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WEL- COME

WEL- COME. STEREOCHEMISTRY OF SOME ORGANIC COMPOUNDS AND DRUG MOLECULES. Dr. A.G. Nikalje. STEREOCHEMISTRY OF SOME ORGANIC COMPOUNDS. DEFINITION OF STEREOCHEMISTRY. Greek term stereos = solid. Science of organic chemistry. Deals with structure in three dimensions.

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WEL- COME

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  1. WEL- COME

  2. STEREOCHEMISTRY OF SOME ORGANIC COMPOUNDS AND DRUG MOLECULES Dr. A.G. Nikalje .

  3. STEREOCHEMISTRY OF SOME ORGANIC COMPOUNDS

  4. DEFINITION OF STEREOCHEMISTRY • Greek term stereos = solid. • Science of organic chemistry. • Deals with structure in three dimensions. • Based on the relationship between molecular structure and properties. Contd.

  5. Study of the static and dynamic aspects of the three dimensional shape of molecules. • Provides foundation for understanding structure and mechanism in organic chemistry.

  6. ABOUT ORGANIC COMPOUNDS Organic Compounds Hydrocarbons (H & C elements) Aliphatic Aromatic Cyclic Alkenes Alkynes Alkanes aliphatic H Benzene H H H C C H C C C H H H H Acetylene H Ethene Methane Cyclohexane

  7. HISTORY • Beginning of Organic Stereochemistry n French Physicist Jean-Baptiste Biot (1815) n Found certain organic compounds / molecules rotate the plane of the polarized light in solution or in gas. n This activity is called as Optical Activity. Contd.

  8. Louis Pasteur (1848) n Describe optical activity is due to the presence of some dissymmetric grouping of atoms in a molecule. n Experiments made on tartaric acid. n Confirmed asymmetric grouping (Chiral carbon atom) is required for optical activity. • Jacobus H. van’t Hoff and J.A. LeBel (1874) n Laid foundation of stereochemistry n Postulated asymmetric carbon model (tetrahedral arrangement) Contd.

  9. n Concept of three dimensional structure n Chemistry Nobel Prize 1901 (First recipient of prize) n Shape and size of Tetrahedral carbon atom in methane n Proposal of tetrahedral carbon base on n Evidence of Isomer number (Greek: isos = identical, meros = part) n Only the tetrahedral structure for methane agrees with the evidence of isomer number. Contd.

  10. Tetrahedral carbon shape and size

  11. CHIRAL CARBON ATOM • Greek word cheir = hand • A chiral object is notsuperimposable on its mirror image. • Any object that cannot be superimposed on its mirror image is said tochiral i.e. it possesses the property of ‘handedness’ (our left hand is the mirror image of our right hand but the two hands are not superimposable) [to superimpose the two hands plane has to be changed]. • Chiral objects – hand gloves, shoes.

  12. CHIRALITY/ASYMMETRY • Chirality or asymmetry • Chirality = handedness/phenomenon shown by chiral carbon atom. Lactic acid

  13. CHIRALITY/ASYMMETRY Lactic acid • Molecules that are not superimposable on their mirror images are chiral. • A carbon atom to which four different groups are attached is called chiral center/chiral carbon.

  14. ISOMERS • Greek word isos = identical, meros = part • Isomers are different compounds that have the same molecular formula. • Isomers are non-identical molecules with the same atomic composition.

  15. Isomers Stereoisomers Constitutional isomers Different functional group Positional isomers Stuructural isomers isomers O H C H C H C H 3 C H 3 3 3 C O H C O H C CH CH 3 2 C H CH 3 2 Br OH 2-Methylpropane Br CH 3 o-Hydroxybenzaldehyde (C H ) 2-Bromotoluene Benzoic acid 3-Bromotoluene 4 10 (C H O ) n-Butane (C H Br ) (C H O ) (C H Br ) 7 6 2 7 7 7 6 2 7 7 (C H ) 4 10

  16. ISOMERISM • ism suffix forming nouns – an action or its results • The phenomenon whereby a single molecular formula can represent more than one compound is called isomerism. Isomers of Glyceraldehyde (C3H6O3)

  17. STEREOISOMERS • Isomers studied with threedimensional aspects are called as stereoisomers. STEREOISOMERISM • The phenomenon whereby a single molecular formula can represent more than one compound in three dimensions is called stereoisomerism.

  18. STEREOISOMERISM • Stereoisomerism occurs in molecules with identical structures, by which is meant they have the same order of atoms but differ in their spatial arrangement (configuration) in relation with chiral carbon atom.

  19. Stereoisomers Configurational Conformational stereoisomers stereoisomers Absolute configuration Diastereomers Relative isomers Enantiomers Geometric isomers (mirror images) (not mirror images) (non superimposable) cis trans Z Meso structures E Optical isomers R S Threo Erythro L D Levo Racemic Dextro ( l )(-) ( dl ) (±) ( d ) (+) CLASSIFICATION OF STEREOISOMERS

  20. CONFIGURATION • Configuration - manner of arrangement/ shape/outline. • Arrangement of atoms that characterises a particular stereoisomer is called configuration. • Probability.

  21. CONFIGURATIONAL STEREOISOMERS • Configurational stereoisomers – manner of arrangement/shape/outline/attachment of atoms in three dimensions. • Configurational stereoisomers, interconverted by inversion (turning-inside-out) at a chiral center.

  22. ENANTIOMERS • Enantiomers Greek word enantio = opposite • Isomers that are mirror image of each other are called enantiomers. • Two stereoisomers that are not superimposablemirror image of each other. 2,3-Dichloropentane

  23. 1,2-Dichlorocyclopentane • Enantiomers have identical chemical properties except towards optically active reagents. • Have identical physical properties, except for the ‘chiral’ direction of rotation of the plane of polarised light.

