FigiMon
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Ethyl acetate preparation - questions
I just finished a synthesis of ethyl acetate. Working on a 2 mol scale, I placed 191 mL 60% acetic acid and 141 mL 95% ethanol (15% excess) in a 500
mL RBF. Since I have no sulphuric acid I put in 31 g NaHSO4 as catalyst (not dissolved - sitting at the bottom of the flask). The boiling flask was
connected via a Dean Stark apparatus to a reflux condenser.
It felt like it took a very long time for the mix to come to a boil, perhaps insulated by the sodium bisulfate as I have no stirring in my heating
mantle. Once boiling the Dean Stark trap filled up fairly quickly with a completely clear condensate. I withdrew some of this condensate and it had a
very strong smell of ethyl acetate. I could not smell ethanol or acetic acid. The mixture in the boiling flask on the other hand was very cloudy and I
thought it was the bisulfate that would not dissolve. After about 45 min more I turned off the heat and the liquid in the RBF rapidly separated into
two layers with the major portion (at least two thirds) in the top layer. At this point I drained the trap of 45 mL and set up for fractional
distillation of the layered liquid in the boiling flask.
144 mL of condensate was collected between 70 and 74 C. The temperature then climbed to 78 C and an additional 46 mL was collected up to 80 C. Here
the experiment was stopped and the remainder discarded.
In total I collected 45 + 144 + 46 = 235 mL of product but this obviously contains ethanol. I haven't had time to wash it yet but I am pleasantly
surprised by how quick and easy this was. I am wondering about a couple of things though:
1. Ethyl acetate forms a ternary azeotrope with water and ethanol that boils at ~70 C. This is roughly what I could measure in the condensation zone
of my reflux setup. But why was I collecting this azeotrope immediately in my Dean Stark trap? Shouldn't I be collecting mostly reactants and water
(at a higher temp) until the reaction had run for quite a while?
2. I could see no layering whatsoever in my trap. Withdrawing 15 mL of this liquid and adding just one drop of water produced immediate cloudiness. Is
the azeotrope "stable" also in the liquid phase?
3. In hindsight I should have just separated the two layers in the boiling flask or kept going with periodic draining of the Dean Stark trap. It seems
like there was a lot more in the upper layer than eventually came over as I switched to distillation. Given that there was quite a bit of water around
is it likely that I ran the reaction in reverse and destroyed some ethyl acetate in the drawn out distillation?
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Praxichys
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The time it took to come to a boil could have been endothermic effects of initial ester formation, which would also explain your immediate collection
of the ternary azeotrope.
The dean-stark setup here is not ideal since the reactants are volatile and soluble in water. Ethanol in the azeotrope will help the ethyl acetate
dissolve in the water you are trying to collect and vice versa, returning a lot of it to the pot.
You can drastically improve yield by putting potassium carbonate in your dean-stark trap. It will grab the water while still allowing the return of
ethyl acetate and ethanol back to the pot. Alternatively (and maybe easier) is to collect the fractions, then shake them over potassium carbonate or
some other desiccant to strip the water and then add them back to the pot. A few iterations of this will remove most of the water and give a much
higher yield.
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FigiMon
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Oh yes of course, that would explain it. I will do the washing and drying when I get a chance and calculate what the yield was. I actually think it
wasn't too bad.
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Dr.Bob
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I am impressed that it worked with only 60% acetic acid. One way to improve it might just be to try to distill the acetic acid to a lower water
content, or maybe you can get away with that and just redistill the product a couple time to purify it.
One reason it took so long to heat is that water has a huge heat capacity, that is part of why it takes so long to heat it up. So if you can obtain
glacial acetic acid, that will help a lot. Also, some HCl or H2SO4 might work better, but if the bisulfate works, that is great. But making some
ethyl acetate is a great example of learning organic technique while also generating a very useful chemical. I use it almost every day. I think
you might even be able to use E85 as a starting material, since it will be mostly dry, with mostly hydrocarbons in it. That would make it very
affordable.
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FigiMon
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I got around to the washing finally. Sodium carbonate wash (no fizzing or bubbling at all), followed by three washings with water. In total 35 mL was
removed in the aqueous phase leaving 159 mL of product. Drying this overnight has left a final 136 ml of dry ethyl acetate. I calculate that this
corresponds to a 69% yield.
I will try this again but this time do a couple of things differently.
1. Try to dissolve as much of the bisulfate as I can to see if that makes a difference in the time it takes to get it boiling.
2. Empty the dean stark trap periodically, shake the condensate with a drying agent and return through the condenser.
3. Separate the non-aqueous phase before going to distillation, not leaving everything in there.
As I side note I discovered that my crappy CaCl2 is full of insolubles, perhaps carbonate and/or hydroxide? I will try MgSO4 next time instead.
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FigiMon
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OK, tried again and made the changes above. Dissolved the bisulfate in the acetic (very hard took lots of stirring) and of course when the ethanol was
added it crashed out as a very fine dust. At least it wasn't in the big hard crystals like first time and getting the solution to boil was much
quicker this time. Kept refluxing for 90 mins. Every 10 minutes or so I would drain the dean stark trap, shake the condensate with MgSO4 and return
through the condenser when the trap was full again.
I ended up with 168 mL of product (washed four times) corresponding to a 85% yield. Big improvement over first attempt.
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