Teaching topics such as partition coefficients in real-life contexts such as pesticides and drug absorption helps make chemistry more relevant to students. 

Partition coefficients and pesticides

[INSERT PE_EU_06_ima]

Pesticides are usually applied in aqueous solution but must pass through the fatty tissue of cell membranes of insects to be effective.

Would you expect pesticides to be more soluble in polar or non-polar liquids?

Pesticides should be more soluble in non-polar liquids that are similar to the fatty tissue in insects.

How will this be reflected in the partition coefficient for the partition of the pesticide between fatty tisue and water?

The partition coefficent will have a very high value. Values of partition coefficients are often quoted as logK because of the high value of K.

Octan-1-ol is weakly polar because of its long hydrocarbon chain and is often used in experiments to simulate the fatty tissue of insects. The partition coefficient K_ow of potential pesticides between octan-1-ol and water can be determined, which gives valuable information about how effective the compound might be.

What would the expression for K_ow look like?

K _ow = (concentration of compound in octan-1-ol) / (concentration of compound in water)

How might you introduce partition equilibria in pesticides into your lessons?

You could ask students to list the characteristics of an effective pesticide.
You could ask students to prepare a short presentation about pesticides.
You might ask students about the advantages and disadvantages of using a compound with a high partition coefficient between octan-1-ol and water as a pesticide.

Partition coefficients and drugs

[INSERT PE_EU_07_ima]

The ability to move from an aqueous solution across cell membranes is equally important in the development of a drug so that it can be absorbed in the body. The relative solubility of a drug in an organic liquid and in water is known as its lipophilicity. A cell membrane has a central layer that is lipophilic. Partition experiments using octan-1-ol are used in drug development to provide essential information about potential drug absorption in the body.

Why is it important that a drug molecule is sufficiently lipophilic?

So that the drug can move from aqueous solution into the central layer of the cell membrane.

Why is it important that the drug molecule is not too lipophilic?

So that the drug is not held in the central layer of the cell membrane but can move into the cell.