Question

Acetaminophen Crystaliztion lab

1. Student A recrystallized acetaminophen but glanced over the part of the procedure where it said to add enough boiling solvent until the solid just dissolves. Instead, excess solvent was used, and a very low yield of purified product resulted. Explain why Student A got a low yield.

2. Where do dark-colored impurities ultimately go when acetaminophen is purified by heating in aqueous sodium dithionite?

3. Why does decolorization not remove all dark-colored impurities?

4. How could you use IR to distinguish p-aminophenol from acetaminophen? If a student tried to fake their results by mixing p-aminophenol with acetone, how would the IR of this mixture differ from authentic acetaminophen?

Answer 1

1. Recrystallisation is a purification technique for solids. It depends on the solubility of the compound in a particular solvent, and with increase in temperature, the solubility increases. The hot solution after it cools becomes supersaturated and the compound crystallises out. In the above case, excess solvent used which does not help in solubility if the compound is not soluble in the particular solvent at room temperature. It is important to use minimum amount of solvent rather than maximum solvent. It does not help in the solubility if the compound is not soluble in the particular solvent at room temperature, rather we can use mimimal solvent under hot condition. In the above case, this would have resulted in very low yield. Further, heating during the reaction and cooling during recrystallisation would have been compromised resulting in low precipitation, which inturn result in very low yield of purified product.

2. This is a decolourisation process for the crude acetaminophen where the dark-coloured impurities are removed by heating in aqueous sodium dithionite and purified. The mixture is heated and allowed to cool in an ice bath. The reprecipitated (decolourised) acetaminophen is filtered throgh buchner funnel by vacuum filtration and collected. The dark-colured impurities of acetaminophen ultimately go into the filtrate during filtration of the decolourised acetaminophen.

3. This may be due the presence of some starting material, p-aminophenol. Further, decolourisation can be done with activated charcoal to remove the colour.

4. Ir stretching frequency for p-aminophenol will be different from acetaminophen. In p-aminophenol, there is a N-H and O-H stretching frequencies whereas in acetaminophen, there is a N-H amide frequency and a O-H frequency.

If the student mixes p-aminophenol with acetone, the IR of this mixture will be different from the authentic acetaminophen. This is because, in the mixture, it shows three stretching frequencies, two for N-H, O-H for p-aminophenol and C-O (ketonic) stretching frequency for acetone whereas in acetaminophen, there is a N-H amide frequency and a O-H frequency. So, we can make out the results in that case whether fake or original.