masonjarmatt
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Hello question about preparing PhMgBr..
Sorry to ask such a basic question but when preparning 1 mole of PhMgBr would someone simply take 157 g of PhBr and 24 g of Mg and mix them in a
flask?? Thank you so much, I love this site. 
I tried a quick search but found nothing.
[Edited on 29-9-2012 by masonjarmatt]
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elementcollector1
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What is "Ph"? I'd assume phosphorus, but who knows.
Hmm. PBr5 + Mg -> P + MgBr would be my guess.
I have no good experience with double salts, but does this one even exist?
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masonjarmatt
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benzene + magnesium -> PhMgBr
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ziqquratu
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"Ph" is standard nomenclature for "phenyl". Thus, PhBr is "phenyl bromide" (or, more correctly, bromobenzene), and "PhMgBr" is phenyl magnesium
bromide. There's a whole range of abbreviations (which, admittedly, do look like element symbols - hell, some are even identical! For example, "Ac"
means, depending on context, "actinium" or "acetyl") used for standard organic groups, and they're worth being familiar with!
Mason, in theory one mole halide plus one mole magnesium gives one mole of Grignard reagent. In practice, however, one usually uses a significant
excess of magnesium (from, say, 1.5 eq. through to five or more, depending on factors such as the reactivity of the halide, the propensity for Wurtz
coupling, etc).
In a practical sense, PhBr in particular can be a little sluggish to react with the metal, so and excess can help there, as can an initiator such as a
small crystal of iodine. I've also had better success with preparing most Grignard reagents (and PhMgBr in particular) using diethyl ether (@EC11 -
Et2O) rather than THF.
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masonjarmatt
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ohhh Im sorry, I got confused.. I have seen a Grinard reaction preformed before, and I know thats what I need to do but the project I am theorizing
is above my head..
I will be back... thank you!!!
[Edited on 29-9-2012 by masonjarmatt]
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masonjarmatt
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Yes the methods I were looking at used diethyl ether synthesized from ethenol.
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masonjarmatt
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It's been a little rough trying to teach myself all the basics of chemistry, but fun. I wish I paid attention in school.. oh well, plenty of time to
learn.... thanks again, great help 
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Nicodem
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Thread Moved
29-9-2012 at 00:26 |
zed
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Assuming you mean Bromo-Benzene, the general method of preparation for Grignard Reagents isn't too difficult.
Don't imagine these instructions are complete, but I've done the procedure a few times with Benzyl-Chloride, and it goes something like this:
1... By means of a standard funnel, pour Magnesium powder or turnings into your reaction flask. This will leave you with a little mountain of
Magnesium at the bottom of your flask.
2...Drop a small crystal of Iodine through the funnel, this should land on the peak of your Magnesium mountain.
3...Set up your flask with an upright cold-water condensor for refluxing, and affix a separatory/dropping funnel at the top of your condensor with an
intermediate "Breathing" adaptor to prevent pressure build-up.
4...Warm the flask gently. Then mix a little Ether with a little Bromo-Benzene, place it in your separatory funnel, and adjust the stopcock to allow
a few drops to fall upon the Iodine crystal at the peak of your Magnesium Mountain.
5...With a little luck, fizzing should begin. When it does, add the remainder of your Ether via the separatory funnel.
Then place your Bromo-Benzene in the funnel, and set the drip rate to maintain a steady reflux.
You must use "DRY" Ether. DRY,DRY,DRY. Moist ether will either prevent the reaction from starting, or kill the reaction once it has begun.
And, oh yes, don't try this on your own, if you have no experience in the chem lab. Quite possibly this little experiment could kill a neophyte.
Worse, you could become a crispy critter. Working with Grignard reagents in Diethyl Ether isn't particularly hazardous, as chemistry goes, but it is
hazardous enough to exterminate the unwary.
Flicking a light switch, in a room full of Ether vapor, ain't healthy. Kaboom! Forgetting to run cold water through the reflux condenser is also
unhealthy. Not understanding the dangers of peroxide formation in Ethers, is equally lethal.
Take a class or find a capable mentor. And, most of all, don't even think of trying something like this in a Mason Jar, Matt.
[Edited on 29-9-2012 by zed]
[Edited on 29-9-2012 by zed]
[Edited on 29-9-2012 by zed]
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masonjarmatt
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Eureka!!! Other than confusing the symbyls; my confusion was mostly stemming from the fact the instruction I was reading didnt make it clear enough
what needed to be added to the sepratory funnel, and what needed to be addd to the reaction vessel.
