Sciencemadness Discussion Board

Reduction of Aromatic Nitro compounds with Zn / HCl A write up

benzylchloride1 - 6-8-2007 at 10:26

Ater the nitration of 30 grams of toluene around 25 mL of a mixed product of para and ortho Nitro toluenes was obtained . No attempt was made at seperating the two isomers.

Reduction with Zinc and HCl
45 grams of granulated Zinc was added to a 500 mL round bottemed flask. The Nitro toluene isomer mixed was added to the flask. A reflux condeser and a addition funnel was attatched. The flask containing the Nitro toluene and Zinc was slowly heated with a hot plate/ magnetic stirrer. Concentrated Hydrochloric Acid was added through the addition funnel 5 ml at a time until 90 ml of acid was added. the reaction mixture turned a very dark color. The reaction mixture was heated after the addition of the HCl for 3 hours.
100 ml of 17.5 M NaOH was slowly added to the reaction mixture after cooling to neutralize the HCl and decompose the Toluidine HCl. The mixture was steam distilled to remove the Toluidine and unreacted Nitro Toluene from the mixture. The steam distillation lasted 6 hours. The steam distillate was strongly acidified with concentrated HCl to convert the Toluidine isomers into the Hydrochloride. The aqueus solution was removed from the unreduced Nitro Toluene. The aqueus solution was neutralized by the addition of strong NaOH solution. The Toluidine isomers Separated on the surface of the aqueus solution as a brown oil.The liquid was decanted into a sepratory funnel to remove the aqueus solution from the toluidine. The aqueus solution was extracted with engine starter fluid (Prestone brand). A total of 7 mL of mixed Toluidine isomers was obtained. This is not an excellent yield. Further experiments will be carried out using more Zinc and longer refluxing times.

benzylchloride1 - 8-8-2007 at 11:46

Another attempt was made at this reaction by increasing the reflux period from 3 hours to 5 hours. No increase in yield of Toluidine isomers was observed.
Zinc / Hydrochloric acid reductions work on aromatic nitro compounds, but are very inefficient compared to reductions with Tin and Hydrochloric acid.

Vitus_Verdegast - 8-8-2007 at 21:21

An option to avoid clean-up nightmares (I shiver when I think back at my first and only reduction of mixed isomers of nitrotoluene using Fe/HCl) is to use electroreduction. No huge metal waste and less solvent, and the yield is generally high for the simple anilines using lead electrodes and dilute H2SO4 as the electrolyte.

There are quite a few references for aromatic nitro reductions in the electrochemical books available in the Sciencemadness library.

For a high current DC power source, use the 5V line of an old PC power supply or a car battery charger. For a diaphragm use an unglazed flower pot or the diaphragm of a car battery. More info:
https://sciencemadness.org/talk/viewthread.php?tid=4145&...

[Edited on 9-8-2007 by Vitus_Verdegast]

PHILOU Zrealone - 9-8-2007 at 08:44

Sorry, but why steam distillation step?
Simple extraction with a volatile non water soluble solvent would have allowed a higher yield, not only because one less step, but mostly because steam distillation is efficient at separating ortho and para isomer. Only one of the two gets through, the other one remains in the origin vessel!

You more than certainly only got one of the two toluidine (three, because there is stil a little meta nitrotoluen in your mix!

Kind regards.

PH Z

S.C. Wack - 9-8-2007 at 14:01

Quote:
Originally posted by PHILOU Zrealone
steam distillation is efficient at separating ortho and para isomer.


Do you have a reference for this statement?

Was there any proof that the 25 ml was o- and p-nitrotoluene and nothing else?

PHILOU Zrealone - 27-8-2007 at 08:49

Most organochem books speak about the increased volatility (sometimes without aid of steam!) of ortho vs para compounds. Obviously because some viccinal group interaction and resonance conformations allows pseudo H-bonding/pi bonding cyclisation.

S.C. Wack - 9-10-2007 at 14:47

And which, exactly, organochem books say such goofy things?

I wonder if perhaps you are getting this confused with the separation with steam of o- and p-nitrophenol, which is quite simply based on a large difference in volatility. Not so with the toluidines, with a bp difference of 1/2 a degree between o and p.

