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Author: Subject: Sodium Triacetoxyborhydrid from NaBH4 in Benzene **HELP**
fckcps20290
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biggrin.gif posted on 15-12-2020 at 08:50
Sodium Triacetoxyborhydrid from NaBH4 in Benzene **HELP**


Hello I have an urgent question I have started to convert my NaBH4 into benzene to STAB.... Unfortunately the slurry has become too thick and I don't have any benzene left to dilute it further so that it can be stirred better by the magnetic stirrer.

I have benzine 40-60*c, DCM, methanol, et20, ipa at my disposal. can one of you tell me what I could use to liquefy it again or do I have no chance?

Would be really great if you could help me.

Best regards

Fckcps202


[Edited on 15-12-2020 by fckcps20290]
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itsallgoodjames
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[*] posted on 15-12-2020 at 08:54


What do you mean by STAB?



Nuclear physics is neat. It's a shame it's so regulated...

Now that I think about it, that's probably a good thing. Still annoying though.
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Antigua
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[*] posted on 15-12-2020 at 09:03


STAB is sodium triacetoxyborohydride. Why were you doing that in benzene though? STAB is usually prepared in situ in your reaction solvent already. You must've used a really large amount of NaBH4 to turn it into a slurry.
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fckcps20290
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[*] posted on 15-12-2020 at 09:15


Because I have a manual for it tells you so.

Do I have then somehow a possibility the process with what I have?

Do not know what to do now ...
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solo
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[*] posted on 15-12-2020 at 09:32


it must be done in an anhydrous environment using acetic acid and NaBH4, .....solo




Reference Information.....enclosed

[Edited on 15-12-2020 by solo]



[Edited on 15-12-2020 by solo]

Attachment: Sodium Triacetoxyborohydride1.pdf (184kB)
This file has been downloaded 45 times





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njl
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[*] posted on 15-12-2020 at 09:41


You need to share more about your preparation and intentions. If you were hoping to use benzene as a reaction solvent, you'll need to add more to thin the slurry. If you can't add more you need to remove some STAB, maybe by extracting with isopropanol or AcOH. If benzene is only a solvent for the preparation of STAB, distill it off and dissolve the residue in a suitable solvent. What is "benzine 40-60*c"? How you should proceed really depends on what you are trying to achieve.
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[*] posted on 15-12-2020 at 09:43


Quote: Originally posted by njl  
You need to share more about your preparation and intentions. If you were hoping to use benzene as a reaction solvent, you'll need to add more to thin the slurry. If you can't add more you need to remove some STAB, maybe by extracting with isopropanol or AcOH. If benzene is only a solvent for the preparation of STAB, distill it off and dissolve the residue in a suitable solvent. What is "benzine 40-60*c"? How you should proceed really depends on what you are trying to achieve.

Extracting with AcOH? That would just ruin the triacetoxyborohydride, it decomposes readily in acidic sol-s. But I agree on the point itself - what the OP wrote doesn't tell us much.
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njl
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[*] posted on 15-12-2020 at 10:04


That suggestion was based on the assumption that AcOH is immiscible with benzene, which is incorrect. Either way, I don't think STAB would decompose in AcOH like it would in other protic solvents.
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[*] posted on 15-12-2020 at 10:21


Actually you're right. Apparently some reactions with STAB are carried out in 1-2 mol eq. GAA, my bad.
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Tsjerk
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[*] posted on 15-12-2020 at 10:44


What a waste of benzene... But if you just want to liquify the thing you can use DCM. You can make STAB in DCM to start with.

Alcohols will react with STAB.
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njl
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[*] posted on 15-12-2020 at 10:59


Alcohols will react with STAB, but apparently isopropanol reacts slow enough to be a viable reaction solvent in some cases. That's why I suggested it as an extraction solvent. To be honest though, tsjerk is right. This seems like a waste of benzene. I think that's the most anyone could say at the moment without any more information.
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valeg96
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[*] posted on 15-12-2020 at 11:37


Quote: Originally posted by njl  
What is "benzine 40-60*c"? How you should proceed really depends on what you are trying to achieve.


