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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:
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chiral center:
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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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Model:
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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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