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Stereoisomerism:- PDF/PPT

Description

Stereoisomerism
• These differ from each other in the
their atoms are connected, i.e., in
way their
• structures. It’s six types signifying the
• main difference in the of the isomers are:
• structural features

• Chain/ Skeletal/ Nuclear
• Position Isomerism
• Isomerism

• Functional Isomerism
• Metamerism
• Tautomerism
• Ring Chain Isomerism
I. Configurational Isomerism
II. Conformational Isomerism
• Enantiomers are known to possess same physical and chemical properties but differ in the way they interact with
polarised light. they plane Substances which can rotate the plane of
polarised light are said to be optically
active.
• Dextrorotatory (Latin: dextre means right) and is indicated by (+) sign.
Laevorotatory (Latin: laeves mean left) and is indicated by (-) sign.
Those substance which do not rotate the
plane of polarised light are called optically inactive.
• To overcome the problem of D-L system, R.S. Cahn(England), Si Christopher Ingold (England), and V.
Prelog (Zürich) evolved a new and unambiguous system for assigning absolute configuration to chiral molecules.
This system is named as CIP (Cahn, Ingold, Prelog) system after their names. It is called as R-S system as the prefixes R-and S-are used to designate the configuration at a particular chirality centre. A racemic mixture is named as (RS). This system is based on certain rules called as sequence rules and also as CIP rules .

Includes Questions

1. State and explain Recemic mixure and write note on different methods of resolution. (Winter-2020)
2. Give Difference between Enantiomers and Diesteriomers (Winter-2020)
3. Give brief note on Conformation of Cyclohexane. (Winter-2020)
4. State and Explain (Summer-2020)
a) Enantiomers b) Diesteriomers c)Specific rotation
d) Mesomers e) Geometric Isomer
5. Write a Short Note on Geometric Isomerism. (Summer-2020)
6. Explain in brief with suitable example – Stereospecific and Stereoselective reactions. (Summer-2020)
7. Define plane polarized light and discuss optical activity in detail (Summer-2019)
8. Differentiate enantiomers and diastereomers, discuss mesomers (Summer-2019)
9. Explain nomenclature of optical isomers. (Summer-2019)
10. Differentiate stereoselective and stereospecific reactions. (Winter-2019)
11. Give short notes on nomenclature of geometrical isomers. (Winter-2019)
12. Discuss the sequence rule in detail to assign configuration. (Winter-2019)

  • Subject:- Organic chemistry 3
  • Course:- B.pharm (pharmacy),
  • Semester:- 4th sem , sem :- 4

STEREOISOMERISM
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 ISOMERISM      isos      meros




Ethyl alcohol (CH3CH2OH) and
Dimethyl ether (CH3OCH3) are isomers.



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          Stereoisomers            CONSTITUTIONAL
                                      ISOMERS
Enantiomers or      Diastereomers
Optical isomers



                  Conformational
    Geometric                        Meso        Epimers
    Ciss/trans       Isomers       compound
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These differ from each other in the way
     their atoms are connected, i.e., in their
     structures. It’s six types signifying the
     main difference in the structural features
     of the isomers are:
I.   Chain/ Skeletal/ Nuclear Isomerism
II.  Position Isomerism
III. Functional Isomerism
IV.  Metamerism
V.   Tautomerism
VI. Ring Chain Isomerism
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    These    have same molecular formula but
   different arrangement of carbon chain within
            the molecule.          CH3

H3C—CH2—CH2—CH3             C4H10            H3C—CH—CH3
       n-Butane         Same molecular 2-Methylpropane (Isobutane)
   (straight chain)        formula          (Branched chain)

                  CH 3 CH 2 CH 2 CH 2 CH 3
                        n-Pentane
        CH 3                                   CH 3

 H3 C   C      CH 3       C 5 H 12    H3 C     CH      CH 2      CH 3
                           Same
                                              2-Methylbutane
         CH 3          molecular formula
                                               (Iso-pentane)
2, 2-Dimethylpropane
(Neo-pentane)
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differ in the position of attached atoms or
groups or in position of multiple (double or
triple) bonds.
                                                OH

