Furboffle
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possible to perform Grignard with no solvent?
I'll be synthesizing some bromo-ethane as soon as my order of sodium bromide arrives in a couple days, and I'd like to work with some Grignard
reactions. I know its typical to use ether or THF as a solvent, but is this even necessary? isn't the main thing just keeping it dry and aprotic until
doing the workup upon completion?
ethyl-magnesiumbromide is a liquid at room temp, if my aldehyde or ketone is a solid at room temp I'm sure solubility in ethyl magnesiumbromide might
be a little limited. however, say I use something like dry acetone as my electrophile. randomly chosen I'm not certain of the solubility/miscibility
of ethyl-magnesiumbromide and acetone, but acetone tends to have pretty promiscuous solubilities.... if they do mix. is it not suitable for Grignard
reagents to either 1. be used as a solvent, or 2. use its counterpart reagent as its own solvent? I plan on doing some experimental work so I suppose
this could very well be one of my tests, but going with what you know I'm sure I'll employ ether or THF. it would just be an interesting way to save
reagents/money. not to mention using bromo-ethane as the solvent when reacting with magnesium to begin with...
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kristofvagyok
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I do not get it., what is this post about?
Grignard reagents are solids when they are dried under inert atmosphere, even EtMgBr, but they are not stable in this form, extremely reactive and
they decompose fast if they are not stored with proper care.
Some Grignard reagents are manufactured without solvent in the chemical industry e.g.: PhMgBr and PhMgCl, but using this method for home experiments
is not a good idea, since a lot heat evolves and because of this the highly toxic, carcinogen bromoethane will evaporate. If you do not have good
labware. And since no solvent is used, also a lot side product will also form, here magnesium-bromide, ethylene and butane, so please, do not try this
method.
You say that acetone would be the solvent of your experiments and also the reactant. This is a good idea, but when the Grignard contacts it, it will
form a nearly insoluble stuff that will settle to the bottom.
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Furboffle
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sorry if the thread didn't have a very clear cut purpose. is it reasonable to perform grignard without a solvent? but I guess you cleared that up, yes
it can be done. but with the whole argument regarding proper setup for safety's sake, I'll stick with solvent. I don't feel like getting any more
cancer than I'm already destined or end up with dolphin babies...
according to the data page below about EtMgBr, it freezes around -116C so I was assuming that meant it was a liquid.... not really sure how to
interpret that any other way... (not trying to be an asshole)
http://www.chemicalbook.com/ChemicalProductProperty_EN_CB869...
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BromicAcid
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There are a number of chemical databases that have cropped up that have no chemists as fact checkers. They harvest their information from around the
web based on different algorithms setup by the creator for those sites. Unfortunately a lot of bad information is propagated in this way. In your
instance, although you see on that page EtMgBr freezes at -116C, note that straight diethyl ether has the same freezing point.
If you look at the density however you note that it is greater than ether itself, however a search reveals that the 3.0 M solution available from
Aldrich has the same density and may be where this value was culled from.
I am not an expert in organomagnesium chemistry but as far as I know the creation of Grignard reagents requires ether to stabilize the Grignard
reagent. Not only does it require an ether but only certain ethers will do, for example dioxane will crash out the magnesium halides and leave behind
alkyl magnesium compounds, In many instances the reaction between an active metal and an alkyl halide will result in the formation of the metal alkyl
and the metal halide but the solvent moderates this reaction and stabilizes this 'intermediate' compound. I do not think that if you ran this
reaction without a solvent it could even rightly be called a Grignard reaction although even if you formed the magnesium alkyl it likely has similar
reactivity to what you are looking for (and it will be pyrophoric as all get out).
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zed
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Generally.......No!
Grignard reagents are complexes that contain some type of ether.
Your chances of forming the Grignard reagent you desire, without using ether, are slim and none. Stop! I was being too generous. None.
Formation and reaction of a Grignard reagent, used to be included in every elementary Organic Chemistry lab text book.
Find one, and follow the procedure EXACTLY.
You seem to be intelligent. Show real intelligence, and perform this experiment, under the supervision of an experienced mentor....or in an academic
environment. As procedures go, forming and reacting a Grignard Reagent isn't super dangerous. None-the-less, Ether can be quite dangerous to work
with. There are lots of critical dos and don'ts. If you screw up, you can die. Worse, you can become a crispy critter.
Note: Grignard reagents may sometimes be formed or at least reacted, in ethers like Anisole. Not common, but possible.
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Furboffle
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I've performed plenty of grignard reactions at the university under supervision in the past, I can safely and properly perform it on my own.
its just studying organic is my hobby and passion, so sure I can learn how to do it and learn the mechanics of the reaction. but I want to know more
than that, not just enough for real world practice. based on the typical requirements it just always calls for ether, but based on any lecture or
textbook explaination it just always calls for anhydrous ether and no protic solvents until the workup. I'd just like to know if more the sake of just
background knowledge if its doable. not that I'm actually going to try it.
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mnick12
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An ether is necessary in most cases because it coordinates the magnesium metal center. However not all ethers are appropriate, stericaly hindered
molecules like MTBE won't work. On one occasion I was able to form n-propylmagnesium bromide in a hexane ether mix, but the yields were hurrendous
(<20%). Really if you want to form a grignard ether needs to be used in sufficient quantities.
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zed
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So, we would use a dangerous solvent like Diethyl Ether, scrupulously anhydrous, just for fun?
Methinks you are jesting.
http://en.wikipedia.org/wiki/Grignard_reaction
[Edited on 3-5-2013 by zed]
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sonogashira
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DCM is a useful solvent for some Grignard reactions. There are many literature examples where it is used successfully.
[Edited on 4-5-2013 by sonogashira]
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BromicAcid
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Sonogashira,
I was able to find a few references quickly to DCM being used as a solvent for reaction of a Grignard reagent however none of them prepared the
Grignard reagent in DCM.
J. Org. Chem. 56, 5736 (1991) EtMgBr in ether, substrate in DCM
J. Org. Chem., 53,729 (1988) Prepared in ether, solvent exchange to DCM (hazy solution)
J. Org. Chem., 69, 7336 (2004) Solvent exchange to DCM
Following the original poster's line of thinking I am more interested in what solvents can be used to form a Grignard. DCM doesn't seem like the best
solvent for making a Grignard as it itself could react with the magnesium metal although it is apparently stable enough to run a reaction in once the
Grignard is made. I would not consider this anywhere near an exhaustive search of the literature but is this what you were saying or have you seen
literature where a Grignard was actually made in DCM?
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Prometheus23
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Glymes are always a useful alternative to the usual options of diethyl ether and THF. Especially butyl diglyme which is sometimes used in gold
refining I believe.
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sonogashira
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No, not from metallic Mg.
The solvent-free Barbier reaction may be more worthwhile to try.
Attachment: Tetrahedron 2011 67 05 00849 - 00855.pdf (729kB)
This file has been downloaded 925 times
[Edited on 4-5-2013 by sonogashira]
Attachment: Grignard reagent formation.pdf (686kB)
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