Summary of Ideas in Chapter 5 - Stereochemistry

  1. Molecules lacking a plane of symmetry are chiral (have "handedness")

    • Such molecules aare nonsuperposable on their mirror images

  2. Stereoisomers that are nonsuperposable mirror images are called enantiomers

    The Jmol windows below contain the enantiomers of:

    A Pair of Enantiomers

    • Enantiomers have identical chemical and physical properties except in a chiral environment

  3. Stereoisomers that are not enantiomers are called diastereomers

    The Jmol windows below contain two of the diastereomers of

    A Pair of Diastereomers

    • Diastereomers have different chemical and physical properties in all environments

  4. One way for a molecule to be chiral is to have an atom with four different ligands

    • Such atoms are called centers of chirality or stereogenic centers

    • The particular arrangement of ligands around the stereogenic center is called the absolute configuration of the stereogenic center

    • Absolute configurations can be specified by using the Cahn-Ingold-Prelog sequence rules

  5. Molecules having n different centers of chirality will have 2n stereoisomers

    • These isomers will compose 2n/2 pairs of enantiomers

    • The enantiomers of each pair are diastereomers of all of the other pairs of enantiomers

  6. If two of the stereogenic centers have the same ligands, at least one pair of enantiomers with be replaced by a single diastereomer with a plane of symmetry

    • This isomer is called the meso stereoisomer

    The Jmol window below contains the meso diastereomer of 2,3-dichlorobutane:

    The meso-Diastereomer of 2,3-Dichlorobutane

  7. In some molecules, an atom can be converted to a center of chirality by replacing one of two identical ligands with a different group, X

    • Such atoms are called prochiral centers

    • Pro-configurations can be specified for prochiral ligands by using the sequence rules

    • If the structures resulting from replacement of the prochiral ligands are enantiomers, the ligands are enantiotopic

    • If the structures are diastereomers, then the ligands are diastereotopic

  8. Molecules with twisted structures can be chiral without containing chiral centers

  9. Enantiomers can be distinguished from each other by the use of plane polarized light, or by interaction with other chiral molecules

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