomers
      65
    • Chirality in the Biological World
      Enzymes are like hands in a handshake
      The substrate fits into a binding site on the enzyme surface
      A left-handed molecule, like hands in gloves, will only fit into a left-handed binding site and
      a right-handed molecule will only fit into a right-handed binding site.
      Because of the differences in their interactions with other chiral molecules in living systems, enantiomers have different physiological properties.
      66
    • Some Examples
      d-Glucose can be used for metabolism for all organisms but not l-glucose
      Almost all of the amino acids in proteins are (S) at the α carbon.
      In most pharmaceutical drugs, only one of the enantiomers are biologically active.
      Eg. (S)Ibuprofen is an active analgestic agent but the (R) enantiomer is biologically inactive
      67
      a carbon
    • Chirality in the Biological World
      A schematic diagram of an enzyme active site capable of binding with (R)-glyceraldehyde but not with (S)-glyceraldehyde.
      68
    • Credits
      Most of these information are taken from online sources[citation needed]
      However due to the fact that the layout was copied from the book, we apologise for the fact that this may be a little boring (for some)
      Done by: Jeff Xu, Luther Mok, Joshua Lay, Wen Song
      Design by Jonathan Yong
      69
    • 70
      Thank You