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Polverone
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high-purity Mg needed for Grignards?
I have heard it said more than once - by people who use this site even, I believe - that high-purity magnesium metal is needed to form the Grignard
reagent. How pure is "high-purity," and where does this knowledge come from? I know that basic chemistry texts often omit important
practical details, but none of my theoretical ochem books mention the purity of Mg being important. Neither do any of my practical ochem books, nor
any of the books on organometallic compounds that I've skimmed.
In fact, I recall some months back reading an older JACS article that indicated modest amounts (5% or so) of various metallic impurities didn't
much impair the ability of Mg to form the Grignard. So what does the mantra "use high-purity Mg" mean? Does it mean "clean surface,
without a lot of oxide inclusions?" Does it mean "don't use magnesium alloys?" Does it mean "don't use magnesium with
nonmetallic impurities?" I would love to hear about personal experiences or published references that indicate magnesium purity is important in
the formation of the Grignard.
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Turel
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Correct
Mg purity is not really important. I have done grignards with 'dirty' Mg shavings. The size of the Mg particles is not of prime importance
either. Obviously smaller particles react faster.
Mg is never clean. It passivates in air nearly as fast as aluminum. Finer particles exhibit this problem even greater due to their increased surface
area. This is what I2 is used for. Addition of seed I2 crystals destroy the passivation on the surface of the Mg.
The most important quality by far, in my opinion, is the quality of the ethers chosen. They should be dry to within 99% for good yields to be
expected. Copper catalysts never hurt either, as they promote carbanion formation and enhance the efficiency of the grignard reaction. Rhodium's
site covers methods for drying ethers using NaOH instead of Na, which apparantly gives superior results.
Even long chain polyethers (glymes) can be used for grignards, with open ended hydroxyl functions, without notable reduction in yield. The main
sensitivities with organomagnesium halides are protonation from water and acid traces, oxidation from air *or* CO2, and Wurtz coupling when working
with benzyl functions.
Ultrasound seems like a neat trick, but I have never witnessed it in person. Grignards are fairly easy to form in decent yield if you use a slight
excess of Mg (1.1x mass) with an I2 crystal, in dry ether. Dioxane is a wonderful solvent for Grignards, but it is poisonous.
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thunderfvck
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My chemistry book fails to mention iodine being used in Grignards. This is kind of sad. If I were to do this, and not have access to the internet, I
would be in a great disappointment if I were to try this reaction.
I don't have much to say about the quality of the Mg, but I don't think purity is a big thing as the whatever halide will pretty much only
be reacting with the Mg...? I've never done this myself, all I know is the reactions it can be used for and how it's done - stuff I've
learned in my class.
On the topic of grignards though, if your ether is wet you'll produce benzene. Is this a good method of producing such a chemical?
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Geomancer
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| Quote: | Originally posted by thunderfvck
On the topic of grignards though, if your ether is wet you'll produce benzene. Is this a good method of producing such a chemical?
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Could you explain this?
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JoeFro84
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Just as in Turel's case, i too have done Grignard reactions with dirty Mg. I used sandpaper on the Mg before I used it though just to clean it
up a bit. This was an ochem lab, we were preparing Triphenylmethanol. Im not sure what yield is good on this procedure, but I got 21.9% with pretty
good purity. After crystallization the products melting point was only a couple of degrees below the textbook melting point.
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thunderfvck
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Sorry.
Woops! My bad. It'll only work for bromobenzene. Or chlorobenzene, etc. A hydrogen replaces the MgBr bond. Damn. I'd still need
bromobenzene, which means I'd need benzene. AND bromine. DAMN. I wasn't thinking when I wrote that. I just remembered the assignment I did
last night (in which the grignard was made from bromobenzene) and H2O to that, yes, benzene. But for anything else, no. Sorry about that!
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Hermes_Trismegistus
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| Quote: | Originally posted by Polverone
I have heard it said more than once - by people who use this site even, I believe - that high-purity magnesium metal is needed to form the Grignard
reagent. How pure is "high-purity," and where does this knowledge come from? |
Perhaps the "high purity" standard is in reference to the purity standards of the turn of the century, when grignard reagents were first
discovered and began to be extensively used.
High Purity then.....and now, certainly has a different meaning. Chemicals back then were just beginning to be standardized, a result of the
frustration of chemists whose "pure" reagents; were anything but.
Arguing on the internet is like running in the special olympics; even if you win: you\'re still retarded.
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chemoleo
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Stupid question
What other solvents can be used for the Grignard reaction?
So far I heard ether and dioxane (both of which I was aware of, but hard to get completely H2O-free).
What about others?
esters (i.e. ethylacetate)? monoethylene glycol (cellusolve IIRC)? pyridine? furane? 100% ethanol? paraffin (hehe)?
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BromicAcid
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Come on chemoleo, it's the ether functional that participates in the Grignard formation. So 100% EtOH and paraffin  are out of the question. However Turel mentioned glymes and that even open
hydroxyls at the end did not significantly affect yields. That is very promising in that you can get diethylene glycol over the counter and according
to the MSDS for it, it is claimed to be anhydrous! But wouldn't that make things too easy?
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chemoleo
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Hehe, that's why I called it 'stupid question'
I wasn't entirely serious on some of the solvents, in case this went unnoticed
- but yes, on some of them I was - good to see you only expect the highest
standard 
Anyway - polyethylene glycols (glymes?), or mono(di?)ethyleneglycol are thus candidates? This is good news, as both are available reasonably easy 
Never Stop to Begin, and Never Begin to Stop...
