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Chirality and Enantiomers
• Classify molecules as achiral or chiral
⟶ Chiral is NOT superimposable on mirror images
⟶ Achiral is superimposable
â–ª Achiral you can pick it up and sit it on top of each other and it is the same,
chiral compounds you cannot pick up and sit on top of one another and it
be the same
• Identify chiral centers
⟶ Chiral centers occur at tetrahedral atoms, a chiral center is bonded to 4
DIFFERENT GROUPS
⟶ There are also stereocentersin which when a group is exchanged with another in
the compound, then making it a different compound
• Identify and draw enantiomers
⟶ ALL CHIRAL COMPOUNDS HAVE AN ENANTIOMER
⟶ An enantiomer is a stereoisomer that is notsuperimposable on its mirror image,
essentially it is just the mirror image
The R,S system
• Rank substituents from highest to lowest priority
⟶ Ranking with depend on the substituents atomic number
⟶ We want H or the lowest priority group to be pointing away from us
⟶ If ranking from 1-4 goesin the counterclockwise direction, it is S configuration.
L=Left-handed
⟶ If ranking from 1-4 goesin the clockwise direction, it is R. R = right-handed
• Assign chiral centers as R or S
⟶ If the lowest priority group (4) is going away from you (like itshould) (thisisthe
easy one), the decision to assign R or S is easy based on the orientation of
clockwise or counterclockwise.
⟶ If the lowest priority group (4) is coming towards you, (as in it is wedged instead
of dashed) (dashed is preferred), you can simply reverse your answer. So
basically, if it is wedged, and the orientation is R, it is actually S. Or vice versa.
⟶ If the lowest priority group is in the plane of the board and is neither wedged or
dashed, you need to visualize it from the correct perspective.
⟶ Draw stickman so that he is oriented so that the H isin the back going away from
him, then go from there.
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