anewsoul
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What in the world happened
Today I decided I'd try to make a bit of ethyl acetate since I have a good bit of acetic anhydride to use up. So I planned on simply reacting ethanol
with acetic anhydride with a little bit of acid catalyst.
I measured out 50 ml of ethanol and 95 ml of Ac2O and mixed them in a flask. Then I added less than a mL of sulfuric acid drop wise. When the drop hit
the mixture, it would make a tiny pop. I thought it would be fine though, just like a local hot spot. I added a few more drops. Probably barely half a
mL. I then went to put the condenser on and was clamping it on. Before I knew it the reaction just blew up. The condenser shot out and a jet of acetic
anhydride and ethanol must of blown out the flask filling my garage with a mist of the stuff. I ran out of the garage and gasped for fresh air and ran
to the hose and drenched myself down. Scared the crap out of me.
Did I do something that should've been common sense not to do or something? Why did the reaction just runaway like this? I thought I would proceed
similarly to the common aspirin synthesis.
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Crowfjord
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Wow. Glad to hear you're okay. I'm not sure what happened, but it sure sounds like a run away reaction. Were the reactants cold when combined or
ambient temperature?
This thread details a similar experience.
Now that I think about it a little more, I think the mistake was adding sulfuric acid. Ethanol and acetic anhydride should react fine alone without
catalyst. Starting temperature may have played a role.
[Edited on 8-4-2017 by Crowfjord]
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anewsoul
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Thanks. When I added the Ac2O to the ethanol it actually got pretty cold. I just find it surprising that such a small amount of acid would cause such
a violent run away. At least I know not to do it now. I'm surprised there isn't anything about this on the internet, unless I haven't looked hard
enough.
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Sulaiman
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I'm only guessing based on the Wikipedia article https://en.wikipedia.org/wiki/Ethyl_acetate
ethanol + acetic acid = ethyl acetate + water
CH3CH2OH + CH3COOH → CH3COOCH2CH3 + H2O
the reaction is accelerated by acid catalysis (you confirmed)
once started, acetic anhydride would remove water from the right side of the equation,
(acetic anhydride + water = acetic acid)
and by Le Chatelier's principle ... more acetic acid and ethanol would react ..
accelerating the reaction even more ... super-duper fast 
the runaway time bomb was just waiting for some acid catalyst
[Edited on 8-4-2017 by Sulaiman]
CAUTION : Hobby Chemist, not Professional or even Amateur
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Magpie
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I have made ethyl acetate using ethanol and acetic acid w/H2SO4 catalyst per Brewster without incident. Solomon's textbook says it can also be made
with acetyl chloride or Ac2O , and for these a catalyst is not needed. In fact he says use of Ac2O is the best.
Using Ac2O the by-product would be acetic acid, which, I suppose would then also be available for converting ethanol to ethyl acetate.
The single most important condition for a successful synthesis is good mixing - Nicodem
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nezza
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I have had the same experience with acetic anhydride and ethanol. Adding acid to the mixture causes violent runaway. I ended up adding the anhydride
to ethanol with a spot of sulphuric acid in and the reaction can be controlled.
If you're not part of the solution, you're part of the precipitate.
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anewsoul
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Sounds like a good explanation to me Sulaiman, thanks. I guess the next time I try something without an exact procedure I won't just mix the reactants
all at once 
[Edited on 10-4-2017 by anewsoul]
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Texium
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Thread Moved
10-4-2017 at 07:18 |
Nicodem
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The reaction behaved exactly as it should have. It always happens that way when you try out a relatively exothermic solventless reaction where at
least one of the components is volatile. As the reaction proceeds, it heats up and as it is not cooled and there is no solvent to dissipate heat, the
reaction temperature increases speeding up the reaction rate, further increasing the temperature... at some point the boiling point is reached and the
heat of the reaction can finally dissipate trough vaporisation, vigorously boiling out and spilling everywhere.
Comparison of a phenol O-acetylation with a primary alcohol O-acetylation is pointless. You are comparing two reactions whose rates are many
magnitudes apart, as well as the enthalpies are quite different. Not only was careless to add sulfuric acid, it was also careless to conduct such a
reaction without proper heat dissipation. If you wanted to do this in a solventless manner, you should have slowly added acetic anhydride into
refluxing ethanol. This way you have full control of the reaction and proper heat dissipation.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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woelen
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I think that there is more to this than only having a solventless exothermic reaction. Acetic anhydride and sulphuric acid really is a special
combination. I have done a few experiments with this myself:
http://woelen.homescience.net/science/chem/exps/exppatt.cgi?...
