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Discussion

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In Part A of the experiment, we compared the polarities of the unknown solution, #96, with two reference solutions, benzophenone and biphenyl, on a TLC plate containing silica gel in order to determine the unknown compound. It is known that benzophenone is largely non polar, but posses a polar carbonyl group, while biphenyl is a very non polar molecule as its structure is symmetrical and it lacks polar functional groups. On the biphenyl TLC plate, the sample and the reference spots were not separated, indicating that the sample compound and reference compound exhibited equal polarities and levels of adsorption. The relativity neutral solvent system allowed the unknown compound and biphenyl compound to travel across the plate based on their affinity for the silica gel adsorbent stationary phase. There was minimal interaction between the solvent and adsorbent and between the solvent and compound. The Rf value found  for biphenyl was 0.72 while the Rf value found for benzophenone was 0.75. This lower Rf value for biphenyl and the unknown compound indicates that it has a stronger affinity for the silica gel adsorbent on the TLC plate than the benzophenone reference compound. Therefore, the unknown compound adsorbed to the silica gel and did not travel as far as benzophenone did on the silica gel, resulting in the separation of the compounds. Thus, the unknown compound #96 is not benzophenone and is more polar than benzophenone.

In Part B, we examined the effect of two different solvent systems on the development of a TLC plate. Based on the polarity scale for solvents provided in Table 2 of the CHM1321 Organic Chemistry Laboratory Manual 2018, pure ethyl acetate (EtOAc) exhibits a greater polarity than hexanes. The pure ethyl acetate solvent strongly interacted with the silica gel due to its high polarity. 

In the elution of the TLC plates in the pure ethyl acetate solvent system, the high polarity of the pure ethyl acetate cause the benzophenone to travel a greater distance than the biphenyl along the silica gel on the TLC plate. This result was unexpected as it was hypothesized that the biphenyl would travel up a further distance on the TLC plate due to its non-polarity. The final outcome may have occurred due to possible sources of error, later explained in the discussion. Another possibility for this outcome is the high polarity of the pure ethyl acetate solvent causing it to interact sternly with the silica gel absorbent. The interactions of biphenyl and benzophenone with the solvent and the silica gel were weak in comparison, therefore, the attraction of the ethyl acetate to the silica gel forced both the biphenyl and benzophenone to travel a great distance on the plate. This can be seen through their high Rf values. The high polarity of ethyl acetate forced the polar benzophenone up the plate by interfering with any possible attraction between the spotted solutions and the silica gel absorbent. 

For the elution of the TLC plates in the hexanes solvent system, the plates displayed dots under the UV light that were relatively close to the starting point, seen through their low Rf values. Since hexane is a less polar solvent, it did not interfere with the attraction of the reference compounds to the silica gel adsorbent. Therefore, the reference solutions remained relatively close to the starting point and did not dissolve int he mobile phase of the hexanes solvent. 

Lastly, an unknown mixture, XX, was spotted in the sample lanes of three TLC plates and visualized with three different reference compounds, ortho-bromonitrobenzene, meta-bromonitrobenzene and para-bromonitrobenzene. This last experiment was preformed in order to determine the mole percent of the compounds in the unknown mixture to then determine the relative ratio of those compounds The use of the non-polar 1:9 EtOAc to hexane solvent system created minimal interactions between the solvent, the compounds and the silica gel. After conducting the experiment, it showed that the unknown compound, XX, had contained two isomers as it displayed two points on the TLC plate. After comparing the Rf values for XX and the three isomers, it was clear that there were two isomers that had the same Rf values as the XX compound. The Rf values of both ortho-bromonitrobenzene and para-bromonitrobenzene were similar to the Rf values of XX, thus, the unknown compound, XX, is a solution of both para-bromonitrobenzene and ortho-bromonitrobenzene. 

Possible sources of error that may have been encountered, creating mistakes in the lab are:
1.) cross contamination of the compounds; the capillaries used may have had remnants of previous compounds in them when putting drops on the TLC plates, causing additional spots on the plate
2.) too much solution may have been added onto the TLC plate, causing the spots under the UV light to become very large; this could have created unclear dots (streaky lines/spots)
3.) leaving the TLC plate in the solvent for a longer time could have cause the length of solvent absorption and the resulting Rf value to be perhaps greater than normal and very near the top of the plate

Questions

1.) The polarity of the solvent affects the molecular interaction of the silica gel with the compound, increasing the distance the compound travels up the pale. Increasing the polarity of the solvent will allow both polar and non polar compounds to travel a greater length up the TLC plate. Increasing the polarity of a solvent may slow down the separation of the compounds as the polarity of the solvent may interact very strongly with the polar silica gel, causing the compounds to experience a relatively small force of attraction and can move quickly up the plate.  Since polar solvents interact better with the silica gel, the Rf value will increase. This is because  interaction pulls the solvent and the polar compound higher up the plate. 

2a) The lowest Rf value will occur for benzyl alcohol since it is the most polar compound out of all three. It contains a hydroxyl group which can create a dipole movement and a hydrogen bond can form. When a compound if polar, more forces and hydrogen bonding occurs with the polar silica gel on the TLC plate. Thus, as the polarity of the compound increases, the less distance the compound will travel up the plate due to the pulling of the solvent as it is attracted more to the silica gel and remains stationary or relatively close to the starting point. This decrease of distance will cause the Rf values to decrease. 

2b) Out of the three given compounds, aniline would have the lowest Rf value. Aniline is the most polar compound because hydrogen bonds can form the with the two hydrogen atoms and it contains one lone pair election on the nitrogen atom where bonding can occurs. As stated before, the more polar a compound, the less distance it will travel up the TLC plate, causing the Rf value to decrease.

2c) Given the three compounds, benzoic acid would have the smallest Rf value as it is the most polar compound. This highly polar substance increases its tendency to form a hydrogen bond with the hydroxyl group. As stated before, the more polar a compound, the less distance it will travel up the TLC plate, causing the Rf value to decrease.

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