An Improved Preparation of a Grignard Reagent Timothy S. Eckerl State University College at Fredonia. Fredonia, NY 14063
Grienard reactions constitute one of the most important classes of synthetic organic reactions. As a versatile means of extending the carbon skeleton this class is without equal. Consequ&ly, it is commonly featured in a first-year organic chemistry experiment. Unfortunately, this experiment can frustrate students because the formation of the Grienard reagent itself is notoriously unpredictable, sometimes resisting all procedural aids. Such techniques as flame-drying glassware to remove adsorbed water, grinding magnesium to expose fresh surface, addition of iodine catalyst, handwarming the reaction mixture, and sonication of the hetero-eeueous mixture have all been tried with variable results. For the last two years, as a first step in synthesizing triphenylmethanol, my organic students have been preparing phenylmagnesium bromide: ether Ph-Br + Mg dP b M g B r Our first experiences with the preparation of this Grignard reagent confirmed the notoriety of the reaction. Despite trying the various aids described above, several students did not achieve the spontaneous boiling of ether indicative of the successfnl exothermic reaction and had to start aeain. Yet after further experimentation i t was found that flame-dryine the aooaratus in the presence of both iodine and maenesium generally resulted in ether-boiling soon after addition of the first nortion of bromohenzene solution to the maenesium-iodine mixture. T o determine more svstematicallv the value of this novel technique, I randomly divided the students into two groups: those who would add iodine to the magnesium before flamedrying and those who would add iodine after flame-drying but before addition of hromohenzene solution. The impressive results of the two groups, as measured by incubation times (i.e., the time elapsed from addition of the first portion of hromobenzene solution until steadv. snontaneous boiling of ether) are presented in the figure. +his figure clearl; indicates that addine iodine before flame drvine . .. meatlv .. shortens the incul~ationtime. The median incubation times (10 vs. 35061and the number of failed reactions (Oof 29 5,s.3 of 24) further accentuate the superiority of early versus late addition of iodine. Furthermore the carlv addition resulted in a 10% greater average yield of the r rig nard addition product, tri~henylmethanol. - lame drying probably served to sublime iodine onto a large surface of magnesium, thereby activating the metal. Althoueh " mv directibns for earlv iodine addition were not so explicit, flame-drying to the point of generating a purple vapor seemed to insure immediate subsequent reaction. Moreover one student, who after flame-drying mistakenly added iodine to a still warm apparatus, observed purple vapor and instantaneous reaction. None of 21 organic laboratory texts found to include a Grignard reaction mentions this technique of sublimed io-
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dine. I did recently encounter an old technique for activating maenesium with heated iodine.' Yet this technique from 1905 has heen either forgotten or overlooked because of its difficultv. The version presented here seems a simple but significant improvemeni in the preparation of rign nard reagents.
Phenylmagnesium Bromide About 1 mg of iodine and 0.68 g (0.028 moll of magnesium tumings, which had been ground with mortar and pestle, were put intoa 50-mL round-bottomed flask fitted with a Claisen connector, addition funnel,reflux condenser, and calcium chloride drying tube. The apparatus was flame-dried from bottom to top (Caution: diethyl ether and other flammable solvents must not he present) while a little purple vapor arose from the iodine. After the apparatus cooled, a solution of 3.92 g (0.025 mol) of hromahenzene in 13 mL (0.13 mol) of commercial anhydrous diethyl ether (Caution: all flames must he extinguished) was placed into the addition funnel and ahout 2 mL was added to the magnesium and iodine. Occasianally hand warming initiated spontaneous boiling. Then the remainins n hromohenzene solution was added slowlv to keeo the reaction mixturr rrflmring gently. When the addirion was ;omplete and ratluring hnd suhiidcd,the mixtdrr was hrnrpd togentle reflux una stram hnrh ior 10 mi". The coultd Crignnrd rpagent, phenvlmagnrsium bromide, was ready to he used ~
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