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Sheddist
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Grignard reagent problem
If my user name sounds unfamiliar, it is because I am a brand new member 
I am working on making 2-phenyl-2-propanol via via PhMgBr and acetone. However, I am having some difficulty in getting my Grignard reagent right: it
always seems to decompose or hydrolyse before I can get it to react with the ketone.
My bromobenzene and ether are general lab grade and 100% water-free. I react the bromobenezene with magnesium turnings, which have been rubbed with
sandpaper to remove the oxide/nitride layer. Dry diethyl ether is the solvent. I reflux the Grignard mixture gently for 45 minutes, under an inert
atmosphere of carbon dioxide gas.
To my disappointment, instead of forming the Grignard reagent, I end up with a white precipitate of magnesium bromide or hydroxide. Where I am going
wrong? This is so puzzling! 
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ScienceSquirrel
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Quote: Originally posted by Sheddist  | If my user name sounds unfamiliar, it is because I am a brand new member
I am working on making 2-phenyl-2-propanol via via PhMgBr and acetone. However, I am having some difficulty in getting my Grignard reagent right: it
always seems to decompose or hydrolyse before I can get it to react with the ketone.
My bromobenzene and ether are general lab grade and 100% water-free. I react the bromobenezene with magnesium turnings, which have been rubbed with
sandpaper to remove the oxide/nitride layer. Dry diethyl ether is the solvent. I reflux the Grignard mixture gently for 45 minutes, under an inert
atmosphere of carbon dioxide gas.
To my disappointment, instead of forming the Grignard reagent, I end up with a white precipitate of magnesium bromide or hydroxide. Where I am going
wrong? This is so puzzling! |
Grignard reagents are not inert to carbon dioxide. They love the stuff and react to form the carboxylic acid salt.
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Sheddist
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| Quote: Originally posted by ScienceSquirrel | | Quote: Originally posted by Sheddist | If my user name sounds unfamiliar, it is because I am a brand new member
I am working on making 2-phenyl-2-propanol via via PhMgBr and acetone. However, I am having some difficulty in getting my Grignard reagent right: it
always seems to decompose or hydrolyse before I can get it to react with the ketone.
My bromobenzene and ether are general lab grade and 100% water-free. I react the bromobenezene with magnesium turnings, which have been rubbed with
sandpaper to remove the oxide/nitride layer. Dry diethyl ether is the solvent. I reflux the Grignard mixture gently for 45 minutes, under an inert
atmosphere of carbon dioxide gas.
To my disappointment, instead of forming the Grignard reagent, I end up with a white precipitate of magnesium bromide or hydroxide. Where I am going
wrong? This is so puzzling! |
Grignard reagents are not inert to carbon dioxide. They love the stuff and react to form the carboxylic acid salt. |
So, this is where I am going wrong!
The reason why I was using carbon dioxide is because it is readily available. I should have known better than to use carbon dioxide! I'll try nitrogen
or helium, depending on which is cheaper and more readily obtainable.
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DrNoiZeZ
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I don't know if it will help you but in general I don't mind using inert atmosphere (it will help all right ). The major concern is water so if you
dry all the glass, the solvent (already dried as you said) and the magnesium, put a CaCl2 tube and a I2 cristal with some soft heat it will form the
Grignard reagent.
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Sheddist
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| Quote: Originally posted by DrNoiZeZ | | I don't know if it will help you but in general I don't mind using inert atmosphere (it will help all right ). The major concern is water so if you
dry all the glass, the solvent (already dried as you said) and the magnesium, put a CaCl2 tube and a I2 cristal with some soft heat it will form the
Grignard reagent. |
What purpose does the iodine serve? Does it have a catalytic effect by lowering the activation energy, Ea? Does it initiate the reaction between the
bromobenzene and Mg?
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Tsjerk
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Yes, the iodine helps the reaction getting started. I don't know how badly your magnesium is oxidised but with a bit of iodine I never had to do
anything to get the magnesium activated.
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DrNoiZeZ
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you can try to start the reaction without the iodine but I always need to use a little. It will not harm your reaction, just use a little cristal to
clean the magnesium's surface and you will see that the iodine's colour will disapear soon and your reaction will start. I always put some metalic
sodium inside the ether to really dry it but you only can do it after dry it well with Mg2SO4 for example. The ether (diethyl is common) is dangerous
and you cannot use sodium if it is wet. The sodium is to "super dry" it, if I can say that. Sometimes it is very difficult to start the reaction even
using all the precautions but with bromobenzene I never had any trouble.
Other thing, you said that you got a white ppt, I don't know but it can be Mg(OH)2 so you have to be shure that your solvent is really water free.
Diethyl ether is hygroscopic and it can have water. I never trust the supplier I always dry it first. When I didn't I lost my stuf.
