I did an esterification experiment of 1-butanol and ethanoic acid.

30 mL of 1-butanol was added followed by 30 mL of ethanoic acid, and then 1.5 mL of concentrated sulphuric acid and some boiling chips.

The mixture was colorless at first. However, it turned brown during the reflux of esterification immediately after heating the solution, and the solution had a burnt fruity smell.

I suspect this is because I didn't swirl the mixture before heating. Also, before heating, the bottom of the round bottom-flask felt warm, and I suspect this could be the reaction between the concentrated sulphuric acid and ethanoic acid.

One thing that I can't seem to understand is, how did the color of the solution turn brown? Could it be that the product of the reaction between sulphuric acid and ethanoic acid reacted with the 1-butanol to produce a colored solution?

P.s: I couldn't obtain any pure ester after the distillation of the ester as the temperature didn't rise above 100 °C.

  • 2
    $\begingroup$ The reaction should have remained clear the entire time, so it sounds like the sulfuric acid reacted with an organic contaminant in your reaction vessel. Such a reaction can be seen when dripping it on paper. $\endgroup$ – ringo May 22 '16 at 6:22
  • $\begingroup$ Would heat be required for the sulfuric acid to react with the possible organic contaminant? Because the color change only occurred when heat was supplied. $\endgroup$ – Jerome May 22 '16 at 10:30
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    $\begingroup$ I do not think that the brown color necessarily must come from the presence of organic contaminants. There might be decomposition/condensation byproducts that are brown. However, I have not done this experiment before, so I'd be happy to let me put right. To get good yields of the ester you probably should do an azeotropic distillation to continuously remove the water formed during the reaction. en.wikipedia.org/wiki/Dean-Stark_apparatus $\endgroup$ – aventurin May 22 '16 at 11:20
  • $\begingroup$ @Jerome Concerning the last remark in the question, note that butyl acetate and the remaining butan-1-ol form an azeotrope, which makes the purification by distillation difficult. Therefore, you should use an excess of acetic acid in order to convert virtually all of the butan-1-ol. $\endgroup$ – user7951 May 22 '16 at 12:01

I don't think the brown color necessarily means that the reaction failed. It could be due to some contaminant in your reagents but in a small quantity that wouldn't cause the reaction to fail. The fruity smell could very well be due to your product see: n-butyl acetate has a sweet banana smell. I would take the distillate you collected (or skip the distillation altogether) and try an aqueous work-up: Add very carefully aqueous $\ce{Na2CO3}$ until basic $\ce{pH}$ and then extract this mixture with ether. This way any ester product will be in the the organic layer and sulfuric/acetic acid will stay in the aqueous together with most of the alcohol.

To finish the work-up dry the ether layers over $\ce{Na2SO4}$, filter and remove the ether (unless you have access to a Buchi evaporator, the safest way is to leave it in an open flask/beaker overnight). You can further purify the final product by distillation.

You might improve the yield if you cool the reaction mixture in an ice bath before adding sulfuric acid

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