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Before we can
discuss stereoisomers, we need to understand chirality. |
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Chirality is the property of
"handness". |
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Consider your hands. They are very similar structures, the fingers
of each hand extend from the top edge of each palm, while the tumbs extend from the side.
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Yet your hands are not identical... they can not be
superimposed on top of each other.
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Look closely... your hands are mirror images of each other.
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Organic molecules can be chiral
if they contain one or more chiral centers. |
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A chiral center is defined as an sp3 hybridized carbon
that is bonded to four different groups.
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Consider the compound 2-bromobutane: |
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Model (cpk / ball & stick) |
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chiral center (on / off) |
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The carbon which is bound to the bromine (the red atom) is a chiral
center.
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It is sp3 hybridized and it is bound to four different
groups: a hydrogen (white), a bromine (red), a methyl (-CH3) group and an ethyl
(-CH2CH3) group.
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The other carbons in this structure are not chiral centers. They are
all sp3 hybridized, but each is bonded to at least two hydrogens.
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2-Fluoro-3-bromobutane has two chiral
centers: |
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The second carbon (C-2) in this molecule (attached to the yellow
iodine atom) is sp3 hybridized and bears four different groups: a hydrogen, a
fluorine, a methyl group and a -CHBrCH3 group.
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Likewise, the third carbon (C-3, attached to the red bromine) is
also sp3 hybridized and bears four different groups: a hydrogen, a bromine, a
methyl group and a -CHFCH3 group.
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The remaining carbons (C-1 and C-4) are both sp3
hybridized but each bears three hydrogens, thus neither is a chiral center.
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If a molecule
possess at least one chiral center the possibility of stereoisomers exists. |
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