Synthesis of Organometallics

Organometallic reagents can be made in many different ways and these include;

1. Reacting magnesium with alkyl halides (in a ether solvent)

2. Deprotonating alkynes

3. Ortholithiation

4. Halogen-metal exchange

Making Grignard Reagents

Reacting magnesium turnings with alkyl (or aryl) halides in ether solvent makes Grignard reagents (or organomagnesiums) e.g. R-MgBr. This reaction then results in a solution of alkyl magnesium halide. (Br, Cl and I can be used.) As below

The alkyl/aryl halides used must not have any functional groups that will react with the Grignard reagent once it is formed.

The mechanism for this reaction is not known what is clear however is that the magnesium is somehow inserted into the carbon-halogen bond. It is known though,that the oxidation state of magnesium changes from (0) to (II). This means that this reaction is a oxidative addition

Making Organolithiums

These are made in a very similar way to the Grignard reagents by oxidative addition of lithium to alkyl halides.

Each reaction needs 2 lithium atoms and produces 1 lithium halide salt. For example:

Transmetallation

This is when the metallic group on a organometallic is swapped with another metallic group. This is simply done by adding the salt of a less electropositive metal e.g. magnesium to replace lithium. This then means the more electropositive metal ion will form an ionic salt while the other one will replace it in the alkyl group.

This is done because some organometallics have a high reactivity and produce unwanted side reactions.

Deprotonating Alkynes To Make Organolithiums

Alkynes are the most acidic of the hydrocarbons because they are sp hybridized. This means that they can be deprotonated by basic organolithiums for example ethylmagnesium. They can also be deprotonated by NaNH₂ which makes a organolithium as Na is a metal.

Halogen-Metal Exchange

This is literally just a reaction where the halogen and metal ion swap places. This reaction works because the end product is less basic and therefore more stable than the starting material. Although the mechanism is not known it is usually drawn as a nucleophilic attack on the halogen by the organometallic.

Ortholithiation

This involves making a organometallic by deprotonating a aromatic ring. This reaction is known as otholithiation because only the protons next to a oxygen or nitrogen functional group can be deprotonated.

This functional group acts as a guide so that the nucleophile attacks the correct protons. This reaction works because protons attached to a sp³ carbons are less acidic than protons attached to sp² carbons.

This is not a popular way to make organolithiums because the aromatic ring must not have certain groups or else the correct reaction will not take place.

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