deltaH
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Electrochemiluminescence lamp with TEMPO
"Electrochemiluminescence or electrogenerated chemiluminescence (ECL) is a kind of luminescence produced during electrochemical
reactions in solutions. ... ... ECL excitation can be caused by energetic electron transfer (redox) reactions of electrogenerated species. Such
luminescence excitation is a form of chemiluminescence where one/all reactants are produced electrochemically on the electrodes.[2]" Wiki
I've always been fascinated by this as it provides a possible route to an electrical 'chemiluminescence lamp', so want to explore
ways that we may construct a setup that could be made to work in the amateur context.
Just like chemiluminescence, this would work very well if you could generate an otherwise poorly-radiating excited species in solution that upon
collision with a fluorescent dye, excites it and then bring about the production of light at reasonable quantum yields.
Consider the hypothetical case where you have a protected/masked form of nitric oxide dissolved in an inert aprotic solvent to which you have
dissolved an inert lipophilic electrolyte.
For example, the commercially available compound (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl radical or its ketone analogue (organic chemists will
know this as TEMPO) and it's typically used in catalytic amounts in organic oxidations.
TEMPO is a stable radical (bright red in colour) that is soluble in organic solvents. It is stable because of the steric hindrance around the N-O
group.
Now if we dissolve TEMPO in an organic solvent and add some of our inert (But)4N PF6 electrolyte, we should be able to readily generate TEMPO+ and
TEMPO- at the anode and cathode respectively (much in the same way as one could do it with nitric oxide hypothetically, but you wouldn't be able to
dissolve nearly as much NO).
Half reactions are:
TEMPO + e- <=> TEMPO-
TEMPO <=> TEMPO+ + e-
Then in solution:
TEMPO+ + TEMPO- => 2TEMPO*
TEMPO* + Dye => Dye* + TEMPO
Dye* => Dye + hv
Then when these meet in solution, charge annihilation possibly results in the generation of excited TEMPO which can then excite a polyaromatic
fluorescent dye in solution, for example rubrene to give a yellow light. As one could dissolve TEMPO in high concentrations, one
ought to be able to run the electrochemical cell at high rates without too much problem (except for having to dissipate heat of course). These
fluorescent dyes are expensive so one want to use them in minimal amounts!
Possibly the 'ultimate' way to make a light source out of such a system would be to dissolve the TEMPO into an inert ionic liquid as both solvent and
electrolyte, for example 1-Ethyl-3-methylimidazolium tetrafluoroborate so as to prepare a saturated solution to which are also added your fluorescent
dyes, however, this is not easy peasy from the amateur point of view!
EDIT: Pruned and cleaned a lot. Also found that TEMPO substitutes are relatively easy to synthesise from ammonia and acetone, see this thread on TEMPO alternatives. I see no reason why the radical derived from 2,2,6,6-tetramethylpiperidin-4-one could not be used here, as the
ketone is nevertheless aprotic and so should be fine to oxidise on the electrodes without gas production, though it might be more susceptible to
degradation over time (less inert than non-functionalized TEMPO).
[Edited on 26-10-2013 by deltaH]
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deltaH
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I think I'm going to have a stab at making TEMPOne as per the acetone/ammonia condensation (but with a twist!) followed by oxidation.
I have quite a bit of urea, so was wondering if I could add urea straight to acetone with sodium hydroxide... should generate anhydrous ammonia for me
in situ? Plus, having carbamate around may help the reaction along?
(NH2)2CO + NaOH ==>> NaCO2NH2 + NH3
NaCO2NH3 + H2O <<=>> NaHCO3 + NH3
The piperidone derivative being formed by:
3CH3COCH3 + NH3 => C9H17NO + 2H2O
Note the formation of water and consumption of ammonia makes pairing it up with the urea hydrolysis reactions above potentially attractive.
Then in a second step, I can oxidise the tetramethyl piperidone to TEMPOne.
UPDATE 1:
Reaction commenced.
Added 0.5 mole NaOH flake and 0.5 mole urea to 3 mole acetone in a PP container with latching lid... closed off 3 latches but left the third in case
pressure builds (unlikely but just in case).
Letting it stand outside at RT and will monitor. I've purposefully left the urea prills and NaOH flakes whole, while I understand this would probably
slow things down drastically, I wanted to easily be able to differentiate between a forming microcrystalline precipitate (sodium bicarb/carbamate) and
also monitor the progression of the reaction by the disappearance of the easily distinguishable prills and flakes.
