Separating Mixtures of Organic Compounds

When chemicals are mixed, several reactions often take place at the same time. Often, only one product is desired. As a matter of routine, chemists must often purify the product of a reaction to get rid of excess starting materials, side products, and degradation products. There are several ways to do this with varying degrees of difficulty.

Distillation

Since chemicals boil at a variety of temperatures, they can be separated by their boiling point.

Recrystallization

Growing crystals and washing them can yield a pure product.

Aqueous Workup

If the product is soluble in a solvent that the contaminants are not

soluble in, it can be extracted into that solvent. This is often performed in a separatory funnel.

Preparative Thin Layer Chromatography

A big plate with silica gel on it is spotted with a line of the mixture. When the edge of the plate is dipped into solvent, each compound in the mixture will run up the plate at a different speed.

Column Chromatography

If a mixture is put into a silica, alumina, or florisil column, polar compounds will move down the column more slowly than nonpolar compounds. Each compound will elute from the column at a different time. Unless the compound is colored, aliquots of liquid must be collected in test tubes every several minutes and checked to see if the sample is in them.

Flash Chromatography

Adding a pressurized inert gas to the column to speed up the separation.

Preparative High Performance Liquid Chromatography

A variation on flash chromatography that uses a mechanical pump instead of compressed inert gas.

Automated Flash Chromatography

Machines like a Biotage Flash Master can do your work for you.