Sciencemadness Discussion Board - Grignard reagent problem

Grignard reagent problem

Sheddist - 25-11-2011 at 16:37

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!

ScienceSquirrel - 25-11-2011 at 16:43

Quote: Originally posted by Sheddist

If my user name sounds unfamiliar, it is because I am a brand new member

Grignard reagents are not inert to carbon dioxide. They love the stuff and react to form the carboxylic acid salt.

Sheddist - 25-11-2011 at 16:50

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

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.

DrNoiZeZ - 27-11-2011 at 09:44

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.

Sheddist - 27-11-2011 at 14:42

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?

Tsjerk - 27-11-2011 at 15:30

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.

DrNoiZeZ - 28-11-2011 at 11:28

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]

SmashGlass - 28-11-2011 at 12:08

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.

SteveJones - 22-12-2011 at 16:25

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.

peach - 22-12-2011 at 19:44

Quote: Originally posted by Sheddist

I'll try nitrogen or helium, depending on which is cheaper and more readily obtainable.

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]

aarcain - 1-2-2012 at 22:38

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.

AndersHoveland - 2-2-2012 at 01:08

Quote: Originally posted by Sheddist

I am working on making 2-phenyl-2-propanol via via PhMgBr and acetone.

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.

entropy51 - 2-2-2012 at 06:14

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.

UnintentionalChaos - 2-2-2012 at 08:44

Quote: Originally posted by AndersHoveland

Quote: Originally posted by Sheddist

I am working on making 2-phenyl-2-propanol via via PhMgBr and acetone.

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.

vulture - 2-2-2012 at 14:47

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.

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.

yankeex - 7-2-2013 at 06:43

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.

DraconicAcid - 7-2-2013 at 09:15

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.

Dr.Bob - 7-2-2013 at 12:44

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.

Adas - 7-2-2013 at 13:17

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.

DraconicAcid - 7-2-2013 at 13:23

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.

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

Adas - 8-2-2013 at 06:11

Sure, thanks

zed - 9-2-2013 at 14:58

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.

ash - 19-2-2013 at 10:11

Quote: Originally posted by zed

Such an animal, might love to be dehydrated into an Alkene.

not that much. if you have skilled right ways of quenching)

SteveJones - 17-3-2013 at 13:04

Quote: Originally posted by vulture

Quote:

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?

Pickardjr - 27-5-2013 at 14:21

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.

brew - 29-5-2013 at 23:08

AMS ?? If so, theres an easier way, Ill hunt it up. cannot recall

Magpie - 3-10-2013 at 16:16

Today I made a Grignard reagent, isopropylMgBr. Since I had no problems I thought I should report my procedure.

I roughly followed the procedure of Drake et al in OrgSyn for making methyl isopropyl carbinol as it used the same substrate, ie, isopropyl bromide.

My ether, recovered from starting fluid, was dried overnight with 3A mole sieves. My isopropyl bromide, which I made last April, was distilled when I made it and therefore was used "as is." My Mg turnings were made from a block of Mg and traded to me by forum member Swany 7 years ago. They also were used "as is." A smidgeon of iodine was added as an initiator.

Edit: I should also add that I washed my glassware with dish soap and water, then dried it for 1.5 hours in an oven set at 110-120°C.

My scale was ~0.1 compared to that in the OrgSyn procedure. I used 53g of isopropyl bromide dissolved in 80mL of ether. This was placed in the p-e funnel. The Mg turnings and iodine were added to 90mL of ether in the 500mL RBF pot. The pot was equipped with a reflux condenser topped with a CaCl2 dryer and a p-e addition funnel by use of a Claisen adapter.

isopropylMgBr Grignard reagent.JPG - 67kB

I placed the pot in a SS bowl and located this on a magnetic stirrer-heater. The bowl was there to quench the reaction by adding ice-water if needed (it was).

I added about 15mL of the isopropyl bromide solution and then because nothing was happening I added a little more. This was a mistake as the reaction took off and flooded the reflux condenser. I quickly added some ice-water to the ss bowl and things quieted down nicely. I'd say the reaction started within 1 minute. From there on out I added the bromide about 5mL at a time over the period of 1 hour. I then refluxed it for 45 minutes.

Tomorrow I will attempt to gas it with formaldehyde, something I've always wanted to try.

[Edited on 4-10-2013 by Magpie]

[Edited on 4-10-2013 by Magpie]

Magpie - 7-10-2013 at 14:51

Here's the results of my attempt to react a Grignard reagent, isopropylMgBr, with formaldehyde from depolymerized paraformaldehyde.