UPDATE 2:
Sadly urea appears to be much less soluble in acetone than I'd hoped (practically insoluble), however, something appears to be happening, the solution
turned yellow within an hour and is now orange a couple of hours later with minute amounts of a fine light coloured precipitate (sodium
carbamate/bicarbonate?). No noticeable difference in the prills and flakes. Solution smells strongly of acetone only, no ammonia discernible.
I'll post a pic tomorrow if it looks worthwhile to do so.
This looks like it's gonna take a few days...
Aside: I am beginning to think this reaction could really benefit from some added methanol, but I'll let it run this way first so that I can have a
benchmark.
A drop of the stuff landed on my hand and evaporated leaving behind a tiny amount of oily residue... smells sweet but also unpleasant at the same
time, rubbing makes sweet smell disappear leaving only a strongly unpleasant one. Hopefully some amines are forming and not just acetone
condensations... just how much remains to be seen!!!
UPDATE 3:
Alas, while initially exciting with the colour changes, little appears to be changing after several hours (still has a similar orange colouration
compared to t~2h). The prills and flakes look resolutely intact, some very small amount of white sediment as well as tiny amounts of brown/red stuff
clinging to the solids here and there (polymeric impurities?) and a thin coating of something on the side of the wall of the container.
[Edited on 27-10-2013 by deltaH]
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deltaH
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UPDATE 4
t = 24h, it's not looking to bad actually. The mixture darkened to a deep orange/red overnight and the flakes are visibly disintegrating. You can see
on the bottom the formation of fine white material which I hope is sodium carbonate. I think this reaction is doing ok...
COMMENT: I think there's a much higher chance that the crystals forming is a sodium carbonate hydrate and not sodium bicarbonate, so that means that I
am 50% short on sodium hydroxide required in this reaction (I used equimolar NaOH and urea).
This inorganic reaction is thus better described by this equation:
2NaOH + (NH2)2CO => Na2CO3 + 2NH3
Water (from the organic reaction) may be absorbed by the anhydrous sodium carbonate to then form a solid hydrate.
I'm going to let this run for a little longer before adding more sodium hydroxide though, because if my theory is correct, it means that I should see
nearly all the NaOH flakes disappear, but plenty of urea prills left. This would confirm it more or less and I'm then happy to add more.
I'm also clipping all four latches now on the container as this reaction will need to run for days and I think I can safely say no gas is evolving.
Furthermore, if you look carefully, you can see the white particles forming on the NaOH flakes only, so I think it's safe to conclude that urea is
slowly dissolving into the solution and then reacting on the flakes to form ammonia and insoluble sodium carbonate.
[Edited on 28-10-2013 by deltaH]
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Random
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I have found urea to be quite resistant to room temperature alkalis. I mixed urea along with excess of Ca(OH)2 in the summer and left this outside in
a bucket, it still doesn't smell much of ammonia. I wonder if any urea got hydrolyzed at all.
Unlike NH4NO3 which on addition of Ca(OH)2 gives off ammonia fumes instantly.
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deltaH
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Hi Random, yeah I'm thinking this experiment is a dud as well, it's now been 45h, there's even more white granules on the bottom and
NaOH flakes slightly smaller, BUT I unfortunately noticed that the urea prills look resolutely the same and unchanged. The solution has also turned
brown. I am pretty certain now that the white stuff at the bottom isn't sodium carbonate, but something else. I can only speculate that it's some kind
of hemiacetal salt of sodium and acetone , I am not sure of the correct name for that, but basically Na+ [CH3O(OH)CCH3]-.
Whatever that may be, the matter remains that my main problem is the urea doesn't appear to be reacting, while the sodium hydroxide does, but very
slowly and probably with the acetone.
I will now play a little with the urea hydrolysis with NaOH in aqueous solution to check what those rates are, then try to figure out the best mixed
solvent system for acetone. As far as I know, in South Africa, we don't have OTC methanol (guys in the US I believe use a product called Heet). So I
will most likely use an acetone/water system.
This would no doubt lead to a two liquid phase system as the dissolving sodium carbonate salt's out as a second liquid phase... perhaps not a bad
thing. I've calculated that if I use 200ml water (compare to 220ml acetone I am using), I should have just enough water to fully dissolve the
stoichiometric amount of sodium carbonate if it forms correctly.
