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Meso compounds are a special
case of stereoisomers which occur when a molecule has two chiral centers and each chiral
center has identical substituants. |
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Consider the stereoismers of
2,3-dibromobutane. |
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Both C-2 and C-3 are chiral centers, they are sp3 hybridized and
they have four different substituants: -H, -CH3, -Br and -CHBrCH3.
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Notice that both chiral centers have the same substituants.
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In this case, we have the possiblity of
stereoisomers called meso compounds. |
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There should be four stereoisomers of
2,3-dibromobutane: |
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This is a pair of isomers are enantiomers:
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(2R,3R) |
(2S,3S) |
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To see this clearly, rotate each molecule so that the Br attached to
C-2 (colored cyan so it is easy to spot) is pointed up and the methyl groups attached to
each C-2 are pointed towards each other.
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These molecules are non-superimposable, mirror images or
enantiomers.
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What about this pair of molecules? |
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(2R,3S) |
(2S,3R) |
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Rotate them as described above... they look like enantiomers, but
they are actually the same compound!
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To prove this, rotate one of the molecules 180o around
the y-axis (i.e. the vertical axis).
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Both absolute configurations shown above (i.e. 2R,3S and 2S,3R)
describe this compound well, yet neither is correct!
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The correct designation of this stereoisomer is (meso)-2,3-dibromobutane.
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The meso compound
is a diastereomer of both of the compounds shown on the top row. |
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Meso Compounds