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Author: Subject: Synthesis of Phenylmagnesium bromide
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[*] posted on 27-8-2008 at 14:35
Synthesis of Phenylmagnesium bromide


Before i start ranting on i would like to point out this is the first grinard i have made so i am a bit nervous about making it.
What i plan to do is this:
First dry 100ml of diethyl ether using plenty of CaCl2. Distill it off.
Into the ether dissolve some bromobenzene- made by bromination of benzene 9any advise on doing this??)
Put this in a 250ml rbf and reflux. whilst refluxing add some magnesium ribbon (wich has been sanded down to remove oxide layer)
Reflux this for a bit.
Do i then keep this Phenylmagnesium bromide soloution in ether or do i distill it out?
How long do i need to reflux it for? until all the Mg has dissolved?
Thanks for any imput,
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[*] posted on 27-8-2008 at 14:42


Try a Google.

Phenylmagnesium bromide must be a standard university preparation somewhere and they will probably have put it on the Internet.
Start off using their conditions etc.
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[*] posted on 27-8-2008 at 14:43
Grignard


You should manage your reflux, since the reaction is exothermic and you do not want it to run out of control (sometimes it may take a while until it starts); depending on the dryness of your ether the reaction could be more or less vigorous and easy to start; a crystal of iodine can be used to initiate the reaction (naphthalene is also good enough, or 1,2-dibromoethane). The Grignard is not distilled off, but it is used "as is". If you add excess magnesium, you should rflux until you see no more reaction progress; otherwise, until all the magnesium has reacted (disappeared). If you wish to know the titer of your solution, you may titrate the stuff. For example you may perform a conventional acid-base titration with HCl (phenolphtalein or any other as indicator) after distroying a known amount of your organometallic reagent with water.
Think answered all your questions.
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[*] posted on 27-8-2008 at 14:46
Grignard (part II)


This recipe may help your

http://members.aol.com/profchm/chap38-4.html
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[*] posted on 27-8-2008 at 14:47


bromination of benzene will just involve adding elemental bromine to benzene and exposing to strong sumlight/UV light?
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cool.gif posted on 27-8-2008 at 14:49


CaCl2 may not be enough. I have had success with *fresh* "anhydrous" (unpreserved) ether, but the yields usually suffer. T his is OK, though, considering what a pain it is to rigorously dry ether. We usually distill it off of Na metal and benzophenone (ketyl).

Flame out you glass (no ether near) to remove adsorbed water. Do this whilst under a flow of inert gas (N2 will do, Ar is better). I always used Mg turnings (small curly bits and chips, tumbled and some small bits of broken glass (which when stirred nick the pieces to expose fresh metal). Quickly add your Mg (and glass bits, if you want), then dry ether though a septum with a needle (or be quick).

Slowly add you bromide with stirring. Keep an ice bath handy, because it will boil if it gets going (maybe more rigorously than you might like), warm it with your hand, if you need to. If it does not start, as a *very* small amount of I2, that usually works. Once the bromide had been added and the spontaneous reflux abates, reflux manually a bit longer. You may need to *add* extra ether.

Use the reagent in the mix as-is in-situ or transfer to another flask(not recommended, but a flask with a stopcock on the bottom, pre-packed with glass wool works well).

Remember, these reagents are very strong bases and have been known to catch on fire (pyrophoric), which is not so good with ether around.

Cheers,

O3

I like how you think, Klute:cool:.


[Edited on 27-8-2008 by Ozone]




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[*] posted on 27-8-2008 at 14:50


There si alot on info on preparaing griganrds in Vogels, etc, aswell as arounf the web.

I don't think CaCl2-dried Et2o will be sufficient, you will have a hell of a time actiavting your reaction... Try NaOh or KOH once you have tested your ether for peroxides (iodine/starch paper), and destroyed them if present (FeSO4, bisulfite), otherwis explosions cna happen (especially with THF).
Better is Na wire or benzophenone ketyl and distn under inert gas but I guess htese might be inaccesable.

Fractionate your bromobenzene carefully, and be sure it is absolutly acid free.

You need to intitiate the recation between the alkyl halide and th emagnesium, a crystal of I2 and slight warming often works, but I think phenyl magnesium halide are quite delicate to initiate. Add 10-15% of your halide in ether, and warm if no reaction (exothermic reaction, disappearance of the yellow iodine colour).

Your reagent must be used immeidately, it isn't possible to isolate grignards. Be sure than your substarte in ether solution is anhydrous and acid free.

Plan on using ice cold water in a lon, long condenser (two even)

I can only recommend reading more on the subject. Grignards aren't that delicate to perform, but it does take "the feeling", and dry dry conditions (glassware at 150°C 1H, CaCl2 guards, dry atmospheric conditions, etc etc.)