I also am inclined to think that the presence of a little water also is of key importance to explain what happens. A similar reaction also occurs with
zinc chloride, acetic anhydride and water.
I agree, however, with Nicodem that doing this kind of syntheses must be done very carefully and you never should mix all reactants in a single step
and then hope that the reaction proceeds smoothly. Either add one reactant slowly to the others or use a solvent (a suitable solvent in some cases can
be the reaction product).
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unionised
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Quote: Originally posted by Sulaiman  | I'm only guessing based on the Wikipedia article https://en.wikipedia.org/wiki/Ethyl_acetate
ethanol + acetic acid = ethyl acetate + water
CH3CH2OH + CH3COOH → CH3COOCH2CH3 + H2O
the reaction is accelerated by acid catalysis (you confirmed)
once started, acetic anhydride would remove water from the right side of the equation,
(acetic anhydride + water = acetic acid)
and by Le Chatelier's principle ... more acetic acid and ethanol would react ..
accelerating the reaction even more ... super-duper fast
the runaway time bomb was just waiting for some acid catalyst
[Edited on 8-4-2017 by Sulaiman] |
That assumes that there's acetic acid present initially- which is unlikely.
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Magpie
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The problem I have with this experiment is the scale - assuming the OP was not following an established procedure. I'm all for experimentation, i.e.,
trying something on your own, but do it on a small scale until some experience is gained.
The single most important condition for a successful synthesis is good mixing - Nicodem
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anewsoul
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I don't believe it's that unlikely. It's pretty humid where I live.
| Quote: Originally posted by Magpie | | The problem I have with this experiment is the scale - assuming the OP was not following an established procedure. I'm all for experimentation, i.e.,
trying something on your own, but do it on a small scale until some experience is gained. |
Yeah this experience really drilled that lesson into me...
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unionised
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That rather misses the point that acetic anhydride reacts rapidly with ethanol.
There's no need to explain it in terms of acetic acid that might be present in small amounts, when you can explain it in terms of the things you
deliberately added.
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macckone
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Acetic anhydride and sulfuric acid catalyst can be combined then drip in ethanol with vigorous cooling. Keep the reaction cool. Doing it this way
won't cause a runaway.
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Melgar
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All these explanations seem to gloss over the question of why ethyl acetate didn't boil as it formed, and thus cool the reaction vessel? They also
seem to ignore the counterintuitive reaction between sulfuric acid and the reactants. Obviously, the simplest explanation is that this reaction was
put in place by a vindictive God, to test the mettle of those chemists with more acetic anhydride than they know what to do with. 
However, it could also be that this reaction doesn't go to runaway until there is a certain amount of water present to catalyze the reaction.
Ordinarily, this isn't a problem, since one of the products is water. But acetic anhydride is an even stronger dehydrator than H2SO4, and what may
actually be happening is [speculation]that the acetic anhydride is abstracting water from H2SO4 and leaving behind SO3 or more likely diethyl sulfate.
Since neither SO3 or AcOH act as an acid until water is present, it may have taken some time to reach that state where water was in excess. But once
this state was reached, a runaway reaction began, as SO3/diethyl sulfate and to a lesser degree acetic acid began actually acting as an acid in the
sense that they catalyzed this reaction.[/speculation]
[Edited on 4/11/17 by Melgar]
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clearly_not_atara
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It seems like maybe EtOAc did boil and it created a foam. That would account for the rapid buildup of pressure. The foaminess may be caused by a
partial polymerization of Ac2O leading to increased viscosity or perhaps the mixture itself simply acts like that.
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macckone
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Runaway reactions usually involve boiling, in this case the boiling
of the ethyl acetate and ethanol. The problem occurs when heat
speeds the reaction faster than boiling can safely remove the heat.
The runaway usually results in jets of boiling liquid of some
temperature erupting from a vessel in some fashion. In a closed
industrial system the vessel explodes. In a lab setting the
glassware usually unseats and/or sprays a volcano of potentially
hazardous material all over the lab and possibly the chemist.
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