Grignard reagent (grēnyärd´ rēā´jənt), any of an important class of extremely reactive chemical compounds used in the synthesis of hydrocarbons, alcohols, carboxylic acids, and other compounds. Chemically, a Grignard reagent is an organic magnesium halide dissolved in a nonreactive solvent (typically dry ethyl ether). The substance is made up of an organic group, e.g., an alkyl or aryl group, joined by a highly polar covalent bond (see chemical bond) to magnesium, while the magnesium is joined by an ionic bond to a halogen ion, e.g., bromide or iodide. A Grignard reagent will react with water, oxygen, carbon dioxide, or almost any electrophilic organic compound. The reaction of Grignard reagents with aldehydes to form alcohols is of particular importance in the laboratory. Because Grignard reagents are so unstable, they are generally prepared just before use by reacting an organic halide, e.g., methyl iodide, with magnesium metal in a completely dry solvent; air is usually excluded from the reaction vessel, e.g., by flushing it with nitrogen. Grignard reagents are named after Victor Grignard, a French chemist, who received a Nobel Prize (1912) for their discovery.