  24. ENANTIOMERISM • The phenomenon of formation of enantiomers is called enantiomerism.

  25. OPTICAL ISOMERS • Optical visual; of or according to optics. • Optic of eye or sight. • Some substances rotates the plane of polarization and such substances are called optically active. • If the rotation of plane is to the right (clockwise) the substance is dextrorotatory (Latin: dexter = right) denoted by (d) or (+). • If the rotation of plane is to be left (counter clockwise) the substance is levorotatory (Latin: laevus = left) denoted by (l) or (-).

  26. OPTICAL ISOMERS • The two forms are called optical isomers. • Mirror image non superimposable. • Optical isomers also called enantiomers. (d) or (+) Lactic acid (l) or (-) Lactic acid • The phenomenon is called optical isomerism.

  27. OPTICAL ISOMERS (d) or (+) Glyceraldehyde (l) or (-) Glyceraldehyde

  28. RACEMIC MODIFICATION • A mixture of enantiomers is called a racemic modification. • Racemic modification is optically inactive if the mixture is 50:50. • When enantiomers are mixed together, the rotation caused by a molecule of one isomer is exactly cancelled by an equal and opposite rotation caused by a molecule of its enantiomer. • (dl) or (±) – lactic acid.

  29. DIASTEREOMERS • Dia means two. • For more than one chiral centers. • Stereoisomers that are not mirror images of each others are called diastereomers. • Diastereomers are stereoisomers but not enantiomers.

  30. DIASTEREOMERS 2,3-Dichloropentane (not mirror image) * chiral carbon atom

  31. DIASTEREOMERS • Diastereomers have different physical properties. • Chemical properties are not identical. • May be optically inactive. • The phenomenon of formation of diastereomers is called diastereoisomerism.

  32. MESO STRUCTURES • Molecules have symmetric end – the term Meso is used. • Meso means combining form middle, intermediate. • A Meso compound is one whose molecules are supeimposable on their mirror images even though they contain chiral centers. • A Meso compound is optically inactive.

  33. MESO STRUCTURES 2,3-Dichlorobutane • Superimposable; turned end-for-end a Meso compound (in same plane).

  34. MESO STRUCTURES • In meso structure one half of the molecule is the mirror image of the other half. 2,3-Dicholorobutane • In meso compounds two chiral centers have opposite configuration.

  35. GEOMETRIC ISOMERS • Science of properties and relations of lines (bond). • Not require chiral carbon atom. • The particular kind of diastereomers that owe their existence to hindered rotation about double bonds are called geometric isomers.

  36. GEOMETRIC ISOMERS • The configurations of the isomeric 2-butenes cis-2-Butene trans-2-Butene (not mirror image) • cis– (Latin: on this side or same side or identical groups) • trans – (Latin: across or opposite)

  37. GEOMETRIC ISOMERISM • The phenomenon of forming geometric isomers is called geometric isomerism. • A pair of geometric isomers, are, then diastereomers, as they are not mirror images. • Geometric isomers, interconverted – in principle – by rotation about a double bond. • Geometric isomers have different physical and frequently different chemical reactivities.

  38. GEOMETRIC ISOMERS CYCLIC COMPOUNDS cis-Decalin cis-Decalin trans-Decalin trans-Decalin

  39. GEOMETRIC ISOMERS Z AND E • ZGerman : Zusammen = together, same side • E German : entgegen = opposite, opposite side (Z)-1-Bromo- (E)-1-Bromo- 1-chloropropene 1-chloropropene Priority CH3 > H Br > Cl

  40. RELATIVE CONFIGURATION Erythro and Threo isomers • Used for molecules having non-symmetric ends. • Generally used in Fischer projection. • Not mirror images. • Resembles with diastereomers.

  41. Erythro and Threo isomers • Two like functional groups Same side Opposite side (Erythro) (Threo) Erythrose Threose

  42. RELATIVE • CONFIGURATION • D and L isomers • Used for molecules having non symmetric ends. • Orientation of –H and –OH on chiral carbon which is farther most from the primary chiral carbon. • Generally used in Fischer projection.

  43. D and L isomers D-Glucose L-Glucose

  44. ABSOLUTE CONFIGURATION R and S isomers • Suggested by Cahn, Ingold and Prelog (CIP). • R Latin : rectus = right • S Latin : sinister = left R.S. Cahn (The Chemical Society, London) Sir Christopher Ingold (University College, London)

  45. R and S isomers • V. Prelog (Eidgenossiche Technische Hochschule, Zurich) • CIP system most frequently used for designating absolute configurations of chiral compounds.

  46. R and S isomers • Sequence rules 1) Arrange the ligands associated with an element of chirality into order of priority. Priority Rules • Higher atomic number is given higher priority. • Higher atomic mass is given higher priority.

  47. R and S isomers cis>trans [(R, R)] or (S, S)] > [(R, S) or (S, S)]. I > Br > Cl 2) View the molecule with the lowest priority group pointing away from the viewer. 3) Count the remaining ligands in order of decreasing priority.

  48. R and S isomers • If the path traced is clockwise, the (R) absolute configuration is assigned. • If the path traced is counterclockwise, the (S) absolute configuration is assigned. RS Bromochloroiodomethane Priority I > Br > Cl > H

  49. Priority I > Br > Cl > H R and S isomers R S

  50. R and S isomers (-)-2-Butanol (+)-2-Butanol Relative configuration (R)-2-Butanol (S)-2-Butanol Absolute configuration Priority HO > CH3CH2 > CH3

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