Good reaction
[Edited on 28-11-2011 by DrNoiZeZ]
[Edited on 28-11-2011 by DrNoiZeZ]
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SmashGlass
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Another improvement could be to sonicate your reaction,
instead of refluxing the diethyl ether, to generate the
Grignard reagent whilst under the inert atmosphere.
Good luck.
If it ain't broke don't fix it....
Now where are my screwdrivers? |
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SteveJones
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Also make sure that you keep the temperature down as low as you can after you're done refluxing the grignard. Don't forget that Grignards are strong
bases as well as nucleophiles, so keep your reaction mixture as cold as you can - salt and ice as a minimum but dry ice - acetone would be ideal.
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peach
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Helium is always the most expensive and, with it being lighter than air, it tends to form poor blankets over things.
Argon is heavier, and nitrogen.
Nitrogen is the cheapest of the three.
I think the smallest cylinder of nitrogen from BOC at the moment is about £70 per year in rental and ?£35? odd to refill it. The rental is annoying,
because a single fill will last a year and the rental is always there, eating into the bank account.
There are a few small companies in the UK (online) who'll sell you the cylinder and refill it, or refund you when you give it back. Personally, I'd
not be too confident on getting the money back after keeping it for a few years, as they'll probably have disappeared in the meantime.
There are disposable argon cylinders. Toolstation.com have them.
They're about £10 each*, which will undoubtedly be more per unit of gas than the rental places, but then you can balance that out against the fact
that you're not going to pay £70 when it'll actually be in use for about an hour per year (it takes seconds to purge something, unlike welding where
the rental cylinders can be emptied within the space of a day by a busy workshop).
*Orders over £10 from Toolstation qualify for free next day delivery, if you don't have one local. If you visit one, be sure to abuse the vegetable
soup button on any vending machine you see in there; it's free.
[Edited on 23-12-2011 by peach]
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aarcain
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In all my lab reactions of a Grignard reagents the crystal of iodine is a must! You also need to dry the glassware in a oven or with a hand held
propane torch prior to use. One molecule of h2o will destroy the regent. I always use a ammonia chloride drying tube.
Your statement of the preparation of the magnesium is the correct procedure. On the other hand a much simpler reaction to the product you seek is a
free-radical alkylation of benzene with acetone.
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AndersHoveland
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Could one not just condense hydroquinone with an excess of acetone, then selectively reduce the hydroxy group on the phenyl group?
Acetone is known to be able to condense with chloroform to form chlorobutanol, with phenol to form bisphenol A, or even with itself to form pherone.
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entropy51
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I know that serious chemists use inert gas over their Grignards, but I have never used inert gas cover and I've mostly had no problems with Grignards
(unless the reagents had water in them).
I was taught that the heavy ether vapor formed a layer over the Grignard and prevented exposure to oxygen.
When I worked in industry we used Argon over LAH reductions, Raney Nickel reductions and alkyl lithiums, but never over Grignard reactions.
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UnintentionalChaos
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| Quote: Originally posted by AndersHoveland |
Could one not just condense hydroquinone with an excess of acetone, then selectively reduce the hydroxy group on the phenyl group?
Acetone is known to be able to condense with chloroform to form chlorobutanol, with phenol to form bisphenol A, or even with itself to form pherone.
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The hell are you talking about? For all practical purposes, you can't remove phenols, period. Second, how would that proposed reaction make the
tertiary alcohol that he's after?
Lastly, it's called phorone, though I believe that isophorone is the dominant product. Mesityl oxide and diacetone alcohol are intermediates.
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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vulture
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| Quote: |
Also make sure that you keep the temperature down as low as you can after you're done refluxing the grignard. Don't forget that Grignards are strong
bases as well as nucleophiles, so keep your reaction mixture as cold as you can - salt and ice as a minimum but dry ice - acetone would be ideal.
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I'm sorry but do you have any practical experience with Grignard reagents? Because this advice is, well, crap.
Sometimes it's necessary to cool the reaction during formation of the Grignard, but this is only done to prevent runaway. At low temperatures Grignard
formation won't start!
Secondly, Grignard reagents precipitate at low temperatures.
Finally, low temperatures = CONDENSATION. You want water in your Grignard reaction? Didn't think so.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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yankeex
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Please tell some easy otc grignard reaction so I can test quickly are my reagents dry enough for main reaction? I have iodine, mg turnings and ether.
The ether dryness is the issue i need to find out.
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DraconicAcid
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| Quote: Originally posted by entropy51 | I know that serious chemists use inert gas over their Grignards, but I have never used inert gas cover and I've mostly had no problems with Grignards
(unless the reagents had water in them).
I was taught that the heavy ether vapor formed a layer over the Grignard and prevented exposure to oxygen. |
This. Ether is several times as dense as air, and makes quite a good inert atmosphere. I've supervised this reaction at the undergraduate level many
times, and we never used anything else.