      CH 3 CH 2 CH 2 OH                   CH 3 —CH—CH 3
 Propan-1-ol (The OH group atC1)   Propan-2-ol (The OH group atC2)


  4    3    2     1                  4          3     2    1
  CH3CH2C         CH                 CH3        C     C    CH3
But-1-yne (Triple bond at C1)      But-2-yne (Triple bond at C2)



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   different functional groups.
        CH 3 CH 2 OH              CH 3       O     CH 3
                                    Dimethyl ether
            Ethanol
              O                          O

     CH3     C   OH                H     C       OCH3
       Ethanoic acid              Methyl methanoate

        O                                    O
                       and
CH3     C   CH3               CH3CH2      C    H
Propanone (Acetone)          Propanal (Propionaldehyde)
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    ( i. e., -O- , -S-, -NH-, - CO- etc.). Metamerism is
    shown by members of the same family, i.e., same
    functional groups.
        O                                        O
                                                                   O CH3
CH3CH2—C—CH2CH3 is a metamer of   CH3CH2CH2—C—CH3 or
                                                             CH3—C—CHCH3
   Pentan-3-one                       Pentan-2-one
  (Diethyl ketone)                (Methyl n-propyl ketone)   3-Methylbutan-2-one
                                                             (Isopropyl methyl ketone)


                                                                      CH3

CH3CH2—O—CH2CH3 is a metamer of    CH3CH2CH2—O—CH3 or         CH3CH2CH—O—CH3
   Ethoxy ethane                   1-Methoxy propane            2-Methoxypropane
   (Diethyl ether)                (Methyl n-propyl ether)      (Isopropyl methyl ether)

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Structural or constitutional isomers existing in
 easy and rapid equilibrium by migration of an
                         tautomers      ( keto-enol
 tautomerism).




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Open chain and cyclic compounds having
the same molecular formula


CH 3 CH   CH 2      and
    Propene                  Cyclopropane

CH 3      C    CH   and
       Propyne             Cyclopropene


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      formula but differ in the manner their
      atoms or groups are arranged in the
      space are called stereoisomers. It is of
      two types:
I.    Configurational Isomerism
II.   Conformational Isomerism




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 interconverted unless a covalent bond is
 broken are called configurational isomers
 . These isomers can be separated under
 normal conditions.
The configurational isomerism is again of
 two types:
a) Optical Isomerism or Enantiomerism
b) Geometrical Isomerism

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                        which are related to
                as an object and its non-
 superimposable mirror image are called
 optical isomers or enantiomers (Greek:
 enantion means opposite).
The optical isomers can also rotate the plane
 of polarised light to an equal degree but in
 opposite direction.
The property of rotating plane of polarised
light is known as optical activity.
The optical isomers have similar physical
and chemical properties .
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                                Mirror
          COOH                                       COOH


H                    OH                  HO                     H


          CH3                                        CH3
    ( -) - Lactic acid                         ( +) - Lactic acid
(Rotates the plane of polarized          (Rotates the plane of polarized
light towards left hand side i.e.        light towards right hand side i.e.
anticlockwise)                           clockwise)


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Geometric isomers




These isomers are also called as cis-
 trans isomers. For example, molecular
  formula C 2H2Cl2 corresponds to two
 geometric isomers as follows:
                                    can   be
 interconverted rapidly at room temperature
 without breaking a covalent bond are called
 conformational isomers or conformers.
  Because such isomers can be readily
 interconverted, they cannot be separated
 under normal conditions.
 Two types of conformational isomers are:
a) Conformational isomers resulting from
  rotation about single bond
b) Conformational       isomers arising   from
  amine inversion
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containing single                    bonds         have many
interconvertible                                 conformational
isomers.e.g, 'boat' and 'chair' forms of cyclohexane.
                          H          H                          H
                               H H                                  H
         H                                   H          H
                     H
     H       H                           H                  H
                         H H
                                     H                          H
                 H
                          H              H                  H
 H
             Cyclohexane                         Cyclohexane
     H       (Chair form)                        (Boat form)
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                           "inside out"
                                                        amine
 inversion     Walden inversion

                                                   R3
                              R3           R1
                                                          R2

     N          R1     N                           N

R1        R3                    R2
     R2
                 Transition state

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Measured with polarimeter
Rotation, in degrees, is []
Clockwise rotation is called dextrorotatory
Anti-clockwise is levorotatory


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                                                         19
The angle between the entrance and exit
planes is the optical rotation.




