4.2 Organometallic compounds of magnesium

While beryllium makes a range of organometallic compounds, their hazardous nature has limited their study. In contrast, the ionic nature of calcium, strontium, and barium compounds limits the number of stable organometallic derivatives. However, the organometallic chemistry of magnesium is rich and extensive. The importance of Grignards (RMgX, where X = halide) and dialkyl magnesium compounds (R ₂ Mg) is due to their use in organic synthesis and as synthons for a range of organometallic compounds.

Grignard reagents

Grignard reagents (and the Grignard reaction using these compounds) are named after Victor Grignard ( [link] ). After studying mathematics at Lyon he transferred to chemistry, becoming a professor at the University of Nancy in 1910. During World War I, he was involved in the field of chemical warfare; however, it is for his major contribution to organic chemistry he is remembered.

Preparation

The general the synthesis of a Grignard reagent involves the reaction of an alkyl halide (RX, where X = Cl, Br, I) with magnesium metal in a suitable ether solvent, [link] .

While diethyl ether (Et ₂ O) and tetrahydrofuran (THF) are commonly used as solvents, other polar non-protic solvents are suitable, including: triethylamine (NEt ₃ ), dimethylsulphide (Me ₂ S), dimethylselenide (Me ₂ Se), and dimethyltelluride (Me ₂ Te).

In general the alkyl halide is added to an excess of magnesium suspended in the solvent. In most cases it is necessary to activate the magnesium, by the addition of iodine (I ₂ ), 1,2-dibromoethane, or sonication. If the halide is very inert reaction can be promoted by the co-condensation of magnesium and THF under vacuum.

There is often an induction period after the initial addition of alkyl halide. However, since the reaction, [link] , is highly exothermic care should be taken to ensure that the reaction does not run-away. For this reason it is normal to initially add a small quantity of the alkyl halide to ensure the reaction initiates. Once reaction is initiated, the addition of alkyl halide is maintained at a suitable rate to ensure the reaction is maintained until all the alkyl halide is consumed. The excess reaction magnesium is removed from the reaction mixture by filtration.

It is not always necessary to use a liquid or solid halide dissolved in the solvent. Bubbling methyl chloride (MeCl) through an Et ₂ O suspension of magnesium yields MeMgCl. The advantage of a gaseous alkyl halide is that the reaction is very clean as all the magnesium is consumed and the excess alkyl halide is bubbled away.

The purity of the magnesium is very important. For his original experiments Grignard used magnesium of a purity of 99.2%. However, it is now more typical to use 99.8% pure magnesium. It is important that the magnesium not be too pure since it is thought that the transition metal impurities catalyze the reaction.

The relative order of reactivity of the alkyl halide follows the trend:

In fact alkyl fluorides are sufficiently inert that highly coordinating polar solvents such as THF or dimethylformamide (DMF) must be used.