Sciencemadness Discussion Board

Toluene Nitration Results (Help)

RxnJackson - 22-1-2008 at 06:36

Yesterday I ran a electrophilic aromatic substitution rxn on Toluene with Nitrogen. Using a mixture of Conc Sulfuric acid and Nitric Acid to create the nitronium Ion.

After the creation of the nitronium ion (addition of sulfiric and nitric acids), the vessel was brought to 20C in an ice water bath and the toluene was added slowly in 3-5ml portions with vigorous stirring. After each addition the reaction turned brown and then back to pale yellow with an exothermic component (Approx 5C for each addition). I never let the mixture get above 30C.

After the addition of about 90% of the toluene, I was adding an additional 5ml portion when all of a sudden the reaction took a totally unexpected turn. A very bright yellow precipitate formed (it looked like lard) and seperated from a very light yellow lower layer that looked to be toluene. The results smelled like a slightly sweeter version of toluene.

I allowed the vessel to warm to 30C to see if maybe there was a phase that could be forced back to miscibility, but with no luck. After an additional hour of stirring and no change I gave up.

During disposal of the materials I noticed that upon addition of H2O the yellow precipitate seemed to go into solution. Is there any chance that all I needed to do was hydrate the reaction a little. This is somewhat confusing because I understood the creation of the nitronium ion to have a by product of water, maybe I needed more.

Any thoughts would be helpful. I would say contamination is a possibility but this is a lab environment and everything was thourougly cleaned, in addition, materials are HPLC grade and the reaction seemed to be going perfectly until the end.

Thanks in advance for pointing out some obvious failure in my lab technique and/or mechanism :P

-RxnJackson

[Edited on 22-1-2008 by RxnJackson]

YT2095 - 22-1-2008 at 09:03

most all of the methylbenzene nitrates are a waxy consistency, you probably had Mono and some Di nitrates there.
I never remember ever seeing a Brown stage though?
but they are slightly soluble in water.

RxnJackson - 22-1-2008 at 11:12

I call it "brown", but it is more of a redening of the yellow, it turns back to yellow in a matter of a few seconds. This is typical in nitrating Methyl-Benzene to my knowledge.

di-nitrating should require substantial heat and fuming nitric acid as the nitrogen substantially deactivates the electrophilic substitution tendancy of the ring. Not allowing the reaction to go above 30C should have maintained o,m, and p nitrotoluene.

What's most interesting to me is that as the waxy precip just appreared quite suddenly. The stirring was vigorous and a nice vortex was maintained, but when this precip formed the vortex stopped as the top layer was very thick.

I'm going to run this again tonight and while I do I will take some pics (if not just to show the color changes and setup as I think it's a cool reaction home chemists might have some fun running). With that said, it can become dangerous but not if your careful about the addition of the aromatic hydrocarbon.


RxnJackson.

PHILOU Zrealone - 24-1-2008 at 05:50

Overfusion and mass cristallisation of orthonitrotoluene once the critical germs concentration was reached.

I ran a test also and with only HNO3 69%, thus without any H2SO4.No heat, simply agitation. I also got a red/orange oil followed by a yellow cristaline mass. This puzzled me because everywhere I read that H2SO4 or conc HNO3 is a must...This is thus a false truth. Orange color is clearly observable after two days. Mix is also self heating by exotherm!

The solid is most likely ortho compound. The remaining orange oil para and traces of meta + tiny amounts of benzoic en nitrobenzoic acids dissolved in the exces toluene.

Your dissolution observation seems stange...maybe is it only the color of some NOx dissolved in the acid. Addition of water should increase the demixion.

not_important - 24-1-2008 at 06:17

Melting point of ortho mononitotoluene is -9,3 C, meta is +16, para is +51,7. Can't see how the waxy solid would be the ortho isomer, unless they're doing this in my dwelling.

PHILOU Zrealone - 24-1-2008 at 08:33

Yep I interverted ortho and para!
There is a time in the lifetime, when you really need some white chocolate (soja lecithin) and fish oil phospholipids in a way to get a sharper brain ;).

so:
"The solid is most likely para compound. The remaining orange oil is ortho and traces of meta + tiny amounts of benzoic en nitrobenzoic acids dissolved in the exces toluene."

Nicodem - 24-1-2008 at 10:04

Quote:
Originally posted by RxnJackson
di-nitrating should require substantial heat and fuming nitric acid as the nitrogen substantially deactivates the electrophilic substitution tendancy of the ring. Not allowing the reaction to go above 30C should have maintained o,m, and p nitrotoluene.

Given that you used the wrong addition sequence when mononitration is the goal (you added the substrate to the nitrating mixture instead of the opposite), then I would expect a high degree of the dinitrotoluenes having been formed. Especially since you heated to 30°C instead of cooling bellow 20°C.
Quote:
Originally posted by YT2095
most all of the methylbenzene nitrates are a waxy consistency, you probably had Mono and some Di nitrates there.

