[Oh_outstanding]

i was wondering about jump starting a Grignard reaction...

1) Apparently this can be done by adding a small crystal or two of Iodine. But I am not sure how this is done. Why Iodine?

A balky Grignard reaction can also be jump started by adding a small amount of previously prepared Grignard. Why and how does previosly prepared Grignard go about doing this?

2) When bromobenzene, Magnesium and ether are combined phenylmagnesium bromide is produced (an ugly brown color).
But I noticed small bubbles form on the surface of the Magnesium along with a cloudy precipitate.
According to my understanding the bubbles would be hydrogen gas (H) that was replaced by the MgBr. As for the precipitate I am not sure that it actually is because the grignard reagent is another color. (ugly brown in my case for phenymagnesium bromide).

3)What would happen if I used Et2O (diethyl ether) instead of petroleum ether to remove the biphenyl from the crude triphenylmethanol.

4) A high concentration of bromobenzene helps to get the reaction started but it can promote a formation of biphenyl on the metals surface. Does anyone have any pointers on how this actually happens (mechanism wise)?

None of these are homework questions! They are formed from pure curiousity! Any pointers would benefit. Thanks

[Chudasama]

iodine can help to "jump start" a Grignard reaction by reacting with magnesium to form magensium iodide, which helps to remove water from the "dry ether" and thus prevents the Grignard from being destroyed by water (acidic proton).

another possible reason for the addition of iodine, though i'm not 100% sure, is to help remove the oxide layer on the magnesium, therefore increasing magnesium surface area exposure and hence increasing the rate of reaction.

hope that helped with one of your queries.

[movies]

I've never heard the explanation that iodine ultimately removes more water from the solution.  I think what really happens is that the oxidative addition of Mg(0) into I₂ is very fast, and then by boring into places where only a small amount of Mg(0) is exposed and therefore the surface area of exposed Mg(0) increases and that increases the rate of Grignard formation.

I will say that I have tried the iodine method and it has never worked for me.  I have found that a better initiator is 1,2-dibromoethane.

[Ψ×Ψ]

Hmmmm.  I've been doing some large-ish scale Grignards, but what I was told is that the iodine is more of an indicator.  I usually add the I₂ along with the Mg, then add a little of the alkyl/aryl halide, then hit it with the heat gun until the brownish color disappears.  At that point, the reaction is pretty well going on its own and the rest of the halide can be added (slowly). 

[Custos]

For really difficult Grignard's you can make a more activated form of magnesium by simply stirring it in ether (or THF) over night... usually under nitrogen or argon. The overnight stirring grinds up the magnesium and exposes a larger surface area I guess. Anyway, the magnesium goes dark grey, even black. Then you cautiously add your halide. I used this method for making tertiary alkyl magnesium halides, when all the other initiator methods failed.

[wintermute]

I usually grind magnesium in mortar which is dried with a hotgun, then I add a small amount of alkyl halide to the reaction mixture and wait a minute or two for turbidity to appear. No initiator needed in most cases.
From your post I see that the role of iodine as an initiator is somewhat unclear to many of us My personal opinion is that it reacts with magnesium oxide and thus recovers the surface of magnesium.

[wintermute]

and personal lesson I got this week is: do not try to prepare MeMgI in THF!

[movies]

My personal opinion is that it reacts with magnesium oxide and thus recovers the surface of magnesium.

I've heard this before from other people, but I've never heard an explanation for how this would actually happen.  I don't see how an oxidizing agent (I₂) would react with oxidized Mg(II), especially when there is perfectly good Mg(0) around.  Does anyone have any insight into this?

Also, as for grinding the Mg, certainly surface area helps, but heating the mortar only solves the H₂O part of the problem.  What about the O₂?  That reacts with Mg(0) just as fast!  I think that is why most people I know prefer to use Mg turnings in favor of Mg powder.  The powder ends up being all oxide, while in the turnings the part below the surface is protected as Mg(0).

[Ψ×Ψ]

and personal lesson I got this week is: do not try to prepare MeMgI in THF!

You too?  I'm sorry!  I tried that about five times (advisor's idea, not mine) before switching to ether.  At least you know you're not alone 

[kipkeston]

just a quick note, i've found using a sonicator helps jump start your grignard as well...

[Ψ×Ψ]

cool!!!  where'd you come across that?

[alphahydroxy]

Sonication used to be quite a common method forthe synthesis of organometallics I think. A bit unreliable though - I think more modern methods are more reproducible and don't depend on finding the "sweet spot" in the sonicator bath!

[Sikhandar]

Sonication used to be quite a common method forthe synthesis of organometallics I think. A bit unreliable though - I think more modern methods are more reproducible and don't depend on finding the "sweet spot" in the sonicator bath!

'tis because you have a fake bath ;P

If i remember, the purpose of iodine is only to remove Mg(II) from surface, this system is used also in methanol dehydratation with metallic magnesium (from the booke: "to activate the magnesium add some iodine"). Tomorrow morning I would know more, now I'm going to bed...(0:35 Italy...)