The Mole Concept Is Useful
Starting the Grlgnard Reaction To the Editor:
To the Editor: The article "An Improved Preparation of a Grignard Reagent" [1987,64,179] suggests that the technique of subliming a crystal of iodine onto magnesium t o start the Grignard reaction "has been either forgotten or overlooked" since 1905. In fact this suggestion appears in the current (6th) edition of Organic Experiments by Louis F. Viewer and Kenneth L. Williamson (Heath: Lexinrton, MA, 1987; p 83). When the Grignard reaction is carried out on a microscale (Williamson. K. L. Microscale Organic Experiments; Heath: Lexington, MA, 1987; p 285) 50 mg i f magnesium are weighed into a 10 X 100-mm reaction tube which has been dried in a 110 OC oven for 30 min. This avoids having flames in the laboratory. The tuhe is capped with a septum, 0.5 mL of anhydrous ether are added followed by dropwise addition of 340 mg of hromobenzene in 0.7 mL of ether. If thereaction does not start after the addition of a few drops of the halide solution, the magnesium is crushed with a stirring rod (an easv nrocess in the lone. -. narrow reaction tuhe). The reaction starts immediately. Even though the iodine sublimation method is also mentioned in this new laboratory manual, we have never found i t necessary, since Grignardreactions seem to start much more easily on a microscale than on a macroscale. Part of the reason for this may be the fact that the ether is drawn by syringe from a septum-capped bottle and is thus not exposed to the air.
