Sciencemadness Discussion Board - Notes on making m-dinitrobenzene

Notes on making m-dinitrobenzene

Keras - 12-1-2025 at 12:41

I need m-dinitrobenzene to synthesise resorcinol by the classical nitro-reduction followed by diazotation.

M-dinitrobenzene is obtained from nitrobenzene, itself from nitration of benzene. This first reaction proceeds quite smoothly, provided your mixed acid is cold (mine was stored outside overnight with 1°C in the morning. I had problems with benzene crystallising in the measuring flask.) and you don’t drip it too fast into benzene. It is even possible to keep the mixture well below 20 °C with a slow-enough rate, but has no benefit since after the addition the flask must be held at 60 °C for about half an hour for the reaction to complete.

Separation of the nitrobenzene by means of a separating funnel is easy, but watch out that after washing the nitrobenzene is really wet. I had to use both calcium chloride and heat to drive water out of it and get it limpid pale yellow. Do this outside or in a fume hood, since nitrobenzene vapours are toxic (it does smell much like benzaldehyde).

If you intend to go ahead and make dinitrobenzene, purifying nitrobenzene further is unnecessary.

You can dispose of the mixed acid, which contains a bit of nitrobenzene by neutralising with sodium hydroxide and boiling away the few milligrammes of nitrobenzene.

The most intricate part is dinitrobenzene. First you have to measure fuming nitric acid, which is scary. You'd better not spill it out. Remove your gloves! Good news is that, provided your nitric acid is really anhydrous, adding 98% sulphuric acid to it doesn’t lead to any heating. There is simply no water for the acid to dilute into, so no exotherm.

Dripping nitrobenzene into that hellish mixture raised the temperature fast, though. I suppose my nitrobenzene was not super dry. Once all the nitrobenzene is dripped in, the temperature is raised to 100 °C (I used a heating mantle, but you can use a boiling water bath) and the reaction is continued for an hour.

This produces a *lot* (a *hell* of a lot) of nitrogen oxide fumes. The whole RBF and the condenser become red/brown. The method to assess the end of the reaction is to extract a small sample (possibly by removing the thermometer adapter and inserting a pipette inside) and drop it into a small quantity of water in a test tube. If dinitrobenzene separates as a yellow solid, the reaction is over. *Do it absolutely outside*. Given the quantity of fumes released, any attempt to do this inside without proper ventilation is suicidal.

The workup entails emptying the RBF hot in a beaker of cold water (because dinitrobenzene solidifies at 90 °C). Needless to say, removing both the condenser and the thermometer adapter leads to a cloud of nitrogen oxides escaping (way more than before, so take no chances). Do it while holding your breath and run at a comfortable distance while the gas dissipates. When I did it, the air was quite moist (and cold) and I’m pretty sure NO₂ turned into a colourless mist of airborne nitric acid.

Once this perilous phase is successfully weathered, the workup is pretty simple (just empty the RBF into a liberal amount of cold water, dinitrobenzenes are insoluble), and filtering the crude dinitrobenzene on sintered glass is easy.

Last thing: don’t push the temperature above 100 °C, or you risk oxidative clivage and a huge mess. Trinitrobenzene cannot form, contrarily to TNT, where the slight donor propriety of the methyl group suffices to lower the energy barrier and render the third nitration possible.

Fery - 12-1-2025 at 21:52

I did the nitration with 67% HNO3 and 98% H2SO4 very long ago and I got some m-dinitrobenzene, just the yield was much lower. I recrystallized the crude product from 95% ethanol to remove small quantities of o- and p- isomers contamination.
Perhaps the nitration mixture NaNO3 + conc. H2SO4 could be tried to nitrate nitrobenzene into dinitro?

Keras - 12-1-2025 at 22:07

Yeah, I didn’t have time to recrystallise it, will do that today.

Nitrosation is not a bad idea, but since it can be done only at low temp I would simply expect the reaction to fail, given the properties of nitrobenzene.

[EDIT] I’m not 100% certain on this, but I expect nitrosation to work only on activated aromatic cycles (phenol, toluene, etc.). Not even sure it works with benzene. Ah, it does, but apparently crawls. The article in attachment claims that the NO⁺ ion is 10¹⁴ times less reactive than the NO₂⁺ ion.

[Edited on 13-1-2025 by Keras]

Attachment: challis1972.pdf (721kB)
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Alkoholvergiftung - 13-1-2025 at 01:53

Fery. Here is an old german patent form 1907 it uses sodiumnitrate and Sulfuric acid. Interesting thing they mix the nitrate with the benzen and than added slowly the sulfuric acid. they same methode was in an other patent used for naphtalene too.

Attachment: NitroBenzol221787A_all_pages.pdf (155kB)
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Keras - 13-1-2025 at 03:21

Yep I agree, see the document I followed yesterday.

Keras - 13-1-2025 at 08:49

Product after recrystallisation.

DraconicAcid - 13-1-2025 at 10:44

I have very vague memories of an undergraduate lab where the students dinitrated chlorobenzene. I'm pretty sure they didn't work with fuming nitric acid- nobody would have trusted these putzes with a bottle of that stuff. The product was a brilliant red/orange, though....

Axt - 13-1-2025 at 14:05

Quote: Originally posted by DraconicAcid

I have very vague memories of an undergraduate lab where the students dinitrated chlorobenzene. I'm pretty sure they didn't work with fuming nitric acid- nobody would have trusted these putzes with a bottle of that stuff. The product was a brilliant red/orange, though....

It's not coloured, they probably tried to aggressively neutralise it and ended up with a salt of dinitrophenol. DNCB is an odd one for an undergrad though its strongly immunogenic, I've broken out in hives going near that stuff.

Keras - 13-1-2025 at 21:20

Quote: Originally posted by DraconicAcid

There is somewhere on YouTube a video of an Indian guy making dinitrobenzene and his final product looks very much the same.

Now I’ll have to reduce it into m-phenylenediamine.

[Edited on 14-1-2025 by Keras]