The reaction of an alkyl, aryl or vinyl halide with magnesium metal in ether solvent, produces an organometallic complex of uncertain structure, but which behaves as if it has the structure R-Mg-X and is commonly referred to as a Grignard Reagent.

The "R" group in this complex (alkyl, aryl or vinyl), acts as if it was a stabilized carbanion and Grignard reagents react with water and other compounds containing acidic hydrogens to give hydrocarbons (just as would be expected for a well-behaved, highly basic carbanion). In the absence of acidic hydrogens, the Grignard reagent can function as a powerful nucleophile, and is most often used in addition reactions involving carbonyl compounds, as shown above. The product of these addition reactions is typically a secondary or tertiary alcohol (primary alcohols can be formed by reaction with formaldehyde), as shown in the examples below; in these the carbonyl and halide portions of the molecules have been colored blue and red, respectively, to assist in understanding the component parts of the final products.

Carboxylate esters also react with Grignard reagents, undergoing the addition of two moles of reagent to give the final product. The reason for this is clearly seen in the example below, where the product of the first mole of addition is the simple carbonyl compound, which rapidly adds a second mole of reagent.

Strained ethers also react with Grignard reagents to give alcohols, as shown below for reaction with epoxides.

Ethers in four-membered rings can also undergo attack, although the reaction of ethers with Grignard reagent is commonly reserved for epoxides. The regiochemistry of the reaction with epoxides is generally dictated by steric factors.