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Stereochemistry

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Published in: Chemistry
11,514 Views

organic chemistry basics

Sunil N / Mumbai

8 years of teaching experience

Qualification: M.Pharma

Teaches: Bio Chemistry, Biology, Chemistry, Pharmacy

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  1. INTRODUCTION TO STEREOCHEMISTRY BY SUNIL PATIL
  2. NO Do the compounds have the same molecular formulae ? NO Not isomers I so mers Do the compounds have the same connectivity ? YES Isomers are compounds with the same molecular formula but not identical structures onstltutlona OH YES Geometric NO Co nfigu ratio nal Is the isomerism at a double bond ? NO Diastereomers Cl H3C H Hac Cl eremsomers Can the compounds be interconverted by rotation about single bonds ? O ptical Are the compounds n on-s upe rim po sa ble mirror images ? YES Co nform ational 3 YES Ena nti omers CH3CHz 3 CHzCH3 H3C Br Br
  3. NO Constitutio nal OH Do the compounds have the same molecular formulae ? NO I so mers Do the compounds have the same connectivity ? NO Not isomers YES Stereo isomers Can the compounds be interconverted by rotation about single bonds ? YES Constitutional isomers are isomers which have the same molecular formula but differ in the way their atoms are connected Is the isomerism at a double bond ? YES Geometric NO Diastereomers Cl H3C H Hac Cl O ptical Are the compounds n on-s upe rim po sa ble mirror images ? 3 YES Ena nti omers CH3CHz 3 CHzCH3 H3C Br Br
  4. Constitutional Isomers Constitutional isomers are isomers which have the same molecular formula but differ in the way their atoms are connected CH3 butane Chemical Formula: C4H10 CH3 CH Cl-13 2-methylpropane Chemical Formula: constitutional isomers CH3CH20H and ethanol CH3CH2CH2CH2CH3 pentane and CH30CH3 dimethyl ether CH3 CH3CHCH2CH3 isopentane CH3CH2CH2CH2Cl I-chlorobutane CH3CCH3 acetone and and CH3CH2CHCH3 2-chlorobutane CH3CH2CH propionaldehyde
  5. Drawing Constitutional or Structural Isomers of Alkanes Example 8.4 Write all the constitutional isomers having the molecular formula C6H14 start by c—c—c—c—c—c connecting —c the carbons —c in a line determine the C skeleton of the other isomers
  6. ?-?-? ?-?-? —? ? ? ? ? ? ? ?— ?— —?—? ? ? ? ?— ? ? ? ? ? ? ? ? ? ? ? ? —?—? ? ? ? ? ? ? ? ? ? ? ? spuoq ? ???? ???? 0) ? ? II!J ? 1?(..???? ] ? 1???I?(-.? ????? SRUJOS! ?? f8 ??????
  7. ?—? —?—? ? ?— ? ? ? ? -3 ? ? ? ?-?-? ? ? ? ? ? ? ? ? ? ? ? ? ? ?— ? ?—?_ ? ? ? —?—? ? ? —?—? ? ? ? ? ? ? ? ? ? )uasaJdaJ spua ??? ??? puaq ???? — ??????? ?????*? uoqno ? 0) ???? ?????? ? 1?(..???? ] ? 1???I?(-.? ????? SRUJOS! ?? f8 ??????
  8. NO Constitutio nal Do the compounds have the same molecular formulae ? NO Not isomers I so mers Do the compounds have the same connectivity ? YES Stereo isomers Can the compounds be Stereoisomers have the same molecular formula, maintain the same connectivity, but differ in the way their atoms are arranged in space Co nfigu ratio nal Is the isomerism at a double bond ? YES Geometric NO Diastereomers Cl H3C H Hac Cl O ptical Are the compounds n on-s upe rim po sa ble mirror images ? Co nform ational 3 YES Ena nti omers CH3CHz 3 CHzCH3 H3C Br Br
  9. NO Constitutio nal OH YES Do the compounds have the same molecular formulae ? NO I so mers Do the compounds have the same connectivity ? NO Co nfigu ratio nal Is the isomerism at a double bond ? Not isomers YES Stereoisomers Can the compounds be interconverted by rotation about single bonds ? ti al YES Co nform ational Conformational isomers (or conformers or rotational isomers or rotamers) are stereoisomers produced by rotation about single bonds, and are often rapidly interconverting at room temperature Diastereomers Cl H3C H Hac Cl Br Br CH3 CH Ena nti omers CHzCH3 H3C
  10. Conformations of Alkanes and Cycloalkanes Conformations of Ethane 3 ethane Staggered conformation of ethane Eclipsed conformation of ethane Newman Projection Sawhorse Representation Newman Projection Sawhorse Representation
  11. Conformations of Alkanes and Cycloalkanes Conformations of Butane CH3 H3C butane Staggered conformation of butane CH3 CH3 CH3 Eclipsed conformation of CH3 H3C CH3 butane CH3 CH3