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                                         specific
rotation,[ ]D

    []D =           obser ved rotation
             (pathlength x concentration)

         =            =    degrees

    Specific rotation is that obser ved for
     1 g/mL in solution in cell with a 10
     cm path using light from sodium
     metal vapor (589 nm)

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                                                              21
Enantiomers are known to possess same
 physical and chemical properties but they
 differ in the way they interact with plane
 polarised light.
Substances which can rotate the plane of
 polarised light are said to be optically
 active.
Dextrorotatory (Latin: dextre means right)
 and is indicated by (+) sign.
Laevorotatory (Latin: laeves mean left) and
 is indicated by (-) sign.
Those substance which do not rotate the
 plane of polarised light are called optically
 inactive.                     www.remixeducation.in
Angle     of rotation ( is the angle
 (degrees) by which the analyser is rotated
 to get maximum intensity of light. It
 depends upon:
(i) Nature of the substance;
(ii) Concentration of the solution in g/ml;
(iii) Length of the polarimeter tube;
(iv)  of the incident monochromatic light
(598nm).
 (v) Temperature of the sample.



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   Enatiomerism depends on whether a
 molecule in not superimposable on its
 mirror image. If it is superimposable, the
 molecule is optically inactive otherwise is
 optically active. The most convenient
 method of inspecting superimposability is
 to determine whether the molecule has
 any of the following three elements of
 symmetry:
1.Plane of symmetry (s)
2. Centre of symmetry (i)
3.Simple      or proper axis of symmetry
  (Cn)                      www.remixeducation.in
                                             an imaginary
     plane which divides a molecule in such a way that
     one half is mirror image of the other half .
         A molecule with atleast a plane of symmetry can
     be superimposed on its mirror image and is achiral.
     A molecule that does not have a plane of symmetry
     is usually chiral; it cannot be superimposed upon
     its mirror image                   COOH


  •A plan of symmetry            H              OH
  may pass through                                    Plane
                                                      of
  atoms, between                                       symmetry
                                  H             OH
  atoms or both.
                                        COOH
                                 meso-Tartaric acid
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The plane has
 the same thing
 on both sides
 for the flask




                  There isnomirror
                   planefor a hand


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                                          42
 A molecule that has a plane of
symmetr y is achiral.
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        CHRIAL
- ral) [Greek : Cheir 'Handedness'] and the
property of non-        superimposability is
called chirality. Thus our hands are chiral.




Similarly, alphabets R, F,J are chiral and A, M, O
 are achiral.
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       RR
               Chiral objects
       A A M M O O
            (Achiral objects)
Chiral objects - human hand, gloves, shoes, etc.
Achiral objects - a sphere, a cube, a button,
socks without thumb, etc.
Chirality or molecular dissymmetry is the
necessary and sufficient condition for a
molecule to be optically active.
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•                        achiral
•                         chiral
•   Enantiomers: nonsuperimposable mirror
    images, different molecules.
•   One enantiomeric form of          limonene smells
    like oranges, while its mirror image smells like
•   lemons. The one enantiome r of carvone is the
    essence of caraway, and the other, the
•   essence of spearmint.
    Most molecules in the plant and animal world are
    chiral and usually only one form of then enantiomer
•   is found.
    Nineteen of the twenty known amino acids are
    chiral, and all of them are classified as left handed.
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D and L Conversation:-




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▪ The relative configuration of the compound were established
  by obtaining them by a series of reactions from either D (+)
  glyceraldehyde or L(-) glyceraldehyde. These do not involve
  the breaking of a bond on chiral atom, the configuration about
  the chiral atom is retained. For example ,;acetic acid obtained
  from L-glyceraldehyde by a series of step has L-configuration
  and that obtained from D-glyceraldehyde has D-
  configuration.




