Grignard reaction

The Grignard reaction is an organometallic process where a Grignard reagent—an alkyl, allyl, vinyl, or aryl magnesium halide—attacks the carbonyl group of an aldehyde or ketone. It must be performed under anhydrous conditions and is a key method for forming carbon–carbon bonds.

The reaction and its reagents are named after French chemist François Auguste Victor Grignard, who first described them in 1900 and won the 1912 Nobel Prize in Chemistry for the discovery. Notably, making a Grignard reagent by reacting an organic halide with magnesium is not itself a Grignard reaction.

Classically, the Grignard reaction means adding a Grignard reagent to a ketone or aldehyde to produce a primary or tertiary alcohol. However, some chemists now apply the term to any reaction between a Grignard reagent and an electrophile. The Merck Index acknowledges this broader view, noting that the classical definition has been extended to include many electrophilic substrates. This has led to some debate within the chemistry community.

In terms of mechanism, the carbon in the Grignard reagent acts as a nucleophile because carbon is more electronegative than magnesium. It attacks the electrophilic carbon of the carbonyl group, typically through a six-membered ring transition state. For aldehydes or prochiral ketones, the stereochemical outcome can often be predicted using the Felkin-Anh model or Cram’s Rule. An alternative single-electron transfer mechanism, involving a ketyl radical intermediate, has also been proposed based on radical coupling byproducts. Recent computational work suggests that whether the mechanism is polar or radical depends on the substrate, with the carbonyl compound’s reduction potential being a key factor.

The reaction requires strictly anhydrous conditions; otherwise, the Grignard reagent acts as a base instead of a nucleophile, picking up a labile proton and failing to form the desired product. To avoid this, the reaction is run in an inert atmosphere to exclude water. If the starting material contains acidic protons, these can be protected by converting them into ethers or silyl ethers before the Grignard reaction.

Several variants improve chemoselectivity. Turbo-Grignards, modified with lithium chloride, are more selective and do not react with esters, amides, or nitriles. Adding other metals also modifies behavior: copper(I) salts yield organocuprates that favor 1,4-addition, while cerium trichloride enables selective 1,2-additions. Nickel and palladium halides catalyze cross-coupling reactions.