Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: rebecca88 on May 25, 2015, 07:49:31 PM
-
Hello everyone,
i'm trying to make methylmagnesium iodide grignard, 2M in THF. I dried everything thoroughly, added 2M methyl iodide in THF to the dry magnesium turnings, at a slow rate to keep the reaction going. After it was all added I gently refluxed to ensure completion.
The problem is, pretty much the entire flask turned into a thick white/grey precipitate. Is this the grignard? I was under the assumption that it was soluble in THF. Is this solid methylmagnesium iodide? Do I have to use diethyl ether instead (I don't have any so i'd rather not)?
Do you think some water was in there? Or the methyl iodide was added too fast?
Any help would be greatly appreciated.
Rebecca
-
If water or an active hydrogen impurity (e.g. alcohol) was there, the problem would have started before refluxing, together with an exothermic reaction.
I have read somewhere, that Grignards mainly form dimers in Et2O , while mainly monomers in THF. There might be a relationship.
However, pure MeMgI in THF or Et2O solutions, has a grey and sometimes a cloudy appearance because it is rather a suspension than a solution. So, add more THF and warm it carefully (Take care that meanwhile, there is no absorbed humidity).
-
just increase the amount of the solvent
-
I'm not sure what is happening here. Without looking anything up, my recollection is that R2Mg compounds are prepared in THF. Obviously, that may imply MgI2 may precipitate. What I don't recall is how this might be done. I think they add it to a prepared solution of the Grignard in ether (I don't know this).
My observation with using Grignard reagents is that as the concentration increases, they also become 'thicker'. It could be as others have suggested, the solution is near the saturation point and you simply need to add more solvent.
For those that may know, can you reflux Grignard reagents in THF, without ill effects? Obviously, butyllithium will react under these conditions. I also know that epoxies can rearrange due to the Lewis acidity of MgI2. Can MgI2 cleave THF?
-
Whether RMgI or R2Mg formation, depends on the molar ratio between RI and Mg. It also depends on the reaction conditions: Vigorous conditions favor the double substitution, while mild conditions favor the quantitative RMgI formation. Mild conditions means a slow addition (< 1 drop/sec) and a gradual heating to reflux.
-
I'm sure what is happening here.
1) Today I have prepared 2M MeMgI in Et2O.
2) I have added the same volume of THF in one portion.
Immediately intensive boiling and precepitation (as a result I have to clean all around :( ). I have filtered a small portion of this suspension and wash with THF. Tis precepitate easy dissolwed in water/ It is MgI2 (or MgI2*nTHF).
-
Good!
-
Schlenk Equilibrium