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Author: Subject: Fractionnal distillation temperature ranges
Serybva
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[*] posted on 4-6-2021 at 05:29
Fractionnal distillation temperature ranges


Hi everyone, today I made ethyl acetate, using:
- 5 mol acetic acid
- 6 mol 94% ethanol
- 50ml 95% H2SO

Then refluxed for 1 hour, let it cool in the heating mantle for about 1 more hour and then transfered to a sep funnel where I neutralized acids using saturated NA2CO3 solution, shaked, let layers separate drain and repeat until no more fizzing was happening.

Then I've pourred the crude ethyl acetate in a RBF with a fair amount of anhydrous CaCl2 at the bottom and set up the apparatus for a fractionnal distillation, first drops came over à 65°~ish and I started collecting at 68°C and stopped the distillation when nothing more seemed to come over and the temperature dropped by 2/3°C

Ethyl acetate has a boiling point of 77°C but it never reached this temperature, while I know impurities can decrease or increase the boiling point is it normal that my product started to distill à 65°c? 12° below seems a lot

It was the same the other day with methyl benzoate (BP: 199°) product started to come over at 160° or so, never went past 180°, 40° to 20° below the BP of the product seems a lot too.
It always surprises me so each time I quickly drop the planned temperature range for collection, and start to collect after a few ml and see where it stops.

I don't know what to think, the product is purer for sure, but is it pure enough yet?
Should I re-distill again and this time be more restrictive with something like a 75-79°c range?

I plan to use ethyl acetate to prepare ethyl acetoacetate, so I guess, the highest the ethyl acetate purity, the better.

Thanks for your inputs
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teodor
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[*] posted on 4-6-2021 at 07:22


Did you make calibration of your thermometer distilling some pure compounds with a sharp boiling point? Also, make sure you have enough stream through your apparatus. I never see difference in 60-80 range on my thermometers but @ 97C I must add 3 degrees and @ 120C+ 8 degrees - it depends on how deep you set your thermometer. You need to add some degrees if the point where your column stops in the thermometer is not inside apparatus but is in a cold air outside. When you make calibration always write down how deep you installed your thermometer.
Also, don't forget to filter-off CaCl2 before distillation.

[Edited on 4-6-2021 by teodor]

[Edited on 4-6-2021 by teodor]
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Hoffit
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[*] posted on 4-6-2021 at 07:56


Unless there is some measurement error (or uninsulated column way under powered) other sources of error can be azeotropes. Ethanol/H2O/EtOAc has an azeotrope at around 70.2C and ethanol/EtAc at around 73C. I did not check composition to see if those are viable or not, just picked up those from notes of a distillation a while ago where I noticed both. (as use used anhydrous CaCl2 water may not be there if you used it a bunch).

If you have very low power in heating you can sometimes get just vapor and measure temperature below boiling point correctly. You need enough power to have boiling equilibrium at the top of the column but not too fast to push vapors through the column or flood the column.

At such a low temperature like 65-68 I'd wonder if temperature is measured well. You may also want to check out if ethanol azeotrope is a possibility. (that is a pain to remove if you have lots of ethanol, boiling point too similar to product). Ethanol can be removed with water extract (EtOAc has pretty low solubility in water).

Edit: you washed it with aqueous soln already, unless you used a tiny amount of it, you probably don't have too much ethanol left. I would guess some kind of measurement error based on reaction and this.

[Edited on 4-6-2021 by Hoffit]
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Fery
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[*] posted on 4-6-2021 at 10:44


Ethyl acetate is maybe the less yielding synthesis from all possible esters. Due to azeotrope formation. Due to loses of ethylacetate when washing (solubility 8,3 g ethylacetate / 100 ml water).
Serybva removed all acids with NaHCO3.
He also removed ethanol by washing and last traces with CaCl2 which binds ethanol.
I wouldn't synthesize ethylacetate but I would buy it instead, if no one chemshop selling it then non-acetone nail polishing remover.
Btw I would use 5 ml of H2SO4 for 5 mol scale of synthesis, it is only a catalyst, so why to use almost 1 mol of H2SO4...
If your thermometer is OK then an explanation of strange temperature could be the position of the thermometer bulb
http://www.chem.ucla.edu/~bacher/CHEM174/glassware/distillat...