Benzine or ligroin 40-60 is light petroleum ether. Basically every alkane that distills over between 40-60°C after you've recovered LPG, butane, ispobutane, pentane and n-hexane from hydrotreated naphtha. In other words, every C5, C6 and C7 isomer you can think of, except n-hexane and n-pentane.

Also, given the staggering cost of something as common and simple as benzene, I wouldn't use it for any bulk preparation... I would have checked the literature if toluene or xylene could be decent substitutes.





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Tsjerk
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[*] posted on 15-12-2020 at 11:45


If you want to isolate STAB, DCM works well. It becomes a tick paste quickly when GAA is added, but with a bit of vacuum the DCM comes off easily and you are left with a brittle powder.
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fckcps20290
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[*] posted on 17-12-2020 at 12:05


Hello all,
sorry for not getting back to you and for my undetailed description of my experiment.

My intention was to make from a paper(Science of Synthesis: Houben-Weyl Methods of Molecular Transformations Vol6) a guide to the preparation of sodium triacetoxyborohydride as a solid with NaBH4/GAA/benzene.

Out of curiosity, I ragged from the paper the data of NaBH4 0.168g GAA:0.860 x 12 in benzene (100ml). Unfortunately, after 5h the slurry became so thick that there was no more talk of stirring. Yield should be 11.532g according to the paper with ser 12 fold quantity.
I have just doubled the solvent in the belief it would be enough, yes, from mistakes you learn ...

I could unfortunately only now report again because my PC unfortunately only conditionally works...

I know that my action was unfortunately much too rash & unprepared I will change in the future ...

Thank you for the answers have luckily still 100ml benzene can get only I am unfortunately not sure whether the product is still properly
could form :)

Copy of the preperation instructions:
a 100ml schlenk flask was fitted with a schlenk filter and filled with nabh4(186mg) and anhyd benzene(50ml).
the slurry was cooled to 10 c and anhyd acoh(0.860ml) was added dropwise,
so that an internal temperature of not more than 20 degrees celsius was maintained.
h2 evolution was measured using a gas burette.
after the addition of acoh, the mixture was heated to rt and stirred at this temperature for 8h.
H2 evolution had ceased at 330 ml (theoretical for 3.00 aquiv. is 331 ml) after 5 h.
The colorless slurry was filtered and the resulting white powder was washed with anhyd et2o (3x20 ml).
The combined et20 filtrate did not release H2 when treated with 1 M aqueous HCl.
The powder was kept under vacuum overnight to give pure NaBH(Oac)3 as a white hygroscopic solid 92%.

PS: Are the quantities as good? I would like to prepare 4.8g NaBH4(Oac³) in sito with DCM. It would be great if someone of you is willing to explain me the preparation in sito.

best regards
Fckcps20290
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Tsjerk
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[*] posted on 17-12-2020 at 12:59


50ml benzene for 0.186 gram NaBH4? That seems a bit excessive. Maybe benzene gels up quickly compared to DCM but still. I don't remember how much DCM I used but I guess about 25 ml per gram NaBH4.

Just powder your NaBH4, suspend it in DCM and slowly add the GAA, if it is about to foam over you went too quick. Also a Slenck line seems a bit overkill, if you want to be sure you can use a drying tube. I think I just stirred over night in a loosely stopped round bottom. If you want to check my procedure you could look for it somewhere here, it was posted in early 2017.

Edit: here

[Edited on 17-12-2020 by Tsjerk]
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[*] posted on 18-12-2020 at 15:00


Benzene has become a hard get in recent years. Tried Toluene? Or, Xylenes?

Benzene got a bad rep as a carcinogen. The others, might be as bad. Don't know yet.

Mostly, you can still find them in commerce. Meaning, at a local hardware store. Toluene is a good bet.
DCM is a hard get now, also.

Of course, I'm in the U.S.. Different regulations in different countries.

[Edited on 18-12-2020 by zed]
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njl
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[*] posted on 19-12-2020 at 07:18


Well for all we know benzene is dirt cheap for OP, point is you need to either extract or dilute the slurry. Or use it as-is, if the reaction solvent is compatible with the slurry.
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