streochemistry by k.v. murali krishna lecturer in chemisry silver jubilee govt. college (a) kurnool...
TRANSCRIPT
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STREOCHEMISTRY
By
K.V. Murali Krishna
Lecturer in Chemisry
Silver Jubilee Govt. College (A)
KURNOOL - 518002
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ISOMERISM
• The phenomenon of existence of various compounds with identical molecular formulae.
• The individual organic compounds are called ‘Isomers’.
CLASSIFICATION
• Structural Isomerism
• Stereoisomerism.
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STRUCTURAL ISOMERISM
• This type of Isomerism is observed in Organic compounds with identical molecular formulae, but differing in their structural formulae.
TYPES
• a. Chain Isomerism
• b. Position Isomerism
• c. Functional Isomerism
• d. Metamerism
• e. Tautomerism
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A. CHAIN ISOMERISM
• The type of Isomerism arises due to difference in the Carbon chain structure in organic compounds with identical molecular formulae.
• The chain Isomers of a compound are designated as
• n- ( normal)
• iso- (one side chain or with a tertiary Carbon)
• neo- ( two side chains or with a quaternary Carbon)
• Ex: The Chain isomers of Pentane.
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32223 CHCHCHCHCH epenn tan
323 CHCHCHCH
3
|
CH
epeniso tan
CH3 3CHC
3CH
3CH
epenneo tan
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B. POSITION ISOMERISM
This type of isomerism arises due to difference in the position of a substituent or a double or a triple bond in organic compounds with identical molecular formulae.
Ex; Position isomers of Idopropane and Butene.
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iodidepropyln
ICHCHCH
223
iodidepropyliso
CHCHICH
33
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FUNCTIONAL ISOMERISM
This type of Isomerism arises due to
difference in the nature of functional group
In Organic compounds with
identical molecular formulae.
Ex; Functional Isomers of molecular formula C3H6O.
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deopanaldehy
CHOHC
Pr52
Acetone
COCHCH 33
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D. METAMERISM
• This type of isomerism arises due to difference in the nature of Alkyl groups present on either side of a functional group in Organic compounds with identical molecular formulae.
• The isomers of this type are called ‘Metamers’.
• Ex;(a). The Metamers of molecular formula C4H10O.
• (b). The Metamers of molecular formula C5H10O.
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etherDiethyl
HOCHC 5252
etheropylnMethyl
CHCHCHOCH
Pr3223
etherpropylisoMethyl
CHCHOCH
333
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52HC
C
52HC
ketoneDiethyl
3CH
223 CHCHCH
OC
ketonepropylnMethyl
CHCH
OC
CH
23
3
ketoneisopropylMethyl
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E. TAUTOMERISM
• This type of Isomerism arises due to the wandering of a labile Hydrogen atom between two polyvalent atoms within the molecule.
• The isomers of this type are called ‘Tautomers’.
• Ex, (i) Nitroalkane and Isonitroalkane
• (ii). Cyanic acid and Isocyanic acid.
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kaneIsonitroaleNitroalkan
OO
NCHRNCHR
OHO
2
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acidanicIsohydrocyacidcHydrocyani
CNHNCH |
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(iii). KETO-ENOL TAUTOMERISM
• The Acetoaceticester exhibits the properties of both Ketones and Unsaturated hydroxyl compound.
• This dual behaviour can be explained by assuming that it exists in two isomeric forms namely,
• Keto form and
• Enol form.
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STREOISOMERISM: (SPACE ISOMERISM)• This type of isomerism is exhibited by two or more
compounds • with the identical molecular and structural formulae,
but with different spatial arrangements of atoms or groups.
• The Isomers of this type are called ’Stereomers’. The CLASSIFICATION
• Conformational Isomerism • Configurational isomerism.
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CONFORMATIONAL ISOMERISM
• In this type of Isomerism, the various structures are non super imposable and are easily inter convertible by rotation about a single bond.
• The isomers of this type are called Conformational diastereomers or rotational isomers or Conformations. Ex, Alkanes
• An Alkane molecule can have three different Conformations namely,
• Staggered• Eclipsed • Skew or Gauche.
