Separating Fluorine and 9-fluroenone with TLC and Column Chromatography
Written by Alex
Discussion:
In this lab, both column chromatography and TLC chromotography were used to separate a mixture of fluorene and 9-fluorenone and detect their separation. In the first part of the experiment, a proper solvent was experimentally found. Through process of elimination, the best solvent for the fluorene/fluorenone mixture was found one at a time by performing a TLC chromatography with each solvent. This is done by spotting a TLC plate on a line penciled in 1cm from the bottom of the plate. Solvent is allowed to run up the plate. Marking the plate quickly for the top of the solvent level, the plate is removed from the solvent and placed under an ultraviolet light. Spots are shown under this light because the compounds that cause it absorb the light at the spectrum and therefore show up as dark spots on the TLC plates. When hexanes was used as a solvent, the spots had small Rfs between them, with the Rfs equaling to 0 and .33 respectively, indicating the solvent was too nonpolar. The next solvent tested, ethyl acetate, showed only one spot on the TLC plate with an Rf of .7, indicating that the solvent was too polar. Finally, the last solvent, 10% ethyl acetate & hexanes proved to be just right, having two spots with Rfs of .20 and .56, an optimal distance from each other, the starting point of the solvent, and the endpoint of the solvent. This makes sense because the mixture being tested has a nonpolar molecule, fluorene, and a polar molecule, fluorenone so naturally the solvent would have to have both qualities as it did.
Next, a column chromatography was performed using the newly found solvent as the mobile phase, the fluorene/fluorenone mixture, and silica gel as the stationary phase. The column chromatography is set up by rinsing a column chromatography tube with solvent after filling it 2/3rd full of solvent as well as with a slurry made up of mixing the silica gel and the solvent. After the column is rinsed and the silica has settled, a protective layer of sand is added on top of the silica gel as well as a column full of solvent that is run until the solvent level is just above the top of the silica. Next, the fluorene/fluorenone mixture is dissolved in 7 drops of hexanes and 7 drops of DCM and added to the column. And finally, after adding more solvent, rinsing until it is just above the silica level, and then adding more, the actual elution can begin. When running the elution, the liquid was collected in small glass test tubes every 2mL or so, totaling 6 test tubes in total. As the elution progressed, a yellow streak in the column separated out, this was the fluorenone. It moved through the column slower than the fluorene because it was much more polar. This differing in speed allows the two compounds’ independent collecting for use in the final TLC chromatography. Once the separated out fluorenone was completely collected from the column chromatography, the process is completed.
The final part of the experiment is one more TLC chromatography test with each of the test tubes collected to confirm the separation and the presense of both fluorene and fluorenone in the initial mixture. A spot from each of the test tubes are added to a TLC plate as well as a spot of pure fluorene and fluorenone. The solvent used in the column chromatography that was earlier determined, 10% ethyl acetate and hexanes, was also used as the solvent in the TLC chromatography. When put under the light, there was no spotting from test tube 1, but the solution from test tubes 2 and 3 showed a spot at 3cm with an Rf of .61, matching with the spot for the pure fluorene. This indicates that the solutions in test tubes 2 and 3 contained solely fluorene. Test tube 4’s solution yielded a faint spot at 3cm and a more visible spot at 1.5cm, indicating a crossover had occured between the fluorene and the fluorenone, which also showed a spot at 1.5cm. Finally, the spots from text tube 5 and 6 showed solely a spot at 1.5cm with an Rf of .31, matching it the pure fluorenone. These results illustrate that the fluorene and fluorenone were properly separated in the column chromatography and the order in which they appear on the TLC plate is also evident of this. Because fluorene moves faster through the column than the fluorenone, it makes sense that it will show up in the earlier test tubes because it is eluted first. Similarly the spots for fluorenone will show up in the later test tubes because of the longer elution time, which it did as illustrated by the results on the TLC plate.
Why Fluorene Elutes Before 9-Fluorenone
The two compounds have nearly the same carbon skeleton. Fluorene (C13H10) is a hydrocarbon with no polar functional groups. 9-Fluorenone (C13H8O) has a ketone (C=O) group at the 9-position, which makes the molecule noticeably more polar. PubChem lists a topological polar surface area of 0 Ų for fluorene and 17.1 Ų for 9-fluorenone, both coming from that carbonyl oxygen.
Silica gel, the stationary phase, is very polar. Its surface is covered with Si–OH groups that can form hydrogen bonds and other polar interactions with a sample. The carbonyl oxygen of 9-fluorenone interacts with these sites, so the molecule spends more of its time adsorbed on the silica. Fluorene can interact only through weak dispersion forces, so it spends more time dissolved in the mobile phase. In normal-phase chromatography, the less polar compound therefore moves faster. It travels farther on a TLC plate (higher Rf) and comes off a column first.
The polarity of the solvent controls how fast both compounds move. In pure hexanes, the solvent was not polar enough to move 9-fluorenone off the baseline. Pure ethyl acetate was polar enough to compete for the silica’s binding sites and carried both compounds together, giving one spot. The 10% ethyl acetate in hexanes mixture fell between these extremes, so fluorene ran well ahead of 9-fluorenone. That matches the results above: fluorene showed the higher Rf on TLC and appeared in the earlier test tubes, while the yellow 9-fluorenone band eluted later.
Sources: PubChem: Fluorene; PubChem: 9-Fluorenone; LibreTexts: TLC Separation Theory
Conclusion:
Through the first TLC chromatography, a solvent that worked well with the fluorene/fluorenone mixture was found through trial and error, the only way to really determine the best solvent. This step is crucial because this solvent will be used throughout the rest of the experiment and if the solvent does not work well, then the data collected later will be inaccurate at best. After the solvent was determined, a column chromatography was performed to separate the fluorene and the fluorenone from each other. The elutions were collected in 2mL increments totaling 6 total collections. Finally, a TLC chromatography was perfomed to compare the spotting from the elutions collected to the spotting of pure fluorene and fluorenone. Test tubes 2 and 3 showed singularly fluorene, test tube 4 showed a cross over between the collection of fluorene and fluorenone as evidenced by 2 spots being present at both positions, and finally test tubes 5 and 6 showed singularly fluorenone. This all in all, confirms the experiment was a success.