I'm going to experiment with this now and report back when there's something interesting to report 
UPDATE: Well this is shit, as you said, the RT reaction of urea and conc. aqueous NaOH doesn't appear to be doing zilch. I am thoroughly disappointed!
By the way, the urea slaked lime reaction works very well if you melt the urea (m.p = 135C), but I think that's because the urea forms ammonia and
cyanuric acid, which then goes on the react with the calcium hydroxide.
Sigh. OK plan C If I can convert my urea to ammonium carbamate, then I think
I'll be back in business (similar to your ammonium salt example except carbamate can also be quickly hydrolysed).
[Edited on 29-10-2013 by deltaH]
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plante1999
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Quote: Originally posted by deltaH  | As far as I know, in South Africa, we don't have OTC methanol (guys in the US I believe use a product called Heet). So I will most likely use an
acetone/water system.
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Funy to see how regulations are applied in each country. Here, ethanol, even detnature is quite regulated and store prefer not to have the hassles, so
they sell 99.9% methanol by the gallon instead (without colorant or anything). It is my standard solvent. It is a shame you can't get it easily. Try
to find cooking fuel maybe? They tend to say something like: Contain wood alcohol on them.
I never asked for this.
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deltaH
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| Quote: Originally posted by plante1999 | | Quote: Originally posted by deltaH | As far as I know, in South Africa, we don't have OTC methanol (guys in the US I believe use a product called Heet). So I will most likely use an
acetone/water system.
|
Funy to see how regulations are applied in each country. Here, ethanol, even detnature is quite regulated and store prefer not to have the hassles, so
they sell 99.9% methanol by the gallon instead (without colorant or anything). It is my standard solvent. It is a shame you can't get it easily. Try
to find cooking fuel maybe? They tend to say something like: Contain wood alcohol on them. |
Yeah it also amuses me, I mean if things are so bad, then surely all countries should have the same bans, the fact that they don't tells me there is
no rationale behind it 
We have methylated spirits of course, but I can't even get a little sodium hydroxide to dissolve into it, so think they add some alkanes, as in it
probably consists of a mixture of ethanol, methanol and something like hexane.
The problem is South African OTC chemicals are notoriously under labeled. In fact I bought some 'battery acid' the other day with no conc.
specification. I even phoned the distributor and he didn't even know lol
I haven't seen any cooking fuel here, people normally just burn methylated spirits for that as far as I know.
EDIT:
Ok here's my plan for making ammonium carbamate from my urea:
I'm going to hydrolyse my urea with a conc. sodium hydroxide solution by heating, eventually it should go with enough time and high enough temperature
to form a solution predominantly of carbamates and ammonia:
(NH2)CO(aq) + -OH(aq) => -O2CNH2(aq) + NH3(aq)
The I'm going to add ammonium chloride to this, upon cooling, I might be able to collect out the majority of the sodium as sodium chloride crystals
which presumably would be poorly soluble in such concentrate ammonium carbamate solutions.
Na+(aq) + NH4Cl(s) => NaCl(s) + NH4+(aq)
This should also help replenish whatever ammonia I might have lost in the prolonged heating of the urea hydrolysis and also really saturate that
solution with ammonia.
Then after decanting/filtering out the sodium chloride, I 'simply' need to crystallise a crop of ammonium carbamate from my pregnant liquor.
Nice plan, except I don't have OTC NH4Cl
If I remember correctly, ammonium carbamate solutions are curious in that they are MORE soluble if you use ammonia solutions than if you used water
alone, appearing to defy a 'common ion' effect.
Ok, tried to find an old BASF technical flyer on its ammonium carbamate product where I recall seeing solubility information and the increase in
solubility by using ammonia solutions. I think the highest solubility for the ammonium carbamate at RT was something around 60 wt.% using saturated
ammonia solutions to dissolve it in, but that's completely off the top of my head.
Sucks, they now make you register with them to get access to their info!
EDIT 2:
I've just also had a thought, I know that for the reaction of polyol's with urea, zinc oxide is used as a catalyst. Perhaps zinc oxide can help here
to catalyse the hydrolysis of urea. I'm lucky that one of the few chemicals I have left over from an experiment in my teens (a long time ago) is some
zinc chloride, so I could prepare the zinc oxide fairly easily. I might fiddle with that a little as well if I have to time out of interest, but the
zinc oxide would form zincate anions in strongly basic solutions.
