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Earlier today I was preparing an ester (propyl ethanoate) and I noticed that instead of putting the propanol, ethanoic acid and $\ce{H2SO4}$ in the test tube and warming it (taking time and external energy) I could instead mix sulfuric acid with the ethanoic acid, which made the test tube go very hot, and then by adding the propanol and shaking it split into two layers. On pouring this into water I ended up with an immiscible layer which appeared to be an ester (it was sweet smelling and considering the reaction it would be likely to be an ester).

I cannot think of which reaction was causing the heating up initially. The test tube was clean so the only reactants were the sulfuric acid and ethanoic acid, and it appeared that a gas was being given off (as shaking it with a rubber cap on caused it to pop out of the tube, but no effervescence could easily be seen). Can anyone help? Furthermore, would the eventual product be propyl ethanoate or something else (it was suggested that it was dehydrogenating the ethanoic acid so the actual product would be something like propyl ethenoate, but from a quick look on the internet this would seem to require an iron catalyst (or another catalyst) and a high temperature, neither of which was present)?

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  • $\begingroup$ Was the ethanoic acid completely anhydrous and was the sulphuric acid concentrated (usually is for esterification)? Concentrated sulphuric acid reacts very exothermically with water. $\endgroup$ – bon Jun 11 '15 at 19:19
  • $\begingroup$ I believe so. It may have contained a little bit of water, which might explain it. Thanks. $\endgroup$ – user1150512 Jun 11 '15 at 19:20
  • $\begingroup$ My guess is sulphuric acid reacting with water to heat up the test tube which evaporated some of the ethanoic acid causing the bung to pop off. $\endgroup$ – bon Jun 11 '15 at 19:23
  • $\begingroup$ Ah, that would make sense. Thank you for your help. $\endgroup$ – user1150512 Jun 11 '15 at 19:27
  • $\begingroup$ I would wait for someone else more experienced to confirm it as I'm not the most experienced but that seems likely. $\endgroup$ – bon Jun 11 '15 at 19:28
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I suspect that the ethanoic acid contains a residual amount of water and that the concentrated sulphuric acid is reacting with the water. Since this is a very exothermic reaction the test tube gets heated up which evaporates some of the ethanoic acid, causing the bung to pop off. If it's just slight evaporation this would also explain why you couldn't see any effervescence.

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Good observation!

You have actually created the strongest acid ever. Stronger than Magic Acid, which is pretty darn strong (it's known as superacid, where superacid are acids that are stronger than 100% sulphuric acid.)

Combining Sulphuric Acid and Acetic Acid results into the formation of a polymeric acid. enter image description here

This link has a great explanation for the reaction that takes place>> http://link.springer.com/article/10.1007%2FBF00908172#page-1

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    $\begingroup$ Magic acid is a superacid. Also, as far as I am aware, the strongest known acid is fluoroantimonic acid, a 2:1 mixture of $\ce{HF}$ and $\ce{SbF5}$. $\endgroup$ – bon Mar 9 '16 at 12:41
  • $\begingroup$ You are not wrong, but sulphuric and acetic acid mixture is also not any weaker. However, the mixture doesn't seem to be as stable as fluoroantimonic acid mixture. Probably that's why it's not considered/used? Superacids aside, tell me what you think about the sulphuric acid and acetic acid reaction. $\endgroup$ – Prerak Mar 10 '16 at 3:42
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I think what happened is additon of conc h2so4 led to esterification of ethanoic acid and release of water. This water must have reacted with another h2so4 to produce heat without effervescence.

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