Sciencemadness Discussion Board

Preparation of Grignard reagents...

evil_lurker - 19-12-2005 at 02:09

Well it appears I might be giving it a shot sometime in the future... especially benzylmagnesium chloride....

I've done quite a bit of research, UTFSE, Orgsyn, Google, etc and the basic theory is down...

But as with some chemical reactions, seeing it on paper and actually doing it are two diffrent things.

My concern is mainly with safety and the possibility of runaway.

Any of the finer details would be greatly appreciated, especially on how to keep the reaction in check.

vulture - 19-12-2005 at 07:44

Be patient. Grignards can sit there and do nothing for five minutes and then suddenly start boiling violently.

garage chemist - 19-12-2005 at 08:28

I made some ethylmagnesium bromide once (just to try out if it works, no real synthesis), in a test tube with Mg granules, 5ml absolute ether (dried for half a year over slices of sodium) and a ml of ethyl bromide.
On adding a fraction of a mg of iodine, it started and the ether was boiling gently. The reaction was smooth and controllable because of the small surface area of the magnesium (didn't have proper grignard grade Mg shavings).

To confirm the presence of the grignard, I added a drop of water and an extremely violent reaction took place. The test tube nearly frothed over.
So be careful, they react very vigorously with water.

Marvin - 19-12-2005 at 15:17

Of course adding water would have liberated ethane, so it should be impessive.

chochu3 - 19-12-2005 at 16:33

reaction doesn't have to be real dry if one were to submerge the contents of the flask in an ultrasonic bath. The yield of course will suffer a little. Keep rection going by steam bath and keep a ice cold bath so if if it starts smoking.

Reference
J. Chem. Educ. 1985, 62, 720.

Drunkguy - 24-12-2005 at 05:28

I was gonna point out that the ether should be sodium dried. A bit suprised that you left it for half a year to dry it. It never occured to me that it would take that long to dry it

NJF - 25-12-2005 at 07:19

It won't need to be dried for that long, I guess that was just how it was stored, and how long it was stored for! Also, sodium isn't necessary, molecular sieves will do it and are easier to get.

Magpie - 25-12-2005 at 10:12

Off topic but wanted to ask if molecular sieves are good for producing absolute ethanol from the 96%ethanol/4%water?

neutrino - 25-12-2005 at 11:49

3A sieves will work because the EtOH molecules are too big to enter the pores. 4A sieves will absorb ethanol as well as the water, so they are unsuitable.

evil_lurker - 25-12-2005 at 19:20

The reason I would prefer sodium is primarily for the reason of convenience and time. It is my understanding that 4A sieves take up to a week or more to work whereas sodium is only 24 hours. Plus there is no "re-charging" to worry about and less time means less peroxide formation...

500 grams of sodium should be enough to handle at least 50 liters of Et2O and can be had for a coupla hundred bux.

A sodium press however costs $$$$... looked at thomas scientific, they wanted nearly $1700 for one... forget that, I'll rig something up for less than a hundred.

NJF - 25-12-2005 at 19:52

Sure, sodium is cheap, IF you can buy it in quantity, if at all! I don't have a good source for it, and if I did, I certainly wouldn't waste it to dry ether. And if you store your solvents with sieves, you don't have to wait for them to be dried, they will always be dry, so time isn't an issue (remember to use good technique for working with dried solvents, or you'll let moisture in every time you use them!).

Also, I would rather dehydrate sieves, or even buy new sieves, instead of buying new sodium.

And peroxide formation isn't a massive issue. It certainly isn't important enough to decide between using sodium or the slightly slower sieves (also, if you're doing it properly, your dried solvents will be under nitrogen anyway).

Maybe I'm being a bit over-the-top, but I just think that sodium shouldn't be used for the lowly task of drying solvents. It's much too cool!!

neutrino - 25-12-2005 at 20:06

$1700 ?!?!?!

I agree, that's a ripoff. You can throw one together yourself for next to nothing. I remember one of the standard reference books we all use had plans for one of these. (It may have been Vogel's 5th, I don't remember.) IIRC, it involved a threaded bolt and another common threaded part with a small hole drilled in the bottom. Ten bucks tops.

sparkgap - 26-12-2005 at 02:43

Yes, the sodium press you speak off was mentioned in Vogel's. :)
Quote:

...The (sodium press) consists of a rigid metal framework, which can be attached to the
bench by means of a single bolt... An adjustable bottle stand is provided so that bottles up to a capacity of one Winchester quart can be used and their necks brought up to the underside of the mould.

The plunger is of stainless steel as is also the one piece mould and die. (A number of dies of various sizes, thus giving sodium wire of different diameters, are usually available for alternative use.) The die is nearly filled with lumps of sodium, then placed in position in the press, and the plunger slowly screwed down. As soon as the sodium wire emerges from the die, the Winchester bottle containing the (substance) is held immediately beneath the die, and the plunger is gradually lowered until all the sodium has been forced as a fine wire...


Page 164 of the book.

sparky (~_~)

[Edited on 26-12-2005 by sparkgap]

Fleaker - 27-12-2005 at 11:00

$1700 seems grossly overpriced but many laboratories are prepared to pay that.

I know of a cheap decently effective home setup that has worked for me. For making fine wire, just use an old pipe nipple with a small hole drilled through the end which is closed and take a smaller endcap to the other end and use that as the plunger part (I simply push down with a 1cm diameter dowel rod). At the end which the wire comes out, silver solder or weld another pipe nipple onto the end with the hole drilled; that is where the sodium wire will go. It may be crude, but it has worked for me. I suppose one could make up a screw based system to make it easier on your hands and back, perhaps even use something like a larg C-clamp attacked to the plunger.

I should mention that this can easily be done in a large HDPE vessel filled with 40C mineral oil. The sodium becomes very malleable and is considerably softer than cold butter and is extruded into wire without *too* much effort. Remember to clean the sodium of any oxide covering (which is a hard white coating that can get rather thick) that has developed otherwise it will not come out of the hole with out great pressure. For that reason, it is just easier to cut the ends off of the rod and disregard oxidation on the sides.

Magpie - 15-6-2006 at 14:46

After waiting for some time to collect all the needed materials I finally performed a Grignard synthesis today. My main goal was just to see if it I could do it with all OTC ingredients. I was especially careful as my only previous attempt at a Grignard was a failure - a synthesis of benzoic acid as a class assignment in organic lab. This had failed for all of the students - most likely due to wet ether. It was "laboratory" grade instead of "anhydrous."

Today's synthesis of isobutyric acid from isopropyl bromide had all the earmarks of success. The reaction started with very little prompting (a little heat), produced a cloudy gray product, and consumed nearly all of the Mg turnings.

Rather than use solid CO2 to form the adduct I used gaseous CO2 by putting my son's paintball cartridge to a higher use. :D

Although I have not done the final workup I was greeted by the distinctive smell (dirty socks, rancid cheese) of the butyric acid right after addition of 6N HCl.

Starting materials (all OTC):

99% isopropyl alcohol
NaBr
vodka
sulfuric acid
4A molecular sieves
compressed CO2
Mg turnings
CaCl2 for drying tubes
HCl

I should add that the homemade Mg turnings were obtained by trade from another member of the SCMD board. :D