chemrox
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distillation indicating too high a temp
I made propionyl chloride the benzoyl chloride way. Yield was 86% which is almost as good as the paper indicated and was slightly lower because I ran
a tad short on the chlorinating reagent (a few percent short).
What is vexing is everything invloved boils a lot higher than the TM (target molecule) and I was collecting between 88-92. The TM is supposed to boil
at 80 deg. A few ml came over above 90 that I have kept separate. After that the thermometer started really climbing and I shut it off. The boiling
should have been between 78-80 not 88 - 90. My thermometer read boiling acetic at 116-118 so that isn't the problem.
I used a new fractionating system this time: 10 cm vigeraux, wrapped with heating wire and asbestos paper (yeah I have some), then Al foil. I tried
to keep the column at the temp of the top of the flask. Could have been a little hotter than that .. I used a point and read thermometer for that
part. I distilled slowly collecting less than a drop/second.
Only thing I can think of is to test the still on another pure liquid. Benzene would be ideal .. don't have any but probably can get without too much
trouble.
If I have missed something due to stupidity and/or ignorance please don't hesitate to say so. The pure acid chloride is needed failrly soon.
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not_important
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What was the flask/column temperature? Were you getting reflux at the top of the column/sidearm?
You shouldn't have needed to heat the column for this low of a temperature and short column, just insulation should have done the trick.
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Furch
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Just out of curiosity... How can you determine the yield if you haven't succeeded in isolating the product yet? *puzzled*
\"Those who say do not know, those who know do not say.\" -Lao Tzu
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py3rhcl3
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What about pressure? Did the paper indicate low(er) pressure? Are you or the authors even on mountain?  (Just kidding!).
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Magpie
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What was the location of your thermometer bulb?
The single most important condition for a successful synthesis is good mixing - Nicodem
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Nicodem
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| Quote: | Originally posted by chemrox
I used a new fractionating system this time: 10 cm vigeraux, wrapped with heating wire and asbestos paper (yeah I have some), then Al foil.
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Heating wire can not only maintain the heat but also heat up. Using it on a Vigreux column if the boiling point is so low is counterproductive.
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Furch
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I would almost certainly say that the villain in this distillatory drama, as not_important pointed out, is the excessive insulation and warming of the
the fractionating column.
Distill slowly, at the same rate as before, without any insulation AT ALL, and let the equilibrium settle in the column at its own
pace.
\"Those who say do not know, those who know do not say.\" -Lao Tzu
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Magpie
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I agree. I never insulate the column. If the column is too hot it would be counterproductive as it would be keeping the high boiling material in a
vapor state instead of letting it condense and drop back down through the packing.
Edit: To say that a column should never be insulated or heated would be wrong, however. This is amply covered in Vogel.
[Edited on 16-2-2007 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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chemrox
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Many of you commented and I'm very grateful for that. The consensus seems to be the column should haven't been heated and from those I'm thinking the
vapor may have been overheated. I thought of that too. Pressure was mentioned too. The article was based on 760 but I may have introduced back
pressure by the following: long condenser and lengthy vapor trap. Overheating was probably the culprit as nicodem and others said. I will save the
heating for high boiling materials. 80 was pretty low huh? Cason & Rappaport went to great lengths about improving separation by equalizing the
temps between the top of the flask and the top of the column. I was playing with the system having just completed it. Thing is my simple
distillation head doesn't provide room enough for a coldfinger (sticks down too far) and I don't have a variable takeoff .
One last embarassment- I forgot my thermometer adapter was rubber. Changed to cork a little late.. at least I have another and now I've fitted a
cork. Everybody needs a cork press.
One last thing, I've had a nasty flu and wish to apologize for recent sensitivity as well as some carelessness.
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chemrox
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not isolated
I collectd additional product, a few mls...that were indicated at between 90-92. Probably overheated chloride. I kept them separate. They're now
isolated. The amount I first collected was 86% of theorectical. The additional would maje the yield 89%. In perfect agreement with the published
procedure even with s shortage of a few% of the benzoyl chloride. I bought 500g and used it all. I have a lot of the acid.
TCT/DCM sounds awfully expensive. I have another factionating column on the way. Different design. Should have a lot of 'plates'.
