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The number of possible
stereoisomers of a molecule is related to the number of chiral centers present in a
molecule. |
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The exact relationship is m = 2n where m is the number of
possible isomers and n is the number of chiral centers.
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Thus, for molecules with two
chiral centers there are maximum of four possible stereoisomers. (Molecules with
more than two chiral centers will not be considered here.) |
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Consider the stereoisomers of
2-bromo-3-fluorobutane: |
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(2R,3R) |
(2S,3S) |
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(2R,3S) |
(2S,3R) |
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The pairs of compounds which share a line
are enantiomers. |
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In order to see this relationship, rotate each molecule so that the
bromine (red atom) of is pointed up and the methyl group pointed towards the center of the
page.
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Notice that these pairs of molecules are not superimposable, but
that they are mirror images.
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Notice, also, the absolute configurations of each molecule:
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The top pair of enantiomers are the (2R,3R) and the (2S,3S)
compounds.
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The bottom pair of enantiomers are the (2R,3S) and the (2S,3R)
compounds.
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In both cases , the pairs have the opposite configuration at both
stereocenters.
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This is a general characteristic of enantiomers.
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What about the relationship between the
rows of compounds? |
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For example, the (2R,3R) compound and (2R,3S) compound OR between
the (2S,3S) and the (2S,3R) compounds.
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These pairs of compounds are not
superimposable (If they were they would be the same compound!) but they are also not
mirror images. |
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In order to see this relationship, rotate the molecules so that the
bromines of each pair are pointed towards each other, and the methyl groups pointed
towards the middle of each page.
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Two molecules that are non-superimposable, non-mirror images are
called diastereomers.
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Again, notice the absolute configurations of each pair of
diastereomers:
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The left pair are the (2R,3R) and the (2R,3S) compounds.
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The right pair are the (2S,3S) and the (2S,3R) compounds.
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In both cases, the absolute configuration at one chiral center is
the same (C-2 in the left pair and C-3 in the right pair) and different at the other.
Again, this is a general characteristic of diastereomers.
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What is the relationship between the
"diagonal" pairs of compounds? |
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That is, (2R,3R) and the (2S,3R) compounds or the (2S,3S) ant the
(2R,3S) compounds.
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Notice that in each pair the absolute configuration at one chiral
center is the same (C-3 for the first pair and C-2 for the second), while the
configuration at the other chiral center is different.
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These pairs of compounds are diastereomers
(see the definition, above). |
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When a molecule has two chiral centers and
each chiral center has identical substituants the possibility of meso
compounds exists. In this case there are only three stereosiomers, not the maximum of
four. |
