Grignard reagents or Grignard compounds are chemical compounds with the general formula RMgX(S)n, where X is a halide, R is an organic group (normally an alkyl or aryl), S is an ether, and n is usually 2. Usually, the ether groups are omitted from the formula. Thus, two typical examples are methylmagnesium chloride ClMgCH3 and phenylmagnesium bromide C6H5MgBr. They are a subclass of the organomagnesium compounds. Grignard compounds are popular reagents in organic synthesis for creating new carbon–carbon bonds. The carbon-magnesium bond in Grignard reagent is a polar covalent bond. The carbon atom has negative excess charge and acts as a nucleophile. Grignard reagents are rarely isolated as solids. Instead, they are normally handled as solutions in solvents such as diethyl ether or tetrahydrofuran using air-free techniques. Grignard reagents are complexes with the magnesium atom bonded to two ether ligands as well as the halide and organyl ligands. The discovery of the Grignard reaction in 1900 was recognized with the Nobel Prize awarded to Victor Grignard in 1912.
Synthesis
From Mg metal Traditionally Grignard reagents are prepared by treating an organic halide (normally organobromine) with magnesium metal. Ethers are required to stabilize the organomagnesium compound. Water and air, which rapidly destroy the reagent by protonolysis or oxidation, are excluded. Although the reagents still need to be dry, ultrasound can allow Grignard reagents to form in wet solvents by activating the magnesium such that it consumes the water. As is common for reactions involving solids and solution, the formation of Grignard reagents is often subject to an induction period. During this stage, the passivating oxide on the magnesium is removed. After this induction period, the reactions can be highly exothermic. This exothermicity must be considered when a reaction is scaled-up from laboratory to production plant. Most organohalides will work, but carbon-fluorine bonds are generally unreactive, except with specially activated magnesium (through Rieke metals).
Magnesium Typically the reaction to form Grignard reagents involves the use of magnesium ribbon. All magnesium is coated with a passivating layer of magnesium oxide, which inhibits reactions with the organic halide. Many methods have been developed to weaken this passivating layer, thereby exposing highly reactive magnesium to the organic halide. Mechanical methods include crushing of the Mg pieces in situ, rapid stirring, and sonication. Iodine, methyl iodide, and 1,2-dibromoethane are common activating agents. The use of 1,2-dibromoethane is advantageous as its action can be monitored by the observation of bubbles of ethylene. Furthermore, the side-products are innocuous:
Mg + BrC2H4Br → C2H4 + MgBr2 The amount of Mg consumed by these activating agents is usually insignificant. When treated with small amounts of mercuric chloride, magnesium pieces become coated with an amalgam, enhancing its reactivity. Activated magnesium, such as Rieke magnesium, circumvents this problem. The oxide layer can also be broken up using ultrasound, using a stirring rod to scratch the oxidized layer off, or by adding a few drops of iodine or 1,2-Diiodoethane. Another option is to use sublimed magnesium or magnesium anthracene. "Rieke magnesium" is prepared by a reduction of anhydrous magnesium chloride with potassium:
MgCl2 + 2 K → Mg + 2 KCl
Mechanism In terms of mechanism, the reaction proceeds through single electron transfer:
R − X
+ Mg ⟶ [ R − X ∙ ] −
+ [ Mg ∙ ] + [ R − X ∙ ] − ⟶ R ∙
+ X − R ∙
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