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unusual chirality > odds & ends > home      

Unusual cases of chirality

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
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)

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