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Topic: Unexpected Precipitate in Esterification Lab  (Read 2730 times)

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Offline aseverson98

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Unexpected Precipitate in Esterification Lab
« on: May 20, 2017, 04:35:50 PM »
I was doing a Fischer Esterification of methanol and salicylic acid to form methyl salicylate. I used 30 mL of methanol and 7 g of salicylic acid. Reactants were mixed in a round bottom flask in an ice bath. 2 mL of sulfuric acid was added drop-wise with frequent stirring. Solution was then refluxed. After refluxing solution was put in a separatory funnel and 20 mL of deionized water was added and solution was shaken vigorously. Then it was left to settle and separate; however, instead of an aqueous layer at the bottom a precipitate began to form. Precipitate was then separated. The precipitate was examined and was paste like. Also the solution was placed back into the separatory funnel and no oil layer formed.
What could this be? Could it be acetylsalicylic acid though the synthesis for that requires acetic acid.

Offline phth

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Re: Unexpected Precipitate in Esterification Lab
« Reply #1 on: May 21, 2017, 05:41:08 PM »
I have seen this before using NaHSO4 at 0°.  I would expect it to show a blank NMR spectrum.

Offline clarkstill

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Re: Unexpected Precipitate in Esterification Lab
« Reply #2 on: May 22, 2017, 06:30:28 AM »
Did you add an organic solvent too during the work-up? You don't mention it... generally for a Fischer I would add a decent amount of ethyl acetate and water, enough that the methanol doesn't form any emulsions. Separate the layers, then wash the organic layer again with NaHCO3 to remove any unreacted starting material as the acetate.

It's not clear to me what the precipitate is - the product should be an oil at RT so not that. I guess it could be unreacted SM since its aqueous solubility is low, so you may have triturated it out of the methanol solution by adding water. There isnt any plausible way you could have formed acetylsalicyclic acid...

Offline aseverson98

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Re: Unexpected Precipitate in Esterification Lab
« Reply #3 on: May 22, 2017, 06:36:50 AM »
Yeah there was no organic solvent added before attempting to separate. Good to know though thanks for sharing! My lab procedure just called for add water, shake, and separate. Followed by a double wash of saturated sodium bicarbonate.

Offline kriggy

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Re: Unexpected Precipitate in Esterification Lab
« Reply #4 on: May 22, 2017, 01:52:03 PM »
Why did you use separatory funnel?
Im not sure how the procedure is worded exactly but maybe it called for addition of water and filtration?

Offline discodermolide

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Re: Unexpected Precipitate in Esterification Lab
« Reply #5 on: May 23, 2017, 02:33:40 AM »
Your procedure is crap. First remove the excess methanol, then add heptane and extract twice, finally wash combined heptane extracts with bicarb, dry and remove heptane. Residue is your ester.
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