The Grignard reaction (pronounced /ɡriɲar/) is an organometallic chemical reaction in which alkyl- or aryl-magnesium halides (Grignard reagents) attack electrophilic carbon atoms that are present within polar bonds (for example, in a carbonyl group as in the example shown below). Grignard reagents act as nucleophiles. The Grignard reaction produces a carbon–carbon bond. It alters hybridization about the reaction center. The Grignard reaction is an important tool in the formation of carbon–carbon bonds. It also can form carbon–phosphorus, carbon–tin, carbon–silicon, carbon–boron and other carbon–heteroatom bonds.
It is a nucleophilic organometallic addition reaction. The high pKa value of the alkyl component (pKa = ~45) makes the reaction irreversible. Grignard reactions are not ionic. The Grignard reagent exists as an organometallic cluster (in ether).
The disadvantage of Grignard reagents is that they readily react with protic solvents (such as water), or with functional groups with acidic protons, such as alcohols and amines. Atmospheric humidity can alter the yield of making a Grignard reagent from magnesium turnings and an alkyl halide. One of many methods used to exclude water from the reaction atmosphere is to flame-dry the reaction vessel to evaporate all moisture, which is then sealed to prevent moisture from returning. Chemists then use ultrasound to activate the surface of the magnesium so that it consumes any water present. This can allow Grignard reagents to form with less sensitivity to water being present.
Another disadvantage of Grignard reagents is that they do not readily form carbon–carbon bonds by reacting with alkyl halides by an SN2 mechanism.
François Auguste Victor Grignard discovered Grignard reactions and reagents. They are named after this French chemist (University of Nancy, France) who was awarded the 1912 Nobel Prize in Chemistry for this work.