Nicodem - 13-10-2007 at 00:55

The compounds that can efficiently H-bond with water deviate a lot from the steam distillation laws. For example, p-methoxyphenol performs badly in steam distillation since its phenolic OH group participates in H-bonding with water without any hindrance (this increases its solubility in water which works against the steam distillation phenomena – calculations based on the Clausius-Clapeyron equation can only be made on ideal biphasic system where the components are not intersoluble and do not interact on a molecular level). On the other hand o-methoxyphenol whose OH is hindered from one side and can furthermore form a intramolecular H-bond is less prone to H-bond with water. As a consequence it is more prone to steam distill in a manner closer to ideality. Similar effects are noted in other such ortho/para substituted phenol&aniline pairs. These differences can be efficiently used for regioisomers separation. Obviously, this can not be extended to regioisomers which have no interaction with water molecules – for example, o- and p-xylene both steam distill equaly efficiently and thus xylenes can not be separated this way.

S.C. Wack - 13-10-2007 at 01:26

My request for an *actual example* of the use of steam for this instance, or perhaps even in other cases since you bring it up, where there is similar volatility otherwise (such as the 16C between the nitrotoluenes), stands. From where I'm sitting I see no evidence. Inter- and intramolecular H-bonding having an effect on boiling points is obviously not a new exotic theory. But your talking of steam not only having additional effect due to being hot water, but essentially making p-nitrotoluene so much less volatile that simple steam distillation is presented as a preparative method, without any need for citing of non-vague references or description, troubles me. You'll need more than a dismissive wave of the hand.

EDIT: or the toluidines. I forgot for a minute where in the reaction scheme the steam distillation happened.

[Edited on 13-10-2007 by S.C. Wack]

Nicodem - 13-10-2007 at 02:31

This actual example (o- vs. p-toluidine) does not fit the basic requirements I described above: it does not form an itramolecular H-bond interaction (the Me group is not an H-bond acceptor). The only factor increasing the ratio of o-toluidine in the steam is its lower solubility in water (intermolecular H-bonding with H2O) which I doubt differ enough for anything close to a clear cut separation.

Higher temperature favors intramolecular vs. intermolecular H-bonding and does thus not inhibit the described effect.

[Edited on 13/10/2007 by Nicodem]

Sauron - 13-10-2007 at 03:49

So separate the nitrotoluene isomers and reduce the o-toluene.

And use Sn not Zn.

Nicodem - 13-10-2007 at 04:00

Reading this thread, I can not help asking myself why people use messy metal dissolving reductions when sodium dithionite can be bought OTC so easily. I can buy sodium dithionite mixed with Na2CO3 as used for clothes discoloring. This mixture is just perfect for reducing nitroaromates since you need to add Na2CO3 anyway. Are there countries where Zn or Sn powder/granules is easier to buy than Na2S2O4?

Sauron - 13-10-2007 at 04:43

Good point. It was the thread author who seems fixated on dissolving metal reductions. You are quite correct that there is an embarassment of options for other reductions and some of them are indeed OTC.

garage chemist - 13-10-2007 at 11:32

Yes, sodium dithionite is definately a very good choice for lab scale reductions of nitroaromatics. There is a german synthesis of luminol on the website of some university that nitrates phthylic anhydride, condenses the nitrophthalic acid with hydrazine, and reduces the 3-nitrophthalhydrazide to luminol with sodium dithionite/NaOH.

PHILOU Zrealone - 28-12-2007 at 04:01

Yes I was confused.
I know the pseudocyclisation effect induced by hydroxy (phenolic) group on vicinal proton acceptor groups(like the case of nitrophenol) ...this effect is reduced when we look at nitrotoluens but the hydrogens form the methyl group of nitrotoluen are acidic too..as proven by the fact toluen reacts with NaOH to form an acinitro red form...aci TNT form is a red compound that is more shock sensitive compound than TNT itself. Hydrogen from the methyl moeity are thus labile and jump to the vicinal oxygen atom. The methyl group becomes a methylene group and you get a pseudo penta ring.
This effect should not be observable with toluidines, because it would induce a tetra ring and NH2 doesnt allow the same kind of transformations as nitro group.

