Method to remove impurities from acetaminophen synthesis experiment

Question

I'd like to know about impurities from acetaminophen synthesis experiment. Are there any impurities produced by oxidation of p-aminophenol? If there is any impurity produced by oxidation of p-aminophenol, I would like to know the formula and impurity removal method. Also, it is said that acetic acid and water wash away when they react, and I want to know more about this.

Answer 1

After the synthesis, your crude solid acetaminophen contains dark impurities, which have been formed from oxidation of the starting material, 4-aminophenol that carried along during the synthesis (if you have used 4-aminophenol as it is without purification). These impurities are intensely colored dyes of unknown structures with highly conjugated chromophore, hence the color. Although the amount of the dye impurities is very small, their intense color is bright enough to impact the color of the crude acetaminophen, which supposed to be colorless. Most of these colored impurities is destroyed by heating the crude product with sodium dithionite $\ce{(Na2S2O3)}$, a reducing agent. The heating with an aqueous solution of dithionite reduces double bonds in the colored dye to produce colorless substances. Thus, decolorized acetaminophen can be collected on a Büchner funnel by filtration. Following is an example for micro-scale decolorization procedure:

Dissolve $\pu{0.2 g}$ of sodium dithionite $\ce{(Na2S2O3)}$ in $\pu{1.5 mL}$ of water in a $5$-$\pu{mL}$ conical vial. Add your crude acetaminophen ($\approx \pu{200 g}$) to the vial. Heat the mixture at about $\pu{100 ^\circ C}$ for $\pu{15 min}$, with occasional stirring with a micro-spatula. Some of the acetaminophen will dissolve during the decolorization process: Cool the mixture thoroughly in an ice bath for about $\pu{10 min}$ to reprecipitate the decolorized acetaminophen (scratch the inside of the vial, if necessary to induce crystallization). Collect the purified material by vacuum filtration on a Hirsch funnel using small portions (about $\pu{0.5 mL}$ total) of ice water to aid the transfer. Dry the crystals for $5$-$\pu{10 min}$ by allowing air to be drawn through them while they remain on the Hirsch funnel. Weigh the purified acetaminophen and compare the color of the purified material to that obtained above.

It is better if you further purified your decolorized materials using a microscale crystallization technique.

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Note: Usually, your limiting reagent in this experiment is 4-aminophemol. The excess acetic anhydride would decompose to acetic acid during the reaction. Acetic acid is also a byproduct of the reaction. The product, acetaminophen (N-(2-hydroxyphenyl) acetamide) is insoluble in cold water and would precipitate. Thus, your first filtration and washing would remove all soluble byproducts including acetic acid. The crude product would be colored if there are impurities present in the initial 4-aninophenol starting material.