Grignard reaction: methylmagnesium bromide and benzaldehyde

Grignard reagents contain a carbon–magnesium bond that is polarised towards carbon, so the carbon behaves like a carbanion. They add to aldehydes and ketones to form new C–C bonds; an acid workup then protonates the alkoxide to give an alcohol.

Try it in the builder →  Change the reactants or conditions and generate your own mechanism.

Nucleophilic addition (Grignard)

The methyl carbon of CH₃MgBr attacks the carbonyl carbon of benzaldehyde, giving a magnesium alkoxide. Adding dilute acid protonates the alkoxide to give 1-phenylethanol, a secondary alcohol.

Overall reaction

1. 1Grignard reagent adds to C=Onucleophilic addition

   methylmagnesium bromide

   +

   benzaldehyde

   alkoxide

   +

   MgBr⁺

   Electron movement

   1. The electrons of the C–Mg bond attack the carbonyl carbon, forming a new C–C bond.
   2. The C=O π electrons move onto oxygen.

   Carbon is more electronegative than magnesium, so the methyl carbon is δ− and nucleophilic. The reaction must be kept dry: water would destroy the Grignard reagent by protonating it to methane.

2. 2Acid workupproton transfer

   alkoxide

   +

   H₃O⁺

   H₃O⁺

   1-phenylethanol

   +

   water

   Electron movement

   1. The alkoxide oxygen takes a proton from H₃O⁺.
   2. The O–H bond of H₃O⁺ breaks, leaving water.

   Dilute acid is added only after the Grignard step is complete. It protonates the alkoxide to give the alcohol.