The Grignard reagent is a highly reactive organomagnesium compound formed by reacting a haloalkane with magnesium in an ether solvent. The carbon atom of a Grignard reagent has a partial negative charge. The Grignard reagent is a versatile material that can be used to form new carbon–carbon bonds. It acts as a nucleophile and attacks the carbonyl carbon atom to give an alkoxide which forms a salt with (MgBr)+. Hydrolysis of the magnesium bromide salt gives the alcohol. To determine how to combine two molecules to give an alcohol in a Grignard synthesis, examine the substituents bonded to the carbon bearing the hydroxyl group. One component of the new compound can come from the Grignard reagent. The other component must have been present in a carbonyl compound.
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Primary alcohols are made from a Grignard reagent and formaldehyde.
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Secondary alcohols can be made by reacting a Grignard reagent with an aldehyde.
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Tertiary alcohols can be made by reacting a Grignard reagent with a ketone.
The use of the Grignard reagent is precluded if there is an acidic hydrogen in the substrate selected to react with the Grignard reagent. This acidic hydrogen destroys the Grignard reagent before it adds to the carbonyl group. If the substrate has a hydroxyl group, it can be protected by forming a trimethylsilyl ether.
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