A class of organometallic compounds of magnesium, with the general formula RMgX, where R is an organic group and X a halogen atom (e.g. CH₃MgCl, C₂H₅MgBr, etc.). They actually have the structure R₂Mg.MgCl₂, and can be made by reacting a haloalkane with magnesium in ether; they are rarely isolated but are extensively used in organic synthesis, when they are made in one reaction mixture. Grignard reagents have a number of reactions that make them useful in organic synthesis. With methanal they give a primary alcohol

Other aldehydes give a secondary alcohol

With alcohols, hydrocarbons are formed

Water also gives a hydrocarbon

The compounds are named after their discoverer, the French chemist Victor Grignard (1871–1935), who discovered them in 1901.