Experiment - Grignard Reaction of Phenyl Magnesium Bromide with Benzophenone to Form Triphenylmethanol

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and the round bottom flask should be filled with purple vapors. BE CAREFUL

HEATING THE MAGNESIUM METAL AS EXCESSIVE HEATING COULD IGNITE THE

METAL.

Close the gas to the burner, put the burner away, and wait for the apparatus to cool.

You may not continue until all of the students have finished this step. Ether is very flammable

and may explode if ignited. Attach the water hoses to the condenser and turn on the water

flow. When the Instructor or TA gives the OK, you can continue the experiment.

Measure 25 mL of anhydrous ether into the large graduated cylinder. Transfer 6 mL of

ether into the small graduated cylinder and add 2.0 mL of bromobenzene to the small

graduated cylinder. Place one-forth (2 mL) of the bromobenzene solution in the small

graduated cylinder into the addition funnel and immediately add this to the magnesium

turnings. This concentrated solution should start the reaction and a more dilute solution will be

added to decrease the amount of coupling product formed. Start stirring the solution at a

moderate rate.

The formation of the Grignard reagent, Ph-MgBr, is accompanied by the formation of

bubbles and by a white precipitate from the surface of the magnesium metal. The reaction

should produce enough heat to boil the ether on its own. If you have difficulty in getting the

reaction to start, get help from Dr. Nichols. After the reaction has started and appears to be

going well, place the remainder of the ether in the large graduated cylinder into the addition

funnel and add to that the remaining solution of bromobenzene from the small graduated

cylinder. Add the dilute solution of bromobenzene from the addition funnel dropwise over the

course of 10 minutes. After all of the bromobenzene solution has been added, heat the

reaction mixture using a hot TAP water bath for an additional 20 minutes. (You may need to

refresh the hot water bath to keep the solution refluxing during the entire period.) At this point

the phenyl magnesium bromide has been prepared and the solution should be a light to dark

brown color.

Addition of the Grignard Reagent to Benzophenone. Into a tared, dry, 50 mL

Erlenmeyer flask, weigh 3.12 g of solid benzophenone. Add 10 mL of anhydrous ethyl ether to

the flask and swirl the solution to dissolve all of the benzophenone. Transfer this solution to

the addition funnel. While continuing to reflux the phenyl magnesium bromide solution, add

the benzaldehyde solution dropwise over the course of 15 minutes. If the reaction becomes

too vigorous, remove the hot tap water bath. After all the benzaldehyde solution has been

added, reflux the solution for an additional 20 minutes, with moderate to vigorous stirring.

Cool the reaction flask in an ice bath, place a glass stopper on the flask and secure it

with some parafilm. We will work the reaction up the following week.

Working Up the Reaction. With swirling, add 2 mL of ethyl alcohol, dropwise to the

round bottom flask containing the reaction mixture. Add with swirling, 10 mL of 3 M HCl, in 1

mL increments. You should see vigorous reactions occur as the white solid (a mixture of

product alkoxide and magnesium salts) and the magnesium metal dissolve in the acid and the

resulting alcohol product dissolves in the diethyl ether. You may need to add a few more mL

of ether and/or HCl until two clear layers are obtained. The top ether layer (which may be

slightly yellow) contains the triphenylmethanol product and the side-product biphenyl.

Separate the two layers, placing the organic layer into a clean, dry, Erlenmeyer flask. Add 2 g

of anhydrous potassium carbonate and 2 g of anhydrous sodium sulfate to dry the ether layer.

Swirl the solution and allow it to dry for at least 10 minutes. Transfer the solution to a 50 or

100 mL rb flask and have the ether removed using the rotary evaporator. The product should

be slightly yellow; this is the biphenyl side product. Wash the solid with 5 mL of hexanes

(petroleum ether) to remove the biphenyl side product. If time permits, you can recrystallize

the product using ethanol. Dry the sample in the oven for 10-15 minutes. Determine the

product’smass,meltingpoint,andanalyzeit by TLC for purity. Turn in the product on a

properly-labeled aluminum weighing dish.

Place all of the solutions in the waste container for proper disposal.

REPORT REQUIREMENTS

A written report will be turned in for this experiment. You will include the following (See the

Instructions for Preparing Organic Laboratory Reports for further details):

(2) Write out the objective of the experiment. Make sure you include the method of

product purity analyses and purifications (if any).

(3) Write the balanced chemical equation for the overall Grignard reaction. Include a

completed table of physical data for each reactant, product or solvent used. Include