Column Chromatography Lab Report

Post Lab #4- Column Chromatography
Organic Chem 3418-2 March 3, 2011

Theoretical Background-

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. The final attempt was more successful, but some of the dried fluorene and fluorenone did not make it into the pipette because the mixture was not completely evaporated, and because some of the mixture fell out of the watch glass. This, unfortunately, affected the final values for mass, though melting point evaluation was not affected.

Question 1-
The mass of the fluorene and fluorenone mixture was 0.268 g. If fluorene and fluorenone were in a 1:1 ratio by mass, the theoretical yield would be:
Fluorene (theoretical yield): ): 0.12 g mixture/2 components (fluorene and fluorenone)= 0.06 g fluorene
Fluorenone (theoretical yield): 0.15 g mixture/2 components (fluorene and fluorenone)= 0.075 g fluorenone.
Assuming that fluorene and fluorenone were in a 1:1 ratio by mass, the theoretical yield both compounds was 0.06 g, and 0.075g respectively. However in the experiment 0.12 g of fluorene were recovered and 0.15 g of fluorenone were recovered. Consequently, the percent yield of each compound was:
Fluorene: 0.12g recovered/0.06 g. theoretical x 100= 200% yield of fluorene
Fluorenone: 0.15g recovered/0.06g theoretical x 100= 250% of fluorenone
The percent yield of fluorene was 200% and the percent yield of fluorenone was 250%. Obviously, either this is a gross miscalculation on my part, or I have violated the laws of physics and thermodynamics by creating mass.

Question #2-
It was necessary to dissolve the mixture of fluorene and fluorenone before loading it on the column so that both fluorene and fluorenone could seep into the silica gel adsorbent.
If the mixture was not dissolved before loading, the fluorene would have dissolved in the heptane (the first eluent used in the column) and the fluorenone would have been left undissoved. It was necessary to dissolve the mixture in as little solvent as possible because it would take much longer for a high volume of dissolved mixture to run through the column. Also, a high volume of dissolved mixture along with the solvents would become unmanageable and require multiple watchglasses to collect a few milligrams of
crystals.
Question #3 Zubrick states that methanol as an eluent displaces all of the other molecules in the mixture (Zubrick: ). This is because methanol is highly polar, because of the alcohol functional group (methanol’s structure is:
And more precisely because of the oxygen atom containing two electron pairs, which creates a dipole moment in the direction of the alcohol group. As a result of methanol’s polarity, it displaces the other molecules in the mixture because those molecules are less polar than methanol and do not form the strong bonds that methanol does with the silica gel. Methanol forms hydrogen bonds with the polar silica gel molecules (O=S=O) because the hydrogen atoms on methanol bond with the negatively charged oxygen atoms on the silica gel molecule. Consequently, all the molecules in the mixture wold move with the methanol solvent front so none of the components would separate because all of the components are less polar than methanol.
Question #4-
In the experiment, the melting point range for a 1:1 mixture of fluorene to fluorenone was relatively sharp at 82-92C. A 3:1 molar mixture of fluorene to fluorenone has a broader melting point range of 60-90C. This is because there is more fluorene in the 3:1 mixture and fluorene has a higher melting point at 116-117C. So more fluorene in the mixture will cause the melting point range to increase due to fluorene’s higher melting point.

Conclusion-
The column chromatography experiment was conducted three times because the first and second trials yielded too much fluorenone, since there was too much ethyl acetate in the fluorene/fluorenone mixture. The third trial was relatively successful because 0.12g of fluorene was collected when heptane was used as the eluent and 0.15g of fluorenone was collected when ethyl acetate was used as the eluent. The initial melting point measured for the mixture was 82-92C. The final melting point measured for the recovered fluorene was 112-117C and the actual melting point of fluorene is 116-117C. The melting point of fluorene may have been somewhat lower because some fluorenone may have flushed out with the fluorene when all of the heptane solvent had run through the silica gel. The melting point range for the fluorenone was 82-85C. The fluorene crystals recovered had a white, grainy-flaky appearance, while the fluorenone crystals were grainy in texture and yellow in color. The percent yields were low, 33.3% for lfuorene and 66.6% for fluorenone (0.01g of fluorene and 002g of fluorenone). Some of the fluorene and fluorenone mixture may have stuck to the watchglass on which the mixture was dissolved, or on the sides of the pipette on the top of the column. As stated before, the experiment had to be started three times, with the last time being the most successful. Even though the last pass was the most successful, there were also errors with mass, since some of the mixture solute was spilled during the watchglass to pipette transfer. The individual fluorene and fluorenone compounds also might have been compromised after the distilled compounds evaporated on the watchglass, and were transferred to the scales.
Essentially, the experiment was a success because fluorene came out first with heptane as the eluent which was expected since fluorene and heptane are non-polar. Fluorenone followed fluorene, with ethyl acetate as the eluent. This was expected because both fluorenone and ethyl acetate are polar and create hydrogen bonds between their molecules.