If there is a heaven, it seems not to be materially based. Does chemistry exist there and if yes, how does it look like? Are there good souls well supplied with laboratory equipment, glass, chemicals and information?
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Serybva
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[*] posted on 9-6-2021 at 03:30


Thanks for your answers, indeed thermometer calibration isn't something I did or even thought of, I'll do a test with deionized water.

I did a quick test with almost boiling water the other day using my 2 thermometers and there was a 2° difference between the two.

I think I placed my thermometer right, actually I took a picture:




Doesn't seem to be the issue

I used approx. 1l of Na2CO3 sol to wash the organic phase so I don't think I had much more ethanol in it at the time of distillation.

Actually Fery I have pretty good yields on this preparation (between 60 and 70% that's good enough for me), and since I need ethanol to prepare the acetoacetate I thought I could prepare ethyl acetate as well instead ordering it and pay shipping fees.

But the next time I thunk I'll just order it indeed and hydrolyze with NaOH it to obtain ethanol.

I used that much H2SO4 to act as a water sequestrant and help push the equilibrium of the reaction to the right
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teodor
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[*] posted on 9-6-2021 at 04:10


The difference between the boiling temperature and what thermometer shows (according to my experiments) is depending on:
1. Boiling temperature
2. How deep the thermometer is inside the glass (immersion depth).
3. How bulb is positioned to output.
4. Where it the highest layer of vapours in the adapter.

I always keep 3 and 4 the same but 2 can be changes when different adapters are used, so it requires different calibration. Also the difference is changing with the boiling temperature, so it is good idea to calibrate thermometer with a test liquid of boiling point +- 10C to what is required to measure.

When you get a lot of practice you can avoid this calibration just by looking on changing temperature - you know for some kind of synthesis it is always pre-fraction, the middle and the post-fraction, and the middle is going with ~2degrees change, so you probably can avoid calibration on some intermediate levels if you know the typical composition of your result - so you can calibrate doing actual distillation and then check that the results are consistent with other cases.

Pay attention to 4 noticing where is the "ring" of vapours in relation to output - I calibrate my thermometers on "1 cm above the output" and I know I will get a proper temperature when I keep the same level during the distillation.

Another hack is to use a thermometer with a fixed glass joint which is usually already calibrated (because it doesn't allow changing of immersion depth) as a reference.


[Edited on 9-6-2021 by teodor]
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Oxy
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[*] posted on 9-6-2021 at 04:15


Thermometer should be calibrated as there is a risk of incorrect readings.
You should check also thermometer immersion depth. Mine are 77 mm and because of that I can't really use them for melting point tests (I could but I should have another one or make calibration curve) as their read is much lower than in reality.

I've checked melting point of extracted acetaminophen to check if I should recrystallise it once more or it's enough. I used glass thermometer and got quite sharp (1.5 deg C) melting point at about 140 deg C. I checked the thermometer and saw information on it's back that it's immersion depth is 77mm. But it was immersed only to 20mm during the test. So I've replaced it with thermocouple and got correct ~169 deg C reading.

So, it's not only about the position but about the thermometer itself
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Amos
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[*] posted on 9-6-2021 at 04:30


Weirdly enough nobody has said it; ethanol and ethyl acetate pretty much can't be separated by fractional distillation, at least not in their entirety.

An azeotrope of 69.2% ethyl acetate and 30.8% ethanol distills over at 71.8 C. A few degrees lower than expected seems to be pretty common when doing lab-scale distillations, and of course theres elevation that needs to be adjusted for as well. I don't think anything is amiss.
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Sulaiman
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[*] posted on 9-6-2021 at 06:53
Not applicable in this case but worth a mention


I recently got temperature readings several degrees above the true temperature during a fractional distillation,
the temperature reading at the top of the column was as expected but the temperature reading for the vapour in the rbf was too high.

The level of the liquid in the rbf became lower than the upper part of the heating mantle,
this caused the glass to reach a much higher temperature than the liquid/vapour,
the heat was conducted to the thermometer.

My temporaty 'fix' was to route the condenser cooling water through a pvc tube wrapped around the neck of the flask holding the thermometer.
Now I have a ptfe thermometer holder (actually an overhead stirrer gland/adapter) and the problem seems fixed.




CAUTION : Hobby Chemist, not Professional or even Amateur
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XeonTheMGPony
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[*] posted on 9-6-2021 at 17:10


Don`t forget altitude, I have to subtract 2c due to it.
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