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Author: Subject: Make Potassium (from versuchschemie.de)
Sedit
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[*] posted on 5-12-2010 at 18:36


I think Naptha would be a better alternative since it is non reactive, can have high BP and on top of that its overall contents can be found thru MSDS.

Does anyone have spare trace amounts of K or Na to test my theory? I don't and I would have to make some thru electrolysis before I could confirm my theory. I have been thinking about this alot and im not 100% sure that the alcohol does squat other then remove the protective coating which could be done better by amalgamating the Mg prior to use.

Preformed Potassium tert-butoxide should be used IMHO instead of the alcohol since it would not evaporate on contact with the high temperature reaction.





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[*] posted on 5-12-2010 at 18:37


Quote: Originally posted by Sedit  
Preformed Potassium tert-butoxide should be used IMHO instead of the alcohol since it would not evaporate on contact with the high temperature reaction.


That was my thought too.
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[*] posted on 5-12-2010 at 19:15


I reckon the water present in the KOH probably also activates the surface and any in situ KtBuO is made into elemental K by reaction with the magnesium.

So this tells me that perhaps I can make K from KtBu in say decalin or some other solvent I have available in the laboratory. Hmm...




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[*] posted on 5-12-2010 at 20:40


Quote: Originally posted by garage chemist  
T- butanol is easily made by the grignard synthesis from MeMgBr and acetone.
PainKilla has outlined how he successfully made MeBr from NaBr, H2SO4 and MeOH and used it to generate a substantial amount of 1,4-dimethoxybenzene from a solution of hydroquinone in aqueous NaOH.
For turning the MeBr into the grignard, it would have to be purified and dried by a H2SO4 washing bottle and CaCl2 drying tube and condensed in a receiver cooled by ice and salt, or simply dissolved into chilled absolute ether and this solution slowly added to Mg and a little iodine crystal.
For this grignard synthesis, the reflux condenser would have to be colder than 0°C (chilled water/glycol or alcohol as the coolant) to condense the evaporating MeBr, as the formation of the grignard reagent is very exothermic.

I have prepared tert-pentanol from EtMgBr and acetone, and find that this synthesis is excellent to practice the generation and use of a grignard reagent at home.
I mix all of the EtBr with the amount of abs. ether that gives a 2,5 mol EtBr/L solution and add some of it to the stochiometric amount of reagent grade Mg to which a small I2 crystal has been added, so that the Mg is completely covered by the solution.
It is carefully heated by a heatgun or small flame until the solution gets turbid and it starts to reflux on its own (reflux condenser with ice water and drying tube). The rest of the EtBr solution is added at a rate that maintains a gentle reflux of the ether. After all is added, the solution is refluxed for 30-60 minutes so that nearly all of the Mg has reacted (I like to use an excess, but it's not really necessary).
The addition of acetone to this solution is highly exothermic, with fizzing and sputtering. It is done drop by drop under magnetic stirring.
Then work up as usual, with cold dilute aqueous HCl (the tertiary alcohol doesn't dehydrate under these conditions, but be sure to give the ether solution a final thorough wash with aqueous Na2CO3 before distillation, as HCl traces would otherwise cause elimination to 2-methyl-2-butene).

This synthesis of potassium is most remarkable. Regrettably I can't try it at this time, due to lack of Shellsol solvent (what is this solvent made up of anyway?). I do have the t-butanol and 99,8% Mg powder, and the KOH which generally is of 85% purity with the remainder being water.
The t-butanol may not be replaceable in this synthesis, but I hope that the Shellsol is.

Maybe I will attempt the synthesis of t-pentanol tomorrow. I have never done a Grignard before.
How dry exactly does the ether need to be? I heard for making Grignards such as PhMgBr you need very dry ether, but when using aliphatic halides this is not necessary right? Is drying with anhydrous CaCl2 suffiencent, or should I also dry with Na? And is distillation after drying with Na necessary? AFAIK hardly any NaOH is soluble.
I will make EtBr by the method GC mentioned here, and wich was nicely documented by smuv:
http://www.sciencemadness.org/talk/viewthread.php?action=pri...
It is required that EtOH and H2O are removed from the EtBr, Et2O contamination is no problem in this case. Is it satisfactory to dry it over CaCl2, wich absorbs both EtOH and H2O? I'd rather not wash with conc. H2SO4, because it costs quite a lot of this reagent to remove relative small amounts of impurities. But if it's absolutely required, I will.
Finally, can I use Mg-ribbon (cut into small pieces) as the magnesium source?

