Autor: yan000 28 January 2010
Words: 1850 | Pages: 8
Grignard Synthesis of Triphenylmethanol from Benzophenone
Purpose: The purpose of this experiment was to synthesize the tertiary alcohol triphenylmethanol from a Grignard reagent, phenyl magnesium bromide. The Grignard reagent was synthesized from bromobenzene and magnesium and then reacted with benzophenone to produce triphenylmethanol. It was important that water be excluded from the reaction, in order to prevent the formation of benzene. The reaction of phenyl magnesium bromide and benzophenone was quenched with sulfuric acid, and an extraction was performed in order to separate the organic phase containing the triphenylmethanol from the aqueous phase. The triphenlmethanol was then isolated and purified by crystallization and vacuum filtration.
Mechanism: The nucleophilic phenyl magnesium bromide attacks the electrophilic carbonyl of benzophenone to form a magnesium alkoxide. This is followed by hydrolysis by an acid to produce the alcohol, triphenylmethanol.
Procedure: The procedure followed was as is described in â€œLaboratory Manual for Organic Chemistry 2311,â€ Fifth Edition, Jane E. Wissinger, Thomas Custom Publishing, Mason, Ohio, 2006. pp. 34-37. Modifications of the procedure included adding an additional 1.25 mL of bromobenzene to flask containing the magnesium before the start of the Grignard reaction, because of a broken stopcock which let all of the original 2.25 mL of bromobenzene into the flask initially. Then, this additional bromobenzene was slowly added to the flask dropwise in an attempt to start the reaction. However, because the reaction did not begin, the entire procedure up until this point was repeated, this time with an intact stopcock and just the 2.25 mL of bromobenzene as specified in the lab manual. A crystal of iodine was added in order to start the reaction. During the crystallization, as the solution was boiling and after four or five full pipets of petroleum ether were added, no cloudiness was observed. Therefore, instead of adding more drops of petroleum ether, the solution was taken off the hot plate and allowed to cool.
Reagent/Product Table (bold= measured, italics = calculated):
1bpt/mpt, ◦ C
130 ◦C /-31◦C
1090◦C /650 ◦C
305 ◦C /47-51 ◦C
337 ◦C /10 ◦C
360 ◦C /164-165◦C
1Solubility in water
Irritant, environmental hazard
Eye and skin irritant
Data for Triphenylmethanol
Solid white, tiny crystals, no noticeable odor
2.25 mL C6H5Br x (1.49 g C6H5Br/1 mL C6H5Br) x (1 mol C6H5Br/157 g C6H5Br) x (1 mol (C6H5)3COH/1 mol C6H5Br) = 0.0214 mol (C6H5)3COH
3.48 g (C6H5)2CO x (1 mol (C6H5)3COH/182.2 g (C6H5)3COH) x (1 mol (C6H5)3COH/1 mol (C6H5)2CO) = 0.0190 mol (C6H5)3COH
0.480 g Mg x (1 mol Mg/24.3 g Mg) x (1 mol (C6H5)3COH/1 mol Mg) = 0.0199 mol (C6H5)3COH
The limiting reagent is benzophenone.
0.0190 mol (C6H5)3COH x 260.3 g (C6H5)3COH = 4.86 grams (C6H5)3COH
1.023 grams (C6H5)3COH Ñ‡ 4.86 grams (C6H5)3COH x 100 = 21 % yield
Observations: When flame drying the apparatus that would be used for the synthesis of the Grignard reagent, there was a large amount of water vapor contained within it, such that it took several long periods of flame drying (about an hour or so in total) before the magnesium could be added to the flask. This, in addition to adding all of the bromobenzene right away as previously stated in the procedure, perhaps prevented the reaction from beginning the first time. The magnesium and bromobenzene mixture in the flask did not become cloudy, even after the addition of a crystal of iodine. On the second attempt at synthesizing the Grignard, after adding a crystal of iodine to start the reaction, the mixture in the flask did begin to grow cloudy and small bubbles arose from the surface of the magnesium, indicating the start of the reaction. As the reaction progressed, the flask became increasingly cloudy and was slightly brownish in color from the iodine. Some bits of magnesium still remained at the bottom of the flask after about half an hour or 45 minutes, indicating that the Grignard reaction was not complete, but the benzophenone was added anyway due to time constraints. The solution became bright pink and after several minutes of swirling the flask at room temperature, it became a whitish pink color. The solution also became a thicker consistency, though it appeared to still be liquid for the most part. Because of this thick consistency, some of the product may have been lost when poured into the sulfuric acid/ice water solution, even with the addition of ether to try to rinse it out from the flask. After performing the extraction and washes, the triphenylmethanol, biphenyl, and ether solution appeared to be a clear yellowish liquid. In performing the crystallization in the hood, the solution never grew cloudy in appearance as the lab manual suggested, therefore after adding about four or five full pipets of petroleum ether, the solution was removed from the heat and after about 10 minutes, small white crystals were observed at the bottom of the flask. Further cooling in the ice bath for another twenty minutes or half an hour led to the formation of more crystals. Some of these crystals were lost in the process of transferring the solution to the Buchner funnel for vacuum filtration. The crystallization was repeated a second time with the remaining solution that was collected in the flask from the filtration, but after letting the flask cool at room temperature and then in an ice bath for another half an hour, no crystals were observed and therefore, the vacuum filtration was not repeated. Perhaps if there had been more time, some crystals would finally have been observed and a second filtration, followed by another crystallization and filtration in order to yield a greater amount of product.
