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Author: Subject: Benzene synthesis
wayne_m
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[*] posted on 22-7-2017 at 16:43


Has anyone ever figured out just what the orange gunk from the benzoateNaOH reaction is?
I can confirm that it is not phenol red (I had heard it proposed as an answer.) I added bromine to my latest batch, which would have turned it into bromophenol blue, but it certainly wasn't that.
Whatever it is, I hope there is a good use for it. I'm growing quite a stock of it in my benzene waste container.

Also, in this last batch, I noticed that the distillate from refining the crude product came over at 73, which indicates an azeotrope of benzene and acetonitrile. (Could be something else, but that's the exact number I found for that mixture.)
I distilled my benzene before washing it with water this time.
I'll wash it out with water and try to isolate it, then post my findings.

[edit]
Apparently not. Acetonitrile should be soluble in water, and 100cc benzene mix + 100 cc water shaken vigorously and allowed to separate yielded: 100 cc of benzene mix and 100 cc of water.

Anyone have any ideas ideas what might have made the BP of the benzene mix so low?

[Edited on 23-7-2017 by wayne_m]

[Edited on 23-7-2017 by wayne_m]
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[*] posted on 22-7-2017 at 17:22


Benzene has an azeotrope with water at 69.3 C.

I'm not quite sure what the pyrolysis reaction between benzoic acid and sodium hydroxide is, but it's something like this:

PhCOOH + 2 NaOH -> PhH + Na2CO3 + H2O







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clearly_not_atara
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[*] posted on 22-7-2017 at 20:48


Benzophenone or triphenylmethanol are my first guesses.
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wayne_m
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[*] posted on 23-7-2017 at 04:37


I actually had two temperature rests; one at 68, which stayed there for several minutes, and one at 73.

I didn't bother fractioning at this point, since it was just to get the crude separated into low-boiling and water / high-boiling components. I stopped it at 85 C, and a lot of yellow came over, which seems to indicate that it isn't any of the really high boiling things.
I suppose it could be something like benzophenone, if it evaporates readily in boiling benzene, somewhat like a steam distillation. I don't have any experience with it, so I don't know.

I doubt it's or triphenylmethanol, or tetracene, considering that the leftover tar has a melting point well under 100 C, but given the unknown impurities, there's no way to be sure. It solidifies at room temperature.
Could tricene form the dye Alizarin in the benzoate reaction? It seems unlikely, but you never know until you know.

I'll dry the tar and try dropping some of my potassium in it. (I'll warm it just until it melts, and wait a bit to see if it runs away before applying any more heat! Made that mistake the other day and lost a stopper - I think it's in the neighbor's swimming pool.) If it turns blue or purple, it should confirm benzophenone. If not, then I'll try something else until an answer is found!
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[*] posted on 30-8-2018 at 10:03


I just ran the NAOH decarboxylation of Sodium Benzoate last night. I used a paint can over a Walmart camping stove as a vessel. I sunk a glass 24 40 adapter into the lid with high temp putty and used the usual glassware to collect and condense. I got a strongly orange colored cloudy product. ( I intend to run several times and then purify the commingled results.) I noticed some orange solid collected at the top of the condenser where the joint had loosened for a short time. (the 3 way is stuck to the adapter in the can lid now, and the joint is orange.)

I believe that the orange gunk IS Chrysene/Tetracene, and that it forms an azeotrope with benezene, thus boiling below its advertised melting point.
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[*] posted on 31-8-2018 at 12:38


Ran another batch. Similar results ~60 ml of orange "Tang". Worked the temperature control more carefully to try and keep the white vapor just spilling over into the condenser.

Next step is the purification of the commingled batches. Once I have the product gin clear and dry, I will proceed to commit nitrobenzene.

Some observations:



  1. Permatex High Temperature putty works well to seal the glass adapter to the paint can top
  2. It is necessary to hammer the paint can lid in place, or you will get wisps of white carcinogenic vapor creeping out.
  3. I added a small amount of water to the hot pot to flush the system.
  4. Buying the benzene may well be cheaper, but this is way more fun.
  5. Having run out of ring stands, a standalone toilet paper stand weighted with a brick made a creditable stand-in.
  6. Grease every joint.
  7. Did I mention grease?



[Edited on 31-8-2018 by digga]

[Edited on 31-8-2018 by digga]
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[*] posted on 1-9-2018 at 06:40


Quote: Originally posted by digga  
Once I have the product gin clear and dry, I will proceed to commit nitrobenzene.
wouldn't it decarboxylate better if you nitrated benzoic acid first ?
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[*] posted on 1-9-2018 at 09:00


Benzene can be obtain if you melt solid benzoic acid (dry sample) with solid calcium-oxide also dry and few grains of sodium hydroxide.
The mixture in the vial after some time of heating will get the smell of benzene and it can be destiled.

