Sciencemadness Discussion Board - Separating DMF from water

Separating DMF from water

nimgoldman - 16-10-2018 at 12:39

After my last synthesis (nitroethane from NaNO2 + EtBr in DMF), I have ended up with DMF and water mixture at about 1:1 ratio (the reaction between NaNO2 and EtBr is quenched by pouring the reaction mixture in ice cold water).

I tried fractional distillation but even with 60 cm column, the mixture boils at steadily increasting temperature and the vapor temperature also steadily rises. Now the mixture boils at 114 °C and the vapor temp. is 106 °C.

The separation is poor and the water in receiving flask strongly smells of DMF. I tried using aluminium foil insulation with no improvement. The solution simply does not boil at all below 113 °C and above that, the separation is already poor.

My theory is that there are some side products in the DMF, like dimethylamine, causing the weird boiling point.

Other than that, I don't know how to remove all that water from DMF. I know to dry and purify it then, but the amount of water is too large for chemical drying.

[Edited on 16-10-2018 by nimgoldman]

nimgoldman - 16-10-2018 at 12:53

One idea could be using packed column (instead of plain Vigreux) or salting out DMF somehow. I haven't found much info on either of these methods.

clearly_not_atara - 16-10-2018 at 12:53

Salting out?

I think you should be able to separate the DMF/water by adding ammonium sulfate, widely available as fertilizer in large quantities. (NH4)2SO4 is very poorly soluble in DMF. Afterwards, ammonium sulfate can be recovered by removing water under vacuum or possibly with heating

unionised - 16-10-2018 at 12:57

I'd wonder if you are making nitrosodimethylamine as a by-product.
Be careful what you sniff at.

How slowly are you distilling it?

Can you salt out the DMF?

nimgoldman - 16-10-2018 at 13:08

I don't know whether I can salt it out. This is why I am asking. I haven't found any info on salting out DMF which amazes me given that DMF/water mixtures must be extremely common in the lab.

The distillation proceeds at moderate speed but given that it's mostly water it does not drip at some measurable speed, it's more like nothing comes out for several seconds, then a short streak of water comes out.

I think I will stop the distillation and continue tommorow with vacuum, it's about liter of DMF/water and it already ate 6 blocks of ice. I hope to find it's boiling point at 100 mbar (about 75 mmHg).

I don't have ammonium sulfate at hand but at this point I think about just disposing it safely since it's not worth all the labour and money already spent on trying to purify it.

Making nitromethane with DMF as a solvent is so low yielding (I got less than 1 ml) and recycling the solvent is so hard this method is basically not worth all the hassle.

[Edited on 16-10-2018 by nimgoldman]

nimgoldman - 16-10-2018 at 13:20

As for the possible nitrosamine contamination, I can treat the water with potassium permanganate in sulfuric acid, which should destroy it.

[Edited on 16-10-2018 by nimgoldman]

nimgoldman - 16-10-2018 at 14:53

I transferred the remaining impure DMF to a 1L flask for further (vacuum) distillation. It became cloudy and a curious thin layer appeared on top. Now I am completely confused.

I think the DMF was already impure since it was smelly (it should have no smell if pure) and I should have purified it beforehand.

I think I will now distill as much DMF as possible with fractional vacuum distillation and treat the residue with KMnO4 in 3M sulfuric acid to destroy any possible nitrosodimethylamine that could have formed.

The product (nitroethane) is strongly green, which is also very confusing (it should be colorless) but I have not disilled it yet, the amount is too small and I will collect more - it can be contaminated with nitrosodimethylamine too unfortunately.

I think I will use DMSO instead of DMF next time. DMF itself have already too many problems.

[Edited on 16-10-2018 by nimgoldman]

clearly_not_atara - 16-10-2018 at 15:01

Quote:

I haven't found any info on salting out DMF which amazes me given that DMF/water mixtures must be extremely common in the lab.

I doubt anyone bothers to recover DMF; it's just not worth the effort. If you're paid for time, it doesn't make sense to spend a long time recovering a cheap solvent. (If you can buy more DMF, that probably makes more sense than anything in this thread!)

Also, this is the second time someone has said they don't have access to sulfate fertilizers, and I'm perplexed. Sulfates are useless for making bombs. Why would they ever be restricted?

[Edited on 16-10-2018 by clearly_not_atara]

nimgoldman - 16-10-2018 at 16:09

Quote: Originally posted by clearly_not_atara

Quote:

I haven't found any info on salting out DMF which amazes me given that DMF/water mixtures must be extremely common in the lab.

I have access to sodium sulfate but I meant it's not worth ordering it just for that single purpose.

Yes I think the best course of action would be to simply dispose it. It's just 500 mL after all.

I originally wanted to recycle it since the nitroethane synthesis requires lots of DMF to make just a tiny amount, hence I would either have to use several liters of DMF or recycle a liter or so several times. But this synthesis route is not very useful after all.

XeonTheMGPony - 16-10-2018 at 16:34

Quote: Originally posted by clearly_not_atara

Quote:

I haven't found any info on salting out DMF which amazes me given that DMF/water mixtures must be extremely common in the lab.

I live in farming territory and I ended up having to order ammonium sulfate! from ebay

clearly_not_atara - 16-10-2018 at 16:47

I guess I'll have to revise that recommendation!

