Question

Organic lab- II

Explain.Why are Grignard reagents usually prepared in ethers as solvent?

Answer 1

Explain.Why are Grignard reagents usually prepared in ethers as solvent?

Answer :

Grignard reagents form via the reaction of an alkyl or aryl halide with magnesium metal. The reaction is conducted by adding the organic halide to a suspension of magnesium in an etherial solvent, which provides ligands required to stabilize the organomagnesium compound.

Diethyl ether is an especially good solvent for the formation of Grignard reagents because ethers are non-acidic (aprotic). Water or alcohols would protonate and thus destroy the Grignard reagent, because the Grignard carbon is highly nucleophilic. This would form a hydrocarbon. But Grignard reagents are stable in ethers.

Another reason that ethers are good solvents for Grignard reagents is that the MgX bond is ionic and thus benefits greatly from being effectively solvated. The formation of ions in very nonpolar solvents, where they would not be effectively solvated is very difficult. Ethers are surprisingly good at solvating cations, because the C-O bond is relatively polar, thus allowing the oxygen end of the ether dipole to solvate and stabilize (electrostatically) the magnesium ion.

Empirical evidence suggests that the reaction takes place on the surface of the metal.

The reaction proceeds through single electron transfer. In the Grignard formation reaction, radicals may be converted into carbanions through a second electron transfer.

R−X + Mg → R−X^•− + Mg^•+

R−X^•− → R^• + X⁻

R^• + Mg^•+ → RMg⁺

RMg⁺ + X⁻ → RMgX