Question

Column Chromatography Lab

1. What would happen if diethyl ether were put in the column first instead of petroleum ether?

2. The recrystallization lab required you to purify naphthalene which contained a small portion of fluorenone. Could you use column chromatography to purify the same mixture? Would the selection of solvents be different from those used in this lab?

Answer 1

1. The separation of mixtures through cholumn chromatography highly depends upon the polarity of solvents (mobile phase). It is advised first to start with non-polar or less polar solvents and then increase the polarity. By doing this, the compound which is less polar separated first from column leaving more polar compounds in the column. Once the first fraction of less polar compound is separated then use high polarity solvent to elute the polar compound. But when polar solvent is used first, both compounds (nonpolar as well as polar) starts moving from column resulting collection of mixture agin. So, in this case diethyl ether is polar solvent than petroleum ether and hence, should be used after petroleum ether.

2. both can be separated by column chromatography as fluorenone contains a C=O polar functionality where as naphthalene has no such polar group and behave as nonpolar compound as compared to fluorenone.