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Chapter 9 INTRODUCTION TO STEREOCHEMISTRY

Chapter 9 INTRODUCTION TO STEREOCHEMISTRY. Isomers are compounds with the same molecular formula but not identical structures. Constitutional isomers are isomers which have the same molecular formula but differ in the way their atoms are connected. Constitutional Isomers.

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Chapter 9 INTRODUCTION TO STEREOCHEMISTRY

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  1. Chapter 9INTRODUCTION TO STEREOCHEMISTRY

  2. Isomers arecompounds with the same molecular formula but not identical structures

  3. Constitutional isomers are isomers which have the same molecular formula but differ in the way their atoms are connected

  4. Constitutional Isomers Constitutional isomers are isomers which have the same molecular formula but differ in the way their atoms are connected

  5. Drawing Constitutional or Structural Isomers of Alkanes Example 8.4 Write all the constitutional isomers having the molecular formula C6H14

  6. Example 8.4 Write all the constitutional isomers having the molecular formula C6H14

  7. Example 8.4 Write all the constitutional isomers having the molecular formula C6H14

  8. Stereoisomershave the same molecular formula, maintain the same connectivity, but differ in the way their atoms are arranged in space

  9. 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

  10. Conformations of Alkanes and Cycloalkanes Conformations of Ethane ethane Staggered conformation of ethane Eclipsed conformation of ethane Newman Projection Sawhorse Representation Newman Projection Sawhorse Representation

  11. ethane Staggered conformation of ethane Eclipsed conformation of ethane Newman Projection Sawhorse Representation Newman Projection Sawhorse Representation

  12. Conformations of Alkanes and Cycloalkanes Conformations of Butane butane Staggered conformation of butane Eclipsed conformation of butane

  13. Conformations of Alkanes and Cycloalkanes Staggered conformation of butane Eclipsed conformation of butane

  14. Configurational Isomers are stereoisomers that do not readily interconvert at room temperature and can (in principle at least) be separated.

  15. Geometric isomers are configurational isomers that differ in the spatial position around a bond with restricted rotation (e.g. a double bond):

  16. Geometric (Cis and Trans) Isomers • Geometric (Cis and Trans) Isomers result from restriction rotation • Compounds with double bonds • cisisomer – 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

  17. Cis-trans (Geometric) isomerism in Alkenes cis-1,2-dichloroethene trans-1,2-dichloroethene (E)-1,2-dichloroethene (Z)-1,2-dichloroethene

  18. 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 1,1-dichloroethene

  19. Identifying cis and trans isomers of Alkenes Example 11.3 Two isomers of 2-butene are shown below. Which is the cis isomer and which is the transisomer cis-2-butene trans-2-butene

  20. Naming cis and trans compounds Example 11.4 Name the following geometric isomers. cis-3,4-dimethyl-3-octene trans-3,4-dichloro-3-heptene

  21. Identifying Geometric Isomers Example 11.5 Determine whether each of the following molecules can exist as cis-trans isomers: 1-pentene 3-ethyl-3-hexene 3-methyl-2-pentene 1-pentene 3-ethyl-3-hexene cis-3-methyl-2-pentene trans-3-methyl-2-pentene

  22. Cis and Trans Fatty Acids cis-9-octadecenoic acid trans-9-octadecenoic acid

  23. Cis and Trans Fatty Acids

  24. 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-1,2-dichlorocyclohexane

  25. Naming cis-trans Isomers of Substituted Cycloalkanes Example 10.6 Determine whether the following susbstitutedcycloalkanes are cis or trans isomers. trans-1,2-dimethylcyclopentane cis-1,2-dimethylcyclopentane

  26. Conformations of Alkanes and Cycloalkanes Conformations of Cyclohexane cyclohexane Chair conformation of cyclohexane Boat conformation of cyclohexane

  27. Chair conformation of cyclohexane Boat conformation of cyclohexane

  28. Chair-Chair Interconversion

  29. Optical isomers are configurational isomers that differ in the 3D relationship of the substituents about one or more atoms.

  30. Diastereomers are optical isomers (stereoisomers) that are not enantiomers.

  31. Enantiomers are optical isomers that are non-superimposable mirror images.

  32. Chirality 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 nonsuperimposable mirror images

  33. 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 (*)

  34. Chiral Molecules with One Asymmetric Center Molecules with one chiral center would have 2 enantiomers *Chiral molecules would have 2nenantiomers (where n is the number of chiral centers)

  35. ChiralvsAchiral

  36. 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

  37. 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

  38. Fischer Projections

  39. 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 1-chloro-1-ethanol as an example. • Chlorine = 36 amu • Oxygen = 16 amu • Carbon = 12 amu • Hydrogen = 1 amu • STEP 2. Rotate the molecule so that the lowest priority group is pointing away from the • reader.

  40. 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”. Step 4. Name the enantiomer in terms of its R or S configuration. - the name for this isomer is (S)-1-chloro-1-ethanol

  41. Naming Enantiomers -R,S System • Assigning Priorities to Groups • Oxygen (from CH2OH) = 16 • Carbon (from CH2CH3) = attached to carbon • Carbon (from CH3) = attached to hydrogen • Hydrogen = 1

  42. Naming Enantiomers -R,S System

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