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• Eclipsed Conformation:-
• The three groups or Hydrogen atoms attached to the front Carbon will be exactly in front of those attached to the rear Carbon atom.
• Staggered Conformation:- • All the six groups or Hydrogen atoms attached to the
two Carbon atoms are clearly seen from the front side of the molecule.
• Skew Conformation:-• This is an intermediate conformation to that of
Eclipsed and Staggerted Conformations.
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CONFORMATIONS – MODELS
(a). BALL AND STICK MODEL:
• This model is quite helpful to visualize the relative positions of different atoms of rotational Conformations.
• But this model only gives a two dimensional picture
. Ex, Conformations of Ethane
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(b). SAWHORSE MODEL• This model is quite easy to draw.
• It gives a three dimensional picture. • The molecule is viewed slightly from above and from
the right.
• The lower left hand Carbon is always taken to be towards the front.
• The Sawhorse drawings for Staggered and Eclipsed Conformations of Ethane are given below.
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Staggered eclipsed
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( c). NEWMAN PROJECTION FORMULAE
• The front Carbon atom is represented by a point.
• The three bonds linking groups or atoms with this Carbon atom are indicated by three lines radiating from this point.
• The rear Carbon atom is represented by a circle.
• Its three bonds are shown by three lines radiating from the edge of the circle.
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Staggered eclipsed
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II. CONFIGURATIONAL ISOMERISM
• In this type of Stereo isomerism the two structures are non super imposable and non inter convertible by rotation around the single bond.
Types
• (i). ENANTIOMERS: These are the mirror images of each other and they are also called Optical or Inversional Isomers. Ex, Enantiomers of Lactic acid.
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OH
2CHOCHO COOHCH 2
OHCO
OH
OCHO
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(ii). DIASTREOMERS
• The Configurational Optical isomers which are not mirror images of each other are known as ‘Diastereomers’.
• Ex, Diastereomers of Beta-dibromocinnamic acid.
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(iii). GEOMETRICAL ISOMERS
• The isomers of this type differ in the arrangement of atoms or groups around two doubly bonded Carbon atoms.
• These isomers are also called ‘Cis-trans Isomers’.
• Ex, Cis-teans isomers of dibromoethene.
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BrCH
BrCH
||
HCBr
BrCH
||
053..
2,1
pm
cisenedibromoeth 065
2,1
transenedibromoeth
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GEOMETRICAL ISOMERISM
• This type of Isomerism arises due to difference in the arrangement of atoms or groups around two doubly bonded Carbon atoms in Organic compounds with identical molecular and structural formulae.
• The Isomers of this type are called Geometrical or Cis-trans isomers. In a Cis-isomer similar atoms or groups lie on the same side of the molecule and in a trans isomer similar atoms or groups lie on either side of the molecule.
• Ex, (i). Cis-trans isomers of 2-butene.
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3
3
||
CHCH
CHCH
CHCCH
CHCH
3
3
||K673
Butenecis 2 Butenetrans 2
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• (ii). 1,2-dichloroethene
• The Geometrical Isomerism cannot exist in Alkene molecule if either Carbon carries two identical groups.
• Thus Propylene, 1-butene and Isobutylene do not exhibit Geometrical isomerism.
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heneDichloroet2,1cis trans
060..pb 048..pb080.. pm
050.. pm
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E-Z-CONFIGURATION
• The Geometrical isomers of a molecule of molecular formula abC=Ccd cannot be shown in the form of cis and trans isomers, but can be assigned E-Z-configuration.
• A priority is assigned between a and b groups of first carbon atom and
• also between c and d groups of the second carbon atom in accordance with Cahn-Ingold-Prelog sequence rules.
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• The isomer with higher priority groups of both the carbon atoms lying on the same side of the molecule is assigned ‘Z’-configuration ( Zusammen in German meaning on the same side)
• The other isomer with higher priority groups of both the carbon atoms lying on either side of the molecule is assaigned ‘E’- configuration ( Entegegen in German meaning opposite).