The second method of obtaining a benzene from benzoic acid is melting benzoic acid and citric acid, but i didnt try it.
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digga
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[*] posted on 5-9-2018 at 10:28


Did a third run. I have been able to reuse the same 1 quart paint can each time. The NAON/Sodium Benzoate method leaves a hard deposit of sodium carbonate which dissolves when the can is heated on the stove with water added.

What I like about this method is that the lid can be re-used over a series of paint cans.

Washed the product with 100 ml of distilled water, then set to dry over a layer of calcium chloride which clumped up. Now it is orange and clear.

As for decarboxylating nitrated benzoic acid, this had not occurred to me. But I also wanted to have some benzene on the bench for the odd task.
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[*] posted on 18-9-2018 at 08:52


I took the three combined runs of raw orange benzene, washed them with distilled water and dried them over calcium chloride, which clumped immediately. I poured off the remaining liquid and distilled off about 4 ounces of clear benzene, which came over around 90o C. I stopped the distillation with about 15 or 20 ml of orange liquid remaining in the round bottomed flask This was transferred to a beaker and allowed to dry at ambient temperature and humidity for several days.

What remained was 4 grams of shiny orange crystals. They had a strong aromatic odor. I am positing that they are primarily tetracene (napthacene) with chrysene impurity.

I am thinking about purifying the tetracene by sublimation or chromatography. A uv lamp is on order. I will check the fluorescence.
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macckone
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[*] posted on 18-9-2018 at 13:12


If your are getting tetracene in the final product before careful redistillation, there is probably naphthalene and anthracene as well other conformers. Short of repeated fractional distillation these would seem to be difficult to avoid.

Obviously on the first run everything is getting condensed.
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S.C. Wack
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[*] posted on 18-9-2018 at 14:48


Biphenyl...go back 14 years...the lowest heat is best IMHO.



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digga
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[*] posted on 19-9-2018 at 07:26


Quote: Originally posted by S.C. Wack  
Biphenyl...go back 14 years...the lowest heat is best IMHO.


Makes sense. As the temperature rises, the activation energy requirements for more reactions are satisfied. I suspect that for each product formed in a heat catalyzed decarboxylation, the equilibrium is way to the right.

For example, once you have napthalene, it will not go back to benzene. Once napthalene goes to anthracene, it doesn't go back to napthalene.

BUT, if the process cannot, for some reason, proceed past a certain compound - like tetracene or its isomer chrysene, that compound will tend to accumulate.


[Edited on 19-9-2018 by digga]

[Edited on 19-9-2018 by digga]
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macckone
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[*] posted on 3-10-2018 at 15:51


I did a little research on the permatex high temp putty.
Looks like it is clay with sodium metasilicate and possibly some aluminium.
They say 'inert material 40-60%'.

This looks like it would be easy to formulate.
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[*] posted on 4-10-2018 at 10:26


Quote: Originally posted by digga  
I took the three combined runs of raw orange benzene, washed them with distilled water and dried them over calcium chloride, which clumped immediately. I poured off the remaining liquid and distilled off about 4 ounces of clear benzene, which came over around 90o C. I stopped the distillation with about 15 or 20 ml of orange liquid remaining in the round bottomed flask This was transferred to a beaker and allowed to dry at ambient temperature and humidity for several days.

What remained was 4 grams of shiny orange crystals. They had a strong aromatic odor. I am positing that they are primarily tetracene (napthacene) with chrysene impurity.

I am thinking about purifying the tetracene by sublimation or chromatography. A uv lamp is on order. I will check the fluorescence.


update: The orange solid product fluoresces bright orange under UV light. The solution in acetone under uv light fluoresces the same as a picture of a tetracene sample I saw. btw, the uv light ALSO shows just how that stuff spreads all over my bench.
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[*] posted on 4-10-2018 at 15:52


Pentacene, higher compunds and analogs/confomers are probably not making it out of the reaction chamber in significant quantity due to the high vapor phase temperature.

The following paper also suggests injecting steam to reduce crosslinking.

Dabestani, R., Britt, P. F., & Buchanan, A. C. (2005). Pyrolysis of Aromatic Carboxylic Acid Salts:  Does Decarboxylation Play a Role in Cross-Linking Reactions? Energy & Fuels, 19(2), 365–373. doi:10.1021/ef0400722

That study provides some good insights. It is done in a sealed environment at 450C for sodium benzoate. Water reduced crosslinking and at 450C. The reaction took 4hrs to decarboxylate 30% of the sodium benzoate.

In a non-sealed environment with added sodium hydroxide as has been used, I would expect cross linking to still take place and higher yields are being reported with slightly lower temperatures since the added hydroxide should have a catalytic effect.

If the chamber top is below 372C then no pentacene should be escaping. This gives a narrow range of 357C - 372C where we expect benzene production since we are getting tetracene (BP 357C) but not pentacene (wiki listed sublimation point 372C) however the lower portion of the chamber could be much hotter.

If steam is introduced in this temperature range, there should be a higher yield. But I don't have the equipment to feed in temperature regulated steam.


[Edited on 5-10-2018 by macckone]
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