12.3: Organometallics

Organometallics are molecules that contain a carbon-metal bond which has a high degree of covalency. There are two important types of organometallic species: Grignard reagents and organolithium reagents. These molecules are stable a low temperature (some of them are even stable at room temperature). We will use them as reagents in chemical reactions later on, but for now let’s talk about how to form them and what they can do as starting materials.

Let’s say that we have a bromobenzene. If we were to treat bromobenzene with magnesium metal, the metal would eventually dissolve and R-Mg-Br would form at the end of the reaction. The covalent R-Mg-Br species is known as a Grignard reagent. The reaction is exothermic and occurs in THF or Et₂O (diethyl ether) as solvent. The mechanism of this reaction involves radical-transfer reactions in which a transient carbon-centered radical is formed in the presence of a magnesium-centered radical. Transfer of another electron would form the stable organometallic Grignard reagent:

If lithium metal was used instead of magnesium metal, only one electron transfer event would occur, and an organolithium reagent would form.

Both of these reagents are highly basic. One can think of the carbon atom of the C-M bond (M = metal) as being strongly negatively charged (d^–). In turn, magnesium and lithium are positively charged (d⁺). In the presence of water (or any protic species), Grignard and organolithium reagents will protonate to give hydrocarbons. This is because the conjugate base (OH^–) is much more stable than a negatively charged carbon species. Remember that the pK_a of H₂O is 15.7, but that the pK_a of benzene is 43. Thus, water is a much better acid than benzene. Following this logic, if D₂O is used instead of water, one can generate deuterium labeled molecules.