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ClBr
C
C
HCH3
BrCl
C
C
HCH3
Z EenechloropropBromo 11
ClBr
HCH
3
ClH
C
C
BrCH3
HCl
C
C
BrCH3
Z EenechloropropBromo 12
HCl
CHBr
3
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CAHN-INGOLD-PRELOG SEQUENCE RULES:
• 1. The group with higher molecular complexity should be given higher priority.
• 2. In the case of individual atoms, the atom with higher atomic number should be given higher priority.
• 3. If the first atoms of the groups are identical then the atomic numbers of the second atoms should be considered for assigning the priority of the groups.
• Ex: Z and E-configurations of 1-bromo-1-chloropropene.
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OPTICAL ISOMERISM• The phenomenon in which compounds with similar
chemical and physical properties differ only in their behavior towards plane polarized light optical activity) is known as ‘Optical Isomerism’.
• The isomers of this type are called Optical isomers.
• The type of light, whose vibrations occur in only in a single plane, is known as ‘Plane polarized light’.
• The plane polarized light can be obtained by passing the ordinary light through a Nicol prism.
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• The optically active substances can rotate the plane polarized light either towards right or towards left by a certain angle.
• The Optical active substances which rotate the plane polarized light towards right are called Dextrorotatory (d or +) and
• those which rotate the plane polarized light towards left are called ‘Laevorotatory’ (l or -).
• The Optical activity of a substance is measured by using a Polarimeter.
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POLARIZER
SOURCE
LIGHT
SAMPLE
TORYDEXTROROTA
CONTAINING
TUBE ANALYZER
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• The Polarimeter consists of two Nicol prisms, which are called Polarizer and Analyzer.
• The Nicol prism is made by joining two prisms by means of a special adhesive known as ‘Canada balsam’.
• A space is provided in between the two Nicol prisms for inserting a tube containing the sample liquid or solution, whose Optical activity is to be determined.
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SPECIFIC ROTATION• This is defined as the observed rotation, when the
polarized light is passed through one decimeter (10 cms) of the solution with a concentration of one gram per milliliter.
• The + or – signs denoted along with angle of rotation indicates the direction of rotation. The (-) sign denotes the rotation towards left, while (+) sign denotes the rotation towards right.
• Specific rotation [ α]dt = αobs / l .C
• Where αobs = Experimental rotation
• l = length of the solution in decimeters
• C = grams of substance per milliliter of solution
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• The magnitude of rotation depends upon the following factors:
• (a). Nature of the substance.
• (b). concentration of the solution.
• (c). Length of the sample tube.
• (d). Nature of solvent.
• (e). the temperature of the solution.
• (f). the wavelength of the light used. Generally the light used is Sodium D-line of wavelength 589 Millimicrons.
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CHIRAL CARBON OR CHIRAL CENTER• A Carbon atom which is linked to four different mono
valent atoms or groups is known as ‘Chiral Carbon’ or ‘asymmetric Carbon’ or a ‘Chiral Center’ (Cheir in Greek means hand, which has no symmetry).
• The Chiral Carbon is responsible for the Optical activity of substances.
• A molecule with a Chiral Carbon is called a chiral or asymmetric or dissymmetric molecule.
• But meso-tartaric acid with two Chiral Carbons is ‘Achiral’ (Symmetric).,
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ELEMENTS OF SYMMETRY - CHIRALITY
Features of the Chiral molecule,
• A molecule with one Chiral center is always Chiral.• A molecule with more than one Chiral centers may be
Chiral or achiral.• A Chiral molecule should not have the following three
Elements of symmetry,• Plane of symmetry• Center of symmetry• Axis of Symmetry
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• The Plane of symmetry is a plane that divides the molecule in ti two equal halves.
• A chiral molecule has no Plane of Symmetry.• A hand is a chiral object and the ball is an Achiral
object.
AchiralChiral
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• Center of symmetry is a point from which if lines are drawn on any group, on both sides to an equal distance, it divides the molecule in to two equal halves.
• Axis of symmetry – Two types.• Simple axis of symmetry (Cn): Symmetry operation
means, an operation which produces an orientation identical with that of the original.