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Dr.Bob
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| Quote: Originally posted by yankeex | | Please tell some easy otc grignard reaction so I can test quickly are my reagents dry enough for main reaction? I have iodine, mg turnings and ether.
The ether dryness is the issue i need to find out. |
Testing the ether for water is best done by simple water tests like treating anhydrous CuSO4 with it, and looking for blue color or other simple
tests. Any water present will simply destroy a small amount of Grignard but will then remove the water, so tiny amounts are not a big deal, if you
have a simple reagent. If bromobenzene works and reacts with ethyl acetate or acetone to form a new product or with CO2 to form benzoic acid, then it
is working.
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Adas
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I don't think if I get this right. You are reacting PhMgBr and acetone to get 2-phenyl-2-propanol? Is it even possible? Where do you get that hydrogen
atom from?
See:
C6H5-MgBr + C3H6O -----> C9H12O + ???
11 H on the left side ------ 12 H on the right side
Don't know what the other product is supposed to be, but look at the hydrogens. They can never fit unless there is other reaction going on.
Rest In Pieces!
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DraconicAcid
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| Quote: Originally posted by Adas | I don't think if I get this right. You are reacting PhMgBr and acetone to get 2-phenyl-2-propanol? Is it even possible? Where do you get that hydrogen
atom from?
See:
C6H5-MgBr + C3H6O -----> C9H12O + ???
11 H on the left side ------ 12 H on the right side
Don't know what the other product is supposed to be, but look at the hydrogens. They can never fit unless there is other reaction going on.
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The Grignard reagent reacts with the ketone to give a mixed magnesium halide/alkoxide.
RMgX + CH3C(O)CH3 --> (CH3)2CROMgX
The product is then hydrolyzed with water or acid to give the organic product and a magnesium salt.
R'OMgX + H2O -> R'OH + Mg(OH)X
R'OMgX + HX -> R'OH + MgX2
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Adas
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Sure, thanks 
Rest In Pieces!
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zed
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The reaction should work. Look it up in the literature. Do keep in mind, that you are forming a tertiary alcohol, with Benzyl as one of the
components.
Such an animal, might love to be dehydrated into an Alkene.
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ash
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not that much. if you have skilled right ways of quenching)
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SteveJones
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| Quote: Originally posted by vulture | | Quote: |
Also make sure that you keep the temperature down as low as you can after you're done refluxing the grignard. Don't forget that Grignards are strong
bases as well as nucleophiles, so keep your reaction mixture as cold as you can - salt and ice as a minimum but dry ice - acetone would be ideal.
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I'm sorry but do you have any practical experience with Grignard reagents? Because this advice is, well, crap.
Sometimes it's necessary to cool the reaction during formation of the Grignard, but this is only done to prevent runaway. At low temperatures Grignard
formation won't start!
Secondly, Grignard reagents precipitate at low temperatures.
Finally, low temperatures = CONDENSATION. You want water in your Grignard reaction? Didn't think so. |
Yes, actually I have extensive experience with Grignard reagents and organolithium reagents, as well as some experience with dialkylzincs. To address
your points:
1) I said to cool the reaction "after you're done refluxing the grignard", indicating that Grignard formation will be complete by the time the
reaction is cooled. So your point about Grignard formation being slow at low temperatures is inconsequential.
2) PhMgBr certainly does not precipitate at temperature ranges 0 to -30 degrees celsius and even if it does precipitate at -78 C, it's not a big
issue, you don't have to saturate the acetone with dry ice, any temperature between RT and -78 C is attainable with acetone - solid CO2. Having used
PhMgBr at -78 C I did experience precipitation of the Grignard reagent, however it dissolved when I added more THF even when the temperature returned
to -78 C, indicating that it was too concentrated a solution more than anything else.
3) As for low temperatures causing condensation, this is nonsensical; should we then do every water sensitive reaction at room temperature?
Ortho-lithiations using secondary or tertiary butyllithium have to be done at -78 C otherwise the intermediates and/or solvent will decompose. The
facts are that if you're doing a reaction that is air and water sensitive all your glassware will (or should) be pre-dried, your apparatus will be
sealed and in an inert atmosphere. Condensation is irrelevant; your reaction mix will not be in contact with the environment.
Grignards, like alot of other organometallics, don't just act as nucleophiles, they can act as bases and in the case of Grignard reagents, reducing
agents. These are not synthetically useful functions, at least not for this reaction in any case, a lower temperature will hopefully disfavour these
unwanted side reactions.
Make sense?
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Pickardjr
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I have extensive experience in this compound and could get it to work in pouring rain , 95%
humidity. you gotta dry everything,,,, the bromobenz, the ether the mg and make sure the iodine is clean and dry.
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