My source of paraformaldehyde was Thetford Aqua-Kem DRI, a chemical sold for RV toilets. This is an unproven source of paraformaldehyde (to my knowledge). In keeping with information obtained from journal articles the standard technique for preparing formaldehyde from paraformaldehyde to produce dry gas is to first dehydrate it in a vacuum dessicator over P4O10 for a minimum of 48 hrs. I attempted to do this with the setup shown below, using a period of about 80 hrs. I will talk more about this later.

The Thetford product (a blue free flowing powder) was then placed in a 250mL flask heated in an oil bath kept at 160°C. The generated gas was then sparged into the Grignard reagent using a glass tube of 5mm ID. Argon was metered into the gas generator at 0.2-0.4 SCFH as a carrier gas for the formaldehyde. This is shown below.

At first everything seemed to be going well. But after a short time the sparge tube began to plug with a white solid, paraformaldehyde. This continued to be evermore problematic requiring frequent shaking of the sparge tube at first and then frequent routing with an ice-pick. Eventually I just gave up, chocking this experiment up as a failure. The reaction vessel did warm up some, indicating some reaction with the formaldehyde had ocurred.

Obviously I had water in my gas. Possible sources include one or both of the following:

1. Improperly dried Thetfford paraformadehyde. My drying effort did not go well. A lot of formadehyde settled on the P4O10, dehydrating and turning into a black goo that pretty well sealed off the remaining P4O10, nullifying any further drying.

2. My welding grade argon may have some water in it but to the best I could learn that would likely be <1ppm. That risk could have been solved by using a drying tube.

It seems pretty clear that the source of the water was in the generation flask as the rubber hose and tube were both filled with the white solid, and a lot of it at that.

I haven't given up yet. Some things I'd like to try:
1. Some reagent grade paraformaldehyde.
2. Formaldehyde from trioxane.

As usual, comments, questions, and suggestions are welcomed.

watson.fawkes - 7-10-2013 at 15:14

Quote: Originally posted by Magpie

My source of paraformaldehyde was Thetford Aqua-Kem DRI, a chemical sold for RV toilets.

Do you know how much methanol is used as a preservative in that product? The MSDS doesn't say. I can imagine that methanol could swamp the P4O10 desiccant. Perhaps a two-stage drying process is called for, the first to remove methanol.

Magpie - 7-10-2013 at 15:43

I have no idea if or how much methanol is used in the Thetford product. It was a free flowing powder when I opened the package and was same when I transferred it to the generating flask 3.5 days later.

I could see that the goo on top of the P4O10 was turning darker and thicker as time progressed.

It would be nice if the paraformaldehyde in this product could be somehow made useable for home chemistry. It's cheap, readily available, and I have a whole box of packets left. I hesitate to do much experimentation with my precious P4O10, however.

Perhaps it could be purified by distillation, sublimation, or even extraction with a solvent.

I should add that it has a perfumy/detergent-like smell.

[Edited on 7-10-2013 by Magpie]

arsphenamine - 7-10-2013 at 22:53

Formaldehyde boils at -19C and is very soluble in diethyl ether.

Conceivably, you could hydrolyze the crude polymer with toilet-cleaner grade HCl solution, bubble it into ether over a generic drying agent for later use.

Magpie - 8-10-2013 at 06:07

Quote: Originally posted by arsphenamine

Formaldehyde boils at -19C and is very soluble in diethyl ether.

Conceivably, you could hydrolyze the crude polymer with toilet-cleaner grade HCl solution, bubble it into ether over a generic drying agent for later use.

I had that same thought. It seems like it would be a good technique. I wonder why I never ran across anyone doing this in my literature search? BTW, it's not really a hydroylsis but is a depolymerization with water as a by-product.

H-[CH2O]_x-OH + heat -----> xCH2O + H2O with x =8-100

testimento - 11-10-2013 at 18:27

Btw, how prone is grignard to impurities? If there are, for ex. ppm amounts of dissolved metals or other gunk?

sonogashira - 13-10-2013 at 01:47

It rather depends upon what those impurities are. Some metal salts will alter the position of Grignard addition. What dissolved metals shall you have?

chemrox - 13-10-2013 at 14:20

ph-Mg-Br.et-O-et is probably the easiest of all the Grignards to make. Forget the sandpaper, as long a the turnings aren't gray. Reflux ether in the setup over the Mg to push out any air and moisture. then run in a little bit of concentrated ph-Br/ether solution and maybe an I2 crystal or two. Should start right up. Have an ice bath and turkey basters available for the first 20-30 minutes to cool the flask. You will need this. Once all the reactants are in reflux for another 45 minutes. Decant into another flask to leave unreacted Mg behind.

