Question

When we perform column chromatography with silica to purify the reaction crude product, we sometimes need to change the solvent system to better extract our target product. For example, changing from pure hexane to 1:4 hexane:DCM. However, if the polarity is not change gradually, it often leads to column cracking, which means there are air bubbles (or is it solvent bubbles?) in the column, and it will affect elution the pattern. Why might changing solvent polarity crack the column? And where is the air coming from?

Answer 1

Changing solvent polarity in column chromatography can lead to column cracking due to differences in solubility and miscibility. The air bubbles in the column are caused by the vaporization of the solvent.

Step-by-step explanation:

Changing the solvent polarity in column chromatography can lead to column cracking, which is the presence of air bubbles in the column. This can affect the elution pattern of the target product. When the solvent polarity is changed abruptly, it can cause air bubbles to form and get trapped in the column.

The cracking of the column occurs because of the differences in solubility and miscibility between the old and new solvents. The rapid change in polarity creates a pressure difference within the column, causing the solvent to vaporize and form bubbles. These bubbles can disrupt the flow of the solvent and affect the separation of the target product.

The air bubbles come from the solvent itself. When the solvent vaporizes due to the pressure difference, the gas phase is formed, creating air bubbles in the column.