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▪ The relative lactic acid also produced from D-glyceraldehyde
  is the same but it is levo-rotatory, while glyceraldehyde it self
  is dextro rotatory,




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▪ It should be noted that the notations D and L refer to the
  relative configuration, while d or(+) and l of (-) refer to
  the sign of optical rotation. A compound with D-
  configuration can be dextrorotatory [D(+)] levorotatory
  [D(-)].similarly a compound with L-configuration can be
  either [L(+)] or [L(-)].Thus the sign of rotation is not an
  indication of D(+) glyceraldehyde. In fact there are large
  number of compounds which have similar configurations
  but different signs of rotation.
▪ In 1951 X-ray diffraction results were employed to
  ascertain the absolute configuration of the molecules,
  which gave the same configuration to D-glyceraldehyde
  ,as was assigned by Emil Fiscer.


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To overcome the problem of D-L system, R.S. Cahn
 (England), Sir Christopher Ingold (England), and V.
 Prelog (Zürich) evolved a new and unambiguous system
 for assigning absolute configuration to chiral molecules.
 This system is named as CIP (Cahn, Ingold, Prelog)
 system after their names. It is called as R-S system as
 the prefixes R-and S-are         used to designate the
 configuration at a particular chirality centre. A racemic
 mixture is named as (RS). This system is based on
 certain rules called as sequence rules and also as CIP
 rules .

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Cahn, Ingold and Prelog helped in introducing a new system
for assigning the configurational form to the molecule. The
system replaced the DL system. This system, adopted by
IUPAC, is called the RS convention or the sequence rule
system.
In the sequence rule system, the four atoms or groups directly
attached to the asymmetric carbon atom, are to be ranked and
to be placed in a priority order. Use of the following rules was
made to decide the priority order of the groups attached to the
asymmetric carbon atom.



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◦ Each atom bonded to the stereocenter is assigned a priority, based on
  atomic number. The higher the atomic number, the higher the priority.
◦ Of the four atoms directly attached to the asymmetric carbon atom, the
  atom with the highest atomic number was given the highest priority and
  the atom with the lowest atomic number the lowest priority.

     1        6          7           8          16       17       35       53
    H        CH3        NH2         OH         SH        Cl       Br       I

                       Increasing Priority

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                                                                                61
◦ 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


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2.   In case, the molecule has more than one group, having the same atom
       through which the groups are attached to the asymmetric carbon atom,
       then to decide the priority order between the groups the next atoms
       attached to the first atom are taken into consideration. For example in 2-
       butanol, CH3CHOHC2H5, the four different groups attached are –
       CH3,

       -C2H5, -OH and H. The highest priority group would be –OH and the
       lowest priority group would be H. As both the groups –CH3 and –C2H5,
       are attached to the asymmetric carbon atom through carbon therefore to
       decide the priority between these two the next atoms attached in the
       groups are to be taken into consideration. In –CH3 the next atoms are
       hydrogen whereas in –C2H5 the next atoms are one carbon and two
       hydrogens; therefore the priority goes to the ethyl group.
◦ Atoms participating in a double or triple bond are
 considered       to be bonded to an equivalent number
 of similar atoms by single bonds

              H                         H
              C O                       C O
                                        O C

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1.
2.    Assign a priority to each substituent from 1 (highest)
      to 4 (lowest)
3.    Orient the molecule so that the group of lowest
      priority (4) is directed away from you
4.    Read the three groups projecting toward you in order from
      highest (1) to lowest priority (3)
5.    If reading is clockwise, configuration is R (from the Latin
      rectus). If it is counterclockwise, configuration is S (from the
      Latin sinister).