[Edited on 29-10-2013 by deltaH]
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froot
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| Quote: | | As far as I know, in South Africa, we don't have OTC methanol (guys in the US I believe use a product called Heet). So I will most likely use an
acetone/water system. |
I have a 210 liter drum of methanol if you want some. I can give you other sources too.
| Quote: | | We have methylated spirits of course, but I can't even get a little sodium hydroxide to dissolve into it, so think they add some alkanes, as in it
probably consists of a mixture of ethanol, methanol and something like hexane. |
You can buy NaOH as 'caustic soda' in little buckets at any Pick n Pay.
We salute the improvement of the human genome by honoring those who remove themselves from it.
Of necessity, this honor is generally bestowed posthumously. - www.darwinawards.com |
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deltaH
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Thanks froot, no I've got plenty of caustic thanks, mine courtesy of Spar
| Quote: | | I have a 210 liter drum of methanol if you want some. I can give you other sources too. |
Thanks so much for
the offer, though I don't think it's necessary just now. I'm more concerned about getting the urea to hydrolyse which as random
correctly pointed out is a bit of a mission. I don't believe the methanol would help in this respect (compared to caustic aqueous solutions).
I simply refuse to give up (just yet) that there might not be a cute urea variation on this affair, but alas, I fear the only thing to do is to
decompose the urea to make anhydrous ammonia and then bubble that through the acetone, it's just such a shame I can't use the urea in some way
straight as a source of 'dry' ammonia.
Using ammonium carbamate could potentially be great, but one would need to go through quite a bit of effort to make it from urea so it hardly seems
worth the effort and for now, the best I can do would require ammonium chloride which I would need to buy from a chem supplier as I don't believe
there is an OTC source for that? That fact alone puts me off it totally as I've found the suppliers here, while affordable, usually require you to buy
20kg bags of the stuff and that's just stupid in this case!
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Lambda-Eyde
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| Quote: Originally posted by deltaH | | I think I'm going to have a stab at making TEMPOne as per the acetone/ammonia condensation (but with a twist!) followed by oxidation.
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Why? Acetone + ammonia has been proven to be an easy route to this TEMPO derivative. Why change it? The NaOH is sure to make a lot of shit out of that
acetone via self-condensations, much more than ammonia alone I would think.
This just in: 95,5 % of the world population lives outside the USA
Please drop by our IRC channel: #sciencemadness @ irc.efnet.org
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deltaH
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| Quote: | | Why? Acetone + ammonia has been proven to be an easy route to this TEMPO derivative. Why change it? The NaOH is sure to make a lot of shit out of that
acetone via self-condensations, much more than ammonia alone I would think. |
It started out because I had a
big bag of urea and wanted to use it I was (wrongly) under the impression that
it could be hydrolysed easily by strongly caustic media. Agreed, the caustic does seem to be making a lot of shit! But half the fun of experimentation
is also trying out new ideas... case in point, this is not working, so shoot me lol
I'm still a little interested in trying ammonium carbamate as I don't think that will make a lot of shit because it's only mildly basic and an
excellent source of almost dry ammonia.
Look, I'm probably going to HAVE to hydrolyse my urea to make the ammonia needed for this reaction because I don't want to go buying 20 litres of
ammonia from my chem supplier and they don't sell smaller amounts.
I figured if I'm going to have to make this effort (boiling up the urea with a strong base), it might not be a bad idea to convert that into ammonium
carbamate.
EDIT:
You know I find it frightening just how many chemicals fall under the 'used in baking' category. Just what the heck are they putting in bread these
days lol.
Anyhow, I mention this because I just read that ammonium chloride is also used in commercial baking. Trying to find an OTC source for it here
in South Africa, preferably one that I don't need to get ripped off with or required to buy 20kg!
Oh gosh, I just realised I'm missing an obvious route to ammonium chloride, boiling up urea with conc. HCl. I've done a similar hydrolysis to make
ammonium phosphate using phosphoric acid! Can't believe I forgot about that!
The reaction would be:
(NH2)2CO(aq) + 2H+(aq) + H2O(l) => 2NH4+(aq) + CO2(g)
This is so classic, boil up urea and sodium hydroxide in one pot to make sodium carbamate, then boil up urea and hydrochloric acid in another to make
ammonium chloride and then combine... I like it!
[Edited on 29-10-2013 by deltaH]
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