So far, all the stereogenic centres discussed involve a carbon atom with 4 different substituents attached to it (asymmetric centre). However, in case of trivalent central atoms having a lone pair of electrons, the lone pair of electrons may be considered as a fourth substituent. Usually, in such cases, the enantiomers will undergo Walden inversion from one form to other. The Walden inversion may however be prevented by locking using bulky substituents. Here, while the trisubstituted aliphatic amines easily undergo inversion, the same is not possible for aziridines as it would require twisting of the 3-membered ring. Similarly, in Troger’s base, the trivalent nitrogen present on bridge head cannot undergo inversion (Figure 1).

Figure 1

It is also to be remembered that for a compound to have enantiomers, it must be non-superimposable on its mirror image. This condition can also be satisfied by certain other compounds which do not have asymmetric centre. As an example, cis-octahedral complexes may exhibit enantiomers even though they lack an asymmetric centre (Figure 2).

Figure 2

Some compounds which do not have asymmetrically substituted carbon atoms (no stereogenic centre) may still be chiral if they feature two perpendicular planes which are not symmetry planes. If these disymmetric (chiral) planes cannot freely rotate against each other, the corresponding compounds are chiral. Compounds of this type are said to be axially chiral. Some examples of this type involve unsymmetrically substituted allenes, biphenyl derivatives and spiro compounds (Figure 3).

Figure 3