Sciencemadness Discussion Board

Reduction of Nitro Toluene

merlic79 - 16-1-2008 at 21:58

I've been researching various methods of reducing nitro compounds using OTC sodium dithionite (Rust Out). I've found some interesting literature regarding this:

"The reduction of the nitro toluene may also proceed with an inorganic reducing agent such as sodium dithionite. Preferably, 3 reducing equivalents are required for each mol of nitro group reduced. At the beginning of the reaction, an aqueous basic reaction medium is employed. Most preferably, the pH is kept between 10-12. Atmospheric pressure is normally used, and the temperature of the reaction medium is kept below 30 C. The medium is then stirred at an ambient temperature."

Is this process feasible? Has anyone attempted something similar to it? Any feedback would be greatly appreciated as I am not a fan of the messy Sn/Fe HCl reduction method.

Drunkguy - 17-1-2008 at 11:22

My experience has been that the reduction of NO2 with Sn/HCl and also of "Schiff bases" with Zn/HOAc is not nearly as "messy" as some people have said.

Follow the procedure for making aniline as quoted in Vogel, substituing nitrotoluene for nitrobenzene.

https://sciencemadness.org/talk/viewthread.php?tid=462&p...

^Read my notes (and others) for information on the technique.

I havent tried sodium dithionite, nor do I know where it is obtained from.

Nicodem - 17-1-2008 at 12:14

Quote:
Originally posted by merlic79
"The reduction of the nitro toluene may also proceed with an inorganic reducing agent such as sodium dithionite. Preferably, 3 reducing equivalents are required for each mol of nitro group reduced. At the beginning of the reaction, an aqueous basic reaction medium is employed. Most preferably, the pH is kept between 10-12. Atmospheric pressure is normally used, and the temperature of the reaction medium is kept below 30 C. The medium is then stirred at an ambient temperature."

I do not know where you got the cited text from since you do not put the reference :mad:, but when using Na2S2O4 for reduction of nitroaromatic compounds, it is generally done by refluxing in MeOH/H2O solution (actually anything from 50 to 90% MeOH will do just fine). Methanol only serves as a cosolvent for the substrate. The basic media is maintained by adding Na2CO3 to the reaction mixture. Generally 3h reflux suffices, but you can nevertheless follow by TLC.

merlic79 - 30-8-2008 at 10:04

Today I will attempt to reduce 300ml of freshly prepared nitrotoluene using Sodium Dithionite (rust out). It will be carried out in small batches in order to deal with the limited solubility of Dithionite in MeOH/H20 solution. I wonder if this reaction in particular needs a catalyst such as a viologen. And if this rxn goes as planned, how would one go about extracting the toluidine from solution efficiently? ....steam distillation, simple distillation hmmmmm....Anyways I'll see how it goes and post an update with some pics in a few days.

Nicodem - 31-8-2008 at 09:20

The Na2S2O4/Na2CO3 mixture does not dissolve in the reaction mixture of your substrate in MeOH/H2O. The reaction mixture is heterogeneous. When the TLC shows the absence of your starting material (usually takes a couple of hours reflux, but depending on the substrate), you just filter off the solids, wash them with some methanol and concentrate the filtrate. Then you do the purification depending on your substrate properties. In your case, if there is no more starting material showing up on TLC you can just do a steam distillation. That would be the simplest solution. Otherwise you can purify by extraction (using the acid/base method) which will however not remove possible basic side products. You can also use a vacuum distillation with a distillation column. The choice depends on what equipment you have.

fuming_nitric_acid - 1-9-2008 at 18:16

I think that steam distillation would work. Just add water and boil. But depending on the equipment you have you can do as Nicodem said!


[Edited on 1-9-2008 by fuming_nitric_acid]

merlic79 - 5-10-2008 at 17:44

I went ahead with the reaction using sodium dithionite. The smell has changed from an almondy shoe polish type, to a completely different piercing, complex ammonia-like smell:o. I believe that this is the smell of toluidine.....I will now try to steam distill the crap and see what the end product looks like.

Sauron - 5-10-2008 at 21:31

Fe filings and hydrochloric acid is much more economical of acid than Sn/HCl. On a lab scale this may not matter, but industrially it surely does.

kmno4 - 5-10-2008 at 23:38

Quote:
Originally posted by Sauron
Fe filings and hydrochloric acid is much more economical of acid than Sn/HCl. On a lab scale this may not matter, but industrially it surely does.

That is it what I wanted to write :)
Fe messy ? Not at all.
Reduction -NO2 to -NH2 requires a lot of hydrogen (six moles of H per one mole of R-NO2) and Fe/HCl is the cheapest reagent.
Some time ago I reduced 1 mole of MeNO2 to MeNH3Cl. At room temperature reaction is slow - it took about 5 days.
But - extremely simple aparaturus: glass bottle with a cap, shaken
a few times per day ( continous strong shaking during ~1 minute - the stronger and longer the better)).
Pressure relised 2-3 times per day (very small amounts of hydrogen). My previous experiments with refluxing (without stirring) gave shitty results. Why? Very compact layer of Fe2O3 on pices of Fe - the hoter, the more compact.
Anyway - reducing NT (and other aromatic nitrocompounds) with Fe would require an addition of cosolvent (methanol, ethanol..... ). With MeNO2 is much simpler....
Do not use powdered Fe, because it will cake in hard, almost unrective mass. Just crystals of Fe2O3 lime Fe paricles into big one.
This is the end of my offtopical digression about reducing with Fe ;)

Sauron - 6-10-2008 at 01:09

I recently posted articles from Industrial & Engineering Chemistry describing the nitration of chlorobenzene and reduction of the mononitrochlorobenzene isomer mix to mixed chloroanilines. The reagent of choice was Fe/HCl. Comparisons with Fe/H2SO4 and Sn/HCl were made and Fe/HCl was best by far. H2SO4 was much slower, and Sn/HCl required a large excess of acid. See the thread on p-chloroaniline.

Nitrobenzene, nitrotoluene, nitrochlorobenzene, same story different chapters.

manimal - 1-10-2009 at 16:10

How's this for separating the toluidine isomers?

1/3 of the toluidine is neutralized with HCl. The p isomer, being the stronger base, is neutralized first and becomes a salt, and the remaining, mostly o-, toluidine is separated. I got the idea from Victor von Richter's organic chemistry.

Nicodem - 1-10-2009 at 22:57

p-Toluidine is only a 4 to 5 times stronger base than o-toluidine (pKa of 4.57 vs. 5.23). This is a small difference which might even be within experimental error, particularly if not measured by the same method. With such a small difference you would not be able to selectively dissolve only the p-toluidine (providing that you use an equimolar amount of HCl). I would estimate you not be able to purify above something like 70-80% purity. What relevant stuff does that reference describes anyway?

manimal - 1-10-2009 at 23:51

It describes separating the toluidine isomers by adding a slight excess of H2SO4 to neutralize the p and distilling in steam. But you are right about the ratio of pKbs determining the distribution of isomers between the aqueous and basic phases.

Specifically, the Kb of o is 2.6*10^-10 and that of p is 12*10^-10. I can't think of how to do a precise analysis of the system to figure out the isomer distribution off the top of my head.

[Edited on 2-10-2009 by manimal]

Nicodem - 12-8-2013 at 07:41

An example of reduction of ortho-nitrotoluene to ortho-toluidine with iron is reported at the Synthetic Pages:
Reduction of nitro arene by Fe/ammonium chloride; o-toludine (DOI: 10.1039/SP538)

Related topic (reduction with Zn/HCl):
Reduction of Aromatic Nitro compounds with Zn / HCl A write up