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clockwise          1          2       2                     counter
                                                    1       clockwise
                       C                   C
                   4                  4
                       3                   3
 view with
 substituent of
 lowest priority
 in back
                       R   (rectus)       S    (sinister)
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           I1                         I1

  F        C                         C
4                        4F
                Br                           Cl
      Cl             2        Br
           3                             3
                              2
           R     Enantiomers        S
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1.   -OH      HO
                   H
                       COOH         HOOC
                                             H
                                                 OH
                   C
2.   -COOH                                   C

3.   -CH3          CH3                       CH3

4.   -H      (R)-(-)-lactic acid    (S)-(+)-lactic acid




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                             3                        2
       4                    F                          COOH
      H                2          1
                                  Br
                                               3H C       1
 1          2          Cl                        3         OH
 Br         Cl
                            H4                        4
       F3                                                 H

R-configuration    R-configuration             S-configuration

Lowest priority 4 on horizontal li
                                       3
      2                                 CH3
       CHO                       4            1
 4           1                    H           OH
  H          OH
                                       2 CH2CH3
      3 CH2OH



 R-configuration             S-configuration
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     CHO              CHO                         CHO

           H   HO           H               H            OH
HO
           H    H           OH            HO             H
HO
     CH2OH            CH2OH                        CH2OH
 L-Erythrose        D-Threose                   L-Threose
  (2S, 3S)           (2S, 3R)                    (2R, 3S)




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  A 50:50 mixture of          two chiral or      enantiomers
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)
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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Racemic mixture, also called racemate, a mixture of
equal quantities of two enantiomers, or substances that
have dissymmetric molecular structures that are mirror
images of one another. Each enantiomer rotates the plane
of polarization of plane-polarized light through a
characteristic angle, but, because the rotatory effect of
each component exactly cancels that of the other, the
racemic mixture is optically inactive. The name is derived
from racemic acid. Racemic acid, or, more properly,
racemic tartaric acid, is a mixture of equal amounts of
dextrorotatory and levorotatory tartaric acids; it is
customarily designated D- or L-, or (+)- or (–)-,
respectively, tartaric acid.
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     1.    Physical methods:
            - Spontaneous resolution
            - Inclusion compounds
            - Chromatography
2.        Chemical methods:
            - Diastereomeric salt formation
3.        Biochemical methods:
            - Enzymatic decomposition


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 Usual methods of separation such as fractional
 distillation, fractional crystallization or adsorption
 techniques cannot be used for the separation of
 enantiomers. Therefore, some special procedures are
 needed for resolution of racemic mixtures. Some of
 the more important methods are:

1 Preferential Crystallization
2 Biochemical Method
3 Resolution through the formation of
  diastereomers:  The Chemical Method
  Chromatographic    Method

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            crystallization          is closely       related
to mechanical separation            of crystals.
A   supersaturated            solution of the racemic
mixture is inoculated with a crystal of one of the
enantiomers or an isomorphous crystal of another
chiral compound. For example, when the saturated
solution of (±) sodium ammonium tartarate is seeded
with the crystal of one of the pure enantiomer or a
crystal of (–) asparagine, (–) sodium ammonium
tartarate crystalises out first.
This method is also called as entrainment
and the seed cystal is called entrainer.
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 Microorganisms              or     enzymes         are       highly
   stereoselective.
   Fermentation of (±) tartaric acid in presence of yeast or a
   mold, e.g., Pencillium glaucum . The (+) tartaric acid is
   completely consumed leaving behind (–) tartaric acid.
  (±) Amino acids can be separated using hog- kidney acylase
   until half of acetyl groups are hydrolysed away, only acetyl
   derivative of L- amino acid is hydrolysed leaving behind
   acetyl derivative of D-amino acid.
Limitations:
(i)) These reactions are to be carried out in dilute solutions, so
   isolation of products becomes difficult.
(ii)) There is loss of one enantiomer which is consumed by the
   microorganism. Hence only half of the compound is isolat d (
   partially destructive method) .

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Basic Principle
Step 1.
optically pure reagent (+) or (–)-B to give amixture of two
products which are diastereomers. The reagent (+) or (–)-B
is called the resolving agent.
(±) - A + (+)-B ,(+)A(+)B + (-)A(+)B

Step 2. The mixture         of diastereomers       obtained
above    can be separated using the methods of
Fractional distillation, fractional crystallization, etc.

Step 3. The pure diastereomers are then decomposed
each into the     corresponding enantiomer     and the
original  optically       active reagent, which    are
then separated.

