Keras - 23-3-2026 at 00:52
Folks,
While I've (temporarily) turned my attention to silver plating (another thread), I’m still trying to convert toluene into cresol via
mono-nitrotoluene and toluidine.
I did obtain a somewhat large batch of p-nitrotoluene from a first experiment, but turning it into toluidine was ineffective. Somehow the reaction
with sodium hydrosulphite worked (the smell of almonds disappeared, replaced by one of – difficult to say what. The French Wikipedia section on
toluidine describes it as ‘wine’). But I failed to isolate it properly. It is there, but elusive. Apparently it doesn’t steam distill. I
hesitate to distill it at 200 °C. The quantity is not much, so maybe I’ll do that using microscale glassware.
In the meantime, I remade some nitrotoluene (using concentrated sulphuric acid and azeotropic nitric acid). I had to stop the experiment midway
through the work-up, namely after separating the compound from the nitrating mixture at the basic washing step. I used sodium carbonate, and … the
nitrotoluene seems to precipitate in small flakes while in basic solution. Did anyone ever experienced this? There is still a liquid organic phase, so
I wonder if only one of the two isomers precipitates?
The organic phase is deep red/crimson, and this colour seems to change with the pH — I say that because when neutralising the nitrating mixture with
sodium carbonate, the colour of the mixture was first dark orange, and then turned yellow when an excess of acid was added. When I added sodium
carbonate to neutralise the excess acid, the colour change was very obvious as local spots of basic pH were created. So I suppose the overall orbitals
are pH dependent.
So I will probably neutralise the excess sodium carbonate with a few drops of hydrochloric acid, because having flakes in the liquid makes the
separation a bit difficult.
Also, I replaced my years old hydrosulphite by a fresh batch, and it seems to work way better to reduce nitrotoluene to toluidine (the yellow colour
fades away almost instantly). Does anyone know if toluidine steam distills, or if there is any way (other than distilling at 200 °C) to isolate
toluidine from a solution of sulphate salts and maybe traces of unreacted toluene/nitrotoluene?
Cheers!
Fery - 23-3-2026 at 02:01
Magpie did it:
https://www.sciencemadness.org/talk/viewthread.php?tid=29729
Using cheap Fe and catalytic amount of HCl does not work? Then steam distillation or maybe Dean-Stark or Clevenger apparatus but it has density
similar to water?
My experiments with nitration of toluene:
https://www.sciencemadness.org/whisper/viewthread.php?tid=15...
My separation using vacuum distillation with Hempel column, variable rate distillation head, 2-stage oil rotary vane chines vacuum pump (so
o-nitrotoluene distilled at 70 C) was not good enough as I did not yet have heating mantle with mg stirring that time and used only hotplate with mg
stirring with with oil bath pot - too high heat loses, the drip ratio was very low, only 2 drops every second, if using more powerful heating mantle
and much higher reflux flow = more drops per second the separation would be better.
Putting mixture of o- and p- nitrotoluene into freezer at -18 C caused complete solidification, but during winter time with outside temperature -15 C
caused best and highest yield of separation of the p- isomer, maximal amount of crystals of p- and maximal amount of liquid phase - though the liquid
phase still contained a lot of p- isomer. You can also gradually crystallize more and more of p- at temperatures -5 C, then -10 C, finally -15 C.
These crystals are covered with a little of o- isomer, require washing with little of very cold (iso)hexanes on sintered glass during filtration and
finally recrystallization from methanol.
But when your goal is toluidine, you do not need to separate o- and p- nitrotoluenes strictly, it is easier to separate o- and p- toluidines as their
salts with oxalic acid, read the last post of this thread:
https://www.sciencemadness.org/talk/viewthread.php?tid=28526
also this video:
https://www.youtube.com/watch?v=FIUgBfWkx7I
Keras - 23-3-2026 at 03:10
Ok thanks! Apparently the red colour that appears during the nitration reaction and persists, especially when the organic phase is washed with basic
water, would be caused by parasitic phenols. Submitting a question to Google Gemini gave me this:
Technical Note: Why Crude Nitrotoluene Turns Deep Red After a Carbonate Wash
If you’ve just finished the nitration of toluene and your organic phase turned from a pale yellow to a deep "port wine" red after washing with
sodium carbonate , don't panic. This is a classic phenomenon caused by three main factors:
1. Formation of Nitrophenolates (The Primary Cause)
Nitration with mixed acids is a highly oxidative environment. A small fraction of the toluene (or the nitrotoluene itself) undergoes side-chain or
ring oxidation, leading to the formation of nitrophenols (such as 2-nitro-p-cresol).
The Mechanism: In acidic conditions, these nitrophenols are in their neutral state and appear pale yellow.
The Color Shift: When you add a base like sodium carbonate, the phenols are deprotonated into phenolate salts. These ions have a highly extended
electron delocalization (resonance).
The Result: This structure absorbs light in the blue/green spectrum, reflecting a deep, intense red to the eye.
2. Charge-Transfer Complexes (CTC)
The reaction mixture contains a cocktail of electron donors (unreacted toluene) and strong electron acceptors (p-nitrotoluene and traces of
dinitrotoluene).
These molecules can stack on top of each other, forming "Charge-Transfer Complexes."
These complexes shift the light absorption toward much longer wavelengths, creating dark orange, brown, or blood-red hues that aren't present in the
pure, isolated molecules.
3. Reversibility with Acid
As observed, this color is pH-dependent.
Adding an acid (lowering the pH) reprotonates the phenolate ions back into neutral phenols. The extended conjugation is broken, and the solution
reverts to the characteristic yellow of simple nitro compounds.
[EDIT: more precisions]
Update: Why the red color appears during nitration (Acidic conditions)
You might notice the mixture turning from yellow to deep red/orange even before the carbonate wash, while still in a highly acidic environment. This
is due to:
Radical-Cation Intermediates: The harsh oxidizing nature of the
mixture creates transient radical species. These intermediates have unpaired electrons that absorb light intensely, leading to dark discolorations.
In-situ Charge-Transfer Complexes: Even in acid, the interaction between unreacted toluene (electron donor) and newly formed nitrotoluene (electron
acceptor) creates CTCs that shift the color toward the red end of the spectrum.
Dissolved Nitrogen Dioxide: Micro-decomposition of nitric acid releases nitrogen dioxide. When trapped in the organic phase, this gas acts as a
powerful red/brown dye.
Note: This "acid-red" usually clears up or shifts during the final drowning in water, whereas the "base-red" (nitrophenolates) only appears after
neutralization.
[Edited on 23-3-2026 by Keras]