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Stereochemistry

Stereochemistry. Handedness. Some things have a “handedness,” that is look at your right and left hand. They look alike, but are not the same. They are mirror images.

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Stereochemistry

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  1. Stereochemistry

  2. Handedness • Some things have a “handedness,” that is look at your right and left hand. They look alike, but are not the same. They are mirror images.

  3. Their research deals with the fact that many molecules appear in two forms that are mirror images of each other, just like the left and right hands. Nobel Prize - 2001

  4. Chirality The mirror image of a chiral object is different and will not superimpose on the original object. Objects which are chiral have a sense of “handedness” and exist in two forms.

  5. Mirror Image

  6. mirror Are these two structures identical?

  7. Stereoisomers

  8. Stereoisomers

  9. Stereoisomers

  10. Stereoisomers

  11. Stereoisomers

  12. Stereoisomers

  13. Stereoisomers enantiomers Stereoisomers that are nonidentical mirror images are called enantiomers.

  14. Visualize, visualize ….

  15. Visualize, visualize …

  16. C C Visualize, visualize … CH3 CH3 H H Br F F Br enantiomer ODD: 1..3..5…etc interchanges = enantiomer EVEN: 2..4..6...etc interchanges = original compound

  17. C C H H CH3 Br 2 1 C C C C F CH3 Br CH3 Br H H CH3 Br F F F ENANTIOMER ENANTIOMER SAME 3 Are these identical or are they enantiomers? CH3 H YOU CAN USE INTERCHANGES CH3 F H Br Br F

  18. Isomers • Isomers: different compounds with the same molecular formula • Constitutional isomers: isomers with a different connectivity • Stereoisomers: isomers with the same molecular formula, the same connectivity but a different orientation of their atoms in space that cannot be interconverted by rotation about a single bond

  19. Chirality • Mirror image: the reflection of an object in a mirror • Objects that are not superposable on their mirror images are said to be chiral, that is, they show handedness • Objects that are superposable on their mirror images are said to be achiral, that is, they do not show handedness. An achiral object has at least one element of symmetry

  20. Chirality • A molecule cannot be chiral if it has a plane of symmetry.

  21. Chirality • A plane of symmetry is a plane that cuts through an object in such a way that one half of the object is an exact mirror image of the other half. • A molecule that has a plane of symmetry must be identical to its mirror image and therefore must be nonchiral, or achiral.

  22. Stereogenic Carbons This is one type of …. stereocenter …. others are possible A stereogenic carbon is tetrahedral and has four different groups attached.

  23. C l C l B r C l r B l C Elements of Symmetry • Plane of symmetry: an imaginary plane passing through an object dividing it such that one half is the mirror image of the other half F F

  24. Elements of Symmetry • Center of symmetry: a point so situated that identical components of the object are located equidistant on opposite sides and equidistant from the point along any axis passing through the point

  25. Achiral The plane of the paper is a plane of symmetry Two identical groups renders a tetrahedral carbon achiral.

  26. C l C l B r C l B r C l Two Views of the Plane of Symmetry F plane of symmetry F side view edge view

  27. Symmetry Plane

  28. ENANTIOMERS Stereoisomers whose molecules are non- superimposible mirror images of each other DIASTEREOMERS Stereoisomers whose molecules are not mirror images of each other CONSTITUTIONAL ISOMERS ISOMERS Different compounds with the same molecular formula Isomers with a different order of attachment of the atoms in their molecules each isomer could have stereoisomers STEREOISOMERS Isomers with the same order of attachment, but a different configuration (3D arrangement) of groups on one or more of the atoms double bond or ring with a ring cis/trans ISOMERS both can apply TYPES OF ISOMERISM (geometric)

  29. Enantiomers • Enantiomers: stereoisomers that are nonsuperposable mirror images; refers to the relationship between pairs of objects

  30. Enantiomers rotate this molecule is chiral note that the fluorine and bromine have been interchanged in the enantiomer do interchanges in class

  31. Lactic acid Enantiomers

  32. Enantiomers • 1,2-propanediol

  33. Enantiomers • 3-Chlorocyclohexene

  34. Carvone

  35. Limonene

  36. Chiral Drugs • Most pharmaceutical drugs are chiral • thalidomide

  37. Optically Active • Refers to molecules that interact with plane-polarized light Jean Baptiste Biot French Physicist - 1815 He discovered that some natural substances (glucose, nicotine, sucrose) rotate the plane of plane-polarized light and that others did not.

  38. Optical Activity angle of rotation, a a incident polarized light transmitted light (rotated) sample cell (usually quartz) a solution of the substance to be examined is placed inside the cell

  39. Plane Polarized Light • Ordinary light:consists of waves vibrating in all planes perpendicular to its direction of propagation • Plane polarized light:consists of waves vibrating only in one plane • Plane polarized light is an equal mixture of left and right-circularly polarized light. These two forms are nonsuperposable mirror images and, therefore, enantiomers.

  40. c n = l Plane-Polarized Light Beam l wavelength . END VIEW SIDE VIEW polarized beam frequency ( n ) NOT PLANE-POLARIZED Sine waves are not aligned in the same plane. c = speed of light unpolarized beam

  41. Plane Polarized Light • Because of its handedness, circularly polarized light reacts one way with a stereocenter with R-handedness, and differently with its enantiomer • The net effect of the interaction of plane polarized light with a chiral compound is that the plane of polarization is rotated • Polarimeter: a device for measuring the extent of rotation of plane polarized light

  42. optical activity - ability of certain molecules to rotate plane polarized light detected using a polarimeter Optical Activity

  43. Polarimeter

  44. Polarimeter  Na vapor lamp sample cell polarizer analyzer l (dam)

  45. Optical Activity • Observed rotation:the number of degrees, , through which a compound rotates the plane of polarized light • Dextrorotatory (+):rotation of the plane of polarized light to the right • Levorotatory (-):rotation of the plane of polarized light to the left

  46. Optical Activity • Specific rotation:Observed rotation of the plane of polarized light when a sample is placed in a tube 1.0 dam in length and at a concentration of 1g/mL. a = observed rotation c = concentration ( g/mL ) l = length of cell ( dm ) D = yellow light from sodium lamp T = temperature ( Celsius )

  47. Optical Activity • For a pair of enantiomers, the value of the specific rotation of each is the same, but opposite in sign

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