acx01b
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use of Na in PhMgBr synthesis
i thought an interesting post on a french forum i traduced it:
bromobenzene is hard to react with magnesium, even when activated with sublimated iodine then iodine in solution.
sometimes even after 20hours it doesnt react.
whereas with the use of 30mg of Na it could always react in 10hours, if oxyded magnesium turnings have been activated, and hydrated ether dried with
MgSO4 then distillation.
1,2-dibromoethane could make it reacting in 20min withouth biphenyl byproduct.
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S.C. Wack
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active Mg
Thats different. I'll try it some time. The usual involvement of sodium is to make Rieke magnesium from MgCl2. Lithium and of course potassium
also precipitate a very active (sometimes too active) Mg or Zn, so they may have some utility as well.
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frogfot
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Isn't main task of Na to remove any present water.. Usually, initiation of grignard formation with Mg can be delayed if water is present.
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acx01b
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the task of 1,2-dibromoethane is to form
CH2Br-CH2MgBr with activated magnesium.
then CH2Br-CH2MgBr + Ph-Br --> Ph-MgBr + 1,2 dibromoethane
and the reaction starts!
and the task of na is:
Ph-Br + Na --> Ph-Na
(very fast because Na has been activated by crushing it in the solution)
Ph-Na + Mg 2+ --> Ph-Mg+ + Na+
Mg2+ probably from MgI2 and MgIO3 made from the reaction of magnesium with iodine !
but probably not from MgO and Mg(OH)2 which is nearly anionic
[Edited on 12-5-2004 by acx01b]
[Edited on 12-5-2004 by acx01b]
[Edited on 12-5-2004 by acx01b]
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frogfot
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Cool, I didn't knew that.
Could R-Na be used instead of grignard?
I've seen R-Li compounds used as nucleophiles, but Na is easier to get than Li..
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acx01b
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Ph-Na reacts very fast with Ph-Br
[Edited on 13-5-2004 by acx01b]
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Esplosivo
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| Quote: |
Ph-Br + Na --> Ph-Na
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Quite strange indeed. Using Na mixed with a halogeno alkane, as far as I know, causes the reaction known as Wurtz reaction. The following should
occur:
2 Ph-Br + 2Na --> Ph-Ph + 2NaBr
This reaction occurs very fast. Why wouldn't it occur in the reaction in prefernce to the reaction of Ph-Na with Ph-Br?!
Theory guides, experiment decides.
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FritzHaber
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"..Quite strange indeed. Using Na mixed with a halogeno alkane, as far as I know, causes the reaction known as Wurtz reaction. The following
should occur:
2 Ph-Br + 2Na --> Ph-Ph + 2NaBr "
-no, it does not as far as you keep an eye on the rxn conditions (inert atmosphere, low temperature /usually about -30°C)
this is because of the stability of the arene anione (aromatic sextet stabilises it)
youre right with the Wurtz reaction by <b>halogenoalkanes</b>, but bromobenzene is an <b>halogenoarene</b>..
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acx01b
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(1) PhBr + 2Na --> PhNa + NaBr
(2) PhNa + PhBr + Ph-Ph + NaBr
(2b) PhMgX +PhBr --> Ph-Ph + MgXBr
(3) PhNa + MgI2 --> PhMgI + NaI
(1) is fast
(2) is fast, very faster than (2b), but (3) is fast too.
(2b) is probably not very fast.
finally we have (3) with probably very low yields, but enough to make the reaction starting.
Maybe this activation of Mg works too:
Mg non activated [ MgO and Mg(OH)2 ]
+ dilute HCl --> Mg non activated [ MgCl2 ]
then Ph-Na + MgCl2 --> Ph-MgCl + NaCl
maybe it works better with HBr or HI instead of HCl.
and for all types of activations of Mg, CH2Br-CH2Br is usefull (makes reaction 1 or 3 or twice faster), but maybe lower amount of 1,2diBromoethane
than Na's amount is needed.
probably a bigger amount of CH2BrCH2Br than Na's makes the wanted reaction not working (the reaction (3)).
[Edited on 15-5-2004 by acx01b]
[Edited on 15-5-2004 by acx01b]
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true_alchemy
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I have prepared PhMgCl many times in THF with complete and responsive reaction. I'm sure the bromide would give a much quicker response.
Chlorobenzene is cheaper. On larger scale (>20L) no external heat was necessary to complete the reaction.
