Antigua
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Formaldehyde bisulfite adduct preparation
Hey,
I'd like to prepare some pure formaldehyde bisulfite adduct as a powder. I've found a couple of sources about it, but none of them describe the
workup or how to crystallize the adduct.
My plan was to prepare a solution of sodium metabisulfite (CAS:
7681-57-4) and add it to 0.5 mol eq. of aqueous formaldehyde 36% solution.
The problem arises with separating the product. It seems really polar and the bulkier the carbonyl compound the less water soluble the adduct is.
The problem here is that formaldehyde is obviously the least bulky carbonyl compounds one could imagine.
Any ideas or experiences you could share, please?
I've searched sciencemadness and other forums without much luck.
Here are the sources I went through:
https://patents.google.com/patent/US3293214A/en (describes preparation of the adduct, but doesn't mention it's crystallization at all)
https://core.ac.uk/download/pdf/9640129.pdf (talks a little about adducts)
Thanks in advance 
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Antigua
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Additional source: https://www.chemicalforums.com/index.php?topic=70428.msg2539...
Information on solubility of the adduct is very sparse: https://pubchem.ncbi.nlm.nih.gov/compound/Sodium-hydroxymeth...
This page quotes it as "water soluble", but doesn't say to what extent. It also says this, though: "Needles from water /Monohydrate/
Budavari, S. (ed.). The Merck Index - An Encyclopedia of Chemicals, Drugs, and Biologicals. Whitehouse Station, NJ: Merck and Co., Inc., 1996., p.
718"
Which implies that the compound is recrystallizable. Do you guys think one could dehydrate it to anhydrous without destroying the adduct by light
heating in warm oven?
I was unable to find the Merck reference in two books I found on libgen.
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njl
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Maybe an isopropanol/water solvent system would be better for recrystallization. Whenever I can't find good info on simple compounds like this I just
do a few test tube experiments ie. dissolve adduct in water and note the solubility changes when various alcohols are added.
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Boffis
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Is this the stuff that is known in the dying industry as Rongalite or hydroxymethylsulphinate?
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Antigua
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No, these are different compounds. Rongalite is the adduct formed from dithionite, not bisulfite.
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Antigua
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I'm going to conduct some experiments - luckily the formation of adduct leads to no side products. Therefore I could just evaporate the solution and
crystallize in the freezer without fear of side products precipitating too.
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dawt
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Oh cool, I was planning on doing the same thing next week (due to me using formaldehyde so rarely it's polymerized whenever I need it) and came up
with zilch info on the workup as well. My plan was using an excess of formaldehyde to ensure all the metabisulfite is used up, evaporating and then
recrystalizing from water. I'll keep you posted and am looking forward to reading about your experience.
Edit: According to the publication below (if I'm reading it correctly, old German nomenclature is all messed up and there was quite an uncertainty
about the identity of the substance until 1926) the bisulfite adduct can be crystalized by addition of ethanol (but not methanol) to the aqeous
solution, and recrystallized by dissolving in warm water and adding ethanol.
https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002...
[Edited on 2020-12-20 by dawt]
[Edited on 2020-12-20 by dawt]
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Antigua
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47.15g (1.1 mol eq.) of 35-38% (I assumed it was 35%) formaldehyde aq. solution (formalin) was poured into a high-walled 250ml beaker. 47.5g (0.5 mol
eq. of metabisulfite, works out to be 1 mol eq. of bisulfite due to the reaction (Na2S2O5 + H2O → 2 Na+ + 2 HSO3−) was added in small portions
with vigorous stirring. Each addition led to a slightly exothermic reaction as evidenced by water condensing on the sides of the beaker and the awful
vapor of formaldehyde reaching the experimenter’s nose... Distilled water from a squirtbottle was used in order to wash off any solid powder stuck
to the walls. The first few additions didn’t prove problematic, as everything dissolved nicely, but after ~half of the metabisulfite was added
further dissolution was difficult, and so additional 3.5g of the formalin solution was added alongside further additions of water. Then additional
2.1g of formalin was added, any cloudiness has been then removed by filtration. At this point the clear solution was similar to thinned out corn syrup
- clear and a bit heavy/viscous.
An aliquot was taken to a test tube and a couple drops were of ethanol were added. Surprisingly, 2 layers formed - probably due to the big differences
in density. Some cloudiness appeared in the aqueous adduct layer, but no precipitate fell out.
The bulk of clear, filtered solution was put into freezer - I’ll update if anything falls out. A seed crystal would be useful, so if nothing happens
in 2 days I’ll evaporate a bit of the solution and seed the mother liquor.
