I want to remark some IMPORTANT points which are from personal experience after 100+ grignard rxns and hope it will help some bee. I think this one is one of the most problematic reactions. But its the most enjoyable to perform
The major disadvantages of the grig. rxn are the huge volumes and its highly exhotermic nature so its a real bitch for bigger scale (cant believe how long such a tiny bit of mg reacts and releases such heat!)
The first princple of a good start is VERY CLEAN Mg
This is the most important aspect and addition of I2 aims to clean the surface of mg.
The variables and their effects:
Water
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It doesnt harden the start of the rxn directly. But indirectly water may cause to form a oxide layer over the surface of the Mg and this may inhibit the rxn start. On these "wet" circumstances the Mg will darken. Adding I2 will reverse these but if you add too much it eases the formation of R-R byproducts (from R-MgX). Water also destroys the formed grignard reag thereby decreases the yield. (H2O has a very low molecular weight. This means that a "small" amount of H2O can destroy a huge amount of reagrnt. For ex: 6mls of h2o is 0,33 mole and it will destroy 50 grams of benzylmagnesium chloride !!)
R-X selection
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The possiblity of byproduct formation goes in this order :R-Cl
The reactivity goes in the same order
Heat
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Violent reactions tend to produce lots of byproduct so reagent is added drop by drop. The low bp of diet.eth. offers an advantage of lower rxn temps avoiding the already mentioned R-R byproducts.
Concentration
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I have seen violent+low yield rxns due to high concentration of reagents which would go smoothly with a high yield if diluted reag was used. My choice is using a 0,5M solution of the halide in ether and covering the Mg as 4M (1mole mg is covered w/250ml ether).
Initiation of a reaction:
I dont prefer starting a reaction by heating. This is a hot start to an exhothermic rxn - not good.
I never add more than 0,02 mole of the halide from the start and wait for it to react. In totally anhydrous conditions where brand new mg is used this happens within 10 seconds! If it doesnt start I continue by doing these in the order written waiting enough between each step
1) Add 1 tiny crystal of I2.
2) stratch the Mg turnings with a glass rod.
3) combine a few pieces of mg with 1-2 I2 crystal over a metal plate and stronngly heat with a flame when satisfied let it get warm and dump it in.
4) warm the rxn very slowly and have an icebath ready
Lastly I strongly reccomend diethyl ether for grignard newbees. It is usually dryer than thf. Its vapors dont let H2O inside. The disadvantage is its low bp and inability of disssolving many grignard intermediates.
THF from an open bottle must be distilled over na w/moist protection and it should never be let stand in open containers while measuring and dripping into the rxn. This brings neccesity of drying tubes.
Have lotsa ice and a fire extinguisher ready. We're headin' for grignard!!
