L 7 structural-isomerism_pch217_2013_2014
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  • Make Molecular Models to Illustrate here!!
  • Plane Polarized Light has Only one Single E-Vector direction

L 7 structural-isomerism_pch217_2013_2014 Presentation Transcript

  • 1. 1 Organic Chemistry Course Number: PCH 1120-217 Lecture # 7 Sunday September 22, 2013 Structural Isomerism-Stereoisomerism with One and Two Chiral Carbons Prof. Oludotun A. Phillips Room # 2-81, 2nd Floor Pharmacy Building Email: dphillips@hsc.edu.kw Tel: 24986070
  • 2. 2 Learning Objectives At the end of the class students should be able to:  describe structural and stereo-isomerism.  identify chiral carbon / stereocenter and achiral carbon.  draw structures of stereoisomers.  identify enantiomers and diastereomers.  distinguish between diastereomers and meso compounds.  describe optical activity in stereoisomers.
  • 3. 3 Relationship Among Isomers Geometric isomers Rigidity in a molecule leads to isomerism
  • 4. 4 Structural (Constitutional) Isomers  Constitutional isomers: compounds with the same molecular formula but different connectivity (order of attachment) of their atoms.  There are two constitutional isomers with molecular formula C4H10. CH 3 CH2 CH2 CH3 CH3 CHCH3 CH3 B u ta n e (b p -0 .5 ° C ) 2 -M e th y l p ro p a n e (b p -1 1 .6 °C )
  • 5. 5 Structural (Constitutional) Isomers  The potential for constitutional isomerism is enormous: 4,111,846,7 63 4,347 75 3 1 Constitutional Isomers Molecular Formula C H 4 C 5 H 1 2 C 1 0 H 2 2 C 1 5 H 3 2 C 3 0 H 6 2 36,797,588C 2 5 H 5 2
  • 6. 6 Assignment Submit to me during the next class:- Questions: 1. How many constitutional isomers would you expect for each of the molecular formulas: C5H12, C6H14, C2H6O and C4H10O? 2. Draw all the possible constitutional isomers for the molecular formulas above.
  • 7. 7 Stereoisomerism  Stereoisomers: compounds with the same molecular formula and same connectivity (order of attachment) of their atoms, but different 3D (3-dimensional) orientations of their atoms in space.  Stereoisomerism relates to the three dimensional structure of the molecule in space.  Stereochemistry is the study of the orientation of molecules in space, relating how the atoms are arranged in space relative to each other.
  • 8. 8 Stereoisomerism  Three aspects of Stereochemistry: 1. Chirality of molecules: with right- or left-handed arrangement of the atoms around a carbon atom.  Chiral (from the Greek: cheir; hand) molecules are compds that are non-superimposable on their mirror images are stereoisomers; e.g Enantiomers 2. Geometric isomers: relates to how rigidity in the molecule can lead to isomerism. 3. Conformation of molecules: relates to how the change in the shapes of molecules can lead to isomerism.
