Additional Chirality Elements
- A molecule with four substituents (a, b and c, d) arranged in pairs around an axis is chiral if these pairs do not lie in the same plane and each pair consists of two different substituents (that is, a is unequal b and c is unequal d).
This type of chirality is illustrated by certain molecules with cumulated double bonds.
In the allene 1,3-dichloropropadiene, for instance, two planes are defined by carbon C1 and its substituents hydrogen and chlorine, as well as C3 and its substituents hydrogen and chlorine. These planes are perpendicular. The molecule shows D2 point symmetry. The main C2 symmetry axis of the D2 point group lies along the cumulated double bonds. In the case of chiral molecules, this is called chirality axis. The two C2 axes of the D2 point group are perpendicular to the main C2 axis, lie within the planes of the formerly mentioned molecule, and pass through carbon C2. A prerequisite of the chirality of allenes, such as 1,3-dichloropropadiene, is the prevention of the rotation of carbon-carbon double bonds. 1,3-Dichloropropadiene contains no asymmetric carbon atom. However, it is chiral. Try to make the interactive molecular models of 1,3-dichloropropadiene superimposable by rotating them with your mouse.
As you have already seen, molecules I and II have a mirror-image relationship and are not superimposable. Therefore, they are enantiomers, and 1,3-dichloropropadiene is a chiral compound. (See animation!)
Additional examples of molecules that contain a chirality axis are ortho-disubstituted biaryls, in which the free rotation around the phenyl-phenyl single bond is restricted by steric interactions of the ortho substituents. If the rotational barrier is high enough that the enantiomers of a compound cannot interconvert rapidly, they can be isolated. The compound is then optically active.
- Separable stereoisomers that emerge from a restricted rotation around a single bond are called atropisomers.
At room temperature, a rotational barrier of about 70 to 90 kJ/mol is required in order for atropisomers to be isolated. In conformers the rotational barrier is much smaller, so that the conformers cannot be isolated at room temperature, as they interconvert rapidly. Butane, for example, has a rotational barrier of 14 kJ/mol.
The ortho-disubstituted biaryls III and IV are atropisomers. Their chirality axis lies along the phenyl-phenyl single bond. They have a mirror-image relationship and are not superimposable. Therefore, they are enantiomers although they contain no asymmetric carbon atom.