Just to say, I'd rather have a little lower yields, than spending a lot of time distilling and wasting chemicals to make the used chemicals very dry for optimal yields.
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[*] posted on 5-12-2010 at 20:51


Wow this is quite a thread. Too bad I don't have anything to add.

I might be ordering some tert-butanol, but I'm not sure how feasible it would to send it to other members




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[*] posted on 5-12-2010 at 20:53


I've always heard that the ether must be dry for a Grignard. I believe this as my attempt at school failed, as did those of all my labmates. We suspected wet ether. The night shift used a new can of ether and they were all successful.

I did a Grignard at home and it worked very well. Made phenyl bromide, IIRC. I used 3A mole sieves to dry my ether. As GC recommended I also added a speck of iodine.

[Edited on 6-12-2010 by Magpie]




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[*] posted on 5-12-2010 at 22:00


The ether must be predried over CaCl2 for several days and then refluxed over thin slices of sodium for 24 hours, using a reflux condenser with CaCl2 guard tube until there is absolutely no more hydrogen evolution at reflux temperature. Then it is distilled form the sodium in an oven-dried distillation apparatus also carrying a drying tube, and used immediately. The apparatus itself also has to be oven-dried, as glassware from the shelf contains adsorbed moisture on its surface that will make starting of the grignard difficult.
The EtBr is predired with CaCl2, dried by shaking with P2O5 and distilled in an oven-dried apparatus over a fresh portion of P2O5.
Using those precautions, my grignards start reliably and quickly.




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[*] posted on 5-12-2010 at 22:08


Thanks. I understand this will work, but isn't this a bit overkill? I hate to all the distillations and refluxing for 24 hours. I don't have the time for it. Will drying for a few hours over CaCl2 or otherwise refluxing for an hour over Na, and distilling be fine?
I will dry the EtBr over P2O5, it doesn't introduce an extra distillation step anyway.
The acetone, should this be dried? I have a bottle 'for analysis' from Acros.
By the way, i have about 100-150mL ether left. If I dry it (I will need about 50mL for the Grignard), I will dry it all, and distill it all. i will then have to stabilise it again. I have done this before, by adding a pinch of diphenylamine. But how much is actually needed?
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[*] posted on 5-12-2010 at 22:26


I agree that your glassware should be well dried in an oven. I forgot to mention that.



The single most important condition for a successful synthesis is good mixing - Nicodem
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[*] posted on 5-12-2010 at 22:38


@Jor: just be careful how you dry your acetone, drying agents which have proton abstraction ability and are far too basic can form an aldol condensation product which is a pain in the arse to get rid of. Threw me off once and I saw it on another blog as well (http://curlyarrow.blogspot.com/2010/04/anhydrous-solvents-pa...) . CaCl2/NaSO4/MgSO4 work fine for a quick dry.
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[*] posted on 5-12-2010 at 23:56


Assuming your ether isn't soaking wet, just a couple of hours is more than dry enough for a Grignard reaction if you use an initiator (iodine, ultrasound, 1,2-dibromoethane). If you have benzophenone, add a little as an indicator - if it's blue, you're water and oxygen free; anything else, reflux it longer then add a little more benzophenone, repeat until it stays blue.

Probably the easiest at-home method, however, is to use molecular sieves - 24-48 h over 10-20 %w/v of 3 (or probably 4) angstrom sieves should get it as dry as would sodium wire. See the attached article from J Org Chem.

You can dry acetone over sieves as well, but only overnight (any longer and the aldol condensation aonomus mentioned becomes significant).

In addition, if memory serves, there was a J Chem Ed article a couple years ago that suggested that a Grignard reaction could easily be performed in undried ether if you sonicated the reaction. The yields were somewhat lower, but with acetone and bromo(m)ethane, it's probably not a huge issue.

Attachment: Efficient drying of organic solvents using molecular sieves or activated alumina.pdf (765kB)
This file has been downloaded 8653 times
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[*] posted on 6-12-2010 at 01:21


what about making the alcohol from MTBE a common additive for fuel



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[*] posted on 6-12-2010 at 06:37


Well if you just had some MTBE sitting around, sure. But otherwise you could as easily order tert-butanol as MTBE, neither is OTC.
Theoretically you could also synthesize a suitable alcohol via Barbieri reaction rather than Grignard (for example, 3-ethyl-3-pentanol from 3-pentanone, zinc, and ethyl iodide... and I think then you don't have to worry quite as much about dryness). But again, unless you just happen to have these reagents (and probably even if you do) it seems easier to just order the tert-butanol.
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[*] posted on 6-12-2010 at 07:19


Read the patent. Shellsol should NOT be essential and a close equivalent should work, assuming the whole thing isn't a hoax, of course! In the other examples of the patent other solvents are used (assuming again these examples are real experiments). An inert, non-polar, mostly paraffinic solvent is what seems to be needed.

