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Important Concepts
1. Isomers – Same molecular formula, different
compounds:
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Constitutional – Individual atoms are connected differently
Stereoisomers – Same connectivity, different 3D arrangement.
Mirror-Image Stereoisomers – Related as image
2. Chiral Molecule – Not superimposable on its mirror
image
3. Stereocenter – Carbon atom bearing 4 different
substituents
4. Enantiomers – Two stereoisomers, each a nonsuperimposable mirror image of the other
Important Concepts
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5. Racemate – A one-to-one mixture of enantiomers
6. Mirror Plane – Chiral molecules cannot contain a
mirror plane.
7. Diastereomers – Stereoisomers not related to each
other as mirror images (i.e. cis/trans).
8. Two Stereocenters In A Molecule – Create up
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to 4 stereoisomers: 2 diastereomerically related pairs.
If the 2 stereocenters generate a mirror plane in the
molecule, the molecule is known as a meso compound
and is achiral.
Physical Properties of Stereoisomers – Most
are the same except for the rotation of plane-polarized
light. One enantiometer rotates the plane of
polarization to the right, the other to the left. This
rotation is expressed as the specific rotation [].
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Important Concepts
10. Absolute Configuration – Determined by X-ray
diffraction. Assignment of R or S, as determined by the
Cahn, Ingold, and Prelog sequence rules
11. Fischer Projections – Standard 2-D
representation of 3-D molecules containing
stereocenters
12. Radical Halogenation – Can introduce chirality
into an achiral compound. Racemic mixtures are often
produced.
13. Radical Halogenation of a Chiral Molecule
Containing 1 Stereocenter –
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Gives a racemate if the reaction occurs at the stereocenter
Gives 2 diastereomers in unequal amounts if the reaction
occurs elsewhere
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Important Concepts
14. Stereoselectivity – Preference for the formation of
one stereoisomer when several are possible
15. Resolution – Separation of enantiomers
• Reaction with a pure enantiomer of a second chiral
compound and separation of the diastereomers
• Chiral chromatography