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Fischer Projections

Fischer Projections. Many molecules of interest (e.g., sugars and amino acids) have more than one chiral atom. An effective way to show structures with more than one chiral C atom is to use Fischer projections. Fischer Projections.

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Fischer Projections

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  1. Fischer Projections • Many molecules of interest (e.g., sugars and amino acids) have more than one chiral atom. • An effective way to show structures with more than one chiral C atom is to use Fischer projections.

  2. Fischer Projections • The main C chain is shown along the vertical axis of the projection, with C1 at the top. • Asymmetric C atoms are where the lines cross. • Vertical bonds are bonds to groups behind the plane of the paper. • Horizontal bonds are bonds to groups in front of the plane of the paper.

  3. Fischer Projections • 1-bromo-1-chloroethane would be shown as C #1 Fischer projection

  4. Fischer Projections: (R),(S) • Which configuration is it? 2 1 3 It looks like (S) but the H is in front, so it is (R).

  5. Fischer Projections • A Fischer projection may be rotated 180°. same molecule

  6. Fischer Projections • A Fischer projection may NEVER be rotated 90° left or right. (R)-1-bromo-1-chloroethane (S)-1-bromo-1-chloroethane

  7. Fischer Projections • This compound would be named… Does it contain a mirror plane of symmetry? S R (2S,3R)-pentane-2,3-diol

  8. Fischer Projections • Fischer projections make enantiomers really easy to draw: just interchange left and right groups. S R R S (2R,3S)-pentane-2,3-diol (2S,3R)-pentane-2,3-diol

  9. Fischer Projections - Testing for Enantiomers • If the mirror images do not look alike, and cannot be made to look alike by a 180° rotation, the two Fischer projections are enantiomers. enantiomers enantiomers

  10. Fischer Projections - Testing for Enantiomers • If the mirror images do not look alike, and cannot be made to look alike by a 180° rotation, the two Fischer projections are enantiomers. mirror plane of symmetry enantiomers same molecule meso compound Are these three molecules related? What are they called?

  11. Diastereomers Revisited • Diastereomers are stereoisomers that are not mirror images. enantiomers diastereomers

  12. Physical Properties of Diastereomers • Diastereomers have different physical properties. • boiling point • melting point • density • index of refraction • (Enantiomers have identical physical properties except for the rotation of plane-polarized light and how they react with other chiral molecules.)

  13. Isomers

  14. Isomers • Can you draw any other isomers of C4H10O2?

  15. Meso Compounds • A compound that has chiral atoms but which is itself achiral. cis-1,2-dibromocyclohexane meso-butane-2,3-diol 2R,3S-butane-2,3-diol 2S,3R-butane-2,3-diol (1R,2S)-1,2-dibromocyclohexane

  16. Relative Configurations - The D-L System • The (R),(S) notation designates an absolute configuration. The position of every atom in the molecule is known. • Before x-ray crystallography, we knew enantiomers existed and we knew one was (+) and the other (-), because that could be measured experimentally, but we did not know the absolute configuration. However, we could pick a benchmark molecule and, if we knew how this molecule was related to it, we could designate its relative configuration. • Compounds with the same relative configuration around the bottom chiral C as (+)-glyceraldehyde were given the D prefix.

  17. Relative Configurations - The D-L System • Compounds with the same relative configuration around the bottom chiral C as • (+)-glyceraldehyde are given the D prefix, and • (-)-glyceraldehyde are given the L prefix.

  18. Resolution of Enantiomers • When a chiral compound is made from achiral reagents, a racemic mixture is the result. • Separating the two enantiomers is called resolution, and both require interaction with a pure enantiomer of a different chiral compound. • Chemical separation • Chromatographic separation

  19. Resolution of Enantiomers • Chemical separation

  20. Nomenclature • This is the template you will use to build the name of any organic compound.

  21. Nomenclature • When naming a compound with a single chiral C, use (R) or (S). (S)-bromochlorofluoromethane (R)-2-chlorobutane

  22. Nomenclature • When naming a compound with multiple chiral C atoms, give the configuration around each chiral C atom at the beginning of the name. (2S,3S,4S)-2,3-dibromo-4-methylhexane

  23. Using Nomenclature to Differentiate Compounds • Naming, or at least identifying the R/S configuration around each chiral C atom, is a good way to determine whether you have enantiomers, diastereomers, or the same compound. • Enantiomers will have R and S reversed for everychiral carbon. • Diastereomers will have some reversed and some not. • Same compound…duh.

  24. Nomenclature • Draw the stereoisomers of 2,3-dibromobutane, name them, and give their relationships to each other. • Which ones are optically active?

  25. Nomenclature • Draw the stereoisomers of 2,3-dichloropentane, name them, and give their relationships to each other. • Which ones are optically active?

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