Lab Report 6

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Feb 20, 2024

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Max Barberito 12174194 CH-231-019 March 7, 2023 Synthesis of Triphenylmethanol and Trityl Carbocation Introduction The Complementary Colors Company manufactures synthetic dyes, including many triphenylmethane dyes. Gilda Lillie, product development director for Complementary Colors Company, would like to develop some new colors to improve its market share in the dye industry. She has learned, for example, that having two para-dimethylamino groups on two of the three benzene rings in the parent structure yields a green dye, while having three of them yields a violet dye. The company’s technicians need to know how the kinds and positions of the substituents on the parent triphenylmethyl ring structure affect the color of triphenylmethane dyes. For that, they need a sample of the unsubstituted parent compound of these dyes, the triphenylmethyl carbocation. The assignment is to prepare triphenylmethanol and convert it to trityl fluoborate, a salt that contains the trityl carbocation.

Results Theoretical Yield Actual Yield Percent Yield Trityl Alcohol .02 mol benzophenone x 1 mol benzophenone/ 1 mol triphenylmethanol x 260.3 g / mol triphenylmethanol = 5.21g 2.24g 2.24g/5.21g=42.3% Trityl Tetrafluoroborate 1g triphenylmethanol / 260.3 g/mol triphenylmethanol x 1 mol trityl tetrafluoroborate / 1 mol triphenylmethanol x 330.1g / mol trityl tetrafluoroborate = 1.27 g trityl tetrafluoroborate .91g .91g/1.27g = 71.7% Crude Product Recrystallized Product Melting Point 113°C-128°C 148°C-152ּ°C

Pure Crude Biphenyl Triphenyl Methanol Benzophenone R f 20/44=.455 31/44=.705 20/44=.455 31/44=.705 39/44=.886 39/44=.886 20/44=.455 31/44=.705 (All length measurements in millimeters) Discussion In the experiment, .22 moles of magnesium, ,22 moles of bromobenzene, and 15 mL of diethyl ether were added to a round-bottomed flask. The magnesium was scratched, so the unoxidized layer beneath was exposed and ready to react. .02 moles of benzophenone were then added to a separatory funnel. This was then dissolved in 15 more mL of diethyl ether. The mixture in the separatory funnel then dripped into the solution over a span of around 10 minutes. The solution was then heated up in a warm water bath then cooled. Water and hydrochloric acid were then added to the mixture. All solids were filtered off. The two layers were then separated into two different flasks. The organic layer was then washed with 20 mL 5% NaHCO 3 . A small amount of magnesium sulfate was added to the organic layer until the mixture was not cloudy. The solids were once again filtered off. The solution was then put in the rotovap to evaporate the solvent. The crude solid was then recrystallized using 2:1 hexane: ethanol. The Grignard reagent was very difficult to produce. Scratching the magnesium was a slow and tedious process because the magnesium resisted the scratches and the unoxidized layer was difficult to expose. The first

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time the Grignard reagent was attempted to produce, the wrong diethyl ether was used, resulting in no reaction. Once the correct diethyl ether was used, producing the Grignard reagent was possible. Once the solution started bubbling, one could tell the reaction had started. The solution then turned murky. When the Grignard reagent was made, the solution turned pink. When hydrochloric acid was added to the mixture, the solution turned yellow, and a white precipitate formed. The melting point of the trityl alcohol found in the experiment was lower than the accepted values for both the crude and recrystallized products. The recrystallized product’s melting point was much closer to the accepted value, though. This could be because the crude product contained more contaminants than the recrystallized product, but the recrystallized product still contained some trace elements. On the TLC tablet, one could have seen that the pure and crude products both contain benzophenone and triphenyl methanol. If one were to increase the concentration of ethyl acetate in the solution system, the R f values would have increased. This is because it would have increased the polarity of mobile phase, causing the other compounds to be carried farther. Based on the results of the TLC tablet, there was biphenyl in the crude product, but there was none in the recrystallized product. No other impurities were found in the analysis. Considering the results, the melting points of the crude and recrystallized products resemble the presence and absence of biphenyl, respectively. Because the crude product contained biphenyl, it had a lower melting point, but the pure product had a higher melting point. The IR spectra of the two compounds does confirm that trityl tetrafluoroborate wis the cation. This is because, on the IR spectrums, most of the peaks and ranges made from the C-H, aromatic, and out-of-plane C-Hs are the same. However, on the trityl tetrafluoroborate, the stretching from the O-H bonds has a much lower peak than that of the pure triphenylmethanol.

This means that the -OH group attached to the potential carbocation appears much less often, resulting in carbocations. If there were no hydroxyl groups, the mark at 3470 cm -1 (the peak of the O-H stretching), would not be there at all, and all the compounds would be carbocations. Conclusion Regarding the initial problem statement regarding the effects of substituents on the color of the dye, the main factor that changes the color of the dye is the electron distribution in the pi- systems in the compound. The final color of the trityl cation was yellow and did not contain any amino groups. When two amino groups were added, the dye was green. When three amino groups were added, the dye became violet. This is because the amino groups add electron delocalization to the compound, resulting in different wavelengths of light produced. Questions 1. What is the white precipitate that forms when benzophenone is added to your Grignard solution? a. The white precipitate that forms after the addition of benzophenone to the Grignard reagent is a metal hydroxide. In this case, the white precipitate is MgOHBr. 2. Why was acetic anhydride used as the solvent in the conversion of trityl alcohol to trityl tetrafluoroborate? a. The purpose of the acetic anhydride was to remove water from the reaction mixture as it was produced, pushing the reaction to completion. 3. Two substances move with the solvent front (R f =1) during a TLC analysis using a silica gel plate with methanol as the developing solvent. Can you conclude that these two

substances are the same compound based on this experiment? If not, what additional experiment(s) would you perform? a. You cannot conclude that they are identical. One could re-run the TLC in a different solvent system and check that they have the same or different R f values. 4. Draw a synthesis of malachite green and crystal violet from commercially available materials (see your book for structures of these dyes). You can look to see if compounds are commercially available by looking in the Aldrich and/or Acros catalogs available in the lab, in the Science Library, or online. a.

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