One of the most frequent pains of almost every chemist dealing with the synthesis of new organic compounds is precisely the fact that almost no known reaction gives pure products that we can just take from the reaction mixture and proceed to do with them whatever we had in mind.

On the contrary, at the end of the reaction, the reaction mixture almost always contains, along with the desired product, some compounds that we didn’t want to get or that we don’t need. These can be some additional substances that are created in the reaction, sometimes it is even the initial compound, the one from which we started, which failed to react all the way through (that is, not all of it was transformed to a product), and also some substances that we may have used as additives or catalysts.

Be as it may, sometimes it is very difficult to extract from such mixtures a product that will be pure enough to be used either in further reactions or for some other intended purpose.

One of the most commonly used methods of purifying organic (and some inorganic) substances, i.e. compounds, is column chromatography. This method is based on the different division of substances in a vertical column with respect to their polarity when passing through the column. Namely, the column is most frequently filled with silica gel or aluminum oxide, which are very polar substances, suspended in some (less polar), usually organic solvent.

The solution of the sample that we want to purify is applied to the column prepared in this way, and it is passed through the column. Since it stands vertically, with the addition of solvent on the upper opening, under the influence of gravity, the sample will go down the column and finally come out purified on the other side.

It is only now that the real story begins! Given that the substances that make up the mixture are almost always of different polarities, the more polar ones will travel more slowly through the column. Why? Because the old, possibly the first rule you learned in chemistry (well, after the one about not licking a spoon in the laboratory…) applies here as well. Of course, the rule about similarity attraction. Thus, substances that are less polar will pass through the column faster, since they will be pulled through it by a solvent that is less polar than the filling of the column, while those that are more polar will be held by the filling more firmly, which will slow down their journey.

In the end, all that remains is to collect different fractions (some parts of the liquid coming out of the column) and determine which of them contains our desired, purified compound… Given that organic compounds are usually colourless, sometimes even this step is a challenge. Nevertheless, in the photos you can see one of the most beautiful examples of column chromatography, by means of which the coloured compounds were segregated, which certainly facilitated their identification, and also the determination of the beginning and the end of each fraction.

Author of the text and photos: Alen Bjelopetrović, Dr.Sc.