Grignard reagent
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Grignard reagents are alkyl, vinyl, or aryl-magnesium halides, with the general formula R-MgX, where R is the organic group and X the halogen (Cl, Br, I).
Contents
Properties
Grignard reagents are solid compounds, having a coloration between white and yellow, mainly due to impurities and side reaction products. They react violently with water, acids, alcohols, aldehydes, amines, esters, halocarbons, ketones, nitro compounds, but are stable and soluble in very dry organic solvents, like ethers (diethyl ether, dioxane, THF) and hydrocarbons like benzene, toluene, hexane. Other incompatible solvents are acetonitrile, carbon disulfide, DMSO, etc.
Synthesis
To obtain a Grignard reagent, you will need to do the following steps:
Preparing glassware and other equipment
- Make sure your glassware is perfectly clean and dry, with no traces of moisture or other contaminants
- Check the glassware for any defects, cracks
- The inert gas tank you're using must be checked before use
- Make sure your heating source works
- Dry your reagents and solvents thoroughly
Drying solvents
- To prepare dry solvents that can be used in the preparation of Grignard reagents, always check the compatibility of the solvents with the drying agent
- Akali metals can be used to dry ethers, saturated hydrocarbons
- Drying solvents works best under reflux
- Drying solvents under inert gas limits the amount of dissolved gases that may interfere with the reaction
Setting up your installation
- Add the magnesium flakes/powder in the reaction flask and a stir bar; a stir bar should also be added in one or both dropping funnels from your installation to aid dissolving the organic halide
- Connect the rest of the final installation
- Using a vacuum pump remove the air inside the installation, close the vacuum valve then introduce the inert gas (nitrogen or argon) inside the installation
- Repeat the procedure 2-4 times
- Turn on water cooling
Obtaining the Grignard reagent
- In a dropping funnel from the installation add the inert solvent (ether) and pour a small amount on the magnesium from the reaction flask; if you don't have a dropping funnel, you can use a septum to cover the glass joint and use a syringe or cannula to inject the ether in the reaction flask
- Add the organic halide or the ketone/aldehyde in the dropping funnel and dissolve it in the leftover ether
- Turn on the stirring and introduce an activator, like iodine, 1-bromopropane or 1,2-dibromoethane in the reaction flask, over the ether-magnesium suspension, which will cause the magnesium the react, and the ether will begin to reflux; an external heat source like a heat gun can be used to control the effervescence
- Once the reaction has calmed, slowly add the ether solution to the reaction flask and add the flask in a heating bath until your reaction has completed
- After completion, turn off the heating, cooling, remove the flask from the bath and installation, then extract your compound
Notes
- The use of inert gas may not be required if you can keep the interior of the installation perfectly dry; however, for most organometallic compounds, having an inert environment is crucial, as many compounds are air-sensitive
- A drying tube filled with calcium chloride can be used to limit the moisture that enters the installation
- If you're using a glovebox instead of a Schlenk line, the process is much simpler
- Any leftover Grignard reagent should be neutralized with alcohol
Uses
Organometallic reagents are used in Grignard reactions to obtain various alcohols, like tert-butanol from acetone.
- CH3-MgX + (CH3)2C=O → (CH3)3C-O-MgX
- (CH3)3C-O-MgX + H2O → (CH3)3C-OH + Mg(OH)X
Reaction with carbon dioxide will give the carboxylic salt of the organic group + the carbon from the CO2.
- 2 R-MgX + CO2 → (R-COO)2Mg + MgX
Reaction with an anhydrous metal halide, like zinc chloride, will give the organometallic compound of said metal.
- 2 R-MgX + ZnX2 → ZnR2 + 2 MgX2
This process is known as transmetalation.
Handling
Safety and storage
Grignard reagents are extremely sensitive to water and moisture. Work must be done under inert atmosphere.
Grignard reagents aren't very stable and should be used as soon as possible. Short term storage can be done in a flask filled with dry argon, kept in a cold place, like a fridge.
Disposal
Grignard reagents are best neutralized by adding them in a non-polar solvent, like toluene, where small amounts of alcohols are added to safely neutralize the Grignard reagent. This procedure must be done in a low oxygen environment, as there's still a small risk of fire.
