Question

Use words and a diagram/structures to explain what role the solvent plays in the formation of the Grignard reagent.

Answer 1

Formation of Grignard Reagents from Organic Halides

q      The carbon atom of organic halide which is directly attached to the halogen is, of course, electrophilic. This electrophilic reactivity can be switched to nucleophilic reactivity by conversion to an organomagnesium halide, i.e., a Grignard reagent.

q      Especially note that the carbon-magnesium bond in a Grignard reagent is polar covalent with carbon being the negative end of the dipole. Thus the nucleophilicity of carbon in a Grignard reagent. Note also that the magnesium-halogen bond is largely ionic, as shown in the structure above.

q      We will consider the synthetic applications of the nucleophilic carbon atom present in Grignard reagents and other organometallic reagents in a later chapter.

q      The mechanism of formation of a Grignard reagent is shown below. Like the other reactions considered in this chapter, it also involves radical intermediates. There is one major difference, however. Grignard formation does not involve a radical chain mechanism. It is a non-chain radical reaction.

q      Note that the first step is rate-determining and involves the transfer of one electron from Mg (which has two electrons in its valence shell) to the carbon-halogen bond. This forms Mg⁺¹, which is a radical. This then couples with the alkyl radical formed.

q      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.

q      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.