0.2 Grignard reaction  (Page 4/5)

Part 1:

Preparation of 1-methylbutyl magnesium bromide:

1. Prepare a 5.0 mL conical vial containing a magnetic spin vane and equipped with a condenser. Weigh and place 75 mg (approximately 3-4 pellets) of magnesium in the vial, and then add a small crystal of iodine (Do not add excess iodine), followed by 500 µL of anhydrous ether.

NOTE: Metal magnesium you will be provided with has a coil shape and has been cut into small pieces. This material should be handled with forceps only. Four to five small pieces of Mg weigh about 75 mg.

NOTE: Plastic syringes tend to get stuck when filled with ethyl ether. Be ready to push a bit harder on a syringe plunger when adding liquid to the reaction mixture.

2. Prepare a solution of 520 µL of 2-bromopentane in 600 µL of anhydrous diethyl ether in a dry 5 mL conical vial. Use a syringe to deliver each solution. TA will do that.

3. After the assembly has cooled to room temperature, then draw the 2-bromopentane solution into a 1.0-mL syringe and then insert the syringe needle through the rubber septum on the top of the condenser. (Do not heat the closed system, it may explode!!!!)

4. While stirring the heterogeneous mixture, slowly add 6-8 drops of the 2-bromopentane ether solution to initiate the formation of the Grignard reagent. The evolution of tiny bubbles from the surface of the magnesium is evidence of a reaction.

5. When the reaction has started, slowly add the remainder of the 2-bromopentane-ether solution drop wise over a 3 to 5-min period. Warm the reactants slightly. Upon completion of this addition, draw the rinse in the capped vial into the syringe and add it through the septum in a single portion to the reaction vial. Stir the resulting solution for 15 min.

Caution

Do not overheat. Overheating will cause loss of ether solvent. (The b.p. of ether is 34.6 °C.) Small fragments of magnesium may remain at the end of the addition of the alkyl halide.

6. Cool the gray-colored solution of Grignard reagent to room temperature.

Preparation of propanal (propionaldehyde)

1. Prepare a solution of the aldehyde by weighing 100 µL of propanal into a tared, oven-dried, 3 mL conical vial followed by the addition of 200 µL of anhydrous diethyl ether. Cap the vial after solution is ready. The propanal is the limiting reagent and therefore an accurate weight should be recorded for the yield calculations. TA

Part 2: grignard reaction

1. Now add the propanal solution carefully, with stirring, to the Grignard reagent over a period of about 30 sec at such a rate as to keep the ether solvent at a steady reflux.

2. Rinse the vial that contained the propanal solution with 100 µL of anhydrous diethyl ether and add to the Grignard reagent. This step insures that all the propanal is added to the Grignard reagent.

3. Stir the reaction mixture for 5 min and then allow it to cool to room temperature. Remove the conical vial and take off the cap. It is recommended that the vial be placed in a beaker to prevent tipping and loss of product.

Part 3: isolation of product

1. Hydrolyze the magnesium alkoxide salt by addition of 2.0 mL of water. Stir the resulting mixture for 5 min. A two-phase (ether-water) reaction mixture develops as the magnesium salt is hydrolyzed. Remove any unreacted magnesium.