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not_important
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You still need a slight gradient along the column I think, and certainly the top of the column should be no warmer than the bottom.
When I am distilling fairly low boiling materials in one of the Vigreux columns I use a jacket I made. It's a piece of copper pipe cut open
lengthwise, with layers of ceramic blanket, aluminium foil, closed cell PE, and another wrapping of foil, going from inner to outer. There's a
inverted funnel-shaped 'hat' of copper sheet at the bottom, soldered to the pipe, also covered with insulation. This is on a 600 mm column, about 8
theoretical plates.
The slot in the pipe lets me see into the column, as well as allowing the pipe to fit over the column as it was just a shade too small. The pipe
conducts heat well enough that the temperature is fairly constant top to bottom. The 'hat' covers the top of the flask, reducing heat lose from there
and funneling some of the heat into the column.
I should have thermocouples or other temperature sensors at several points, but I don't 8-( I thinking of putting in a couple of sensors like these
http://www.national.com/pf/LM/LM76.html
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4253
for low boiling stuff, up to 120 C.
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chemrox
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acid chloride synthesis issue
Last year I reported an anomlously hight temp for propionyl chloride made with benzoyl chloride. HC Brown method. here's the thread:
(Edit by Nicodem: Merged both threads and removed the dead link)
Anyway, I ran it again using 3 moles benzoyl chloride to 2 moles acid as recommended in the paper. It's a reactive distillation. I set it up with a
high amount of reflux, also as recommended. This is to cut down on the carry over of the reactants with the HCl being generated. So I ran it with a
six cell Snyder column and Claisen adapter with a cold-finger on top and the thermometer in the elbow. No jacket, no heating on the column. It
didn't flood the column but made a lot of return. here's the anomaly: this time the temperature is too low; its coming off at 55-60 deg.
I ran an IR on the product and got strong absorptions for acyl Cl. I'm thinking maybe the HCl is involved in bring the product over below its boiling
point.
Indeed the paper says just this:
"In refractionating the
distillate care should be taken to maintain a very
high reflux ratio at first; otherwise the dissolved
hydrogen chloride tends to carry over a considerable
amount of the acid chloride below its true
boiling point."
So- how can I increase the refulx ratio? Do I need to? The temperature is holding copnstant at 55-57 so its probably pure stuff. I can always
re-distill later on. Yield is looking good so far. I had to shut down for an early am trip to the airport.
[Edited on 23-2-2008 by chemrox]
[Edited on 23/2/2008 by Nicodem]
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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Sauron
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So, you have no doubt that the product is propionyl chloride, but you are bothered by the low apparant bp?
I would say, you are right, and you are entraining some propionyl chloride with HCl gas. This suggests that the 6-cell Snyder column is inadequate,
and you should replace it with a longer and more efficient column. Try the specific column dimensions and design that was described in the Brown
paper. At worst, just condense and collect, eventually the still head temp ought to reach the bp of propionyl chloride, and then keep going till
there;ss nothing coming over, at which point there will be nothing in the pot but benzoic acid and a little excess benzoyl chloride. IIRC this
procedure yields >90%.
You just want to avoid stripping off propionic acid with the HCl but, that is so much nigher boiling that it seems unlikely.
Sic gorgeamus a los subjectatus nunc.
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chemrox
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So the worry is needless. The Snyder is a 600 mm column and the six bulbs should be eq to six plates, neh? What would be better? An 800 mm Hempel
with packing of beeds or rashig rings? I reduced the efficiency by not insulating my column to get more reflux. Mistake? Anyway, since its coming
over low this time and nothing boils lower than the product everything should be great. I hopes so, I used all my Benzoyl Cl for this.
I would tyhink you would need acid chlorides for your amino acid work (?)
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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Sauron
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Too high a boilup rate could also be a villain here, in conjunction with some or all of other factors mentioned.
Lag the column but do not heat it.
Run 90-95% reflux (1:20 to 1:10) at least till your still head vapor temp is stable and where it ought to be. Then you can gingerly lower the RR but
only as long as the vapor temp stays stable. If it changes return to a higher RR.
Sic gorgeamus a los subjectatus nunc.
