Sciencemadness Discussion Board

What in the world happened

anewsoul - 8-4-2017 at 11:35

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.

Crowfjord - 8-4-2017 at 11:58

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]

anewsoul - 8-4-2017 at 12:22

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.

Sulaiman - 8-4-2017 at 15:05

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 :D

the runaway time bomb was just waiting for some acid catalyst

[Edited on 8-4-2017 by Sulaiman]

Magpie - 9-4-2017 at 13:53

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.

nezza - 9-4-2017 at 23:39

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.

anewsoul - 10-4-2017 at 07:01

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]

Nicodem - 10-4-2017 at 07:32

Quote: Originally posted by anewsoul  
Why did the reaction just runaway like this?

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.

woelen - 10-4-2017 at 12:09

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).

unionised - 10-4-2017 at 12:15

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 :D

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.

Magpie - 10-4-2017 at 14:00

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.

anewsoul - 10-4-2017 at 18:51

Quote: Originally posted by unionised  

That assumes that there's acetic acid present initially- which is unlikely.


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...

unionised - 11-4-2017 at 10:55

Quote: Originally posted by anewsoul  
Quote: Originally posted by unionised  

That assumes that there's acetic acid present initially- which is unlikely.


I don't believe it's that unlikely. It's pretty humid where I live.


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.

macckone - 11-4-2017 at 14:07

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.

Melgar - 11-4-2017 at 14:25

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. :P

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]

clearly_not_atara - 11-4-2017 at 15:36

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.

macckone - 7-5-2017 at 07:56

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.