  12. NO Constitutio nal OH Do the compounds have the same molecular formulae ? NO I so mers Do the compounds have the same connectivity ? NO Co nfigu ratio nal Is the isomerism at a Not isomers YES Stereo isomers Can the compounds be interconverted by rotation about single bonds ? YES Co nform ational H3C Configurational Isomers are stereoisomers that do not readily interconvert at room temperature and can (in principle at least) be separated. NO Diastereomers Cl H3C H Hac Cl Are the compounds n on-s upe rim po sa ble mirror images ? CH3CHz YES Ena nti omers 3 CHzCH3 H3C Br Br
  13. NO Constitutio nal OH YES Geometric Do the compounds have the same molecular formulae ? NO I so mers Do the compounds have the same connectivity ? NO Configurational Is the isomerism at a double bond ? NO Not isomers YES Stereoisomers Can the compounds be interconverted by rotation about single bonds ? O ptical Are the compounds n on-s upe rim po sa ble mirror images ? YES Co nform ational 3 YES Geometric isomers are configurational isomers that differ in the spatial position around a bond with restricted rotation (e.g. a double bond): H Hac Cl CH3CHz 3 CHzCH3 H3C Br Br
  14. Geometric (Cis and Trans) Isomers Geometric (Cis and Trans) Isomers result from restriction rotation Compounds with double bonds cis isomer— have same substituents on the same side of the double bond (= Z with more complex molecules having high priority groups on the same side) trans isomer — have the same substituents on the opposite side of the double bond (= E with more complex molecules having high priority groups on opposite sides) Compounds with bonds in a ring: cis isomer — have the same substituents on the same side of the ring trans isomer - have the same substituents on the opposite side of the ring
  15. Cis-trans (Geometric) isomerism in Alkenes cis-1,2-dichloroethene (Z)-1,2-dichloroethene trans-I,2-dichloroethene (E)-1,2-dichloroethene
  16. Cis-trans (Geometric) isomerism in Alkenes *If one of the two carbon atoms of the double bond has two identical substituents, there are no cis-trans isomers for that molecule 1, I-dichloroethene
  17. Naming cis and trans compounds Example 11.4 Name the following geometric isomers. H3C H3C H3C CH3 trans-3,4-dichloro-3-heptene CH2 CH3 CH3 cis-3,4-dimethyl-3-octene
  18. Identifying Geometric Isomers Example 11.5 Determine whether each of the following molecules can exist as cis-trans isomers: (1) I-pentene (2) 3-ethyl-3-hexene (3) 3-methyl-2-pentene CH2CH2CH3 I-pentene CH3 H 3CH2C cis-3-methyl-2-pentene H3CH2C CH2CH3 CH2CH3 3-ethyl-3-hexene H 3CH2C CH3 trans-3-methyl-2-pentene
  19. Cis and Trans Fatty Acids OH cis-9-octadecenoic acid OH trans-9-octadecenoic acid
  20. Cis-trans (Geometric) isomerism in Cycloalkanes Cis-trans isomers are molecules having the same arrangement of atoms but differ in the spatial orientation of their substituents. cis-1,2-dichlorocyclohexane trans-I,2-dichlorocyclohexane
  21. Naming cis-trans Isomers of Substituted Cycloalkanes Example 10.6 Determine whether the following susbstituted cycloalkanes are cis or trans isomers. CH3 CH3 trans-I,2-dimethylcyclopentane CH3 CH3 CH3 CH3 cis-1,2-dimethylcyclopentane CH3 CH3
  22. Conformations of Alkanes and Cycloalkanes Conformations of Cyclohexane cyclohexane Chair conformation of cyclohexane Boat conformation of cyclohexane
  23. Chair conformation of cyclohexane Boat conformation of cyclohexane
  24. Chair-Chair Interconversion axial H equatorial equatorial chair—chair interconversion axial
  25. NO Constitutio nal OH YES Geometric Do the compounds have the same molecular formulae ? NO I so mers Do the compounds have the same connectivity ? NO Co nfigu ratio nal Is the isomerism at a double bond ? NO Not isomers YES Stereo isomers Can the compounds be interconverted by rotation about single bonds ? O ptical Are the compounds n on-s upe rim po sa ble mirror images ? YES Co nform ational 3 YES Optical isomers are configurational isomers that differ in the 3D relationship of the substituents about one or more atoms. CH3CHz CHzCH3 3 H3C Br Br
