Rando1234
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The Preparation of Benzotriazole
I just finished a synthesis of benzotriazole, which doesn't appear to have any preparations here. I figured I'd post mine and get some feedback.
Intro
Benzotriazole is a quite useful compound. It's used as a photographic developer, a corrosion inhibitor, a precursor to explosives, and much more.
Boffis used it to make triazoledicarboxylic acid, which was cool. It's usually bought as a photographic chemical, but since I had some time to spare
and a jar of o-phenylenediamine laying around, I figured I'd give it a go. It works decently well.
Safety
-Acetic acid glacial is corrosive and can cause chemical burns if used improperly.
-Sodium nitrite is an oxidizer and a poison. Ensure that it doesn't touch your skin or eyes. Do not pour it down the drain, as it is harmful to
waterways. It's also incompatible with acids.
-o-Phenylenediamine is a suspected carcinogen, poison, and irritant. Avoid contact, as it may cause an allergic skin reaction. Avoid any exposure to
it. Do not pour it down the drain, as it is harmful to waterways.
I'm making phenylenediamine sound like the devil, because it kind of is. And being too safe is better than being not safe enough.
Chemical Needs
-10.8g (0.1mol) o-phenylenediamine from HiMedia
-11.5ml (0.2mol) glacial acetic acid from Photographer's Formulary on eBay
-7.5g (0.108mol) sodium nitrite from HiMedia
-0.25g activated charcoal from Starwest Botanicals on Amazon
-Distilled water en masse
Methods
The acetic acid is mixed with 30ml water, and with gentle stirring the o-phenylenediamine is added. I used my hotplate to warm it up and stirred until
most of it dissolved, forming a brown solution.
Separately, the sodium nitrite is dissolved in 15ml water. This forms a slightly yellow solution, and dissolution is endothermic.
The phenylenediamine solution is chilled in an icebath to 10C, then the nitrite is poured in in one portion. The temperature rapidly rises to 65C then
cools to 30C. The color goes from brown to reddish-brown to a turbid tan-yellow.
The solution is allowed to cool until a red oil (which is crude benzotriazole) separates. The solution is then stirred until the oil solidifies into a
brown solid and the solution thickens with precipitated benzotriazole.
The solids are gravity filtered off and washed with 4x30ml cold water. The solids are then pressed dry, giving a tan-brown product.
Recrystallization is immediately effected by dissolution in ≈120mL boiling water. This gives a brownish-red solution. The activated charcoal is then
immediately added, and stirred for about 1 minute.
The mixture is then filtered, giving a yellow solution. At the beginning, some precipitate forms (benzotriazole crashing out) but quickly redissolves.
The filtrate is allowed to slowly crystallize. This didn't exactly go too well, as my lab was 18 degrees C at the time. The cold bench caused some of
the triazole to oil out as a yellow oil, which crystallized after 1 hour into a tan solid with white crystals of triazole above.
It was then placed in a lab-use-only freezer and allowed to chill for 20 minutes.
Gravity filtration was then used to collect the product, which was washed with 2x20ml cold water.
After air-drying overnight and breaking up any lumps with a mortar and pestle, a yield of 4.46g of a slightly yellow fluffy powder is obtained. This
corresponds to a percent yield of 37.47%. This isn't great, but I'll explain my reasons for that in the next section.
The melting point was determined at a range of 84-86 ±2C. Despite being inaccurate, it had a tight range, which leads me to believe there was some
water left over in the product.
Conclusion
The yield was poor. However, my phenylenediamine was evidently at least had been in storage for a few years before I bought it. The flakes were very
brown. I didn't want to recrystallize it.
The fast-and-loose recrystallization and washings undoubtedly lost me some yield.
This reaction is used as a teaching demo in other countries (India) as is clearly evident by the many YouTube videos of Indian people doing it.
I can see why: it's easy, and is a good example of a diazonation.
The procedure was taken from https://labmonk.com/synthesis-of-benzotriazole.
Despite the incorrect mechanism, it worked pretty well for me.
Orgsyn also has a procedure: doi.org/10.15227/orgsyn.020.0016. However it requires vacuum distillation at a pressure my current water aspirator-based
system cannot attain.
I don't have any uses for it, but I may try using it as a corrosion inhibitor for lack of a better idea.
Here's the video I made about it: https://youtu.be/O6QBLRx5GVA
-Jacob
P.S. I'm working on some other preps, which I'll post in a few days.
[Edited on 16-1-2022 by JacobM]
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Fery
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Very nice.
You are worried about your yield. In orgsyn they suggest to reach 80 C, they used cold bath at 1 mol scale, but suggest not to use cold bath at
smaller scale to reach 80 C after NaNO2 addition:
| Quote: | | 1. Too efficient cooling and stirring are to be avoided. It is essential that the temperature rise to 80°. Too rapid cooling after this temperature
has been reached results in lower yields. With runs one-tenth this size it is advisable to remove the mixture from the cooling bath as soon as the
sodium nitrite has been added in order to ensure the rise in temperature. |
They also suggest vacuum distillation, but m.p. is 96-97 C and if the product starts to crystallize in condenser and clogs it (vacuum take off adapter
is the narrowest part) then vacuum distillation is stopped. Maybe at small scale vacuum sublimation could be performed instead?
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Rando1234
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I didn't read the orgsyn method thoroughly enough apparently. Thanks, Fery.
[Edited on 17-1-2022 by JacobM]
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Fery
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Of course lower temperature prevents diazonium intermediate decomposition. Here they performed reaction at 5-10 C with ultrasonic cleaner and got
yield 91% of benzotriazole:
https://sci-hub.ee/10.2174/157017807780037504
Attachment: 157017807780037504.pdf (31kB)
This file has been downloaded 169 times
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Fery
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JacobM another hint concerning you melting point and possible purity / effectivity of crystallization. In the link you posted the researchers were
certainly fighting against oiling out as was also in your case.
It was necessary to keep a little of impure crystals as a seed and introduce them into crystallization process.
| Quote: | | Dissolve the solid in about 130 ml of boiling water, add decolourising charcoal, filter and allow the filtrate to cool to about 50°C before adding a
few crystals of the synthesized product (benzotriazole) which have been retained for seeding. Allow the mixture to attain room temperature slowly (to
avoid separation of the material as an oil) and then thoroughly chill in ice |
I was fighting against oiling out during 4-nitrophenol crystallization and the only way to go was to scrap crystals and dissolve the bottom solidified
oil and repeat the crystallization.
https://www.sciencemadness.org/whisper/viewthread.php?tid=15...
https://www.sciencemadness.org/whisper/viewthread.php?tid=15...
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Rando1234
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I didn't bother a second recryst because it was getting late and I tried seeding it, but that didn't work. I don't care too much about the purity
because it's not being used for anything.
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Fery
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In your case I would just scrape crystals, filter them and wash with a little of fresh cold solvent (water in your case). Then I would heat the bottom
solidified oil with mother liquor til dissolution. Then slowly cool and introduce a little of crystals. That way the solution would be less
concentrated, seeds added so less chance of oiling out. Just an idea, which would cost extra work and time, but may be helpful in better purity.
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