  • 9. 9 Stereoisomers with One Chiral Carbon Atom  A carbon with four different bonded groups is referred to as a Chiral Carbon (a Stereocenter, Chiral center; Stereocenter, Stereogenic center): I C C l F B r H 3C C C H 2 C H 3 C H 2 C H 2C H 3 C H 2 C H 2C H 2C H 3 C hiral C arbons (Stereocenter) all four groups are different H 3C H 2C C C H 3 F H
  • 10. 10 Stereoisomers with One Chiral Carbon Atom  On the other hand, a carbon with all the four or two of the groups the same is referred to as an Achiral carbon: H C H H H achiral Carbons all the four groups are the sam e H3C C CH3 CH3 CH3 H3CH2C C CH2CH3 CH2CH3 CH2CH3
  • 11. 11 Stereoisomers with One Chiral Carbon Atom  A Chiral carbon can exist in either of two 3D structural arrangements, because of the tetrahedral geometry.  The Two 3D representations are non-superimposable, mirror-image configurations referred to as Enantiomers.  Configuration refers to the orientation of the groups around the chiral carbon (stereocenter).  For example the two structural representation of Lactic acid, shown in next slide are in 3D, tetrahedral geometry of the chiral carbon:…
  • 12. 12 Stereoisomers with One Chiral Carbon Atom The two structural forms are referred to as a pair of Enantiomers.  Structure (i) is different from Structure (ii), they are both non-superimposable mirror images.  Each structural form exists independently: e.g.  isomer (i): L(+)-lactic acid - is produced in muscles of the body during exercise and responsible for the soreness.  while isomer (ii): D(+)-lactic acid is found in sour milk. C O 2 H C H H 3 C O H C O 2H C H C H 3 H O Lactic A cid: m irror im ages, both structures are different Structure (i) Structure (ii) (S) (R)
  • 13. 13 Stereoisomers with One Chiral Carbon Atom  Enantiomers of 2-butanol shown below:  To confirm that the mirror image of 2-butanol is not superimposable on the original; rotate the mirror image by 180o O H C H3 C CH 2 CH 3 H HO C CH3 H CH3 CH2 O ri g in al m o l e cu l e M i rror i m ag e O rigin al m o le cu le OH C H 3 C CH2 CH3 H OH C CH3 H CH3 CH2 OH C H3 C H CH2 CH3 M irro r im ag e Th e m irro r im ag e ro tate d b y 180° 180° rotate th e m irro r im ag e b y 180° ab o u t th e C -O H b on d
  • 14. 14 Stereoisomers with One Chiral Carbon Atom  now try to fit one molecule (original on mirror image rotated 180o) on top of the other so that all groups and bonds match or align exactly:  The original and mirror images are non-superimposable….  they are different molecules with identical physical properties  They are enantiomers (non-superimposable mirror images) OH C H3 C CH 2 CH3 H OH C H3 C H CH2 CH3 T h e o rigin al m o le cu le T h e m irro r im ag e tu rn e d b y 180°
  • 15. 15 Stereoisomers with One Chiral Carbon Atom Enantiomers:  are non-superimposable on their mirror images  are chiral molecules, having a center of chirality (the carbon with all the four different groups attached is the center of Chirality) – i.e. show handedness  contain stereogenic center, stereocentre, chiral center or asymmetric carbon atom.  do not have a plane of symmetry
  • 16. 16 Stereoisomers with One Chiral Carbon Atom  The Physical properties of Enantiomers, such as melting point, boiling point, and refractive index are identical.  Enantiomers differ in only one physical property, the direction in which they rotate Plane-Polarized light.  Hence, Enantiomers are Optically active molecules.  Racemic mixture or Racemate: a mixture of equal (1:1) parts of enantiomers.  Racemic mixtures are Optically Inactive.
  • 17. 17 Plane of Polarization of Plane Polarized Light  Ordinary light waves - oscillate in all E-vector planes perpendicular to its direction of propagation.  Plane-Polarized Light waves – oscillate in a single E- vector plane to its direction of propagation.
  • 18. 18 Schematic diagram of a polarimeter Measurement of Optical Activity – Polarimeter:  Polarimeter: instrument for measuring the rotation of plane-polarized light by a chiral compound.  Optically active: indicates that a compound rotates plane-polarized light.
