Chemistry in your cupboard

Making the active ingredient soluble (2 of 3)

The answer to increasing the solubility of PCMX is to use a soap.

Soaps and detergents have ‘tadpole-shaped’ molecules in that they have a non-polar ‘tail’ and a polar or ionic ‘head’. The ‘tail’ can form van der Waals bonds with non-polar molecules whilst the ‘head’ can form hydrogen bonds with water. This is an example of the ‘like dissolves like’ rule.

The soap used in Dettol is made from castor oil which contains ricinoleic acid (systematic name (12-hydroxy-(cis)-9-octadecenoic acid), Figure 11.

Figure 11: Structure of ricinoleic acid

Historically, the positions of substituents around double bonds have been indicated by the terms cis (on the same side) and trans (on the opposite side).

An alternative notation makes it possible to deal with more complex cases. The E-Z notation is based on atomic numbers. We look at the atoms attached to each of the carbon atoms in the double bond. When the two atoms (of each pair) of higher atomic number are on the same side of the C=C, the isomer is described as Z, from the German word for together, zusammen. If not it is E, from the German word for opposite, entgegen

So, if the two atoms with the greatest atomic number are on the same side of the double bond as in ricinoleic acid, the configuration is Z.