CH3-Ar-NO2 --> CH2=Ar=N(O)-OH
CH3-Ar-NH2 -/-> CH2=Ar=N-H

Anyway playing with trimmed pH may be a solution in a way to get one of the two toluidine to be water soluble and the other one not...that way steamdistillation would be a separative option
As a side note separation with different melting point is common for o- and p- nitrotoluenes this should be even more true for o- and p-toluidines...in a slightly alkaline media, para toluidine will precipitate from the mix.

o-toluidine:
mp: -23°C
bp: 199-200°C

p-toluidine:
mp: 43°C
bp: 200°C


o-nitrotoluene:
mp: -9.3°C
bp: 222°C

p-nitrotoluene:
mp: 51.7°C
bp: 238.3°C

o-nitrophenol:
mp: 46°C
bp:215°C

p-nitrophenol:
mp: 114°C
bp:279°C

Drunkguy - 28-12-2007 at 06:21

Zinc is a more mild reducing agent than iron, which in turn is milder than tin.

If you had used tin, the chances are that your reduction would have been complete.

Steam distillation is nowhere near as elaborate as its name suggests. Really, u just add water and distill, then the product is salted out into a small amount of organic solvent.

On the topic of the H-bond principle with the ortho and para products. I have not heard of this before. My friend pointed me to this:

http://members.aol.com/organichem/my_pages/distill.htm

This is an interesting thread but it does have to be said the choice of reduction conditions was a mistake.

[Edited on 28-12-2007 by Drunkguy]

solo - 28-12-2007 at 07:06

Quote:
Originally posted by Nicodem
Reading this thread, I can not help asking myself why people use messy metal dissolving reductions when sodium dithionite can be bought OTC so easily. I can buy sodium dithionite mixed with Na2CO3 as used for clothes discoloring. This mixture is just perfect for reducing nitroaromates since you need to add Na2CO3 anyway. Are there countries where Zn or Sn powder/granules is easier to buy than Na2S2O4?


................a very interesting gem of information!......solo

Reference Information



Reduction of Aldehydes and Ketones by Sodium Dithionite
Johannes G. de Vries and Richard M. Kellogg
J. Org. Chem. 1980,45,4126-4129


Abstract
Conditions have been developed for the effective reduction of aldehydes and ketones by sodium dithionite, Nafi204. Complete reduction of simple aldehydes and ketones can be achieved with excess NafizO4 in H20/dioxane
mixtures at reflux temperature. Some aliphatic ketones, for example, pentanone and 4-heptanone, are reduced only sluggishly under these conditions. Good conversions can be achieved, however, by adding dimethylformamide to the reaction mixture, again held at reflux. The reductions of 17 compounds are described and the scope of the reaction is discussed. a-Hydroxy sulfinates are suggested as probable intermediates in these reductions.

Attachment: Reduction of Aldehydes and Ketones by Sodium Dithionite.pdf (560kB)
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Reference Information

solo - 28-12-2007 at 07:21

Note:Sodium dithionite (aka sodium hydrosulfite or sodium hydrosulphite


The reduction of aromatic nitro groups on solid supports using sodium hydrosulfite (Na2S2O4)
Randall A. Scheuerman and David Tumelty
Tetrahedron Letters Volume 41, Issue 34, 19 August 2000, Pages 6531-6535



Abstract
An improved method for reducing aromatic nitro compounds on solid-phase supports using sodium hydrosulfite is presented. Conditions have been optimized to enable the use of this reagent for reductions on both polyethyleneglycol-polystyrene (PEG) resins and traditional polystyrene (PS) resins.

---------------------------------------------------------------------------

see also,

Reduction of Imines and Cleavage of Oximes by Sodium Dithionite
Peter M. Pojer
Aust. J. Chem. 32, 201-4 (1979)

http://designer-drug.com/pte/12.162.180.114/dcd/chemistry/re...

[Edited on 28-12-2007 by solo]

[Edited on 28-12-2007 by solo]

Attachment: The reduction of aromatic nitro groups on solid supports using sodium hydrosulfite (Na2S2O4).pdf (341kB)
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