RMgBr

RMgBr, or a Grignard reagent, is an organomagnesium bromide with the general formula RMgBr, where R is an alkyl or aryl group. It is a highly reactive species used to form new carbon–carbon bonds and to introduce the R group into various substrates. It is typically prepared by reacting an organohalide (R–X) with magnesium metal in an anhydrous ether solvent such as diethyl ether or tetrahydrofuran (THF).

Preparation and handling of RMgBr require rigorously dry conditions because the reagent is highly reactive toward moisture and air. Initiation can be slow and is often aided by small amounts of iodine or by using preformed Grignard solutions. The solvent choice (ether solvents) stabilizes the organomagnesium center through coordination with the Mg atom.

Reactions and scope: RMgBr acts as a source of nucleophilic RM units that add to electrophiles. With carbon dioxide, it forms a carboxylate that on workup gives a carboxylic acid (RCO2H). With aldehydes and ketones, it adds to yield secondary or tertiary alcohols after acidic workup (for example, reaction with formaldehyde gives a primary alcohol, reaction with acetaldehyde gives a secondary alcohol). Epoxides undergo ring opening with subsequent protonation to yield alcohols with extended carbon skeleton. Grignard reagents react with esters to give tertiary alcohols upon use of two equivalents of RMgBr. In the absence of suitable electrophiles, proton sources (such as water or alcohols) can protonate RMgBr to form the corresponding hydrocarbon (RH).

Applications and limitations: RMgBr reagents are versatile in organic synthesis, enabling formation of alcohols, alkanes, and various substituted products. They are incompatible with water, oxygen, and many functional groups (e.g., nitriles, nitro groups) under typical conditions. Storage is typically as dry solutions under inert gas.

Safety: RMgBr compounds are highly reactive, pyrophoric in some forms, and can ignite upon exposure to air or moisture. They require handling in inert, dry environments with appropriate protective equipment.