Post Lab #4- Column Chromatography
Organic Chem 3418-2
March 3, 2011
The fluorene and fluorenone mixture was separated by first dissolving the mixture in heptane. Since “like dissolves like”, fluorene dissolves with the non-polar heptane and the polar fluorenone dissolves in the polar ethyl acetate solvent. This phenomenon was illustrated in class before the experiment, when it was pointed out why water will not dissolve fluorene, fluorenone, or transstilbene as readily as a nonpolar solvent such as heptane or toluene, or a polar eluent such as ethyl acetate. Those solvents have a much more similar structure to the solutes, whereas water’s structure is much different, making it not as ideal as a solvent for the materials with which we have been working. Heptane was added first to the column because it is a nonpolar solvent and the nonpolar molecules in fluorene were attracted to those in heptane. Dispersion forces (London forces) were stronger between the fluorene and heptane molecules (intermolecular) than between the molecules of each compound (intramolecular). As a result, the fluorene crystals were the first to appear in the heptane. Fluorenone followed the heptane because it is a polar molecule and formed hydrogen bonds with the silica gel (structure of O=S=O). In order to acquire the fluorenone crystals, ethyl acetate was introduced second because it is a polar solvent. Hydrogen bonds were formed between the ethyl acetate and the fluorenone molecules, so the fluorenone molecules were displaced from the silica gel adsorbent. Because ethyl acetate is held to the silica gel by hydrogen bonds between the oxygen atoms of the silica gel and the hydrogen atoms of the ethyl acetate, the ethyl acetate effectively competed for the surface of the silica gel. Discussion-
This lab began with two missed attempts at pipetting the fluorene/ fluorenone mixture because the incorrect (ethyl acetate) eluent was used first....
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