• A molecule when rotated through an angle of 360 0, if presents identical appearance by ‘n’ times then the axis is called n-fold axis .
• Ex, Two, three, four and six fold axes are observed in crystals.
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AxisFoldFour AxisFoldTwo
AxisFoldThree AxisFoldFive
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COOH
OHCH
OHCH
COOH
|
|
|
|
Symmetry
Plane
)
(
symmetryofplaneato
dueinactiveOptically
acidTartaricmeso
Symmetry
ofCentre
HOOC
COOHH
H
)
(
4,1
symmetryofcentrea
ofpresencethetodueinactiveOptically
acidicdicarboxyleCyclohexanTrans
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’Alternating axis of symmetry’
• The rotation of the molecule by 360/n 0 about an axis and then its reflection through a plane perpendicular to the axis of rotation produces an orientation identical with the original.
• This is ‘Improper rotation’ and such an axis is called ’Alternating axis of symmetry’ (Sn) ( S = Spiegelung, Spiegel = mirror)
• Ex, 1,2,3,4 - tetramethylcyclobutane
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a b c
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ENANTIOMERS
• The non super imposable pairs of a compound and its mirror image. ( Enantio- Gk = opposite, morph = form).
• Ex, The enantiomers of Lactic acid
Characteristics
• Physical properties are identical.
• Differ in their action on plane polarized light.
• Laevo (l or - ), Dextro ( d or +)
• Possess identical Chemical properties except when they are reacting with optically active reagents.
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COOHCOOH
HH
OH HO
3CH 3CH
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• Ex, The dextro Tartaric acid gets consumed by the mould penicillium glaucum, but not the laevo tartaric acid.
• The racemic form is an equi molecular mixture of the dextro and laevo forms of a compound.
• It is inactive due to internal compensation of rotation.
• Enantiomers are represented either as Tetrahedral models or as Fischer projection formulae.
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)( tetrahedraorformulaeePerspectiv
formulaeojectionFischer Pr
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Single chiral Carbon – Lactic acid• The central carbon atom in lactic acid is linked
to four different groups namely,
• Hydrogen, Hydroxyl, methyl and Carboxylic acid.
• Three Isomers of Lactic acid are
• d or (+) – Lactic acid ( + 2.24o )• l or (-) – Lactic acid ( - 2.24o )
• dl – Lactic acid –Optically inactive due to external compensation of rotation.( 0.00 )
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COOHCOOH
HH
OH HO
3CH 3CH
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NUMBER OF ENANTIOMERS
• 2n will be the number of isomers for a molecule with ‘n’ number of Chiral Carbons.
• These can be shown as 2n-1 pairs of enantiomers and same number of racemic modifications.
• Ex, dibromocinnamic acid with two Chiral centers exists in four Optically active forms.
• Enantiomers: I and II , III and IV = 2 Pairs• Racemic modification: Equi mloecular mixtures of I
and II, III and IV.
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56
|
|
|
HC
BrH
BrH
COOH
56
|
|
|
HC
HBr
HBr
COOH
56
|
|
|
HC
BrH
HBr
COOH
56
|
|
|
HC
HBr
BrH
COOH
sEnantiomer
sEnantiomer
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Two Similar Chiral Carbons – Tartaric acid
• Tartaric acid with two similar Chiral Carbons exists in the forms given below,
• d or (+) – Tartaric acid ( + 12o )
• l or (-) – Lactic acid ( - 12o )
• Meso – Tartaric acid, optically inactive due to internal compensation of rotation.
• dl – Lactic acid –Optically inactive due to internal compensation of rotation.( 0.00 )
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COOH
OHCHO
OHCH
COOH
|
|
|
COOH
OHCH
HCHO
COOH
|
|
|
COOH
OHCH
OHCH
COOH
|
|
|
|
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Two dissimilar Chiral Carbons – 2,3-dibromopentane:
• It exists in four optically active forms.
• Enantiomers/ Racemic modifications: I and III, III and IV.
• Diastereoisomers: I and III, I and IV, II and III, II and IV
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RACEMIZATION
• The process of converting an optically active dextro or laevo compound in to Racemic modification by means of heat, light and Chemical reagents.
