Grignard Reagents

A Grignard reagent is an organomagnesium compound with the general formula RMgX, where R is an alkyl or aryl group and X is a halogen (usually Br or I). Examples are methylmagnesium iodide, CH₃MgI, and ethylmagnesium bromide, CH₃CH₂MgBr. They are named after Victor Grignard, who won the Nobel Prize for discovering them.The key feature is the carbon-magnesium bond. Magnesium is much less electronegative than carbon, so the bond is polar with the carbon atom carrying a partial negative charge:R^δ−-Mg^δ+XThis is unusual. In most organic compounds, such as halogenoalkanes, the carbon attached to the functional group is δ+ and is attacked by nucleophiles. In a Grignard reagent the carbon is electron-rich and behaves as a nucleophile itself, attacking δ+ carbon atoms in other molecules.When a Grignard reagent attacks a carbon atom, a new carbon-carbon bond forms. This makes Grignard reagents one of the most useful ways to lengthen a carbon chain in synthesis. With carbon dioxide they give carboxylic acids, and with aldehydes and ketones they give alcohols (covered in the next lessons).Because the R group is so strongly nucleophilic and basic, Grignard reagents react rapidly with water, which protonates the carbon and forms an alkane:CH₃CH₂MgBr + H₂O → CH₃CH₃ + Mg(OH)BrWorked example: which alkane forms when methylmagnesium iodide meets water? The CH₃ group gains a hydrogen, giving methane, CH₄. This is why Grignard reagents must be kept completely dry.