Formation of Grignard Reagents from Alkyl Halides
Formation of Grignard Reagents from Alkyl Halides: Grignard formation turns an alkyl halide (R–X) into a Grignard reagent (R–MgX) by reacting it with…
What happens
Grignard formation turns an alkyl halide (R–X) into a Grignard reagent (R–MgX) by reacting it with magnesium metal. The magnesium inserts between the carbon and the halogen; X can be Cl, Br or I. It is usually performed in dry ether, such as diethyl ether or THF. The carbon attached to Mg behaves as if it is strongly nucleophilic and carbanion-like (Rδ⁻–MgδXδ⁺), which makes Grignard reagents extremely useful for forming new C–C bonds. Notice how the carbon changes character: in R–X it is relatively electrophilic because the halogen pulls electron density away, and in R–MgX it becomes strongly nucleophilic. It is basically a personality swap for carbon.
Reaction summary
Alkyl halide R–X + Mg → Grignard reagent R–MgX (C–X → C–MgX)
Reagents and conditions
- Starting material: Chlorocyclohexane (alkyl halide)
- Reagents: Mg (magnesium metal)
- Conditions: Dry diethyl ether or THF
- Products: Cyclohexylmagnesium chloride (Grignard reagent)
Common mistakes
- Grignard reagents hate water. They are destroyed by water, alcohols, carboxylic acids and other compounds containing sufficiently acidic hydrogens.
- Functional group warning: groups such as –OH, –COOH, –NH or –SH can protonate and destroy the Grignard reagent. Always check for acidic hydrogens before trying to make or use a Grignard reagent.