• Racemization involves the formation of an unstable ‘enolic’ intermediate which is responsible for the loss of chirality.
• Lactic acid• Malic acid• Tartaric acid.
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COOH
OHCH
CH
|
|2
OHCHO
OHC
CH
||
|3
acidLacticd
ytemporaril
disappears
Chirality
formEnolic
COOH
OHCH
CH
|
|3
COOH
OHCHO
CH
|
|3
acidLacticd acidLacticl
form
Racemic
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RESOLUTION
• The process of separation of the Racemic modification in to two pure enentiomers.
METHODS• Mechanical separation:- By Pasteur. The individual
isomers of Sodium ammonium tartarate can be picked by forceps while observing under a magnifying lens.
• Bio-chemical separation:- The bacteria, yeast, moulds and fungi when grown in a dilute solution of Racemic modification selectively digest one isomer and do not affect the other isomer.Penicillin glaucum digests dextro Ammonium tartarate.
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• Diastereoisomers formation:- An optically active acid or base is used to convert Racemic modification in to diastereo isomers ( salts).
• These salts are separated by fractional crystallization.
• Then they are hydrolysed by acids or bases to obtain original active compounds.
• Dacid + Lacid + 2Dbase ( Dacid – Dbase) + ( Lacid – Dbase)
Racemic acid Active base Diastereo isomers
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• Selective adsorption:-
• The optically active substances get selectively adsorbed by certain optically active acids such as Camphor sulfonic acids, Methoxy acetic acid, Tartaric acid, Malic acid and so on .
• The active constituents of Racemic Camphor can be separated by selective adsorption over dextro lactose adsorbent.
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Specification of configuration
D – L – Notation• Glyceraldehyde is taken as the reference.
• D – Glyceraldehyde - -OH on right hand side and –H on the left hand side.
• The compounds which can be produced from D – Glyceraldehyde or which can be converted in to D – Glyceraldehyde are assigned D – configuration.
• The compounds which can be produced from L – Glyceraldehyde or which can be converted in to L – Glyceraldehyde are assigned L – configuration.
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CHO
HOH
OHCH 2
CHO
HOH
OHCH 2
I
hydeglyceralde II
hydeglyceralde
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OHCH
OHCH
CHO
2
|
|
OHCH
OHCH
COOH
2
|
|
hydeglyceraldeD acidGlycericD
O
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OHCH
OHCH
CHO
2
|
|
BrCH
OHCH
COOH
2
|
|
2
|
|
CH
OHCH
CHO
hydeglyceraldeD
3PBrii
oxidationi reduction
acidLacticD
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R & S Notation ( Rectus and Sinister System)
Cahn – Ingold – Prelog system: Assigning the configuration to O. Isomers;
1, The four different groups attached to the chiral carbon are assigned priority in accordance with the sequence rules.
a. If four different atoms are linked to a Chiral Carbon, The atom with maximum atomic number gets highest priority.
Ex, I > Br > Cl > H.
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ClI
CH
Br
|
ICl
CH
Br
|
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(b). If the first atoms of two or more groups are same then the atomic number of the next atom is considered for assigning the order of priority.
Ex, 2 - butanol
OHandCHHCHCH 323 ,,
OH
CHCCHCH
H
|
|
323
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( c ) . A doubly or triply bonded atom ‘x’ is considered as equivalent to two or three “x’s.
But –X > =x . Examples,
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OHCH
OHCH
OCH
2
|
|
OC
H
OC
H
|
|
toequivalentis
O
OC
H
|
|
OHCHOHH 2,,
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toequivalent toequivalent
CHCHC
C
|
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II. Assigning the configuration:
• The optical isomer in which the different groups are arranged clockwise in the decreasing order of priority is assigned ‘R’ ( rectus = right) configuration.
• The optical isomer in which the different groups are arranged anti clockwise in the decreasing order of priority is assigned ‘S’ ( sinister = left ) configuration.
• The group of least priority is supposed to be away from the rest of the molecule.
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3
|
CHHO
CH
COOH
OHCH
CH
COOH
2
|
R S
acidLactic