Sciencemadness Discussion Board - Methanesulfonic acid for nitration mixtures

Methanesulfonic acid for nitration mixtures

Keras - 18-1-2026 at 08:39

Folks,
I’m attempting to nitrate toluene using a mixture of 70% methanesulphonic acid and potassium nitrate.

Well, to be 100% true to facts, I mixed up those (cold) reactants, stirred them for 15 minutes then filtered the resulting syrupy slush, assuming I would get a mixture of methanesulphonic acid and nitric acid.

However, that mixture doesn't seem to nitrate anything when mixed in turn with toluene. So far, keeping the mixture under 10 °C and slowly dripping the acid didn't get me anything: no yellow colour, nothing.

I’m wondering if 70% methanesulphonic acid is strong enough to turn potassium nitrate into nitric acid. Did anyone ever attempt this?

- Addendum: A drop of the mixture on a small copper bit didn't produce anything: no bubbles, no blue colour. So I suppose that’s it. It doesn’t work

Keras - 19-1-2026 at 06:41

I redissolved potassium nitrate into methanesulphonic acid and let the mix heat up to 50/60 °C and stir for 30 mn circa. It was then filtered. The resulting syrupy liquid smelt somewhat of nitric acid, but it’s difficult to say. Copper didn't seem to be attacked by it. At any rate, mixing it with toluene and stirring didn't seem to do anything.

Not that I was more successful when adding 1 or 2 mL of 68% nitric acid to my acid mix.

I suspected that the biphasic system didn't help, so I added a few crystals of TBAB. And bingo! The toluene layer became yellow in a few seconds. However, I’m now debating if this is nitrotoluene or oxidised bromine from TBAB + nitric acid…

The pKa of methanesulphonic acid is given as -1.9, which is lower than that of nitric acid, so mixing the former with potassium nitrate should release nitric acid. Apparently iif it does, it either a long process, or it doesn’t work at all.

[Edited on 19-1-2026 by Keras]

bnull - 19-1-2026 at 14:10

Did you use an excess of potassium nitrate?

Since we're at it, is potassium methanesulfonate less soluble than potassium nitrate? I can't find a solubility table for it.

Keras - 19-1-2026 at 22:37

Quote: Originally posted by bnull

Did you use an excess of potassium nitrate?

Not really. The goal was to make a mixed acid mixture, so I roughly used a ratio of 2:1 in favour of methanesulphonic acid, hoping that half the starting acid would produce nitric acid and the rest would be left untouched.

Quote: Originally posted by bnull

Since we're at it, is potassium methanesulfonate less soluble than potassium nitrate? I can't find a solubility table for it.

I don’t think there is any published data for methanesulphonic salts. The use of methanesulphonic acid as a readily available stand-in for plain sulphuric acid is recent, IIRC the industrial production of methanesulphonic acid didn't really take off before the 90s when BASF opened a dedicated plant. Apparently, someone offers it as an aqueous solution tagged as 200 g/L, so its solubility must be higher than that.

[EDIT] I'd like to add that I don’t intend to make TNT, but p-cresol via toluene → p-nitrotoluene → p-toluidine → (diazotisation) → p-cresol.

[Edited on 20-1-2026 by Keras]

RU_KLO - 20-1-2026 at 06:53

From this paper, it works with silver nitrate.

"An alternative method for synthesis of silver(i) methanesulfonate is by reaction between AgNO3 and MSA, where the HNO3 formed is removed by distillation

AgNO3 + CH3SO3H → Ag(CH3SO3) + HNO3

"

Methanesulfonic acid (MSA) in clean processes and applications: a tutorial review
Koen Binnemansand Peter Tom Jones
https://pubs.rsc.org/en/content/articlelanding/2024/gc/d4gc0...

I would try with lead or mercury nitrate (very hazadous and poisonous), because they are in the same group of chloride precipitation in analytic chem.

Use other metals nitrates (iron nitrate for example)

It seem key, to remove by heat (destilation) the HNO3 (dont know how much will be decomposed....) also maybe explosion risk.

Be careful.

clearly_not_atara - 21-1-2026 at 16:59

The problem is you are using the aqueous pKas, but you actually have methanesulfonic acid as the primary "solvent". So basically we have to ask whether nitrate anion is more stable in methanesulfonic acid solvent or whether methanesulfonate ion is more stable in nitric acid solvent. And I suspect the former may be true. Methanesulfonate's aqueous pKaH+ might be lowered by better coordination to water relative to nitrate. It might also be possible to obtain nitric acid by using a third solvent in which nitric acid is not strong (i.e. not water) and in which methanesulfonate is more stable than nitrate. Possibly acetic acid? For this you would prefer anhydrous methanesulfonic acid in acetic acid.

Keras - 22-1-2026 at 01:26

Thanks to both of you for your input.

