Analysis A fellow student somehow mixed an aspirin sample with paracetamol after

Question

Analysis A fellow student somehow mixed an aspirin sample with paracetamol after synthesising both. Explain how you would separate aspirin from paracetamol using liquid-liquid extraction. pH speciation plots for the two components are shown below 1.0 0.9 0.8 1.0 0.6 0.5 0.4 0.3 0.2 0.1 0.9 0.8 0.7 0.6 a 0.5 0.4 0.3 0.2 0.1 0 1 2345 6 7 89 10 1112 13 14 pH 0 1 23 457 8 10 11 12 13 14 pH a. pH speciation plot for aspirin b. pH speciation plot for paracetamol The structures shown are for the predominant species at pH 1 aspirin paracetamol

Explanation / Answer

From the given structures, it is clear that aspirin is a carboxylic acid (-COOH) while paracetamol is an amide (-CONH2). We can exploit acid-base chemistry to separate the two compounds.

i) The mixture of aspirin and paracetamol is dissolved in ether and extracted with aqueous sodium hydroxide (NaOH).

ii) Aspirin, being a carboxylic acid, will react with NaOH to form the sodium salt of aspirin while paracetamol doesn’t react.

iii) The sodium salt of aspirin is soluble in aqueous NaOH while paracetamol is less soluble. The aqueous layer contains aspirin as the sodium salt while the organic layer contains paracetamol.

iv) The organic layer is evaporated to remove the ether, leaving behind solid paracetamol.

v) The aqueous layer is treated with dilute hydrochloric acid (HCl) when aspirin is precipitated out. The precipitate is filtered, washed with brine and dried with Na2SO4 to get pure aspirin.

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