Question

1. Predict the order of elution of fluorene and fluorenone from an alumina chromatography column. Explain your answer in terms of each compound’s structure and polarity and its interaction with the alumina stationary phase

2. Upon development, the fluorene standard usually shows a spot due to fluorene and a second spot which has a much lower Rf value. What is the second component and why does it form?

Answer 1

1) Fluorene is a non-polar aromatic hydrocarbon while fluorenone is a less polar aromatic ketone.

Alumina (Al₂O₃) is a polar stationary phase. In case of chromatography, it must be remembered that “polar dissolves more”. The mobile phase used is a non-polar or less polar solvent like hexane or ethyl acetate, etc.

Since fluorenone is slightly polar then fluorene, fluorenone is retained by the stationary phase more. Fluorenone sticks on to the polar alumina particles more and spend considerable time in the stationary phase. The non-polar fluorene is more soluble in the non-polar solvent and is easily eluted. Consequently, the order of elution is fluorene and then fluorenone.