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    (±) - Tartaric acid                    +               ()-Cinchonidine
    (Racemic modification)                               (Resolving agent)


               (+) - Tartaric acid - () - cinchonidine         Diastereomers
                                  +
               () - Tartaric acid - () - cinchonidine          (separable)
                                               Separated by crystallization


(+) - Tartaric acid - () - cinchonidine             () - Tartaric acid - () - cinchonidin

             Dil.H 2 SO 4                                                 Dil.H2 SO4

    (+) - Tartaric acid                                      () - Tartaric acid
     (crystallizes out)                                       (crystallizes out)

Similarly resolution of a (±) base with an optically active acid.
                                                                           HO
         (+) - base                                    (+) - (+) - salt            (+) - base
                          +   (+) - acid                                HO
         () - base                                    () - (+) - salt     () - base
                                                       Diastereom ers
          Racemic                                      (separable) www.remixeducation.in
          m odification
                                             widely
  used                                     both the
  enantiomers are obtained.
  successful    if the following conditions are
  fulfilled:
(i)          The resolving agent should be
  optically   pure.
(ii)  The substrate       (racemic    mixture) and the
      resolvingagent     should       have suitable
  functional groups      for reaction to occur.
(iii) The resolving      agent should be cheap and
  be           capable        of    regeneration    and
  recycling.
(iv) The resolving       agent should   be such which
      produces             easily         crysta llizable
  diastereomeric         products.
                       Asymetric synthesis:
▪ Whenever the achiral compound is treated with an achiral
  reagent it results in the formation of racemic mixture. This is
  due to the reason that there are equal probabilities for the
  formation of the two enantiomers. For example when pyruvic
  acid is reacted with hydrogen and nickel the product is racemic
  lactic acid. This is due to the fact carbonyl group is planar and
  the addition of hydrogen from either side of the carbonyl group
  is equally possible, As a result, we get 1:1 mixture of
  enantiomers and hence racemic mixture is obtained.




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▪ However, if pyruvic acid is first esterified with an optically
  active alcohol such as (-) methyl alcohol and the optically active
  thus obtained is first reduced and then hydrolysed, we get
  predominantly (-) lactic acid. This is due to the fact that the
  direction of attack by the reagent is determined both the chiral
  centre already present in the molecule.




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▪ Thus, a method of directly preparing an optically
  active isomer from an achiral molecule under the
  influence of some other optically active substance
  is called asymmetric synthesis.
▪ It may be mentioned here that even in asymmetric
  synthesis both (+) and (-) enantiomers are formed
  but one of these is obtained in predominating
  amounts so that the product exhibits a net optical
  rotation.


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1. Retention : Bond at chiral carbon do not break : Configuration
   of substrate and product remains same
2. Inversion : Follows SN2 Mechanism. Isomer gets converted to
   other form
3. Racemisation : Second chiral center forms Diasteromer (SS ,
   RS)



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Stereoisomers - Compounds that have the same molecular formula and
the same connectivity, but different arrangement of the atoms in 3-
dimensional space.
Stereoisomers cannot be converted into each other without breaking
bonds. Stereoisomers can be subdivided into two general categories:
enantiomers and diastereomers.

Enantiomers are Stereoisomers whose molecules are non super
imposable mirror images of each other.
Diastereomers - Stereoisomers which are not enantiomers (or mirror
images). Chiral or asymmetric carbon - A tetrahedral carbon atom
bearing four different substituents.
Chirality centers, or stereocenters - Asymmetrically substituted
atoms in a molecular structure.
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Meso compounds, or meso forms - Symmetric, or achiral molecules that contain
stereocenters. Meso compounds and their mirror images are not stereoisomers, since
they are identical.

Optical activity - The ability of chiral substances to rotate the plane of polarized light
by a specific angle.

Dextrorotatory - Ability of chiral substances to rotate the plane of polarized light to the
right.

Levorotatory - Ability of chiral substances to rotate the plane of polarized light to the
left.

Specific rotation - The measured angle of rotation of polarized light by a pure chiral
sample under specified standard conditions.
Racemic mixture, racemic modification, or racemate - A mixture consisting of equal
amounts of enantiomers. A racemic mixture exhibits no optical activity because the
activities of the individual enantiomers are equal and opposite in value, therby canceling
each other out.                                           www.remixeducation.in
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