Bromobenzene reaction with, magnesium

Inspection of Table 11 shows that p for the reaction of aryl halides with tri-u-butyltin hydride was not solvent-dependent, whereas p for the reaction with magnesium was. The small values of p for the reaction of aryl bromide with magnesium in a polar solvent were again interpreted [81c] in terms of a mass transport-limited process. Thus reactions of aryl bromides would be transport-limited in THF and more polar solvents, but not in diethyl ether and less polar solvent. On the other hand, the rate of reaction of aryl iodides with magnesium seemed to be transport-limited, even in diethyl ether, whereas the rate of reaction of chlorobenzene with magnesium, which was lO slower than that of bromobenzene, was not. 

C for a further 45 minutes or until all the bromine has disappeared (no red vapours visible) and the evolution of hydrogen bromide has almost ceased. Transfer the dark-coloured reaction product to a separatory funnel and shake successively with water, with sufficient 5-10 per cent sodium hydroxide solution to ensure that the washings are alkaline to litmus, and finally with water. Dry with magnesium sulphate or anhydrous calcium chloride. Filter through a fluted filter paper into a small distilling flask and distil slowly. Collect the crude bromobenzene at 150—170 °C pour the residue while still hot into a small porcelain basin. Redistil the liquid of b.p. 150-170 °C (2) and collect the bromobenzene at 154-157 °C the yield is about 60 g (60%). 

Grignard reagents—phenylmagnesium bromide in this case—are always prepared by reaction of magnesium metal and the corresponding halide. Starting with bromobenzene, a suitable synthesis is described by the sequence... 

Organophosphorus compounds find wide use in the chemical industry as catalysts, intermediates, complexes, and end-use products. Arylphosphines and phosphine oxides are often produced by the reaction of a preformed Grignard reagent with a halophosphine or phosphine oxide. Yields are reduced by the production of unwanted side-reaction products such as biaryls. These unwanted products are reduced when the reaction is conducted under Barbier conditions. When alkyl and aryl halides are reacted with magnesium metal, a trihalophosphine or phosphine oxide, a metal halide or amine catalyst, in THE benzene mixtures, at reflux, good yields of phosphines or phosphine oxides are obtained [74]. For example, triphenylphosphine can be prepared in a 97.2% yield from the reaction of bromobenzene, trichlorophosphine, magnesium metal, aluminum chloride, and sodium chloride in THF-benzene at 70 80 C. 

Table 2.27. Yields of products from Barbier reactions of bromobenzene with magnesium and diethyl carbonate in various solvents [53]...

In 1895, four years before Barbier s report [6] on the one-step synthesis with magnesium, sodium was used in a one-pot reaction with bromobenzene and ethyl oxalate, the unforeseen product being triphenylmethanol [7]. 

Reaction of Bromobenzene with Magnesium Detailed Reaction Scheme 1963. Accurate analysis of the reaction mixture of bromobenzene and magnesium in diethyl ether [25], by yet another research group led to the rejection of an ionic mechanism which had been suggested earlier [26] a rather detailed reaction scheme for the Grignard reagent formation was presented which-in the authors view-was close to the mechanism proposed by Kharasch and Reinmuth. 

Indeed the formation of more than 20% benzyl alcohol was reported in the reaction of bromo- or iodoethane with magnesium and benzaldehyde (Table 2.23) and 30% in the uncontrolled reaction of bromobenzene with magnesium and benzaldehyde (Table 2.24). 

Tissier and Grignard, in one of the earliest publications on Grignard chemistry [6], found that for the reaction of magnesium with bromobenzene, iodobenzene and analogous organo halides, the metal usually had to be activated by the addition of a crystal of iodine. 

Sonication was ejffectively used [37] for synthesis of N-substituted benzamides in one-step reactions of bromobenzene or 2-methylbromobenzene, tert-huiyl isocyanate and a metal in diethyl ether or THF. With magnesium yields were almost quantitative. However, chlorobenzene proved to be unreac-tive under the same reaction conditions. 

Scheme 9.163. A representation of the formation of benzenecarboxylic acid (benzoic acid, C6H5CO2H) by reaction of the Grignard reagent [formed from bromobenzene (C6H5Br) and magnesium metal (Mg) in ether, (CH3CH2)20 solution] with carbon dioxide (CO2).

Now let s draw the forward scheme. Benzene is converted into bromobenzene upon treatment with Br2 and AlBrj (via an electrophilic aromatic substitution reaction). Bromobenzene is subsequently converted into phenyl magnesium bromide, which is then treated with carbon dioxide, followed by an acidic workup, to give benzoic acid. Conversion to the acid chloride, followed by reaction with dimethylamine, yields the desired amide. 

Fit a 50 ml. round-bottomed flask to a reflux water-condenser fitted with a calcium chloride tube. Dissolve 1-05 ml. of dry bromobenzene in 5 ml. of dry ether and add this solution to 0-25 g. of magnesium contained in the round-bottomed flask. Now add a crystal of iodine so that it rests on the magnesium. Warm if necessary to start the reaction if the latter becomes too vigorous immerse the flask in cold... 

Method 1. Arrange the flask containing the reaction mixture for steam distillation as in Fig. II, 40, 1. Proceed with the steam distillation until crystals of p-dibromobenzene appear in the condenser. Change the receiver and continue with the distillation until all the p-dibromobenzeiie has passed over from time to time run out the water from the condenser so that the crystals melt and run down into the receiver. Reject the residue in the flask. Transfer the first distillate to a separatory funnel, wash it with a httle water, and dry the lower layer with a little anhydrous magnesium sulphate or anhydrous calcium chloride filter. Distil slowly from a small distilling flask use a wire gauze or an air bath (Fig. II, 5, 3). Collect the fraction which passes over at 150-170° pour the residue (R), while it is still hot, into a small beaker or porcelain basin for the isolation of p-dibromobenzene. Redistil the fraction of b.p. 150-170° and collect the bromobenzene at 154-157° (3). The yield is 60 g. 

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