CHEM 234: Laboratory Procedure

THE GRIGNARD REACTION

Adapted from Microscale and Miniscale Organic Chemistry Laboratory Experiments, by Schoffstall, Gaddis, and Druelinger

Mg0 Ph Br Ph MgBr

O O OH Ph MgBr Ph MgBr Ph OEt Ph Ph Ph Ph Ph

Week 1

All glassware must be dried prior to starting this experiment. Glassware can be dried in an oven at 110°C for at least 1 hour prior to use. Do not put plastic parts in the oven. Dry a 50-mL round-bottom flask, a stir bar, a condenser, a 125-mL addition funnel, a Claisen adapter, a graduated cylinder, and several small Erlenmeyer flasks. Allow the glassware to cool to room temperature before beginning the experiment.

Weigh 960 mg of magnesium turnings and place in a dry, round-bottom flask that contains a stir bar. Assemble glassware. In an addition funnel, place 4.18 mL (6.28 g) of dry bromobenzene and 20 mL of anhydrous from a freshly opened can. Swirl to mix. Prepare an ice bath to cool the solution if the reaction becomes too vigorous. Add approximately 1 mL of the bromobenzene/diethyl ether solution to the round-bottom flask. Cloudiness and bubbles at the surface of the metal are indicators that the reaction has begun. This may take 5 minutes. If no reaction is observed at the end of 5 minutes, add a small crystal of iodine. If the reaction still doesn't start, consult the instructor.

Once the reaction has started, turn on the cooling water to the condenser and the stirrer. Add the remainder of the bromobenzene solution dropwise to maintain a steady reflux. The addition should take approximately 45 minutes. Rinse the addition funnel with 3 mL of anhydrous diethyl ether and add the rinse to the round-bottom flask. Fit the flask with a heating mantle and reflux the reaction mixture gently for 15 minutes. The final Grignard reaction mixture should be cloudy and most of the magnesium metal should be gone. Cool to room temperature and add more anhydrous diethyl ether as necessary to maintain the original volume. The Grignard reagent must be used immediately in the next step.

Dissolve 1.41 mL (1.48 g, 10.8 mmol) of ethyl benzoate in 10 mL of anhydrous diethyl ether in the addition funnel. Add the ethyl benzoate solution dropwise to the stirred Grignard solution at a rate to keep the reaction solution at gentle reflux. Have an ice bath available to cool the solution if the reaction becomes too vigorous. The addition will take approximately 20 minutes. After all of the ethyl benzoate has been added, reflux the mixture, with stirring, for 15-30minutes, or let stand until the next lab period if there is not enough time to complete the procedure. This is a good stopping point. Turn off the stirrer, disassemble the apparatus, stopper the flask, and seal it with parafilm until the next lab period. The flask should be nested inside a beaker in your drawer so that it doesn't tip over.

Week 2

Replace any diethyl ether lost by evaporation. The diethyl ether need not be anhydrous. Remove the stir bar. Pour the reaction mixture into a 125-mL Erlenmeyer flask that contains 8 g of ice. Rinse the round- bottom flask with additional diethyl ether and add the rinse to the Erlenmeyer flask. While stirring, carefully add 3 M hydrochloric acid dropwise to the solution. Have an ice bath available if the reaction becomes too exothermic. Use a stirring rod or spatula to break up the solid. Continue to add 3 M HCl until the pH of the aqueous layer (the bottom layer) is just acidic to litmus. Don't overacidify (it is unlikely that you will need more than 15 ml of HCl). Add more diethyl ether as necessary to dissolve the product.

Carefully pour the reaction mixture into a separatory funnel, leaving any unreacted magnesium in the flask. Rinse the flask with 5 mL of diethyl ether and add the rinse to the separatory funnel. The reaction mixture should separate into two layers as the alkoxide salt dissolves. The product is in the diethyl ether (upper) layer. Separate the layers. Wash the lower aqueous layer with 5 mL of diethyl ether. Combine the diethyl ether layers and wash with 5 mL of water. Remove the lower aqueous layer. Transfer the diethyl ether solution to a clean, dry Erlenmeyer flask and dry over anhydrous sodium sulfate for 10 minutes. Decant or gravity filter the dried solution into a tared 50-mL Erlenmeyer flask that contains a boiling chip. Rinse the drying agent with an additional 1 mL of diethyl ether. Carefully evaporate the diethyl ether under the hood to leave an off-white solid, which is a mixture of triphenylmethanol and biphenyl. Weigh the solid mixture.

Add 10 mL of (30-60°C). Stir the mixture thoroughly and vacuum filter. Rinse the crystals of triphenylmethanol with small amounts of petroleum ether and then air dry. Record the melting point of crude triphenylmethanol.

Recrystallize the crude product from isopropyl . This will require approximately 10-15 mL of . Weigh the purified triphenylmethanol. Prepare the product to be turned in.