  • 19. 19 Rotation of Plane Polarized Light Optical Activity:  Dextrorotatory: clockwise rotation of plane-polarized light  Levorotatory: counterclockwise rotation of plane- polarized light  Specific rotation (): the observed rotation of an optically active substance at a concentration of 1g/100mL in a sample tube 10cm long; for a pure liquid, concentration is in g/mL (density):  Specific rotation ()= ________observed rotation “degrees”__________ length of sample tube “dm” X conc of sample “g/cm3”
  • 20. 20 Stereoisomers with One Chiral Carbon Atom  Enantiomerism: is the presence of a carbon with four different groups bonded to it.  A compound having one chiral carbon with four different attached groups will exist in two non- superimposable mirror-image forms.  However, If a compound and its mirror image are superimposable, they are identical and there is no possibility of enantiomerism.  Such molecules are referred to as Achiral (without chirality):
  • 21. 21 Isomers without Chiral Carbon Atom  A carbon having the two or more identical groups around it is referred to as Achiral carbon:  E.g. Propanoic acid:  Here two groups on the center carbon are similar, therefore both compounds are identical. CO2H C H H CH3 Propanoic acid: Both structures are identical Structure (i) Structure (ii) CO2H C H H H3C
  • 22. 22 Stereoisomers with Two Chiral Carbon Atoms 1. Molecules with Two Dissimilar Chiral Carbon Atoms – will give Enantiomers and Diastereomers:  e.g. 2,3,4-Trihydroxybutanal have two stereocenters.  Maximum possible number of stereoisomers = 2n; (n= number of chiral carbons)  22 = 4 stereoisomers are possible HO CH2 -CH -CH- CH O H O H O * *
  • 23. 23 Stereoisomers with Two Chiral Carbon Atoms 1. Molecules with Two Dissimilar Chiral Carbon Atoms: Enatiomers and Diastereomers:  Four Stereoisomers of 2,3,4-Trihydroxybutanal are possible:  Structures AC, AD, BC and BD are Diastereomers. C C H O H C H O O H C H 2 O H H C C HH O C H O H O C H 2 O H H H C H 2 O H H O C C H O H C H O C C HH O C H O H C H 2 O H O H A p a i r o f e n a n ti o m e rs (E ry th ro s e ) A p a i r o f e n a n ti o m e rs (T h re o s e ) A. B. C. D.
  • 24. 24 Stereoisomers with Two Dissimilar Chiral Carbon Atoms  Diastereomers are stereoisomers having more than one chiral carbon atom, but they are not mirror images.  Diastereomers differ in physical properties such as:  melting points,  boiling points,  densities refractive indices, and  if they are chiral, specific rotations can differ.
  • 25. 25 Stereoisomers with Two Chiral Carbon Atoms 2. Molecules with Two Similar Chiral Carbon Atoms - gives Enatiomers, Diastereomers and Meso Compounds:  Meso compound: an achiral compound having two or more stereocenters that is superimposable on its mirror image.  e.g. as may be found in tartaric acid  Have two stereocenters; hence the expected number of stereoisomers 2n = 22 = 4; but only three stereoisomers truly exist CC O H CO O H O H H H O O C H
  • 26. 26 Stereoisomers with Two Similar Chiral Carbon Atoms  Meso compounds (E and F) are achiral compounds therefore they are optically inactive molecules (they are the same molecule).  Compounds (G and H) are Enantiomers.  While compounds (E and G), and (E and H) are Diastereomers. E. F. G. H. C C H OH COOH OH COOH H C C HHO COOH HO COOH H C C H OH COOH H COOH HO C C HHO COOH H COOH OH A pair of enantiomersA meso compound (plane of symmetry) C C H COOH OHH COOH OH 180o identical C C HO COOH OHH COOH H different 180o Has a Plane of Symmetry!!
  • 27. 27 Rotation of Plane Polarized Light Optical Activity:  Like melting point and boiling point, Specific rotation is a physical property of a compound e.g. DD H 3 C C O H H CO O H CH3 C H O H COO H [ ] 21 = -2.6°= + 2.6° 21 [ ] (R )-(-)-L actati c ac i d(S)-(+ )-Lacti c aci d
  • 28. 28 Questions - Stereoisomerism Submit to me during the next class:-  How many stereoisomers are possible for the carbohydrate molecule 2-Deoxyribose shown below?  Draw all the possible stereoisomers of 2-Deoxyribose shown above, and identify the enantiomers and diastereomers. CH2 OH H C OH H C OH CH2 C H O 2-Deoxyribose [A carbohydrate that is a structural component of the genetic material Deoxyribonucleic acid (DNA)]