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Author: Subject: Liquid Lewis acids
chornedsnorkack
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[*] posted on 19-9-2018 at 09:22
Liquid Lewis acids


Aluminum halides are popular Lewis acid catalysts.
However, they have the inconvenience of being solid.

How convenient and widely used Lewis acid is boron bromide?
Boils at +91, freezes at -46.
In contrast to boron chloride (boils at +13), boron iodide (melts at +50), aluminum chloride (melts at +193) and aluminum bromide (melts at +98).

Also, what are good solvents for boron bromide?

Bromine itself is apolar due to symmetry. But boron bromide shares the lack of polarity due to symmetry (B-Br bonds are polar, but flat equilateral triangle cancels the dipoles). Are liquid bromine and boron bromide miscible?
Does the mixture undergo any special reactions?
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Heptylene
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[*] posted on 19-9-2018 at 11:09


Boron tribromide reacts with atmospheric moisture to product HBr and boric acid. So handling it is tricky without inert atmosphere technique. Making it in the lab is probably not easy and its not easy to find. I've seen it on onyxmet however.

So unless you have a commercial source I can't see how it could be more handy than aluminum trichloride, unless the procedure calls for BBr3 specifically.
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JJay
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[*] posted on 19-9-2018 at 11:38


I want to know if boron tribromide and sodium hydride react to make sodium borohydride.



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weilawei
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[*] posted on 19-9-2018 at 11:38


Boron tribromide looks like a nightmare to make. Boron carbide and liquid bromine at 300°C? Whoa. I don't even really mind working with bromine, but I keep it frozen or as cold as possible. The idea of heating it so strongly is disconcerting.
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JJay
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[*] posted on 19-9-2018 at 11:39


300 C is the minimum temperature for the reaction to take place... it is usually run at 800-900 C.



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markx
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[*] posted on 20-9-2018 at 00:30


Titanium tetrachloride is a liquid....vanadium tetrachloride being another example. Both display Lewis acidity properties on a remarkable level.
Synthesis of these compounds might prove to be challenging though.

For example starting from respective carbide:

TiC + 2Cl2 -> TiCl4 +C (amorphous)
at 600-1000C in argon atmoshpere.

The chlorination temperature can be brought down into the 400C range by using catalysts (iron or nickel salts for example).




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SWIM
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[*] posted on 20-9-2018 at 08:34


Titanium tetrachloride has industrial uses as a smoke generator, and might not be that hard to just buy.

I've seen small sealed ampules used for leak finding, even the occasional bottle.

Never tried to buy any myself though.





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DraconicAcid
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[*] posted on 20-9-2018 at 09:09


Quote: Originally posted by Heptylene  
Boron tribromide reacts with atmospheric moisture to product HBr and boric acid.


Most good Lewis acids are moisture-sensitive.




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chornedsnorkack
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[*] posted on 20-9-2018 at 12:27


Liquid or low-melting, moisture-sensitive halides include:
SiCl4, m -69, b +58
SiBr4, m +5, b +153
GeCl4, m -50, b +86
GeBr4, m +26, b +186
SnCl4, m -34, b +114
SnBr4, m +31, b +205
PCl3, m -93, b +76
PBr3, m -41, b +173
AsCl3, m -16, b +130
AsBr3, m +31, b +221
SbCl5, m +3, b +140
SbF5, m +8, b +150
SCl2, m -121, b +59
S2Cl2, m -80, b +137
Se2Cl2, m -85, b +127
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fusso
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[*] posted on 20-9-2018 at 14:05


SnCl4, S(2)Cl2 maybe the easiest to make.



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clearly_not_atara
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[*] posted on 20-9-2018 at 14:13


Liquid Lewis acids are almost always volatile, and, with the exception of fluorinated organoboranes, tend to react with water to produce toxic gases. This makes them dangerous to handle and therefore not preferred.
Quote: Originally posted by JJay  
I want to know if boron tribromide and sodium hydride react to make sodium borohydride.

The answer is "yes", but IIRC the answer is also "yes" for the same reaction with trimethyl borate, which is vastly easier to produce, handle and store.




[Edited on 04-20-1969 by clearly_not_atara]
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macckone
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[*] posted on 21-9-2018 at 09:32


Tin in a chlorine gas stream at 115C+.

That would actually be pretty easy heat a tube with tin in it and flow chlorine gas through it. Stannic chloride will condense in the outflow. That actually seems easier than anything but in situ aluminum chloride.

You could use straight lead free solder which is >90% tin as the other components won't be volatile chlorides at such a low temperature. The usual 'other' components are silver, copper and zinc but other combos are possible.
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chornedsnorkack
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[*] posted on 23-9-2018 at 02:55


Quote: Originally posted by macckone  
You could use straight lead free solder which is >90% tin as the other components won't be volatile chlorides at such a low temperature.

Not important. PbCl2 melts at +505 and boils at +950.
Now, PbCl4 is a liquid that freezes at -15. But it decays above 0, and seems to decay completely above +50. It usually is produced from PbO2, not just chlorination of PbCl2. So does Pb tend to be appreciably volatilized by chlorine as PbCl4 vapours?
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Eddygp
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[*] posted on 23-9-2018 at 03:46


I'm thinking some kind of trialkylborane might work? Alternatively there might be a way to synthesise an ionic liquid with a borane on the sidechain, dodging the problem of volatility straight away while being a liquid.



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Heptylene
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[*] posted on 23-9-2018 at 06:35


Quote: Originally posted by DraconicAcid  
Quote: Originally posted by Heptylene  
Boron tribromide reacts with atmospheric moisture to product HBr and boric acid.


Most good Lewis acids are moisture-sensitive.


Yes that's true, but boron tribromide is particularly reactive. I assume this is the case for most liquid Lewis acids. I've seen it used in the lab (not by me) and you get a large cloud of HBr and boric acid in the air very rapidly.
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