And other t-alcohols are also mentioned. I'll be trying 2-methyl-2-burtanol (t-amyl alcohol).
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[*] posted on 6-12-2010 at 07:28


Barbieri reaction seems a nice alternative.
Say I want to make t-amyl alcohol from acetone and EtBr. I have the zinc powder as well. But the problem is, a quick search indicates that THF is most often used, wich I don't have.
Are there alternative solvents?
And what would be a common procedure in this case for anyone who has experience with this reaction (acetone + EtBr).
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[*] posted on 6-12-2010 at 07:58


Quote: Originally posted by watson.fawkes  
Quote: Originally posted by Pok  
- the glaspipe (25 cm long!) was surrounded by wet toilet paper (which cooled the pipe so that pipe acted like a reflux condenser)
Did you notice, perchance, if there were two condensation limit heights in the tube?—one for D70 and the other for tert-butanol. In the configuration you're using, you might have had both. Any observation you've got here would inform how the tert-butanol is behaving in its vapor phase.


The D70 only seems to form fog within the reaction vessel which doesn't rise up into the pipe. But the t-butanol indeed condenses in the pipe during the whole time (especially at the beginning of the t-butanol addition...after some time only small amounts condense) and is refluxed by outside cooling with wet toilet paper around the glas pipe. This are just observations. Of course, you can't identify the two solvents in a gas or fog phase by eye. But something liquid always condenses (no water, of course) which really should be the t-butanol. As I kept the glas pipe quite cool by the wet toilet paper, only t-butanol can rise up - the D70 would have been condensed much earlier on its way through the pipe.
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[*] posted on 6-12-2010 at 08:45


Quote: Originally posted by Pok  
The D70 only seems to form fog within the reaction vessel which doesn't rise up into the pipe. But the t-butanol indeed condenses in the pipe during the whole time (especially at the beginning of the t-butanol addition...after some time only small amounts condense) and is refluxed by outside cooling with wet toilet paper around the glas pipe. This are just observations. Of course, you can't identify the two solvents in a gas or fog phase by eye.
Identification by eye isn't so much the problem, given the large boiling point differences. At risk of putting words in your mouth, it seems like you're reporting that the reflux is fractionating the two volatile reagents. This is indicative that they're not forming a high-boiling azeotrope. I find it interesting that the top part of the Erlenmeyer flask has enough contact area with air to condense the D70.

The other observation you make is that the reflux rate of tert-butanol seems to diminish over time, which means it's being consumed as a reactant, even if an intermediate reactant. This reminds me of the behavior of AlCl3 as a catalyst, which can end up in a complex with the product. I see several possibilities:
  • Tert-butanol complexes with an intermediate product, adding yet another rate limiting step. If this is happening, is this the reason the reaction proceeds only at high temperature? Is there another catalyst to add that could break up such a complex?
  • It's reacting away in a side reaction. It is feasible that residual aromatics or alkenes in the solvent could scavenge tert-butanol?
  • It's complexing with an initial reagent. This one seems less likely, since it would seem that such a complex would form immediately and not show a gradual tapering-off in reflux rate.
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[*] posted on 6-12-2010 at 08:55


Quote: Originally posted by blogfast25  
Something like a Banbury internal mixer, huh? [...] It don't hurt to dream! :D
You get right on that, will you? kthnx.

This idea did give me another idea this morning for some reactor engineering. Use a column as a reaction with a tube to lead distillate to the bottom of the column. The idea is that you could then add a perforated pressure plate on the reagents. As potassium is generated you could press it out of the solid reagents and cause it to flow upward past the perforations. The plate would separate product from reagent and would further promote intimate contact between the solid reagents.
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[*] posted on 6-12-2010 at 09:58


I was only half joking, Watson. But you do seem to have an awful lot of faith in pok's results. I prefer to be agnostic for the moment. Someone needs to corroborate this first...