Conclusion/Discussion: The synthesis of phenyl magnesium bromide from 0.46 grams of magnesium and bromobenzene, followed by the addition of 3.68 grams of benzophenone and then 50.1 mL of sulfuric acid yielded 1.023 grams of triphenylmethanol. This was a 21 % yield. Furthermore, the melting point range of 162.7-164.4 degrees Celsius, several degrees lower and wider than the literature range of 164-165 degrees Celsius indicate that although the product was indeed the expected triphenylmethanol, it contained some impurities. The impurities were most likely the unwanted side-product, biphenyl, caused by the radical coupling of the phenyl magnesium bromide with the unreacted bromobenzene.
The rather low percent yield can be attributed to a number of things. To begin with, the setup for the Grignard reaction had to be carried out twice, due in part to a faulty stopcock, which resulted in the addition of too much bromobenzene, which in turn probably prevented the reaction from starting by causing the formation of too much biphenyl. During the second attempt, however, which involved replacing the stopcock and acquiring all new reagents, the process of flame-drying the apparatus was overlooked. As a result, there was most likely some water or water vapor that protonated some of the carbanion and formed benzene. Another potential problem was probably not pounding the magnesium enough to create fresh active surfaces on the metal as well as not adding the magnesium to the flask quickly enough, resulting in some oxidation of the magnesium metal and preventing it from reacting with the bromobenzene. This further contributing to the small amount of phenyl magnesium bromide produced and the low percent yield in the end. In actually carrying out the Grignard reaction, the bromobenzene in the separatory funnel may have been added too quickly, also causing the production of a lot of biphenyl and not as much of the phenyl magnesium bromide. In addition, due to time constraints that arose out of having to perform the Grignard reaction twice, the benzophenone was added to the flask before all of the magnesium had reacted. This undoubtedly led to a lower amount of phenyl magnesium bromide that could be used to synthesize the triphenylmethanol.
More difficulties arose when trying to quench the reaction with sulfuric acid First of all, the note in the procedure to add the acid slowly to the water was overlooked. Then, it turned out to be quite difficult to completely transfer all of the Grignard reaction mixture to the acid/ice solution. The mixture was quite thick and seemed to have solidified to some degree, therefore even after rinsing the flask with ether, some of the mixture still remained stuck to the walls of the flask. More of the mixture was probably lost during the extraction and washes in the separatory funnel, simply by not being careful enough when removing the aqueous layer to ensure that some of the organic layer was not accidentally drawn off along with it. Some of the triphenylmethanol may also have been lost when filtered to remove the drying agent or not enough drying agent might have been used.
Problems encountered during crystallization that may have contributed to the low percent yield and impure product included not adding enough petroleum ether. Petroleum ether was supposed to be added first by full pipets and then dropwise after the solution turned cloudy in order to remove the biphenyl. However, because the solution never turned cloudy, the dropwise addition of the petroleum ether was not carried out, and instead the solution was simply removed from the heat and allowed to stand at room temperature and then in an ice bath to crystallize. Allowing more time for this crystallization probably would have led to less biphenyl in the product and therefore a greater purity. When the product was transferred to the Buchner funnel in order to vacuum filter it, it was poured too quickly so that some of the product spilled out of the flask or onto the sides of the funnel instead of onto the filter paper. Although the crystals were scraped from the original flask as well as from the funnel and added to the solution that fell through the funnel in order for a second crystallization to be performed, no other crystals formed during this second attempt. Perhaps not enough time was allowed for the small amount of triphenylmethanol remaining in this solution to crystallize.
Overall, this experiment was a very good lesson in the importance of careful transfers, careful setup of the equipment, and patience. Most of the problems encountered could have been avoided by being more careful and also with practice. If this experiment were performed again, precautions such as better flame-drying, a slower and more careful addition of bromobenzene and a quicker transfer of the magnesium would result in the synthesis of a larger amount of the Grignard reagent. More careful addition of the Grignard reaction mixture to the acid, followed by a more careful extraction, crystallization, and vacuum filtration would have yielded a greater amount of as well as a more pure product.