Good luck!


EDIT: damn, hardly time to write thsi down and there was already 6 answers! :)

[Edited on 27-8-2008 by Klute]




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[*] posted on 27-8-2008 at 14:54


Thanks Ozone and Klute, that was amazingly helpfull,
Other than sodium (i dont yet have a supply of it and i feel making it myself would be a pain) what other salts could be use to dry ether?
I didnt realise Grinards are this nasty:P pyrophoric :)
I knw this sound stupid but how to i destroy them? i dont really want to make them for anything, just for the fun of the synthesis and i dont know what you can use grinards for...
Picric-A

ps. Klute- i know what you mean, whilst i was answering to Ozone you popped up :)

[Edited on 27-8-2008 by Picric-A]
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[*] posted on 27-8-2008 at 14:57


Add water or alcohol (slowly) to destroy your grignard. Shame, we hardly knew thee. Look up triphenylmethanol for a textbook synthesis (Fieser, I think).

A nice bit (University lab series) is attached.

Cheers,

O3

[Edited on 27-8-2008 by Ozone]

Attachment: grignard lab prep_01.pdf (72kB)
This file has been downloaded 16139 times





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[*] posted on 27-8-2008 at 15:02


cool thanks guys for all the help! you have been great :)
god this thread got answered extremly quickly :P
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[*] posted on 27-8-2008 at 15:05


Honestly I think you should find something more interesting to make.

Grignard reagents are standard undergraduate preparations but making them outside a good lab is quite hard.

You are going to have to invest quite a lot of time and money in preparation and solvent drying.

Adding water or alcohol to your Grignard will make a few mL of benzene which is worth almost nothing.
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[*] posted on 28-8-2008 at 03:19


yes fair point... I just love the synthesysing but with other more complex things i just dont have the equiptment, reagents and most of all money.
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[*] posted on 28-8-2008 at 03:32


You could have a go at chromium II acetate.
It is a slightly air sensitive brick red powder and a strong reducing agent. It has a metal metal bond as well.
I have done it at home using scrap zinc sheet and everything else is readily obtainable.
If you get it right you end up with sparkly crystals, if you get it wrong you end up with goo.

Making a sample of this at home would impress your chemistry teacher no end.

http://en.wikipedia.org/wiki/Chromium_acetate

http://alpha.chem.umb.edu/chemistry/ch371/CH371_Experiments/...
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[*] posted on 28-8-2008 at 05:31


cool, i think i will try that!
thanks a bunch!
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[*] posted on 28-8-2008 at 07:58


Magnesium Sulfate is said to work
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[*] posted on 28-8-2008 at 09:38


If preparing a Grignard reagent just for practice then I would suggest to use either an alkyl bromide or iodide instead of any aryl bromide. Without experience and Na/benzophenone ketyl distillation still to dry your ether it is unlikely you could ever start a Grignard with bromobenzene. Aryl halides are notorious for being difficult in this reaction.

Benzyl bromide or chloride, if you have access, reacts with magnesium much more easily and drying your ether with molecular sieves and using I2 to start the reaction is generally enough. These two are perfect for beginners by being almost full proof and not requiring terribly dry ether. Unless you work on a <15mmol scale you need to add just a little alkyl halide until the reaction "ignites" (the iodine color disappears and the magnesium starts dissolving) and only then dropwise add the solution of the remaining halide. Adding all the substrate at once can result in a geyser of ether out your flask (the reaction is very exothermic and ether very volatile). With benzyl bromide you need to cool your reaction or you obtain mostly just the Wurtz coupling product (PhCH2CH2Ph). Good luck.




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[*] posted on 28-8-2008 at 10:28


If you want to practise making a Grignard can I suggest this, also it shows the sensitivity towards water:

C2H5I + Mg > C2H5-MgI

C2H5-MgI + H2O > C2H6 + Mg(OH)I

"The apparatus and magnesium should be oven dried, ethyl iodide should be dried with anhydrous calcium chloride and the ether dried with sodium wire. Set apparatus for reflux, place 1.5g of Mg in the flask with 5mL of ethyl iodide and 20mL of ether. Warm on a water bath gently to initiate the reaction, if it doesn't start within two minutes add a small crystal of iodine. The reaction is complete when all of teh magnesium has dissolved. Convert apparatus for gas collection over water. place 20ml of water into a dropping funnel and add dropwise to the Grignard collecting the ethane produced in a gas jar (1.5L of ethane!) The reaction can be carried out using ethyl bromide instead." Rendle, Vokins, Davis, Experimental Chemistry - A Laboratory Manual, ISBN 7131 2129 7 pages 65-66.