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Magpie
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ether purity important
I've only done one Grignard reaction but want to do more as it was fun even tho mine failed. All the day lab's failed, but the night
lab's all worked well. We traced it to a bad can of ether.
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Turel
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Glymes
I'm sorry, I failed to specify that as far as I know, open OH polyethers should be of high molecular mass, as the OH functions *will* react with
the grignard reagent. Solvents such as PEG-1265 and other high mass polyethylene glycols make suitable grignard solvents, and function even better
when methoxylated.
Ethylene glycol can most certainly *not* be used, it will destroy/protonate grignard reagents. Ethyl acetate will react at the carboxyl carbon, with
extrusion of an ethoxyl upon quenching of the reagent, resulting in a ketone.
I advise many read up on Grignard Reagents online and at the Hive, there have been many many posts on them there.
Most basic organic chemistry texts will not mention the finer details of grignard or organilithides, or any other organometallics for that matter.
Since the reaction mechanisms can be obscure or debatable, and the practice sensitive, they are viewed as an advanced topic, with special books on
organometallic chemistry and synthesis dedicated to them. I have a few such books, and they were quite helpful.
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Organikum
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Xylene + ether is a useful solvent system for Grignards - it avoids the nastyness of pure diethylether and xylene is very esy to dry - just boil away
10% of the volume and done.
Drying the ether by NaOH is told to be superior by people I believe they know what they talk about. Those people also say that pure Mg is important -
99,8% - the often named sacrificial anodes wont work.
But pure Mg is easy if you know where to look.
EBAY - "firestarter" and you are at it. 
Initiating a Grignard:
- ultrasound but even vibrating at much lower frequencies will work. fetch the ghettoblaster man!
- a single crystal of iodine (ONE small crystal - not more!)
- crushed glass and stirring
- praying
The size of the Mg pieces is not so important - everything from turnings to powder will work. (powder has to be "dephlegmatized" thats
understood)
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Polverone
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firestarters work?
From various things I've read, it appears that Mg firestarters are often Mg alloys. What are the sacrificial anodes alloyed with (not that
I've ever seen them)? What about magnesium welding rod/wire (uncommon, but orderable)?
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Organikum
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Not the military firestarters - those dont work at all....
...people sell pure Mg scrap/turnings/pieces at EBAY as "firestarter" - cheap and plenty.
(its a kind of "code" ya know? psssst!)
This costed about 45 lumen in your gloriole Polverone, tz, tz.....
If this happens more often you will need a lamp to read your books at night soon!
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acx01b
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the I2 crystal
I always wondered:
what is a small crystal of iodine, about 1 mg?
1 mm ?
will it work if you use a too big amount of iodine compared to the amount of MgO you have on your magnesium turnings ?
I've read somebody saying that You must not do the reaction of iodine on magnesium oxyde in ether, but just do it in a dried tube, and heat it to
sublime iodine.
is it true ?
tks by advance.
[Edited on 6-5-2004 by acx01b]
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vulture
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A fellow student suggested that one should first activate the Mg by shortly treating it with 0.1N HCl then washing it with methanol and finally ether.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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acx01b
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but then, is iodine still needed?
iodine sublimated or iodine in ether ?
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Magpie
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I2 crystal mechanism
Turel states:
| Quote: |
Mg is never clean. It passivates in air nearly as fast as aluminum. Finer particles exhibit this problem even greater due to their increased surface
area. This is what I2 is used for. Addition of seed I2 crystals destroy the passivation on the surface of the Mg.
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The addition of an I2 crystal to a recalcitrant reaction is what my textbook recommends also. Turel, do you know the chemical reaction(s) for the I2
activation of the Mg? Please show us if you do.
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acx01b
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I have it:
6 MgO + 6 I2 --> 5 MgI2 + Mg(IO3)2
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Magpie
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very interesting
Thanks acx01b. Part of the iodine is oxidized and part is reduced. This is called disproportionation, I believe.
I'm not a chemist by training so don't understand why that happens, and when I see it occuring I'm fascinated. I was wondering how
iodine was going to displace oxygen, which is significantly more electronegative.
It would be interesting to calculate the change in chemical potential (Gibb's Free Energy) of this reaction. I'll have to do that some
time.
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Hjalmar_Poelzig
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check out this patent
US2816937 : Preparation of phenyl magnesium chloride
Not only do they use the much less reactive chlorobenzene, they also claim that the purity of the Mg used is of no importance, all this by adding a
catalytic amount of an inorganic nitrate or an organic nitrate/nitro-compound to the reaction mixture.
Very interesting.
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acx01b
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use of nitrate salt in grignards ???
omg I learn new things every day lol!
can you explain more ?
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Hjalmar_Poelzig
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have a look at the patent
at gb.espacenet.com or uspto.gov
I couldn't say for sure why a catalytic amount of nitrate eliminates the need for ultra clean Mg, in addition to making it possible to use cheap
technical, normally unreactive as hell chlorobenzene.
I will try to search the literature to see if the author published some papers on this route before.
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acx01b
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but you tried it? or you founded other sources ?
like 1mg or AgNO3 (i have) ?
or nano3 ? (i have too)
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