Preparation of the Grignard Reagent

The preparation of the bromobenzodioxole or bromobenzene is going to be the same no matter which one is used and no matter which precursor the chemist wishes to make. This means that this first part needs to be done correctly. This first part of preparation that Strike is talking about is the creation of a Grignard reagent out of the bromo compound starting material [125,131-134]. Mr. Grignard earned a Nobel prize for this in 1912 so you can bet that it's a pretty good procedure.

"Br

Bromobenzene

"Br

Bromobenzene

'MgBr Grignard Reagent

'MgBr Grignard Reagent

The procedure calls for the chemist to place a magnesium atom onto the bromine of the bromobenzodioxole which makes the complex amenable to exchange which you are going to learn about in just a bit. This means that the chemist is going to buy either magnesium powder or magnesium turnings which are thin curly-Qs of Mg that look like they came from a scouring pad. The procedure can be done in either tetrahydrofuran (THF) or ether, but THF is preferable. All the components need to be as dry as possible, which means that the solvent is dried through Na2S04 or distilled, the Mg is dried in an oven at 100°C for an hour and the glassware is baked in an oven at 425°C for at least 20 minutes. The apparatus to use is seen in figure 15.

[Figure 15]

In the three neck flask is stirred 12g Mg and 200mL THF. In the little separatory funnel is placed a mixture of either 100g anhydrous bromobenzodioxole or 78g bromobenzene (for speed) in 200mL THF. What is not pictured is a plastic tray that the apparatus will be resting in on top of the stirplate because the chemist may need to throw some ice water in it during the reaction. Although not entirely necessary, it is probably a good idea to squirt a little nitrogen into the head space of the three neck flask before the reaction begins. Another thing to consider before beginning is whether or not to place a catalyst into the reaction flask. What this means is that it is sometimes difficult to get a reaction going between the magnesium and the bromo compound and chemists often add a little insurance policy in the form of a tiny crystal of iodine or, as in ref 110, exactly 2 drops of dibromoethane into the reaction flask liquid. Like Strike said, it is not always done but it is best that the chemist does so anyway.

The reaction is started by dripping the bromo/THF mix from the separatory funnel into the stirred reaction flask. A vigorous reaction will begin to occur after a little addition. This will give off heat so the chemist adds ice to the ice tray and/or controls the dripping so that the temperature of the reaction stays below 55°C. After addition, the solution is stirred for another hour, during which time it will take on a dark, yellow-green color and just about all the Mg will have dissolved into the solution. That's it. The chemist now has her Grignard reagent which is the entire solution in the reaction flask. She needs to process this into the final product soon because the reagent won't keep forever.

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Responses

  • martha
    What mixed with mg to create a grignard?
    1 year ago

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