UnintentionalChaos - 19-12-2010 at 07:36
Yesterday, I tried running the "classic" destructive distillation of tartaric acid to yield pyruvic acid per http://orgsyn.org/orgsyn/orgsyn/prepcontent.asp?print=1&...
So, the problem is this. At 210C, the melt of tartaric acid and in my case previously fused NaHSO4 (The original prep uses the potassium salt) is
viscous and then the tartaric acid starts dehydrating and decarboxylating. This makes foam. Horrible foam that slowly fills the reaction flask
(despite me giving it extra space relative to the original prep) and climbs into the condenser.
The original prep suggests this foam can be controlled with a free flame on the upper portion of the flask, but in my case, with a downscaled
reaction, that is inches away from visibly fuming 210C mineral oil. Not wise.
I suspect that the issue is the melt viscosity and if anyone has suggestions for thinning it out with a non-reactive, non-water containing compound,
I'd love to hear it.
My one thought is that the NaHSO4/KHSO4 eutectic may well be lower viscosity than either of it's components at the reaction temperature. Preferably,
I'd like to salvage this batch even if it means reduced yield.
Thanks for any help.
[Edited on 12-19-10 by UnintentionalChaos]
Nicodem - 19-12-2010 at 07:53
Whoa, that's foamy!
Foam can be controlled in various ways, not always successfully general to all cases though. In the industry commercial defoamers are used, but you
can try with paraffin oil or candle wax. Though I doubt it would be efficient for such a bad case and the somewhat volatile paraffin oil would also
contaminate your product, though you will probably want to redistil it anyway (but still, rather chose a less volatile paraffin based wax). Another
possible solution (doubtfully completely effective) is to put a wire mesh in the distillation adapter, or better yet, a mesh followed by a vigreux
column.
To salvage this batch, just redistil the crude product.
Magpie - 19-12-2010 at 08:40
I would first try a drop or two of silicone oil. This was first suggested by Tacho, and I think it is a good one, but personally have not yet tried
it.
My usual strategy is to insert a Claisen adapter to increase the time/path for foam dissipation. Also magnetic stirring may beat down the foam.
ziqquratu - 19-12-2010 at 22:40
What about a heat gun? As long as you pay attention to what you're doing, you shouldn't have any flammability issues.
Otherwise, I agree with the column idea - although I'd choose an unfilled column (or simply a vertical condenser) rather than a vigreux, with the hope
that the foam would be less likely to stick.
aonomus - 19-12-2010 at 23:23
Does the original paper specify any agitation? Older papers tended to use overhead mechanical agitation before teflon coated stir bars, and mechanical
agitation is alot better at breaking up foams from my experience than magnetic stirring, particularly for larger scale reactions and flasks.
Also, silicone oil is used for breaking up foams in solutions by altering surface tension, the OPs case seems to be decarboxylation with no solvent.
Surface tension isn't as much of a role since the decarboxylation makes a foam from the tartaric acid melt, as the foam rises, it cools and
solidifies. I think your best bet would be to simply heat the entire apparatus to encourage the tartaric acid and other compounds to melt. Steam or a
heat gun, combined with switching your heating bath to either silicon oil or sand should give you the result you need.
vulture - 20-12-2010 at 13:40
Loose glass wool inside the joint might work, but I'm doubtful whether it'll stay in place.