Your second idea? Think Soxhlet with a fused glass filter as 'pressure plate'...
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[*] posted on 6-12-2010 at 10:33


I think this paper might be of interest, can anyone get it;

http://pubs.acs.org/doi/abs/10.1021/ja01690a014

I have compared their results with the electrochemical series here and it is a bit contradictory.
http://en.wikipedia.org/wiki/Electrochemical_series
Reaction 1 seems OK and what I would expect but the others seem a bit odd.
In my opinion potassium should displace magnesium from its salts, not the other way round unless something very strange is happening.

[Edited on 6-12-2010 by ScienceSquirrel]
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[*] posted on 6-12-2010 at 12:32


Quote: Originally posted by blogfast25  
I was only half joking, Watson. But you do seem to have an awful lot of faith in pok's results. I prefer to be agnostic for the moment. Someone needs to corroborate this first...

Your second idea? Think Soxhlet with a fused glass filter as 'pressure plate'...
I'm more interested in developing hypotheses under the assumption that the synthesis works than believing them myself. I feel like I'm just following the logic of what my turn out to be a counterfactual assumption. It's been profitable as a way of developing things to try and ways to think. I don't have any particular investment in being right, as such. It's a way of promoting creative thought about the subject.

As for using a frit as a pressure plate, I'm guessing the surface tension of molten potassium would require too much pressure to be particularly feasible. Maybe one could locate a plate made of very coarse frit.
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[*] posted on 6-12-2010 at 12:40


Quote: Originally posted by ScienceSquirrel  
I think this paper might be of interest, can anyone get it;

http://pubs.acs.org/doi/abs/10.1021/ja01690a014

I have compared their results with the electrochemical series here and it is a bit contradictory.
http://en.wikipedia.org/wiki/Electrochemical_series
Reaction 1 seems OK and what I would expect but the others seem a bit odd.
In my opinion potassium should displace magnesium from its salts, not the other way round unless something very strange is happening.

[Edited on 6-12-2010 by ScienceSquirrel]


Personally I don’t think the paper you cited will throw much light on the question.

We know that 2 KOH + Mg --- > 2 K + MgO + H2O should work thermodynamically, just calculate the heat of reaction (HoR) from the heats of formation. Similarly 2 KOH + Mg --- > 2 K + Mg(OH)2 works thermodynamically even slightly better (as shown above)…

When solids are reacted and others formed, lattice energies (that make up most of the heat of formation of ionic solids) play a very important part in determining ΔG (and thus also ΔH).

Typical ‘confusing’ situations arise when you rely on the potential series alone:

AlCl3 + 3 Na --- > Al + 3 NaCl:

Estimated HoR: - 649 kJ/mol

AlF3 + 3 NaF --- > Al + 3 NaF

Estimated HoR: - 215 kJ/mol

But NaOH + 2/3 Al --- > Na + 1/3 Al2O3 + ½ H2

Estimated HoR: - 132 kJ/mol

So depending on the solid ionics involved, Na can reduce Al salts but Al can also reduce Na salts.

Here we suspect that the reaction is rather between a dissolved K salt (K t-butoxide) and the solid reagent, yielding K and solid Mg(OH)2…


[Edited on 6-12-2010 by blogfast25]
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[*] posted on 6-12-2010 at 13:00


Fair points, Watson...
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[*] posted on 6-12-2010 at 13:22


I’ve been thinking a bit about the problem of metal coalescence in the case of a typical set up (conical or round flask as reactor) because pok mentioned a test where the metal didn’t seem to want to coalesce into larger globules.

A load of small globules store potential energy which is wholly or partly released when the globules coalesce into larger ones. This is reflected in the thermodynamical definition of surface tension s = dW/dS (J/m2) with W energy and S surface area. To increase the surface area (make smaller, not larger globules), dW = s x dS (with dS < 0). To promote the formation of larger globules you’d have to do something that actually increases surface tension, so the opposite of adding a surfactant. What could achieve that?
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[*] posted on 6-12-2010 at 14:08


thermodynamic back and force, I tried it with low boiling paraffin and isopropanole.
first I heated up the paraffin with the Mg and KOH, noething happened as expexted, then I added the alcohol, bubbles at the Mg have been formed, most likely H2(not tested) after about 30 minutes the gas evolution stopped nearly completely.

I used a piece of Mg, not just powdered Mg, so I took one peace out of the oil and dropped it into water, noething happened, no K at all and the piece of Mg was eroded quite well.

I tried aluminium foil as well, which has become attacked at the addition af new isopropanole but once again, no K with the water test.

It seems that only the isopropanole was consumed in my experiment, bacause the gas evolution started always after adding the alcohol, but then again the potassium propoxid should has been formed and should have reacted to give the potassium, but there was no K at all.
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