I suspect you’re both right: the pKa difference is not big enough (-1.2 for nitric/-1.9 for methanesulphonic) to warrant a complete forward reaction. There must be some sort of equilibrium reached, and distilling away the nitric acid as it is formed seems a good idea since it would constantly displace the equilibrium in favour of the formation of new nitric acid.

I’m going to try that. I don’t think there is any risk involved. I already made fuming nitric acid by mixing concentrated sulphuric acid and potassium nitrate and distilling the resulting nitric acid without any problem. Methanesulphonic acid being milder than pure sulphuric acid, the risks are even lower (it is *not* oxidising and *not* explosive).

If that doesn’t work, I’ll try and heed Atara's advice, which is slightly more complicated to implement.

Thanks again!

semiconductive - 25-1-2026 at 13:12

Quote: Originally posted by clearly_not_atara

So basically we have to ask whether nitrate anion is more stable in methanesulfonic acid solvent or whether methanesulfonate ion is more stable in nitric acid solvent. And I suspect the former may be true. Methanesulfonate's aqueous pKaH+ might be lowered by better coordination to water relative to nitrate. It might also be possible to obtain nitric acid by using a third solvent in which nitric acid is not strong (i.e. not water) and in which methanesulfonate is more stable than nitrate. Possibly acetic acid? For this you would prefer anhydrous methanesulfonic acid in acetic acid.

How would you test?

I have potassium nitrate, and can easily re-crystallize and dry it.

I have MSA, which was anhydrous before opening cap, but I've used it several times. It now has a slight color to it along the red/purple side. Would I need to buy a new bottle?

I also have glacial acetic acid, although it's not truly anhydrous like acetic anhydride is. Also, formic acid (98%), propanoic acid (99.9%), and esters like butyl-buterate, and MSM, 98% DMSO, ...

I'm wondering:
Would dipping copper in such solutions (without water) produce the brown gas as a test for nitric? or is there a more sensitive reagent I could buy that would tolerate the organic acids better?

If, as you suggest, this is a relative stability issue that depends on bond or coordination strength; I'm wondering if there is a way to measure/estimate strong the difference in bond/coordination energy IS by finding at what dilution in water the reaction begins to fail? ( That would be very useful for other projects I'm trying to figure out! )

Thanks for your comments, Clearly_not_atara, they're very helpful.

Keras - 30-1-2026 at 13:01

So I did mix methanesulphonic acid with potassium nitrate, heated and distilled the resulting product as it was formed. It seemed to work. Unfortunately I had to move quite quickly from inside to outside when fumes of nitrogen dioxide started to appear, and during the dismounting/remounting probably some traces of water got in, and my final product is green!

I’ll redistill it tomorrow and see if it’s really nitric acid.

[Edited on 30-1-2026 by Keras]

Metacelsus - 31-1-2026 at 09:45

Green is from N2O3.

Keras - 1-2-2026 at 01:25

Quote: Originally posted by Metacelsus

Green is from N2O3.

I thought this was only stable at low temps?

The fact is, this green tinge seem to develop only in a nitrogen dioxide rich atmosphere, as the following picture demonstrate: the nitric acid being distilled is crystal clear, but as it collects in the NO₂-full flask, it turns to green.

In any case. I redistilled this (the blue/green colour went more or less away). The density of the nitric acid solution obtained after redistillation pointed towards 50% concentration (d ~ 1.3). Starting from 70% methanesulfonic acid, it seems a little low.

In any case I took this solution and mixed it with more 70% methanesulfonic acid in the hope of making a nitrating mixture for toluene. The result is: it didn't work. Methanesulfonic acid (at least diluted to 70% w/w) doesn’t seem to be strong enough to protonate nitric acid. The layer of toluene became somewhat yellowish, but after the first washing that colour went away, so it was probably only nitrous oxide in solution. I drove the solution to 40 °C after the ‘nitrating mixture’ addition was over.

Funny thing however: when I washed a second time with sodium bicarbonate saturated water, the aqueous phase became golden yellow. I assume this is due to the bromide ions coming from the TBAB I used as phase-transfer catalyst in an attempt to prod the reaction.

I’m going to recycle the leftover toluene and try a nitrosation.

[EDIT]: That didn't work either. I got tons of dinitrogen trioxide in my RBF (see blue RBF picture), but none of it reacted with the toluene.

I’m quite baffled. Toluene is supposed to be slighted activated by the methyl group w/r to benzene, but nitration does not seem to occur so easily. In comparison, phenol is thousand times more reactive, it seems (understandably).

[Edited on 1-2-2026 by Keras]

clearly_not_atara - 12-2-2026 at 15:31

In order to protonate nitric acid (-> NO2+ ion), you need the moles per liter of acid to be greater than the moles per liter of water. Methanesulfonic acid has a MW of 95 and water has a MW of 18 so a 1:1 molar ratio is about 83% methanesulfonic acid. That's before we account for any water in the nitric acid.