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Chirality

Chirality. Chirality - the Handedness of Molecules. Isomers. we concentrate on enantiomers and diastereomers. Enantiomers. Enantiomers: nonsuperposable mirror images as an example of a molecule that exists as a pair of enantiomers, consider 2-butanol. Enantiomers.

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Chirality

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  1. Chirality Chirality - the Handedness of Molecules

  2. Isomers we concentrate on enantiomers and diastereomers

  3. Enantiomers • Enantiomers: nonsuperposable mirror images • as an example of a molecule that exists as a pair of enantiomers, consider 2-butanol

  4. Enantiomers • one way to see that the mirror image of 2-butanol is not superposable on the original is to rotate the mirror image

  5. Enantiomers • now try to fit one molecule on top of the other so that all groups and bonds match exactly • the original and mirror image are not superposable • they are different molecules • nonsuperposable mirror images are enantiomers

  6. Enantiomers • Objects that are not superposable on their mirror images are chiral (from the Greek: cheir, hand) • they show handedness • The most common cause of enantiomerism in organic molecules is the presence of a carbon with four different groups bonded to it • a carbon with four different groups bonded to it is called a stereocenter

  7. Enantiomers • If an object and its mirror image are superposable, they are identical and there is no possibility of enantiomerism • such an object is achiral (without chirality) • An achiral molecule, consider 2-propanol • notice that it has no stereocenter

  8. Enantiomers • to see the relationship between the original and its mirror image, rotate the mirror image by 120° • after this rotation, we see that all atoms and bonds of the mirror image fit exactly on the original • the original and its mirror image are the same

  9. Enantiomers • To summarize • an object that is nonsuperposable on its mirror image is chiral (it shows handedness) • the most common cause of chirality among organic molecules is the presence of a carbon with four different groups bonded to it • we call a carbon with four different groups bonded to it a stereocenter • an object that is superposable on its mirror image is achiral (without chirality) • nonsuperposable mirror images are called enantiomers • enantiomers, like gloves, always come in pairs

  10. Drawing Enantiomers • Following are four different representations for one of the enantiomers of 2-butanol • both (1) and (2) show all four groups bonded to the stereocenter and show the tetrahedral geometry • (3) is a more abbreviated line-angle formula; although we show the H here, we do not normally show them in line-angle formulas • (4) is the most abbreviated representation; you must remember that there is an H present on the stereocenter

  11. Drawing Mirror Images • on the left is one enantiomer of 2-butanol • on the right are two representations for its mirror image (in this case, its enantiomer)

  12. The R,S System • To assign an R or S configuration • assign a priority from 1 (highest) to 4 (lowest) to each group on the stereocenter; • orient the stereocenter so that the group of lowest priority is facing away from you • read the three groups projecting toward you in order from (1) to (3) • if reading the groups is clockwise, the configuration is R (Latin, rectus, straight, correct) • if reading the groups is counterclockwise, the configuration is S (Latin: sinister, left)

  13. The R,S System • The first step in assigning an R or S configuration to a stereocenter is to arrange the groups on the stereocenter in order of priority • priority is based on atomic number • the higher the atomic number, the higher the priority

  14. The R,S System • Example: assign priorities to the groups in each set

  15. The R,S System • Example: assign priorities to the groups in each set

  16. The R,S System • example: assign an R or S configuration to each stereocenter

  17. The R,S System • example: assign an R or S configuration to each stereocenter

  18. The R,S System • Because enantiomers are different compounds, each must have a different name • here are the enantiomers of the over-the-counter drug ibuprofen • the R,S system is a way to distinguish between enantiomers without having to draw them and point to one or the other

  19. The R,S System • returning to our original three-dimensional drawings of the enantiomers of ibuprofen

  20. Chirality in Biomolecules • a molecule and its enantiomer or one of its diastereomers elicit different physiological responses • as we have seen, (S)-ibuprofen is active as a pain and fever reliever, while its R enantiomer is inactive • the S enantiomer of naproxen is the active pain reliever, but its R enantiomer is a liver toxin!

  21. Stereocenters • A molecule with n stereocenters has a maximum number of 2n stereoisomers • a molecule with one stereocenter, 21 = 2 stereoisomers (enantiomers) are possible • for a molecule with two stereocenters, a maximum of 22 = 4 stereoisomers (two pair of enantiomers) • for a molecule with three stereocenters, a maximum of 23 = 8 stereoisomers (four pairs of enantiomers) is possible • and so forth

  22. Fischer Projection Formulas • Fischer Projection: show configuration of chiral molecules in two-dimensional representation • Vertical bonds are directed away • Horizontal bonds are directed toward you

  23. Enantiomers & Diastereomers • 2,3,4-Trihydroxybutanal • two stereocenters; 22 = 4 stereoisomers exist

  24. Two Stereocenters • 2,3,4-trihydroxybutanal • diastereomers:stereoisomers that are not mirror images • (a) and (c), for example, are diastereomers

  25. Meso Compounds • Meso compound: an achiral compound possessing two or more stereocenters • tartaric acid • two stereocenters; 2n = 4, but only three stereoisomers exist

  26. Stereoisomers • example: mark all stereocenters in each molecule and tell how many stereoisomers are possible for each

  27. Stereoisomers • example: mark all stereocenters in each molecule and tell how many stereoisomers are possible for each • solution:

  28. Three Or More Stereocenters • how many stereocenters are present in the molecule on the left? • how many stereoisomers are possible? • one of the possible stereoisomers is menthol • assign an R or S configuration to each stereocenter in menthol

  29. Three Or More Stereocenters R S * R * * 23= 8 possible stereoisomers

  30. Stereoisomers • The 2n rule applies equally well to molecules with three or more stereocenters

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