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Topic 5F Stereochemistry

Topic 5F Stereochemistry. 71. Stereochemistry. Study of three-dimensional shape of molecules and how this affects their chemical and physical properties Very important in biology

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Topic 5F Stereochemistry

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  1. Topic 5FStereochemistry

  2. 71 Stereochemistry • Study of three-dimensional shape of molecules and how this affects their chemical and physical properties • Very important in biology • Isomers that have the same formula and connectivity but differ only in the way the atoms are arranged in space are STEREOISOMERS • Constitutional isomers having different connectivity and are joined up in a different way are NOT stereoisomers

  3. 72 Optical isomerism • Optical isomers:differ in the way they rotate plane polarised light. • Enantiomers: isomers that are nonsuperimposable mirror images of each other. • Diastereomers: stereoisomers that are not enantiomers. Isomers with more than one chiral carbon that are not enantiomers but differ only in configurations about chiral carbons.

  4. CH CH 3 CH 2 CH 3 3 H O CH O 3 CH 2 Carvone H (caraway) Spearmint essence 72 Optical Isomerism • Carvone and the essence of spearmint are optical isomers or ENANTIOMERS • They possess CHIRAL carbon and they are chiral molecules • They interact with plane-polarised light differently

  5. 73 Mirror images • Mirror images that can be superimosed are ACHIRAL The mirror image of this fork can be stacked on top of the real thing since the fork is symmetrical. It is achiral. Mirror image Real fork

  6. 73 Chirality or handedness • From the Greek word “cheir” — hand • Hands are non-superimposable — they are chiral • Any molecule that cannot be superimposed on its mirror image is said to be chiral

  7. 74 Chiral carbons • Have four different substituents attached to them • Chiral carbons have no symmetry they are asymmetric

  8. Mirror Mirror image A A A A D D D D D D B D B B D B Mirror image Real molecule 74 Achiral molecules • Carbons bearing two identical substituents are ACHIRAL • They can be superimposed upon their mirror image

  9. CO H CH OH CH 2 2 3 CH 3 NH CH OH CH CH 2 2 2 3 1,2,3-propanetriol 2-hydroxy-2- (glycerol) methylbutane 75 Examples of achiral molecules • Each have two identical substituents H OH OH H H Glycine

  10. M irror 75 Chiral molecules • A chiral molecule is unique and can not be superimposed upon its mirror image A A C C D D B B Real molecule Mirror image Mirror image Mirror image C A A A A C C D B C B D D B D B

  11. 76 Examples of chiral molecules

  12. 77 Ranking groups (1) Higher atomic number of atom bonded to the chiral carbon means higher priority.H C N O F S Cl Br I, Increasing priority • (2) If two atoms are the same, proceed to the second atom or even farther along the chain to the first point of difference. • (3) Groups with pi bonds are given single-bond equivalents by duplication or triplication of multiply bonded atoms. You will learn to use this rule in second year. • (4) E/Z isomers use the same priority rule

  13. (S) (R) 77 (R) and (S) nomenclatureCahn-Ingold-Prelogsystem Steps: • Align smallest group away from you • Assign priorities to all groups attached to the chiral centre • Determine in what direction priorities decrease, clockwise or anticlockwise

  14. 78 Assigning (R) or (S) • If groups descend in priority clockwise then (R)“R” from Latin word “rectus” • If groups descend in priority anticlockwise then (S)“S” from Latin word “sinister”

  15. I OH H * H * C C HO CH 3 CH CH CH CH 2 3 2 3 1 I 1 OH * H H * C 4 C 4 HO CH 3 3 2 CH CH CH CH 2 3 2 3 3 2 1 1 OH I C C CH CH CH HO CH CH 3 2 3 2 3 R S 3 2 2 3 (S)-1-iodopropanol (R)-2-butanol 78 Examples

  16. 78 Enantiomeric pairs • Enantiomers (from Greek enantio, “opposite” and merso ,“part”) have opposite configuration Br Br H H Cl Cl I I Mirror Enantiomers

  17. Mirror CO H CO H 2 2 * H * H CH CH 3 3 NH NH 2 2 Enantiomers of alanine Mirror NH NH 2 2 H * H * CH CH 3 3 CH CH CH CH CH CH 2 2 3 3 2 2 Enantiomers of 2-aminopentane 79 Examples of enantiomers • Enantiomers come in pairs:

  18. A A C B D B D C C C Fischer projection 79 Fischer projections • Configurations at carbon can be represented on paper by Fischer projections

  19. HOCH CH OH 2 2 CHO CHO C HO C H H OH CH OH CH OH 2 2 CHO CHO HO H H OH CH OH Fischer projections CH OH 2 2 80 Example • Enantiomers of glyceraldeyde as Fischer projections: Enantiomers of glyceraldehyde Mirror O H O H C C H * H * C C OH HO

  20. 81 More than one chiral carbon • Each chiral carbon treated separately (2R,3R)-2,3,4-Trihydroxybutanal

  21. 81 Numbers of isomers • 2n Isomers, n = number of chiral centres 2 3

  22. 82 Numbers of isomers • 2n Isomers, n = number of chiral centres • Isomers NOT mirror images are DIASTEREOMERS 2 3

  23. 82 Numbers of isomers • 2n Isomers, n = number of chiral centres • Isomers NOT mirror images are DIASTEREOMERS 2 3

  24. 83 Meso compounds • Two centres but possess a plane of symmetry • Tartaric acid has only three streoisomers

  25. 83 Properties of enantiomers • They behave differently in chiral environments • Chiral reactants • Chiral catalysts • Biological environments • Interaction with plane-polarised light • The name optical isomerism stems from this

  26. Plane Polarised light: 84 Interaction with polarised light • 1815 Jean-Baptiste Biot discovered that some substances rotate the plane of polarised light. • Sugar cane rotates light through 60° • Such compounds are OPTICALLY ACTIVE • All such compounds contain chirality.

  27. 85 Interaction with polarised light

  28. Plane polarised Rotated light Unpolarised light light a Sample solution =-20° polarising a cell filter Counterclockwise Levorotatory =+35° a Clockwise Dextrorotatory 85 Dextro- and levorotation • Compounds that rotate the plane of polarised light to the LEFT are LEVOROTATORY or the (–) form • Compounds that rotate the plane of polarised light to the RIGHT are DEXTROROTATORY or the (+) form

  29. T [a]D 85 Specific rotation • Magnitude of rotation depends upon • Nature of substance • Concentration of the solution • Temperature • Wavelength of light • Solvent • Enables comparison of measurements under different conditions

  30. a T [a]D = (solvent) l x c 85 Specific rotation • ais the observed rotation in degrees • T is the temperature at measurement • D is the sodium D line (589.3 nm) • l is the cell length in decimeters (dm) • c is the concentration in g/mL

  31. 85 Example • Specific rotation of cocaine: • 1.80g dissolved in 10.0 mL of chloroform in a 5.0 cm cell gave an observed rotation at 20° with (sodium D line) of -14.4° Solution: a = -14.4° l = 5/10 = 0.5 dm c = 1.8/10 =0.18 g/mL [a]TD = -14.4/0.5 x 0.18  = -160.0° (chloroform)

  32. O O OH H CO H CHO 2 C C oxidation H OH H OH H * H * C C CH OH CH OH CH OH 2 CH OH 2 2 2 HO HO Sameconfiguration (-) Glyceric acid (+) Glyceraldehyde 86 Configuration • There is no relationship between the actual configuration at a carbon and the direction of rotation of plane polarised lightFor instance:

  33. 50 : 50 No net rotation 87 Racemic mixtures • Enantiomers rotate the plane of polarised light by equal amounts but in opposite directions. • A 50:50 mixture of enantiomers does not rotate the plane of polarised light. No net rotation • RACEMIC MIXTURE

  34. S-fits protein shape R-doesn't fit protein shape CO H 2 CO H 2 H * too big NH * 2 CH 3 S CH 3 R NH 2 H too small Chiral protein Chiral protein 88 Chirality in the biological world • Example:Only one form of the amino acid alanine (S) is incorporated into protein molecules.The enantiomer (R) is oxidised and metabolised Enantiomers of alanine

  35. 89 Chiral drugs • Radically different roles of enantiomers. (S) (S) (S) form is a Teratogen (auses foetal deformities)(R) form is anAnti-depressant (S) form is an analgesic(R) form inactive

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