Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: FleurDEspoir on March 07, 2019, 05:42:58 PM
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Today in lab we synthesized acetylsalicylic acid (aspirin) by combining salicylic acid and acetic anhydride. To purify our product from any unreacted salicylic acid and the byproduct acetic acid, we used recrystallization with a 95% ethanol-water solution. How was this able to work? Ethanol and water are both quite polar, and acetylsalicylic acid has more hydrogen bond-accepting groups than the other two molecules, so shouldn't it have been more soluble in the solution than them? My only guess is that ethanol is 2/3 nonpolar, so it has less of an affinity for aspirin than it does for salicylic acid and acetic acid, but I didn't think this would matter as much due to the strength of hydrogen bonds.
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I think you have it a little backwards, the product has worse water solubility than the starting materials, so adding some water will dissolve the starting materials better than the product.
Pure ethanol probably would be a poor solvent choice, it would dissolve everything more equally.
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So you're saying that the desired product comes out of the solution more readily because it has less of an affinity for water than the other two materials? This still does not make sense to me because aspirin has the same benzene ring as salicylic acid and is capable of receiving more hydrogen bonds due to the replacement of a hydroxyl group with an ester.
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A carboxylic acid is both a hydrogen bond donor and acceptor. The ester is only an acceptor.
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If compound A has a similar solubility to compound B in a given solvent but A is in much higher concentration, recrystallization can still purify A. Most of compound A will crystallize out, but compound B might not.
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You can probably google the solubility of each material. You want the starting materials to stay dissolved and the product to come out yes? then choose a solvent system that solubilizes the reactants more than the product. To prevent crystallising out the starting materials, keep cooling slow until crystals start to appear, then slow cooling even more to grow the crystals nice and big.