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Noob Vacuum Distilation help
Lately I've been experimenting with vacuum distillation. I'm not a chem major but i'm not a complete crayon eater and have UTFSE and my google-fu, and
I've found mixed answers to one of my following questions and no answers at all for my second question. Sorry if i appear to babble on, I'm just
trying to provide as much information as possible so people much smarter than I have something to work with.
Background:
I'm using a water aspirator for my distillations and filtration. So far I've vac distilled MeOH and HNO3 (H2SO4 + recrystallized KNO3) in a standard
24/29 all GG distillation setup with the only difference being applying vacuum to the vac adapter (no capillary or mag stirring) - from this I've
deducted that I'm getting around 500torr from one of my aspirators, I've recently changed to another aspirator that works much better however I have
not had the time to test this one with known, pure, solvents/liquids to ascertain a ball park figure of it's torr rating, but I know it's better than
500 just from putting my thumb over the hose for a couple of seconds.
Question 1:
I was recently given some benzaldehyde, I wanted to distil this before using it for a reaction (No, not THAT reaction!) since I have read it's quite
unstable and it is a bright yellow colour and one of the two containers had crystals form on the walls which I assume are bezoic acid. Now from what i
have read benzaldehyde has a B.P of 178C at 760 and an auto ignition temp of 192C which seems exceptionally dangerous to distil at atmospheric
pressure? My question is, should I setup for simple distillation: single neck flask on an oil bath with no stirring and a bunch of boiling chips,
still head, thermo adapter, thermo, condenser, vac adapter, receiving flask and then run a vac hose to the aspirator so it's a closed setup? Or should
I have a capillary in place of the thermometer?
Question 2:
I tried this today and had a rather unexpected result. I began with simple distillation to draw off anything boiling over at under 150C. From 285ml of
starting material I had a fraction come over at 74C - 78C of approximately 35-40ml of what I'm assuming was EtOH (Smelled like it, burned with an
invisible flame). After about 15min barely 1 drop per minute was coming over and the temp at the still head was still reading 78C so I attached the
aspirator/vacuum and the temp fell to around 65C then rose again to 70C with a couple of more drops coming over, after around 15min the temp at the
still head had reached 76C (bath temp 160C) and nothing more seemed to be coming over however the liquid in the boiling flask was becoming very dark,
almost black.
The receiving flask was removed and put into a stoppered bottle, the receiving flask was washed, dried, and reattached and the thermometer was
replaced with a capillary and vacuum reapplied, the bubbling was quite intense so the top of the tube was closed almost completely up (first time
using one of these). Bath temperature was taken to 180C to 190C over the course of about 2 hours however NOTHING entered the condenser in this time,
some milky white oil refluxing in the still head, but nothing coming over, liquid in the boiling flask at this time had turned completely black.
At this point i changed my approach, I washed everything thoroughly, rinse with acetone, dried in the oven, let cool to check for condensation to make
sure everything was 100% dry, then instead of the standard setup or capillary, i set a thermometer IN the solution in the boiling flask. Long story
short, I had the flask full of tar looking black stuff that still smelled like Benz, showed the first signs of boiling at 150C, I kept jacking up the
temps until the bath temp was 260C, the solution temp was OVER 190C AND under vacuum and yet absolutely nothing would distil over. I'd get a reflux,
quite heavily at times in the flask and still head of a clear or white oil but nothing at all would enter the condenser or receiving flask. 7.5 hours
after beginning what i assumed would be a "simple" procedure I have nothing but an "Epic Fail" T-shirt for my efforts. Could someone enlighten me as
to what may have happened here? I can take a fail as long as i learn from it lol. Also is the black tarry sludge recoverable?
Sorry for such a long post.
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gdflp
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This might be useful as it has a process for distilling benzaldehyde. http://www.sciencemadness.org/talk/viewthread.php?tid=26075 I'm not sure what went wrong with your distillation though, it sounds as though you
did everything correctly. I'm also not sure why there would be ethanol in benzaldehyde, it wouldn't have any stabilizing effects.
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Tried again using the other bottle of benzaldehyde that was given to me about 6 months back.
92ml was put into a 500ml RB flask and lowered into an oil bath, everything was setup for distillation under aspirator vacuum (No capillary - closed
system, no mag stirring but boiling chips seemed to promote boiling). My water pressure wasn't as good as usual so no doubt vacuum suffered but I had
the same thing happen as before. Bath temp hit 80C and I had a slight reflux inside the flask of clear, oily looking drops that left a white residue.
Bath temp hit 130C and the first drop hits the receiving flask with a still head temp of 65C. Bath temp rose to 138C over the next 10 min or so with
more distillate coming over at 66C , after around 20 minutes the rate has fallen to 1 drop per 10+ seconds and for some reason the temp at the still
head has DROPPED to 61C even though the bath temp has climbed higher. Can anyone shed some light on why this happened??
The Bath temp sat at 150C for quite a while (15 min maybe?) and nothing more would come over even though it was boiling and refluxing in the still
head. At this point it was noted that the benzaldehyde was begining to turn darker in colour and the heat was turned off and flask removed from the
bath. From what began as a bright yellow liquid it was now an amber/orange colour - Is this characteristic of benzaldehyde? to discolour when heated
above say 140-150C?
A couple of tests were carried out, like i said in my OP I'm no chem major, so I'm hoping these may be helpful.
1) A couple of crystals of KMnO4 were added to a test tube and dissolved in around 5ml H20, to this a drop of the benz from above was added and after
a few seconds the solution turned from purple to brown and what looked like table salt like crystals precipitated immediately upon the colour change
followed a few seconds later by a fine brown precipitant, on standing for a minute or two the solution become pretty much clear and the initial 'table
salt like' precipitant formed what looked more like oily/gel like blobs.
2) A solution of around 6ml sodium metabisulphite in H2O was made in a test tube and 3 drops of the benz was added. The benz formed oily blobs in the
solution and lightened in colour but no other reaction noted.
3) 10ml of 5% Sodium Bicarbonate added to a test tube and 5 drops of benz added, benz formed oily dispersed globs which wouldn't join together to form
a solid layer. Solution was shook and then left to stand for 5 min - same result.
4) 5ml 3% H2O2 was added to a test tube and 3 drops of benz added, the benz formed a layer on the bottom of the tube, solution was shook, on standing
layer mostly reformed after a few minutes. No other reaction noted.
I'm ripping out hair on this one. WTF is going on??
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