Grignard Reactions and Mg

[Entropia]

Main Question or Discussion Point

Why are "Mg turnings" used in Grignard reactions instead of "regular" Mg?

What is so special about Mg turnings?

 

[Chemicalsuperfreak]

Originally posted by Entropia
Why are "Mg turnings" used in Grignard reactions instead of "regular" Mg?

What is so special about Mg turnings?

Mg turnings are regular Mg. It's just chopped and scraped up into little bits. Why do you think little bits would be better then one big ingot of magnesium?

 

[himanshu121]

as it may inc surface area for more effective rxn

 

[Chemicalsuperfreak]

Originally posted by himanshu121
as it may inc surface area for more effective rxn

Yup.

It can be experimentally hard to get the oxidative addition step of a grignard reaction to go. Cutting the Mg into smaller and smaller strips increases the surface are and can often speed this up. If this doesn't work, you can use Reicke Mg, which generates the elemental magnesium in situ, resulting in basically a very, very fine powder.

 

[Monique]

What is a Grignard reaction?

 

[Chemicalsuperfreak]

Originally posted by Monique
What is a Grignard reaction?

You may remember it from your sophomore O. Chem. class. Basically, a magnesium inserts itself into a bond between an alkyl group and a halide, then adds to a carbonyl. So methyl bromide reacted with magnesium will react with acetaldehyde to produce isopropyl alcohol.

 

[movies]

Elemental magnesium is very reactive and the surface oxidizes almost immediately in air. If you use the turnings then you can squash them a little bit in order to expose some of the interior metal which has not yet been oxidized. It's a nifty trick and works great!

 

[shrumeo]

Usually if the bottle of Mg turnings is old you'd have to activate it somehow, usually with dilute HCl and washing, but the exact method is tedious. I like to get a fresh bottle of Mg powder (200 mesh+) and try not to open it in air if I can help it, or keep the exposure to a minimum. Fresh, active powder beats big turnings any day.

edit: I see someone mention Riecke Mg. Making this is an extreme pain (reducing MgCl2 with potassium in THF), but it's as active as you can get, maybe too reactive even. Can you buy this stuff yet? Surely it would be extremely hazardous to ship.

 

[movies]

Sure the powder is better if you have access to a dry box to keep it in. A lot of labs don't have those though, and it's really not pratical for instructional labs.

Another trick is to add a few drops of 1,2-dibromoethane. The dibromoethane inserts much faster than most substrates and essentially digs pores into the Mg, thereby increasing the surface area of un-oxidized Mg. You can also try adding a couple of crystals of iodine to essentially do the same thing. The iodine trick has never worked for me, but the 1,2-dibromoethane works very well and has never failed me.

You really just need to get the oxidative insertion started, then it tends to propagate on its own because the reaction continually exposes more Mg metal.

 

[Chemicalsuperfreak]

edit: I see someone mention Riecke Mg. Making this is an extreme pain (reducing MgCl2 with potassium in THF), but it's as active as you can get, maybe too reactive even. Can you buy this stuff yet? Surely it would be extremely hazardous to ship.

Actually, I think you can buy it now. Not that I'd trust it, I'd just make my own if other ways of activating Mg wouldn't work. Usually stirring the turings under argon overnight works fine. If not that, then 1,2-dibromoethane usually does the trick.

 

[Entropia]

Elemental magnesium is very reactive and the surface oxidizes almost immediately in air. If you use the turnings then you can squash them a little bit in order to expose some of the interior metal which has not yet been oxidized. It's a nifty trick and works great!

this is the kind of answer i was looking for. what i meant by 'regular mg' was elemental mg.

-thanks!

 

[Ostwald]

So ... what is Riecke Mg?

 

[shrumeo]

Rieke Mg (along with Riecke Zn, Al, and other metals) is simply a nanosized powder with huge surface area and virtually no oxide (it's named after Rueben Rieke)

you start with a halide salt of your metal suspended in dry THF (or glyme or diglyme) under Argon

then you gradually add potassium or lithium metal (sometimes with some napthalene or anthracene as a charge transfer agent to make it a little less vigorous) to reduce your metal and leave potassium or lithium halide

you are left with a suspension of your metal in dry THF along with the alkaline halide

usually the K or Li halide is quite insoluble in the organic solvent and inert to whatever organic reaction you are doing (like a grignard) so it's usually just left in to be washed away later

Here is one of his webpages http://www.unl.edu/cmra/faculty/rieke.html [Broken]