Lab #11
Grignard Reaction

Procedure: Attach a 50 mL RBFlask to a reflux condenser. At the top of the condenser place a drying tube. DO NOT SEAL THE SYSTEM; moisture must be excluded, and this can be accomplished with a drying tube.

Remove the flask and place in it 0.8 gms of magnesium turnings (flammable), 2.0 mL of bromobenzene (toxic), and 5 mL of anhydrous ethyl ether (listed/flammable/toxic) (commercial anhydrous ether can be used without further treatment). Start the reaction by placing in the flask a piece of unpolished tubing; twist the tubing between the thumb and the forefinger so as to cause a scratching effect on the magnesium. Reaction will start when some fresh surfaces have been exposed on the metal. If the reaction doesn't start within 5 minutes, the equipment or reagents probably contain water, a substance that inhibits the reaction. Check with your instructor before you start the experiment over. If your instructor cannot get the reaction started, begin again. After the reaction has started, fit the flask to the condenser and carefully add, through the top of the condenser, an additional 5 mL of ether. Keep a beaker of cold water at hand for moderating the reaction if it becomes too vigorous. When the reaction ceases, or is proceeding at a very slow rate, cool the flask and decant the liquid portion into a dry Erlenmeyer flask, leaving unrelated magnesium in the flask. Lightly cork the flask and cool its contents in an ice bath. Add, in about 6 portions, a mixture of 4 gms of benzophenone (toxic) in 15 mLs of anhydrous ether. When the addition is complete, stir the mixture with a glass rod for a few minutes to insure the completion of the reaction.

 Acidify the addition compound as follows:

 Place the flask in a beaker of ice and water, and add 5 mL of ice water and then,in portions, enough dilute (6M) Hydrochloric acid ( corrosive) to dissolve the magnesium hydroxide which has formed ( an additional 5 mL of ordinary ether can be added at this point if necessary). Separate the ether layer which contains the carbinol, wash it with 5 mL of water, then with 5 mL of 0.6M sodium bicarbonate, and finally with 5 mL of water. Separate the ether layer, and dry it over anhydrous magnesium sulfate.

The ether solution contains, in addition to the carbinol, some diphenyl (biphenyl) which forms during the preparation of the Grignard reagent. The impurity is removed by crystallization of the carbinol from ligroin. Add 8 mL of ligroin (flammable) to the mixture; then remove the ether by distillation from a hot water bath (no flames in the laboratory at this point.) Cool the receiver flask in a cold water flask.

 After removal of the ether, place the flask in a cold-water bath, then in an ice water bath until crystallization of the carbinol is complete. Filter and dry the solid.

CHEMICALS:
magnesium
Ethyl Ether (listed)
Benzophenone
(6M) Hydrochloric acid (corrosive)
0.6M sodium bicarbonate
magnesium sulfate anhydrous
ligroin ( flammable) Petroleum Ether

WASTE Spill & Disposal F: Ethyl Ether (listed), Benzophenone, (mixture of bromobenzene, ligroin, magnesium, benzophenone, and washes)

WASTE Spill Only: (non-listed)Bromobenzene, Ligroin use Disposal D:

WASTE Spill Only Use Disposal B1: 6 M HCl, 0.6 M Sodium Bicarbonate
DO NOT PUT USED WEIGH BOATS IN THE HW CHEMICAL CONTAINER. CONTAMINATED PAPER TOWELS AND WEIGH BOATS ARE PUT INTO THE CLEAR PLASTIC HW BOX IN THE HOOD. PAPER ENVELOPES W/CHEMICALS SHOULD BE FOLDED MULTIPLE TIMES AND SEALED BEFORE BEING PUT INTO THE SPECIFIED HW CONTAINER.