1. In table format, provide the Rf values you recorded for each of the 3 compounds in Part I. List compound on one axis of your table and solvent system on the other. If multiple spots were present for a compound, give Rf value for each component and state if the component was major or minor. | 100% hexane| 25% EtOAc in hexane| 10% EtOAc in hexane| Fluorene| 0.34 cm| 0.94 cm| 0.67 cm| 9 – fluorenol| 0.00 cm| 0.49 cm| 0.05 cm| 9 – fluorenone| 0.00 cm| 0.69 cm| 0.21 cm|
2. Explain how Rf values were generally affected by polarity of the solvent/eluent systems. As part of your answer, rank the solvent/eluent systems in order of decreasing polarity (this can be done according to structure of the individual solvents and % composition of the mixtures). Also rank the 3 compounds in order of decreasing polarity (this compound ranking can be based on structure as well as on these TLC results). Explain each of your rankings. Compounds: 9-fluorenone, 9-fluorenol. Fluorene Solvent: Ethyl acetate 10% in hexane, Ethyl acetate 25% in hexane, 100% hexane Polarity of each compound reacted differently to each of the solvents used.
As we know, ‘like’ interacts with ‘like’. Fluorene has the least amount of polarity based on structure and when it was mixed with non-polar and slightly polar solvents, its mobile phase moved the furthest. Such that when Fluorene was put in 25% EtOAc, which is 75% polar, its mobile phase was the furthest, followed by 90% polar in 10% EtOAc in hexane, and its most solid phase was in 100% hexane. 9–fluorenol is the highest polar compound based on structure and when it was mixed with a polar solvent its mobile phase was the slowest, therefore, it did not travel very far up the silica gel plate.
When 9-fluorenone was combined with 10% EtOAc, given about 90% polar, there was a slight mobile phase, moving just slightly up the silica gel plate. Its mobile phase moved the furthest when it was combined in 25% EtOAc in hexane, given about 75% polar, as the remaining 25% was able to move the compound up the polar silica gel plate. Based on the compounds and the Rf values, the compounds with larger Rf values means that the mobile phase traveled the furthest on the polar silica gel plate. Also, the compounds with the larger Rf value indicates that it was a less polar structure, as it interacts less strongly with the polar absorbent on the TLC plate.
3. Based on structure, which of the 3 compounds can hydrogen bond to the silica gel? Explain your answer. Do compounds that can hydrogen bond to the silica gel generally have lower or higher Rf’s than compounds that can not? The 9–fluorenol is the only one out of the three compounds that can hydrogen bond to the silica gel plate. Based on Rf values shown in question one, it had a lower Rf value, generally meaning compounds that can hydrogen bond to the silica gel will have the lower Rf values.
4. If you attempted to separate a mixture of the 3 compounds using liquid chromatography (see Ch. 18, Lab Techniques book), which compound would theoretically elute from the column first if 25% EtOAc in hexanes was used as eluent? Briefly explain your answer. Fluorene would elute from 25% EtOAc in hexane, since polar compounds are least likely to a have a mobile phase dislodge the fastest or furthest from the silica gel plate, which is polar.mIf the eluent were more non-polar, then there would be a mobile phase further from the solid phase, as non-polar compounds are able to move away from polar solvents because those compounds are not attracted to its polarity.
5. Based on TLC, is your dibromide pure and different from trans-stilbene? In your answer give Rf values for each compound (make sure to give solvent system as well!). Do your TLC results prove that your product is really trans-stilbene dibromide? Explain. Based on my results, my dibromide compound is different than trans-stilbene.
When both compounds were mixed in 100% hexane and 10% EtOAc in hexane, the Rf values were not so different. In 100% hexane, trans-stilbene’s Rf value was 0.39 cm and the dibromide had a value of 0.17 cm. In 10% EtOAc in hexane, trans-stilbene’s Rf value was 0.44 cm and the dibromide had a value of 0.65 cm. Since there was ‘lag’ the Rf value can be considered ambiguous and it is not very telling if the compounds are in fact different. When mixed in 25% EtOAc in hexane, the Rf value differed by 0. 02 cm (trans-stilbene value of 0.96 cm and dibromide value of 0. 4cm). The trans-stilbene and the dibromide do not share identical Rf values, but we can assume since there is only a 0. 02 cm different, it is not enough to say that they are two distinct substances; rather there is some mixture of trans-stilbene in the dibromide, because some of the characteristics may be similar.
6. Briefly discuss your TLC results for the “headache relief” tablets. Do the tablets seem to contain caffeine? Explain. Does TLC prove your answer? If not, what other “evidence” would you need to convict Wiley Coyote (chemical evidence that is)? Pure Caffeine| Unknown tablet| Acetaminophen| Acetylsalicylic Acid| EtOAc/Hexane/AcOH66:33:1| 0.01 cm| 0.02 cm| 0.26 cm| 0.26 cm| Based on the above results, one could assume that the tablet did indeed contain caffeine. When the pure caffeine was combined with EtOAc/Hexane/AcOH the Rf value was 0. 01 cm and the unknown tablet had an Rf value of 0.02 cm. Since there was a difference of 0.01 cm, it is not clear to say that there was no caffeine present. Also, since the difference was so minimal between the two compounds it can be suggested that there were significant amounts of caffeine in the unknown tablet.mBased on the TLC results, the Rf values can conclude there was caffeine in the unknown tablet since there was not a big enough difference in values to say that the unknown tablet contained any other substances.
7. Explain the key/simple difference between gas chromatography and liquid chromatography with regards to eluent (see Lab Techniques book). The mobile phase is an inert gas, generally He2 or N2 does not compete with the stationary phase like TLC or LC, where the mobile phase actively competes with the stationary phase. The mobile phase does not interact with the compounds during GC, while they do interact in liquid chromatography and TLC.
8. Later, we will cover mass spectrometry, and will likely also mention a combined analytical technique called GC-MS (gas chromatography-mass spectrometry). A good overview of GC-MS can be found on Wikipedia (http://en. wikipedia. org/wiki/Gas_chromatography-mass_spectrometry). Briefly, but clearly, explain how GC-MS would be better at solving the “caffeine crime” compared to simple TLC. GC-MS combines gas-liquid chromatography and mass spectrometry to identify different substances within a test sample.
It is used to perform specific tests that can positively identify the actual presence of a particular substance in a given sample. Although both GC-MS and TLC can identify substances in a given sample, the TLC is a more non-specific test that could lead to false positives identification. If the Rf values are identical, one can assume that the substance are the same and the GC-MS would be better to differentiate the Rf values for two separate substances in a given sample, giving a more accurate identification.