As a general
rule of thumb, chiral molecules must have at least one chiral
center--that is, a carbon that has four unique substituents
coming off of it. However, like most rules of thumb, there
are exceptions, and there are indeed examples of chiral molecules
that have no chiral centers, a few classes of which are mentioned
below.
The
Allenes
Allenes,
for example, compounds containing side-by-side double bonds,
can be separated into enantiomers, even though there are no
chiral centers. The central carbon in an allene is sp hybridized
and the substituents at either end are orthogonal to one another,
with one side of substituents going up and down in the vertical
plane, and the other side coming into and out of the horizontal
plane. Because of the rigidity of the double bonds, free rotation
cannot occur at normal temperatures and pressures, and there
is no interconversion between the two enantiomers, and they
can be separated from each other.
The
Biaryls
Another
class of compounds that can be chiral despite a lack of a
chiral center is the bi-aryls. Bi-aryls, compounds that have
two aromatic rings joined by a single bond, can be chiral
if they have bulky groups in their ortho positions that provide
a barrier to the single bond free rotation.
A biphenyl that can be separated into enantiomers
These
compounds are chiral because the steric bulk of the groups
in the ortho positions provide a large energy barrier to the
free rotation around the carbon-carbon single bond, and the
conformations are not easily interconverted. Such stereoisomers
that can be interconverted through a single bond rotation
are called atropisomers. Butane, for example, has conformations
that are atropisomers; however, unlike the biaryls, the barrier
to rotation is so small that they are interconverted rapidly
at room temperature, and they are, for practical purposes,
achiral.
Interconversion between the enantiomers of this biphenyl
is extremely
slow at room temperature because of the high barrier to rotation.
A practical
application of such enantiomerism is gossypal, a binapthalene
derivative, and a natural product that can be separated into
enantiomers because the barrier to free rotation around the
single bond is so high. One enantiomer of this compound (shown
below) was investigated by pharmaceutical companies for use
as a male contraceptive drug. Unfortunately , it was abandoned
because while it was shown to be succesful as a contraceptive,
it was also found to be toxic (so in a sense unintended by
the pharmaceutical companies, it is perhaps the ultimate contraceptive!).
Gossypal, a natural product
The
hexahelicenes
The hexahelicenes
are also an interesting case of chirality. Hexahelicenes are
large polycyclic aromatic hydrocarbons, that, in theory, are
flat because each carbon is sp2 and planar. In reality, the
aromatic rings are not flat, and wind into helices of either
a clockwise or counterclockwise rotation. In that sense, they
are similar to screws, which can be twisted in using either
a clockwise or counterclockwise motion of the screwdriver
depending on which way the threads are aligned.
|
|
Flat
(theoretical) |
Screw-like
helix (actual) |
|