  26. NO Constitutio nal OH YES Geometric Do the compounds have the same molecular formulae ? NO I so mers Do the compounds have the same connectivity ? NO Co nfigu ratio nal Is the isomerism at a double bond ? Not isomers YES Stereo isomers Can the compounds be interconverted by rotation about single bonds ? O ptical YES Co nform ational 3 Diastereomers are optical isomers (stereoisomers) that are not enantiomers. NO Diastereomers Cl H3C H Hac Cl YES Ena nti omers CH3CHz 3 CHzCH3 H3C Br Br
  27. NO Constitutio nal OH Do the compounds have the same molecular formulae ? NO Not isomers I so mers Do the compounds have the same connectivity ? YES Stereo isomers Can the compounds be interconverted by rotation about single bonds ? NO Co nfigu ratio nal YES Co nform ational Enantiomers are optical isomers that are non-superimposable mirror images. YES Geometric ou e on NO Diastereomers Cl H3C H Hac Cl O ptical Are the compounds n on-s upe rim po sa ble mirror images ? YES Ena nti omers CH3CHz 3 CHzCH3 H3C Br Br
  28. Chirality chiral objects right hand left hand achiral objects Chiral objects are objects with left-handed and right-handed forms Achiral objects - objects that have superimposable mirror images Nonsuperimposable mirror images - a mirror image that is not the same as the image itself - chiral objects have nonsuperimposab/e mirror images
  29. Assymetric Center Chirality is not reserved just for objects - molecules can be chiral Chiral molecules - generally molecules containing an asymmetric center Asymmetric (chiral) center - tetrahedral atom bonded to four different groups - indicated with an asterisk ( * ) an asymmetric center CH3CH2CH2CHCH2CH2CH2CH3 OH 4-octanol CH3CHCH2CH3 2-bromobutane CH3 CH3CHCH2CHCH2CH3 CH3 2,4-dimethyIhexane
  30. Chiral Molecules with One Asymmetric Center Molecules with one chiral center would have 2 enantiomers *Chiral molecules would have 2n enantiomers (where n is the number of chiral centers) CH3CHCH2CH3 2-bromobutane CH3CH2 CH3 CH2CH3 CH3 mirror the two isomers of 2-bromobutane enantiomers
  31. How to Represent Enantiomers: Perspective Formulas Perspective Formulas - shows two bonds of the asymmetric center as lines in the plane of the paper, another bond as a solid wedge protruding forward out of the paper, and the fourth bond as a hatched wedge extending behind the paper CH2CH3 CH3CH2 perspective formulas of the enantiomers of 2-bromobutane
  32. Fischer Projections Fisher Projection - representation of an asymmetric center as the point of intersection of two perpendicular lines Horizontal lines represent bonds that project out of the plane of the paper Vertical lines represent bonds that extend back from the plane of the paper away from the viewer
  33. Fischer Projections CH2CH3 CH3CH2 perspective formulas of the enantiomers of 2-bromobutane H3CH2C CH2CH3 CH3
  34. Naming Enantiomers -R,S System STEP 1. Rank the groups/atoms bonded to the asymmetric center in order of priority. - rank in terms of atomic mass. Higher atomic mass, higher priority. - let's have I-chloro-l-ethanol as an example. CHs 1. Chlorine = 36 amu 2. Oxygen = 16 amu 3. Carbon = 12 amu 4. Hydrogen = 1 amu STEP 2. Rotate the molecule so that the lowest priority group is pointing away from the read Prioritize the substituent groups CH3 HO Rotate lowest priority group to back CHs HO @ CH3 Cl OH
  35. Naming Enantiomers -R,S System Step 3. Trace your finger around the three highest priority groups in order of the priority. - If the circle is moving counterclockwise the CIP designation is "S". If the circle is moving clockwise the CIP designation is "R". CH3 Counterclockwise order of priority indicates the S absolute configuration Step 4. Name the enantiomer in terms of its R or S configuration. - the name for this isomer is (S)-l-chloro-l-ethanol
  36. Naming Enantiomers -R,S System Assigning Priorities to Groups 1. 2. 3. 4. Oxygen (from CH20H) = 16 Carbon (from CH2CH3) = attached to carbon Carbon (from CH3) = attached to hydrogen Hydrogen = 1 Figure 4. Some example groups in order of priority (highest to Iou est). CH3 -Cl-la CH3 CHO OH INH2 H3C-C-OH H-C-OH -I-LCF Highest Figure 5. Some groups with double bonds H 40 Highest Aldehyde OH H2C Alcohol 01-12 Alke ne Lowest CH3 Lowest
  37. Naming Enantiomers -R,S System -o Aldehyde (2) Flac s -o H-C c-c c-c o o Alkene (3) Alkene (4) c-c-c H Ketone (1 )

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