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Author: Subject: Yellow impurity in nitrobenzene?
spong
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[*] posted on 13-7-2010 at 04:03
Yellow impurity in nitrobenzene?


Hi guys, I've just made a little batch of nitrobenzene I'm planning on using to oxidize isoeugenol to vanillin and it's a bright lemon yellow colour. I was expecting perhaps a yellow tinge but not this, it's also milky looking, it looks like banana flavoured milk pretty much.
What is causing the colour? It also coloured the water I was using to wash it bright yellow too.
I made the benzene from destructive distillation of sodium benzoate/hydroxide but it was redistilled twice (although I never got a perfect boiling point, perhaps because of its azeotrope with water)
I'm worried because if this is another impurity that could have been nitrated it might not be the safest thing when I distill the nitrobenzene :/
Also, the colour only really showed when I dumped the acid/NB mix into water at the end, before that it was just a pale but clear yellow/orangeish colour.
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Picric-A
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[*] posted on 13-7-2010 at 09:13


The yellow colour is almost certainly dinitrobenzene, a yellow solid. I suggest distillation under strong vacuum to remove the impurity however be carefull! Slight explosion risk!
Otherwise i doubt the impurity will have a big effect on your oxidation so you could think about ignoring it?
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zed
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[*] posted on 13-7-2010 at 11:24


I'm clearly not an expert. I've only seen Nitrobenzene a few times.

But, I've never seen Nitrobenzene which wasn't yellow.
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Picric-A
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[*] posted on 13-7-2010 at 13:16


Nitrobenzene itself is pale yellow however darker yellow samples are indicative of the dinitro impurity
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JohnWW
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[*] posted on 13-7-2010 at 15:50


I once made some nitrobenzene by nitration of benzene; it was a pale yellow to yellow-green liquid.
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DJF90
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[*] posted on 13-7-2010 at 16:04


It sounds to me from the "milky" discription that your nitrobenzene is wet. It should be a clear (but not necessarily colourless) liquid IIRC.

[Edited on 14-7-2010 by DJF90]
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spong
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[*] posted on 13-7-2010 at 16:58


Ahh yes dinitrobenzene would make sense as I accidentally let the temperature get to about 80*C when it was first heated so that could be the cause.
Hopefully it is cloudy from the water in it although I thought it was a bit too cloudy, my benzene is cloudy after it is distilled and clears up after a few days but it wasn't nearly as cloudy as this.
Here's a photo of it in the separatory funnel with the yellow water, could the water be yellow from dinitrobenzenes slight water solubilty?

Here it is by itself:


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entropy51
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[*] posted on 13-7-2010 at 17:08


Quote: Originally posted by spong  
Hopefully it is cloudy from the water in it although I thought it was a bit too cloudy, my benzene is cloudy after it is distilled and clears up after a few days but it wasn't nearly as cloudy as this.
Are you by chance drying any of this stuff you're making? It looks like a wet mess. I've prepared Bz from benzoates and it's never appeared cloudly.

Did you fractionate the alleged Bz by boiling point? Something is wrong here!

IIRC the nitration of Bz is supposed to be done at less than 50 C.
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spong
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[*] posted on 13-7-2010 at 17:48


Well the second time I distilled the benzene I was drying it but it still remained cloudy, I used CaCl2. I didn't fractionate it though (the column was upstairs and I was too lazy to go and get it) but it seems like it will never come over at a sharp 80*C, it just starts coming over at a bit above 70 and finishes by about 78.
I read 60*C for the nitration but yeah, I let it get far too hot but before it had to be cooled off in an ice bath for a bit.
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kmno4
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[*] posted on 14-7-2010 at 10:54


The first fime I nitrated benzene I kept temperature max 60 C, adding benzene to acids mixture. Yield ~50% of nitrobenzene (distilled) + large amount of dinitrobenzene.
Another time I kept temp. as low as possible (pieces of ice added from time to time) with adding acids to benzene.
Yield of nitrob. ~80% (distilled) with small amout od dinitrob.
My distilled (normal pressure, no column) nitrobenzene is almost colourless. I am sure that under low vacuum and with simple colum it is possible to obtain colourless nitrobenzene.

[Edited on 14-7-2010 by kmno4]
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Picric-A
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[*] posted on 14-7-2010 at 11:34


@Kmno4- colourless nitrobenzene does not exist... the actual compound itself is yellow in colour.
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spong
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[*] posted on 14-7-2010 at 17:14


Gosh I'd probably have loads of dinitrobenzene if you got that much. I'll try distilling it under the best vacuum I can pull which is only from an aspirator but if you were fine after distilling at atmospheric pressure then all should be good.
I'm going to give up on the nitrobenzene oxidization and switch to p-nitrobenzoic acid as I've just found a supplier of toluene. Apparently it works much better because it's water soluble (in base) and the potassium salt of isoeugenol is as well.
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andre178
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[*] posted on 17-2-2011 at 10:44


thought there was some debate about Nitro-Bz, I made some fresh this morning and just took a shot with the iphone.

IMG_0057.jpg - 124kB
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