Dipole-Induced-Dipole Interactions

Certain compounds, (e.g. those containing the aromatic nucleus and thus (p) electrons) are polarizable. When such molecules come into close proximity with a molecule having a permanent dipole, the electric field from the dipole induces a counter dipole in the polarizable molecule.

Figure 32. Polar Interactions: Dipole-Induced Dipole Interactions

This induced dipole acts in the same manner as a permanent dipole and the polar forces between the two dipoles result in interaction between the molecules. Aromatic hydrocarbons are typically polarizable compounds and an example of their separation using induced dipole interactions to affect retention and selectivity will be given later. A diagrammatic impression of a dipole-induced-dipole interaction is shown in figure 32. Just as dipole interactions must take place coincidentally with dispersive interactions so must induced dipole interactions always be accompanied by dispersive interactions. Thus, aromatic hydrocarbons can be retained and separated purely on the basis of dispersive interactions, for example in GC using a hydrocarbon stationary phase. Alternatively, they can be retained and separated by combined induced-polar and dispersive interactions using a polar stationary phase such as polyethylene glycol. Molecules need not exhibit one type of polarity only. Phenyl ethanol, for example, will possess both a permanent dipole as a result of the hydroxyl group and be polarizable due to the aromatic ring. More complex molecules can have many different interactive groups.