palico - 24-10-2013 at 12:29

I did Grignard reactions, I can claim they are very sensitive to humidity and the dryness of the solvent.
My advice is wash the magnesium with dilute hydrocloric acid solution and dry it in an oven.
Then set up the apparatus with magnesium inside and with the aid of a torch warm the glass while a vacuum pump is turned on. This for removing all water molecule.
Protect the reaction envoironment by sulfuric acid trap and pass dry nitrogen gas before starting the reaction, just for restore pressure.

vulture - 26-10-2013 at 05:08

Quote:

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?

1) I don't see the point in cooling the reaction after it's done, it does no good and only creates potential for air to be sucked in.

2) You specifically mentioned using dry ice and acetone as the best option to cool (which shouldn't be done in the first place). Controlling the temperature of such a bath is not easily done and results in severe fluctuation. Furthermore, according to you precipitation of the reagent is not a problem? If you want to do anything useful with it, it IS a problem. You mention adding THF to redissolve the reagent, this is an unnecessary complication of the reaction and introduces another possible source of water.

3) This is not a nonsensical issue. Chemical apparatus is never perfectly sealed to air and the effects of water diffusion through septa and balloons are measurable (we did). Furthermore, the experimenter is doing this in a home environment with limited or no access to inert atmosphere, in which case condensation can become a serious issue.

Organolithiums have nothing to do with this. The reaction is between PhMgBr and acetone, where the potential for sidereactions is extremely limited. Your suggestions only unnecessarily complicate the procedure.

Quote:

Protect the reaction envoironment by sulfuric acid trap

That keeps your stuff dry, but suckback of sulfuric acid in a grignard reaction is not something I'd like to witness.

[Edited on 26-10-2013 by vulture]

palico - 26-10-2013 at 12:36

Yes of course, me too I don't wish anyone get a suckback of sulfuric acid in a grignard reaction.
The problem is solved easily, putting an empty flask between the reaction and the acid trap.

S.C. Wack - 5-1-2014 at 13:41

Looking for paraformaldehyde drying references, it was noted that the OS prep of cyclohexylcarbinol from cyclohexyl chloride that came up
http://www.orgsyn.org/demo.aspx?prep=cv1p0188
has several practical hints for using it BTW.

There's a post of mine here somewhere displaying not liking Thetford's products in the past...pretty sure not all Magpie's vapor was monomer...that powder is full of crap...distillation and water removal may be the only "practical" method using it though this eventually yields a wet product...just now noting Purification of Lab Chems suggests drying with sulfuric acid in vacuo...

Dr.Bob - 5-1-2014 at 19:47

I would suggest trying to sublime the paraformaldehyde. That might works well. You might be able to throw the solid paraformaldehyde (purified by sublimation) into the Grignard reaction, as it will react in many other reactions, but I have not tried it in this one. I have used that for reductive aminations as well as other reactions, and it works out OK for me. Might not optimize yield, but very easy.

I have always found Grignards to be hard to start, but then they work well, as long as you add the bromide slowly. When we did them in org class, we did not have drying tube, inert gass, or dry glassware, and they mostly still worked, other than for the students that broke the flask trying to stir the reaction or crush the Mg with a glass stirrod. I prefer just using a Teflon oval stirbar, and if you stir it enough, that will break up the Mg some. Just stir the first bit for 10-15 minutes and it will work fine.

chemrox - 5-1-2014 at 22:37

and since this isn't chem class and you have all the time in the world and not 3 hrs don't bother with the I2. I dry the concentrated (initiator) solution before I add it to the *refluxing* ether. If you start by heating the ether you don't need a special atmosphere. Ether forms an inert blanket over the reax. The reaction is quite vigorous once started and you have to be set up so you can remove heat and bring in an ice bath with a pair of Turkey basters without delay. I would not use and oil bath for this reason. You will be switching between heat and cold more than once if you haven't done this before.... I can't think of a more efficient method of heating than a mantle. The reaction will bubble a little as it starts with the bubbles starting on the Mg turnings. Once the solution is turning brown the reax is well underway. After all the addition of phBr is complete reflux the mixture for an hour and then cool enough to decant in a well ventilated area. You must remove the unreacted Mg before the next reax other wise the allylBr will react with it and start a competing Grignard.