The Synthesis of Triphenylmethanol Via a Grignard Reagent

The Synthesis of Triphenylmethanol via a Grignard Reagent

In this experiment, the organometallic regent, phenyl magnesium bromide, a Grignard Reagent, was reacted with benzophenone and hydrolyzed to synthesize triphenylmethanol. The final product, triphenylmethanol, was weighed and the melting point and percent yield were determined. The limiting reagent in the reaction was benzophenone. The melting point measured was between 153 degrees Celsius and 157 degrees Celsius. The literature value for the melting point of the product is 164.2 degrees Celsius. The percent yield determined was fifty-nine percent. Although, the melting point is close to the literature value, it can be determined that there are impurities in the product. These impurities could be a result of the byproduct biphenyl, which occurs when two phenyl radicals combine during the synthesis of the Grignard Reagent, or because of the presence of water in any of the reaction tubes, which destroys the Grignard Reagent causing no reaction to occur.

Magnesium chips were crushed with a mortar and pestle to rid the magnesium of any oxide coating and placed in a microscale reaction tube. The oxide coating deactivates the surface of the magnesium chips. An iodine chip was placed in the reaction tube to activate the surface of the magnesium. The reaction tube was also equipped with fresh anhydrous calcium chloride, to ensure that the product was kept dry, and an ethanol-wetted pipe cleaner to act as a condensing mechanism for any vapors of the reaction. A bromobenzene/ether solution was added to the reaction tube slowly to initiate the reaction. Adding this solution too quickly caused the reaction to get out of control.

The first step in the reaction begins with the transfer of one electron from magnesium to phenyl bromide. The transfer of this electron produces two radical ions: a magnesium bromide radical and a phenyl radical via an intermediate free radical. A second one electron-transfer then occurs between the phenyl radical and the magnesium bromide radical to produce phenylmagnesium bromide, the Grignard reagent. If the phenyl radical migrates away from the magnesium bromide radical, it can react with another phenyl radical to form biphenyl, a common byproduct of the reaction. Evidence of the reaction was the formation of a brownish gray, cloudy material in the reaction tube and bubbling at the surface of the pieces of magnesium. Figure 1 illustrates the exothermic formation of the Grignard Reagent discussed above.

Figure 1- Formation of the Grignard Reagent