The fertilizer section at a store like Lowe's, Home Depot etc should contain plenty of compounds capable of salting out. See:

https://en.wikipedia.org/wiki/Hofmeister_series

You just have to pick a compound with high solubility that's high in the series. Fire extinguishers usually use a blend of diammonium phosphate, monoammonium phosphate, and ammonium sulfate as the "filler", so you might be able to use the powder from a fire extinguisher (but don't waste your only one!).

I don't know if it'll be dipotassium phosphate, diammonium phosphate, potassium sulfate, ammonium sulfate, potassium carbonate, or sodium carbonate, but the anion should be selected from (HPO₂^-, SO₄^2-, CO₃^2-) and the cation from (NH4+, K+, Na+, Rb+). Salting out of most organic compounds from water uses the same techniques.

Na2CO3 technically counts and is (by far) the easiest to acquire but it has a low solubility which makes it not preferable. K2SO4 also works but has a low solubility. Dipotassium phosphate has a high solubility. Nitrates are not recommended here.

Elrik - 16-10-2018 at 22:57

Quote: Originally posted by XeonTheMGPony

Quote: Originally posted by clearly_not_atara

...this is the second time someone has said they don't have access to sulfate fertilizers, and I'm perplexed. Sulfates are useless for making bombs. Why would they ever be restricted?...

I live in farming territory and I ended up having to order ammonium sulfate! from ebay

Ammonium sulfate has been used in making methamphetamine.
Controlled heating will produce ammonium bisulfate and anhydrous ammonia. Anhydrous ammonia being a very popular meth precursor chemical.

nimgoldman - 17-10-2018 at 02:41

Based on Armarego & Chai: "Purification of Laboratory Chemicals, 7nd Edition", large amounts of water in DMF can be removed by azeotropic distillation with benzene (10% v/v). The benzene must be already dry (preferrably pre-dried with calcium hydride).

My DMF now contains only a small amount of water, thus the appropriate purification is drying with calcium sulfate, magnesium sulfate, silica gel or 4A molecular sieves, followed by vacuum distillation. The powdered drying agent will likely trap part of the impurities.

The resulting distilled DMF won't be completely dry, but at least the nasty impurities will be removed, namely the dimethylamine and the original reaction side products.

Distillation should not proceed to dryness hence I am thinking about redisue disposal. One option might be mixing with a strong base (aq. NaOH) that should hydrolyze DMF into sodium formate and dimethylamine.

Another option could be adding strong acid (namely sulfuric) which will hydrolyze DMF into formic acid and dimethylamine. The acids can be neutralized and finally disposed.

The nitrosamine formation is however still a concern since this is super toxic and should not enter sewer system (not sure about landfill). In the stronly acidic conditions, potassium permanganate is used to destroy nitrosamines into less toxic class of compounds. However, I am not sure how this would react with other side products (possibly small amounts of NaNO2, NaBr, EtBr, EtNO2) - of course adding oxidizer to a nitrate and possibly incompletely decomposed flammable solvent is a no no.

I like to learn how to destroy the toxic waste into less harmful, disposable waste. This is valuable for amateur chemistry and disposal services are costly and require otherwise unnecessary paperwork.

XeonTheMGPony - 17-10-2018 at 03:57

Quote: Originally posted by clearly_not_atara

I guess I'll have to revise that recommendation!

The fertilizer section at a store like Lowe's, Home Depot etc should contain plenty of compounds capable of salting out. See:

https://en.wikipedia.org/wiki/Hofmeister_series

You just have to pick a compound with high solubility that's high in the series. Fire extinguishers usually use a blend of diammonium phosphate, monoammonium phosphate, and ammonium sulfate as the "filler", so you might be able to use the powder from a fire extinguisher (but don't waste your only one!).

If you do that keep on mind they all so contain fumed silica, it makes a royal mess and is a pain to clean and is impressively hydrophobic, it can work but is a head ache.

Ubya - 17-10-2018 at 05:15

Quote: Originally posted by XeonTheMGPony

Quote: Originally posted by clearly_not_atara

If you do that keep on mind they all so contain fumed silica, it makes a royal mess and is a pain to clean and is impressively hydrophobic, it can work but is a head ache.

i have 3 or 4kg of powder from a fire extinguisher, it's hydrophobic as you said and has a really light blue color. i didn't know it had fumed silica in it, i thought it was some kind of silicone hydrophobic coating, anyway, i thought a eay to remove it would be to melt the powder (melting point of monoammonium phosphate 170°C, diammonium phosphate decimposes to monoammonium phosphate at a lower temperature), the fumed silica and other silicones are removed by dissolving the melted (and cooled) MAP in water or suitable solvent of choice

Panache - 17-10-2018 at 18:48

If you have access to decent vac pump try pulling a room temperature vacuum on it, making sure your condenser is ice water cold. I have not seen the phase diagram for DMF/water mixtures so I cannot predict anything but systems distill unusually under high vacuum.
Worth a try.

nimgoldman - 18-10-2018 at 13:06

Quote: Originally posted by Panache

If you have access to decent vac pump try pulling a room temperature vacuum on it, making sure your condenser is ice water cold. I have not seen the phase diagram for DMF/water mixtures so I cannot predict anything but systems distill unusually under high vacuum.
Worth a try.

My diaphragm vac. pump is able to achieve only 100 mbar (about 75 mmHg), so this is not enough for room temperature boiling.

The fractionation might work better under reduced pressure.

But now I will just distill most of the solvent (this will go to disposal) and safely neutralize the possibly nasty residue.