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chemrox
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column efficiency report
Anyone interested in distillation columns should read this paper. What I learned is twofold: My Snyder columns are very efficient but only at the
proper reflux ratio. The efficiency drops off rapidly when they approach flooding. So I used the best column I have for the job but I sabotaged it's
effectiveness. Not to the detriment of the project however since my bp stayed below the bp of the TM. This could all have been handled in PMs but I
thought others might benefit from the experiment since acid chlorides are something we all need from time to time and fractioanl distillations are
part of the chemist's life unless he's one of the bug-soup designers; yechh!
[Edited on 23-2-2008 by chemrox]
Attachment: Dist Columns Efficiency Test Report.pdf (595kB)
This file has been downloaded 910 times
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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Sauron
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An efficient column is one with a very good exchange between the liquid and vapor phases, and the measure of efficiency is HETP, right? Of course
efficiency drops off as flooding is approached, because the liquid phase overwhelms the vapor phase.
But flooding is a matter of column capacity (which is a function of diameter and free volume and also depends on the throughput and the RR being
correct. A properly sized column of a given free volume, with the correct boilup rate, will not flood even at 100% reflux, right?
All discussions of column efficiency and parameters assume equilibration, and equilibration can't possibly occur when a stream of a gas like HCl is
admixed with the vapor and liquid. So the best you can do is to condense all your target material and let the HCl go. That's 100% reflux. Eventually
the HCl will slow and then the column can start to equilibrate. Then and only then can you fractionate. Until then you may as well be doing a simple
distillation.
For tolerance of a high boilup rate you need an Oldershaw column, preferably a 12 plate one or more, and such columns are not cheap! The boilup rate
can be higher because they are mostly empty! But at every plate the vapor is forced through a layer of liquid. Liquid is constantly descending, vapor
constantly ascending. When equilibrated (boilup and RR in steady states) the gradient of lowest boiling vapor at top and highest boiling liquid at
bottom has formed. And that is when you can slowly start taking product off. As long as you do not disturb the equilibrium you can just let the still
head temp be your guide as to when to take a cut.
Conversely, Hempel columns with inefficient packing such as glass spheres, have very little relative free space and therefore can only tolerate very
low boilup rates and RRs. Which is why burl saddles are better, Raschig rings are better etc. Snyder and Vigreaux columns fall in between the extremes
of Oldershaw and Hempel.
And no, I have not yet read yhe paper you posted but yes, I will do so, thanks.
Distillation procedures are a great place to cultivate patience. They simply can't be rushed.
Sic gorgeamus a los subjectatus nunc.
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Panache
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i generally take a boiling point of fractions after the distillation has finished, just a few mL in a test tube with a thermocouple, this isolates
your distillate from your complex system, thermocouples are much better for accuracy. i don't know what other people techniques are but i boil the
sample to fairly vigorous then read the temperature, the point at which boiling stops is usually fairly sharp, i take this as the minimum bp.
The temperature at your still head is a measure of many things and in a perfect world yes it should correspond to the bp of your liquid however just
because it doesn't does not mean that the fraction you are distilling is tainted or impure, unless of course the second test of the bp confirms the
gross increase experienced at the top of the column.
10C is more than a 10% increase on what's expected and i feel this increase could not be reached by back-pressures within the system unless your gear
is grossly mismatched. The heating tape is an effective tool for small columns and small systems as it allows you to quickly bring the temperature of
the column to a point close to that which the vapour would heat it to ordinarily, thus saving time, however it insulates poorly and once your column
is at equilibrium stablity is everything. It surely should never be used once the column is at equilibrium because this defeats entirely the point of
the column, namely to separate the point at which vapour is produced (your flask) and the point at which that vapour is recondensed to liquid. By
adding heat to the system (in the column) you're effectively moving your flask to the still head, you may as well not use your column in some
respects.
I find taping two small thermocouples adhered to the column with al foil tape a very useful tool. At any one point during your seperation, if you're
experiencing something odd at the stillhead, you can plug these into your digital thermometer and understand what the column is doing. I wrap
insulation over them and they remain permanently part of the column. They generally lag around 2C to the temperature inside the column when the column
is experiencing a heating or cooling gradient however once stable they accurately represent the temeprature inside the column (assuming a glass
column).
There however could also be a maximum boiling azeotrope system but as i am unfamiliar with the chemicals or reaction i will leave the notion of this
for others to speculate on.
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