No, I'm not trying any of this stuff. I have been going back and forth with a freind, just conversation and we both were having trouble understanding
some reading material. I have used this forum before as a refrence many time but never needed to post to find an answer. I know there is lots of
great info on here and all over the web about Grinard reactions. I was just too far off base with my understanding to be able to effectivly search for
my answer...
I start classes next spring; I cant waite. I have been intersted for years and dabbling with friends over the past 2 years . I have succecfully
completed a few small but great projects which has drove my enthusiams through the roof..
I appreitiate the help, it has cleared thing up a great deal for me. At least now I will be able to find more information!!!!
Talk to you all soon, thank you!!!
[Edited on 29-9-2012 by masonjarmatt]
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masonjarmatt
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OK yes Bromo benzene... YEESSSSS you guys are soo fucking awsome
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kristofvagyok
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Grignards are easy to prepare, but there are a few things that should be noted, especially when aromatic Grignards are made.
At first: aromatic Grignard reagents industrially are made in-situ, without solvent. The only problem is that here will be a high percent of side
reaction: PhBr + PhMgBr = Ph2 + MgBr2 where Ph is PHENYL and not phosphorous and Ph2 is biphenyl. And because of this side reaction the yield will be
decreased to 70-80%.
Second: the magnesium could be activated simply without iodine by simply crushing it. Also Mg powder is NOT good for Grignard reactions, always use
shavings granulates.
Third: if you use solvents, then always distill them from sodium/potassium or any other thing what will REMOVE the water and not just adsorbe it like
K2CO3 ect.
I have a blog where I post my pictures from my work: http://labphoto.tumblr.com/
-Pictures from chemistry, check it out(:
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ziqquratu
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matt - definitely don't dive in to this project without a firm grasp on what's involved. Grignard reagents are reasonably sensitive and can be a pain
to work with, and there are quite a few things that can go dangerously wrong if you don't know what you're doing. Mind you, they are by far the best
entry to reactive organometallics once you have the experience and knowledge - far more forgiving than, for example, organolithium reagents.
zed - benzylic halides are much more reactive than aryl halides, meaning they begin to react much more readily. As I mentioned, bromobenzene can take
a bit to get going, but a bit of I2 and gently heating after, say, 5-10% of the bromobenzene/Et2O solution has been added to a
suspension of Mg in Et2O will usually get it going (if not, I sonicate until it does! If this is necessary, though, it usually indicates
the Mg is crappy). After it starts, you can then add the halide solution at a rate sufficient to maintain a steady reflux.
kristofvagyok - you're right about crushing the Mg to enhance its reactivity. However, this should be done under a blanket of argon, to prevent
formation of magnesium nitride from reaction with nitrogen (which defeats the purpose of grinding to activate). I find a nice big stir bar, along with
a little broken glass, and stir dry magnesium under Ar for an hour or two does the trick nicely - gives a nice, fine, grey powder that reacts very
well (too well, sometimes - as you say, powder can be less than ideal, particularly for more reactive halides). Also, for drying solvents, if you
don't have sodium or potassium, 20 % (w/v) of activated 4A molecular sieves for 48 h dries ether at least as well as do the metals.
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masonjarmatt
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trust me noo plans on trying this.. Im just trying to educate myself.. I appreciete and respect your concers. I am aware of the dangers of fre and
explotion when preforming a reaction that is water sensitive, and the dangers of gas build up when not working under a vent hood. All of the
information you are giving me is really helpful.. Hearing it explained from several diffrent people can really clear things up sometimes. I am going
to do a little more research and let you guys know how things turn out. I will not preforn anything the end goal is to have a basic understanding of
every step..
This is some great information that I probably wouldnt have found from reading text on how to preform a Grignard reaction. Although I would probably
have it all in my back pocket if I had a formal education.
"Second: the magnesium could be activated simply without iodine by simply crushing it. Also Mg powder is NOT good for Grignard reactions, always use
shavings granulates."
Crush the shavings or shavings are fine??
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zed
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What mesh size the granular Mg powder was, that I used to use, I can no longer recall. It worked well though.
In these troubled times, you will probably have few purchase options. Turnings are still widely available, as are mixtures of turnings with coarse
powder. But, there are now often restrictions on the sale of Metal powders, and I am not entirely sure what those restrictions are.
These restrictions do not arise from chemical uses of those metals, but rather from their explosives/pyrotechnic uses.
The ideal mesh size, or mixture composition for your Grignard reaction, is information you will have to acquire from someone with more contemporary
experience.
http://www.firefox-fx.com/ChemM.htm
[Edited on 30-9-2012 by zed]
[Edited on 30-9-2012 by zed]
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