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ChemichaelRXN
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Yea maybe rotovap half of the solution not crystallizing or add NaCl to make a brine to reduce the solubility might work.
We are the same perception looking out, from the same elements around the universe.
We are everything to be anything to begin with.
Https://syntharise.com
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Antigua
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I feel like crystallization won't work here easily. After sitting in the freezer nice, white, thick needle like crystals do indeed start to form, but
they redissolve very easily into the original solution and aren't therefore easily separated. Maybe ethanol would indeed work well? I'm going to split
the syrup into two parts. One will be evaporated on low heat and the other will have 50ml of ethanol added to encourage precipitation. We'll see!
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dawt
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No luck here either. I added the formaldehyde to a concentrated solution of sodium metabisulfite. Adding ethanol to that created a second layer, but
no crystals formed after a night in the fridge. Mixing the layers also didn't worked so i removed the ethanol (as well as the methanol used to
stabilise the formaldehyde) at the rotavap and tried isopropanol instead ... still nothing after a few hours in the fridge.
At least the hydroxymethanesulfonate appears to be stable at rotavap temperatures (no gas was evolved and no smell of formaldehyde or SO2 observed),
so I guess concentrating in vacuo would be the next logical step.
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dawt
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Alrighty, so I removed the isopropanol and approx. 40 mL water at the rotavap until the solution turned opaque and quite viscous. Added back 20 mL of
water to redissolve everything after it had cooled to room temperature, added 99 % denat. EtOH and let it stand in the fridge for a day. The ethanol
has a slight yellow tinge to it and some needles have formed in the aqeous phase. I'll be gone for a few days and will process them when I'm back.
[Edited on 2020-12-26 by dawt]
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Antigua
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Surprise!
The flask sat in my freezer for a week now and wasted precious cold space. So I removed it and let it sit outside. Today I was greeted with snow-like,
wet crystals. Filtered them, washed with ~50ml of DCM and put into a makeshift NaOH/MgSO4 dessicator.
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Antigua
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After a couple weeks of dessicating the solid over sodium hydroxide (replaced it once) beautiful, sparkling crunchy white crystals were weighed
(17.322g, 0.129mol, 26% yield, yikes). These have a very faint smell of formaldehyde to them, the melting point (or rather the decomposition point, as
the adduct is supposed to break down at 200C) is going to be measured soon.
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Fyndium
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Too much ethanol causes bisulfite to crash out. I had this happen when I attempted to extract phenylacetaldehyde too eagerly and presumed it was the
adduct. Another test with adduct forming compound that was not water soluble produced two layers, but administering few squirts of ethanol with
pipette caused a quick adduct cake formation, which was easy to filter and wash and produced over 80% yield when regenerated.
Formaldehyde might be a little tricky because it's in a solution already, and it likely causes large losses due to many factors, not only due to
dilution. Apparently paraformaldehyde'd be better in this purpose, but this was a test of it's own. Don't know if metabisulfite could be dumped
directly into formaldehyde solution.
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Benignium
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I, too, needed some adduct so I made it in the following way:
100 grams of paraformaldehyde was first dissolved in 170 mL of water at 60 deg. C with a few small flakes of KOH. The resulting solution was added
dropwise from an addition funnel into a well stirred slurry consisting of 368.4 grams of potassium bisulfite and 108 mL of water over an hour or so.
Once addition was complete, >150 mL of water was distilled off and the clear viscous solution cooled to near 0 deg. C. To the cooled mixture ~70 mL
of ethanol was added. The reaction flask was shaken vigorously for a few seconds causing it to crystallize. Crystals were filtered off and the flask
was rinsed with the filtrate to get everything out. The crystals were carefully dried in an oven first at 50 deg. C for a few hours to avoid having it
melt into a puddle and finally at 100 degrees overnight. The product seems stable at these temperatures.
Potassium metabisulfite was used because it's what I have on hand. The product has a mildly unpleasant odor like dog food. Not quite like
formaldehyde, in my opinion.
Damp crystals
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Antigua
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Important!
The melting point of what I obtained was measured. The product started fizzing and effervescing in 200-201°C (lit. 200°C). A sample of sodium
metabisulfite was also put to test. Nothing happened to the solid in the capillary tube even at 220°C. I think this concludes that what we were able
to obtain is in fact the formaldehyde sodium bisulfite adduct. I'd love Benignium to test their product too, as I think your method is far more
efficient.
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