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Author: Subject: Vacuum Distillation of SOCl2 (thionyl chloride)
pleiades
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[*] posted on 19-6-2012 at 18:05
Vacuum Distillation of SOCl2 (thionyl chloride)


I'm currently doing a synthesis where I convert a carbolic acid into the corresponding acid chloride by refluxing in SOCl2. The problem is getting rid of the SOCl2 when finished. I have tried doing the distillation using the vacuum attached to my Schlenk line, and this completely ruined the tygon tubing I was using to attach the distillation setup to my line. It turned purple/black.

I have also tried to distill at atmospheric pressure, but as soon as the excess SOCl2 is gone my product quickly breaks down. Any suggestions on how to get rid of this stuff at low temperature without ruining 3 feet of tubing or killing my yield?

Just FYI the carboxyllic acid is 3,4-ethylenedioxythiophene-2-carboxyllic acid. Thanks!
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watson.fawkes
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[*] posted on 20-6-2012 at 07:47


Sounds like you need a trap train before the vacuum line. Start with a cold trap to get the bulk of it and then a filter canister with some kind of medium to trap what's left. Dry ice / acetone should suffice for the cold trap; LN2 would be better. The important bit is that you're trying to trap the vapor not condensed by the condenser, so you want the trap temperature significantly lower. An adsorptive medium such as activated charcoal would work if the cold trap is effective enough. An absorptive / reactive medium such as water, with a bubbler, might also work. Condensing the thionyl chloride first is preferable, since it lowers the mass load for later stages. Also, since its decomposition products condense at much lower temperatures, it's easier to trap it prior to decomposition. If necessary, use more than two stages.

The important point is that this train can be rigged up entirely out of glassware. At the far end of the train, the exhaust is depleted of corrosives and you can use your ordinary vacuum hose to hook up to your line.
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ziqquratu
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[*] posted on 20-6-2012 at 19:05


I think regular flexible PVC tubing is somewhat more resistant to acids (for a time) than is tygon, too. Even if you only get a single use, it's probably much cheaper anyway. It's not great for really high vacuum, of course - but for the pressures needed to distill SOCl2, it should do OK.

Also, watson's point regarding a trap is important for another reason, too - think what all that corrosive vapour is doing to your pump if you let it go through untrapped!
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peach
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[*] posted on 21-6-2012 at 01:30


What type of Tygon are you using? They make various grades of it and there is one specifically for high chemical resistance, 2375 Ultra Chemical Resistant Tubing. I've tried exposing this, or their last version of it (2075), to hydrogen chloride and the tubing turns opaque, but doesn't stiffen or crack.

If you can only get that in none vacuum styles, you can reinforce it yourself to some extent by winding some suitably resistant wire around a pen or pencil, unspooling it and then pushing it into the tubing, to prevent it collapsing under vacuum. Not ideal, but better than unusable. Grip the ends of the wire with some pliers and poke it back inside the spool, so it doesn't pierce the walls.

I was chatting with one of the guys at Saint Gobain about the ultra resistant tubing, hydrogen chloride and he said it was a known effect and recommended PTFE; bit of a pain in the ass actually connecting that to glassware without the special connectors (since it's polymer not elastomer, so it won't stretch or seal over barbs, stems etc).

However, I think Watson has a better suggestion, try trapping it before any tubing is involved and before it gets near the line.

Recovery bend out of reaction flask -> port on the side of a spare flask -> dewar on the top of that flask -> connect the top of your dewar to the line via your normal tubing.

Your volatiles should end up, largely, trapped in the second flask, and all of the path to that point will be glass / grease.

SO2
Melt -72C
Boil -10 C

HCl(g)
Melt -114C
Boil -85C

Depending on the scale you're running this at, you could just use the one trap and fill it with LN2. That will trap both, but you'll end up with them icing up on the dewar's finger; so make it a big'un. You could also place the bottom half of the recovery flask in a dry ice / LN2 bath to start cooling / capturing it prior to it meeting the cold finger of the dewar above.

Via the wonders of MS Paint comes;



With the recovery bend, you may find it a lot easier to set this up if you put a ball and socket (spherical) adapter either on the reaction or recovery flask neck, so there are a few degrees of flexibility.

Watson's other suggestion of including a neutralising trap is also good.

[Edited on 21-6-2012 by peach]
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pleiades
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[*] posted on 21-6-2012 at 12:16


Thanks a lot guys. I'll try this.

I'm not sure what grade the tubing is. We basically have a drawer with various leftover tubing.
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