Sorry YT2095, but I have to correct you in that no toluene nitrates form during the nitration of toluene. Only nitrotoluenes and some very minor side products resulting from the action of nitric acid on the methyl group form. Ring substituted nitrotoluenes are nitro compounds (nitroaromatics, to be exact) and not esters of nitric acid (alkyl nitrates). I know a lot of beginners confuse these even though they have totally different chemical properties, so I just had to add a correction.

RxnJackson - 24-1-2008 at 10:10

Thanks all,
I ran the reaction again last night and was successful!! This time I added approx 30ml of distilled H20 after the H2SO4 and the Nitric acids were mixed. Interestingly enough I found that upon that addition there was a substantial exotherm. I'm farmiliar with why there would be an exotherm upon the additions of the two acids as the nitronium ions are created, but any thoughts as to why it was observed during the addition of the H2O?

I'm not convinced the addition of the H2O had any effect, it was more likely the fact that my addition rate was much slower this time around as I was a little concerned about getting the same results.

I took some pictures to share with the board and assuming I can figure out how to post them I will do so in this message (any hints, Ill UTFSE as well).

All stages after the initial reaction (mostly separations wen't well). One question though, after the completion of the toluene addition, I allowed the reactants to stir and sit for about 3 hours, after discontinuing stirring there was an obvious separation between the acid layer and the toluene layer. The toluene appeared a transparent yellow. I separated off the acid layer and then placed some NaOH in with some water to wash out the remaining acids (Which turned a very neat red you can see in the pics). After vigorously mixing this and applying about three more washes with distilled H2O my collected Toluene was opaque and yellow. Is this likely due to emulsified H20 in with the oil which can be dealt with by some light heating and another separation?

I tried to post the pictures but had a hard time, I have attached a PDF with a number of pictures taken in the lab of the reaction and the results if anyone is interested.

[Edited on 24-1-2008 by RxnJackson]

Attachment: Nitration of Toluene.pdf (175kB)
This file has been downloaded 742 times

RxnJackson - 24-1-2008 at 10:33

Nico
In most of the papers I have seen on this reaction, this was the addition order, if you have any workups or refs showing otherwise could you share, I'd be interested to see how it's done. Though I cannot understand how it would make a difference it wouldn't be the first time I was way off :)

garage chemist - 24-1-2008 at 16:35

Nitrating toluene to the mono stage requires adding the mixed acids slowly to the toluene with cooling, not the other way around. Look in Gattermann (available in the Sciencemadness library).
Also, the HNO3 used must not be stronger than 65%.

RxnJackson - 24-1-2008 at 17:55

GC.......
Would this be the same for mono-nitration of Benzene? The workups I have seen on that also refer to addition of the Hyrdocarbon to the nitrating agent (though the web is FULL of disinformation, so that's not surprising).

Until I get a password to the reference library, could you answer this.......

If the acids are to be added to the toluene, can I use the same workup I used previously. Mix acids with cooling slowly, setup ice bath for toluene, vigorous stirring, 2-3ml additions slowly while maintaining 20-25C temps. Wash with H2O and NaOH after preseparation, then H2O washes?

Any thoughts on what I made? It looks to be nitro toluene, and smells like NT or did I just a mix of mono-nitrated toluene and di-nitrated?

garage chemist - 24-1-2008 at 18:01

Benzene is less easily dinitrated (dinitration requires fuming HNO3 and higher temperature) than toluene, therefore one can add the benzene to the mixed acids. Not so with toluene.

The order of addition has no influence on the workup.

not_important - 24-1-2008 at 18:32

Gattermann L. The practical methods of organic chemistry

http://eknigu.org/info/Ch_Chemistry/Gattermann%20L.%20The%20practical%20methods%20of%20organic%20chemistry%20(Macmillan,%201909)(600dpi)(T)(372s).djvu

http://www.archive.org/details/methodspractical00gattrich

http://books.google.com/books?hl=en&id=F1k3AAAAMAAJ&...

Oh, and wash with a little water first, then the alkaline wash followed by water again, you'll need less alkali that way.



[Edited on 25-1-2008 by not_important]

RxnJackson - 25-1-2008 at 08:00

GC....

Thanks makes total sense as I got my workup from Vogels and they used Benzene....


Thank you NI, excellent reference info.

Anyone understand why you would need to use 65% Nitric as opposed to conc? Or why it should matter? Wouldnt there just be an ion reaction until one acid or the other is converted? What kind of impact would the conc of the nitric have aside from rate of reaction and final ionic yield?

[Edited on 25-1-2008 by RxnJackson]