Question

a student loaded their mixture of acetylferrocene and diacetylferrocene onto column using 5ml of dichloromethane as the loading solvent. They used about 7 g of SiO₂ as the stationary phase. The student then eluted the column using petroleum ether. After collecting 8 fractions, the student analyzed them by TLC to find all of the compounds came off in the first two fractions, with no separation. Where did the student go wrong and what should they do differently when they try the column again?

Answer 1

There are 2 reasons for this.

When the student used 5 mL dichloromethane to load the column, the polarity was increased due to the presence of dichloromethane. When the petroleum ether was added, the eluting solvent was a mixture of petroleum ether and dichloromethane. This combination has increased polarity. The 2 compounds will be highly soluble in this solvent mixture and will not get partitioned on the silica gel. These compounds will move with the solvent front itself without separating on the column. In order to avoid this issue, the column should be loaded with petroleum ether itself and not with a polar solvent.

A second reason (not the main reason) is that depending on the quantity of the compounds, the quantity of silica may not be sufficient. Even if you increase the quantity of silica, you still have to address the issue of dichloromethane.