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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
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 projections1
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.
fischer projections2
Fischer Projections
  • 1-bromo-1-chloroethane would be shown as

C #1

Fischer projection

fischer projections r s
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).

fischer projections3
Fischer Projections
  • A Fischer projection may be rotated 180°.

same molecule

fischer projections4
Fischer Projections
  • A Fischer projection may NEVER be rotated 90° left or right.

(R)-1-bromo-1-chloroethane

(S)-1-bromo-1-chloroethane

fischer projections5
Fischer Projections
  • This compound would be named…

Does it contain a mirror plane of symmetry?

S

R

(2S,3R)-pentane-2,3-diol

fischer projections6
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

fischer projections testing for enantiomers
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

fischer projections testing for enantiomers1
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?

diastereomers revisited
Diastereomers Revisited
  • Diastereomers are stereoisomers that are not mirror images.

enantiomers

diastereomers

physical properties of diastereomers
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.)
isomers1
Isomers
  • Can you draw any other isomers of C4H10O2?
meso compounds
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

relative configurations the d l system
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.
relative configurations the d l system1
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.
resolution of enantiomers
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
resolution of enantiomers1
Resolution of Enantiomers
  • Chemical separation
nomenclature
Nomenclature
  • This is the template you will use to build the name of any organic compound.
nomenclature1
Nomenclature
  • When naming a compound with a single chiral C, use (R) or (S).

(S)-bromochlorofluoromethane

(R)-2-chlorobutane

nomenclature2
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

using nomenclature to differentiate compounds
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.
nomenclature3
Nomenclature
  • Draw the stereoisomers of 2,3-dibromobutane, name them, and give their relationships to each other.
  • Which ones are optically active?
nomenclature4
Nomenclature
  • Draw the stereoisomers of 2,3-dichloropentane, name them, and give their relationships to each other.
  • Which ones are optically active?