My typical procedure is about 1.5-2:1 THF/PhCl with a small amount initiated w/ a little MeBr, EtBr or n-PrBr. Commercial Mg chips, granules or
turnings are used. Care must be excercised to make sure the PhCl is reactiing out. Of 30-50 times or more running this prep it took off on me once
because I wasn't paying attention and the mixture was too cool and not reacting well and the PHCl built up too much. It may be wise to use
external heat during the addn. Addn. is regulated to control the reflux. It is then stirred and refluxed for 30-90 min after the addn. to react any
residual PhCl and then diluted to 2M or less.( > 2M precipitates). More dilution during the addn. doesn't allow it to stay hot enough.
My yields are 80 -90% using little or no N2.
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thefips
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A little question about grignard-reactions.
What is the difference between normal Mg and Mg especially for grignard reactions?
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BromicAcid
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| Quote: | | What is the difference between normal Mg and Mg especially for grignard reactions? |
In my experience? About 5% surface oxidation and about 350% difference in price.
There is a thread on this somewhere around here, as long as the turnings are not terribly corroded they will work to some extent.
Edit:
Relevent post
[Edited on 10/1/2004 by BromicAcid]
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JohnWW
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Who is making the fantastic profit out of Mg, then? The oxide layer could be removed by dipping briefly in a dilute acid and then washing and
completely drying before use.
John W.
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true_alchemy
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I have never had Mg that didn't work except Mg recovered from previous grignard reactions. I tried washing it w/ dilute HCl and drying etc but
it still didn't work but that experiment was very rough and I'm sure it could be optimized. I had considered remelting, removing the dross
and having it reground.
I don't think I have ever used Mg labeled "for grignard reactions" and all chips, powder, granules, turnings have worked for me. Once
when I was out I made about 4 pounds in an hour from Mg rods drilled lengthwise with a 3/4 drill bit and drill press. It worked very well. Even the
chunks left over worked.
If you do this you want to keep taking it away and not letting the pile get too big in case it ignites from heat or sparks.
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Leo Szilard
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Regarding the treatment of magnesium with 1,2-dibromoethane. I believe that the reason that this activates the magnesium has been properly identified.
The mechanism has nothing to do with forming Br-Et-MgBr as the reactive species. The thing with this species is that it will not last long enough to
appreciably react with anything--it is very unstable. The C-MgBr bond is very nucleophilic and it is able to donate into sigma* of the neighboring
C-Br bond. So Br-CH2-CH2-MgBr QUICKLY eliminates and you get ethylene and MgBr2.
The reason that this dibromoethane compound is used is because it chemically etches the surface of the magnesium turnings and thus there is a high
surface area of reactive metal for the reaction with PhBr.
Generally, try etching the Mg with I2 under an inert atmosphere before adding solvent. If you then add the solvent and aryl halide and the reaction
does not commence, you can then try adding some 1,2-dibromoethane. If sodium or potassium were also present, the alkali metal will reduce the
resulting magnesium salt to Mg0 (as was mentioned by others) resulting in highly active magnesium.
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Dr.Bob
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I have made PhMgBr a couple of times recently on a 100 g scale. While the yields were not steller, the starting bromobenzene was available in large
amounts, and I had a big bottle of Mg turnings, so that made the reaction cheap to do. I did find it slow to start, the key might be to add a small
amount of PhBr first, warm it up to start the reaction, then cool back down and then slowly add the PhBr to keep the reaction going. I did see some
biphenyl (PhPh), but not so much that it was a big problem, as that is easy to remove later from the final product.
Once I made some of the grignard, I used a bit of it to start the next reaction, and it did work better and faster. I did not let it go for 10 or 20
hours, only about 3-5 hours each. The nice part was that the product of the Grgnard and the ketone was pretty crystalline, so once I worked up the
reaction I was able to push it through a plug of silica to remove most of the really non-polars (biphenyl, benzene, etc) and then elute my product
with ethyl acetate, leaving the most polar crude behind. Then the product was crystallized to decent purity from the crude elutant.
But I have to say that buying it pre-made from Aldrich, or better yet, Rieke, is often the simpler way to go, and sometimes the cost from Rieke is
less than the SMs would cost me, once I add in the costs of their shipping, etc. So if I need an odd one that they sell, and they have it already
made, the cost buying buying the Grignard or Zinc reagent pre-made is not that much more and it always is very good if used promptly. Of course I
have also gotten lazy in my old age as well.
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skip
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I have experience w/grignards. Conditions have to be really dry. Dry the glassware in the oven at 100 C along with the mg turnings. Dry the bromo with
anhydr. calcium chloride. dry the ether with sodium/lithium metal. Iodine made from tincture and sublimed to initiate. Induction in 10-15 sec. Never
used anything other that I2 to get it going, when I was green horn I screwed up at least 4-5 times before I got it to work right every time. I could
get it to work in 90 % humidity , but would wait for a nice dry day.
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