This should be easier to perform than with bromobenzene and the ethyl bromide or iodide is easy enough to make.

You could make a mercury-sodium alloy by electrolysis of salt water with a mercury cathode. I have used sodium-mercury and sodium-lead alloys to dry solvents before though not for Grignards but it should work fine if left over night.




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[*] posted on 28-8-2008 at 11:37


thats a good idea but isnt the concentration of soidum in the mercury like 5%? or around that...
that wouldnt dryit well and you would jut end up getting mercury poisoning :P I try not to work with my mercury whenever possible... thanks though
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[*] posted on 28-8-2008 at 11:56


Well do a preliminary drying with say MgSO4 then use the Na/Hg alloy. let it stand overnight with shaking occassionally then decant how much you want to use from that and then distil the solvent if desired (I wouldn't myself if this is no critical experiment). Chance of mercury poisoning is slim so long as you don't mix it with PhBr to get organomercurys! Mercury is one of those chemicals that scares people... much worse would be your exposure to benzene and bromine whilst making bromobenzene (which co-incidently gives me a headache whenever I use the stuff!). Alternatively if you have plenty of lithium why not dry the THF with lithium metal?



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[*] posted on 29-8-2008 at 09:29


Yea i will use my Lithium in place of sodium.
Thanks guys for the help!
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[*] posted on 1-9-2008 at 18:53


To synthesise BromoBenzene you must carry the reaction in the ABSENCE of direct sunlight since sunlight causes Bromine to add directly onto the Benzene and if the Benzene is warm giving Benzene HexaBromide! You must carry the reaction in a vessel that's PROTECTED from the sunlight as much as possible and you have to use a halogen carrier such as Iron, Pyridine, Iodine. When you do these a rapid reaction would occur giving firstly BromoBenzene and then mainly p-DiBromoBenzene!
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[*] posted on 9-10-2008 at 19:03


I find the difficulty of Grignards is often overstated. I've made Ph-mg-Br in ether many times with little difficulty. The glassware was never oven dried .. just acetone and then rinsed with dry ether. Anhydrous ether was used out of the can. Bromobenzene was reagent grade and on occaision made in the lab. A little concentrated ph-Br/ether solution, all the Mg and sometimes I2 crystal to get started. One trick I developed that made them start without a hitch was to heat the glassware with a little ether inside. Fills it up with vapor. When working with larher volumes I used a jacketed cold-finger condenser and the ether never went higher than 20% up the column. I did pre-dry the first solution with CaCl2 some of the first times but gave it up as unnecessary. I think some of the difficulties reported are from old, opened ether containers or oxidized Mg but that's just a guess. Do NOT put the addition funnel on top of the condenser or any other silly shit like that. Unnecessary and hazardous. Unbelievably, I've seen this in a few lab manuals.

I edited to add one other trick I use. Not really a trick even. I have a bowl of ice water ready and a couple of turkey basters for the initial take off of the reaction.

@kclo4: Thanks for the Barbier reaction information.
The video showed apparatus way beyond what's needed for ph-Mg-Br. Normal reflux setup with addition funnel is fine and N2 is not needed for organo Mg bromides.

[Edited on 9-10-2008 by chemrox]




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[*] posted on 9-10-2008 at 20:07


I believe this is a good video for the Grignard: http://www.youtube.com/watch?v=ni8fzSLCd1I

Also, depending on what your making, perhaps check out the Barbier reaction, its like a the wimpy version of the Grignard, leaving an OH where the carbons bound to each other.
http://en.wikipedia.org/wiki/Barbier_reaction


Would MgO work to dry the ether?
Seems to me if you made some fine MgO, and mixed it around with the ether for a while, then distilled, the MgO would take out a decent amount of water. I really don't know how well MgO turns into Mg(OH)2. Considering how easy it is to make, and the fact that it takes out the water via a reaction and not a hydration like CaCl2 does, it seems like it would be a better option IMO.
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[*] posted on 10-10-2008 at 23:13


The use of a sonicator make doing grignards easier even if your chems are still a little wet.



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[*] posted on 11-10-2008 at 02:43


Grignards crap out in water but the risk can be overstated.

I dry my glassware using a hair dryer or something similar. Then just use Et2O dried over molsieves and add a small crystal of iodine, once dissolved add the halo-benzene (if possible also over molsieves but frankly, who cares) and then the Mg turnings. As quickly as possible add a CaCl2 tube at the top of the reflux column and leave the whole thing alone for a while until all the Mg turnings have dissolved. Then add what ever has to react with the Grignard slowly so as not to let the reaction go runaway. Then to kill the reaction add water and wash with a bunch of water and rotavap to get the product.

duh.




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