07-09-04 14:13
No 518381
After discussion with some people I have a question, which I was unable to find an answer to:
What is the maximum allowed water content in diethylether when starting a Grignard reaction, using clean Mg turnings and the usuall shebang? The initiation should occur without the use of ultrasonics and other super fancy gadgets like that. I'm looking for a quantitative answer here...
Are 4A sieves enough to dry it such that it will initiate, or are 3A required?
Thanks!
Have a lovely weekend!
Rainy day regards from
Bandil
Nuke the whales!
(Stoni's sexual toy)
07-09-04 19:51
No 518403
The answer is "as soon as the local water concentration on the surface of the Mg is roughly zero". If you use methods that either continually remove or otherwise penetrate the surface oxide layer from the metal (ultrasound, I2 etching, CH3I) then the Grignard will start as soon as there is no water left. If there is still water around in the solvent above the metal then this will quench the already formed grignard reagent as soon as they meet. This is not necessarily a bad thing since that's some sort of solvent drying too.
There is a ref on Rhodium's that uses wet ether in combination with ultrasound:
../rhodium /grignar
A reaction was performed using diethyl ether that had been shaken with water, the mixture then being allowed to stand for one hour. The ether was so wet that 1-bromobutane formed a cloudy mixture when it was added. The reaction of this mixture with magnesium proceeded satisfactorily after sonication for about 50 min. The yield of product alcohol was about 50% (GC).
Read it and shake your head in disbelief. I sure did when I saw it for the first time, remembering those fucking bromobenzene nightmares that would refuse to start. Your 4A mol sieve is not even necessary when you have an ultrasonic bath.
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(The Archetypical "Good Guy")
07-09-04 20:38
No 518412
Osmium>
Thanks for the answer, but as I stated in my original question:
The initiation should occur without the use of ultrasonics and other super fancy gadgets like that.
So what I´m looking for is a maxiumim amount of allowed water present in the reaction mixture, which will allow the Grignard reaction to start.
Thanks
Regards
Bandil
Nuke the whales!
(Moderator)
07-10-04 01:42
No 518461
Without more details or knowledge gained by experience, it's impossible to answer your query, unless using special circumstances such as ultrasound, the concentration of water must be effectively zero, as water will cleave the Grignard reagent
Some Grignard reactions are far harder to start than others; if you're using an ethereal solvent then the percentage of water present must be below .01% 
You could try flame drying your apparatus, and a test tube, then attempt to start the reaction in a test tube with the reagents in the same proportions. If it starts reacting, then measure your reagents out and put them in the reaction flask, then add the contents of the test tube, which will initiate the Grignard reaction 
Chemistry is our Covalent Bond
(Hive Bee)
07-11-04 02:37
No 518628
I guess Bandil means Grignard in ether - and the reaction he talks about (if I assume right) is usually initiated by warming the flask with bare hands, although using big amounts of ether which has been dried over 3A mol sieves...
Sieves. Back to the topic. As you teach us that the max. allowed water content is 0.01%, I assume that drying with 4A sieves should suffice - the minimum residual water is 0.001% with 4A, so thorough drying with 4A should work, right?
(BTW Thanks Bandil for opening this thread)
Greetz A
"..ein Trank von unterschiedlicher Farbe, in ihm ist Heilung für die Menschen."
(Hive Adickt)
07-11-04 22:21
No 518728
Well the wather will destroy some of the grignard reagent, but if that dosen#t matter 1-3% wather is ok. Iw have done grignard reactions in ether there had been standing for a long time in a not so tight bottle.
A funny thing is that it is hard to start a grignard in totaly dry ether(and here I mean more dry than sives/Na), the forming of some Mghydroxide seams to bee important.