The Melting Point Analysis of Crude and Recrystallized Acetaminop...

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The Melting Point Analysis of Crude and Recrystallized Acetaminophen Synthesized from P-Aminophenol and Acetic Anhydride

By | Jan. 2013
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Abstract
Acetic Anhydride and p-Aminophenol were heated in a vial attached to an air condenser to synthesize crude acetaminophen, resulting in 0.097 grams (47.48% yield). The crude acetaminophen was then recrystallized in a solvent of water and methanol over heat resulting in 0.082 grams (39.61% yield) of pure acetaminophen. Melting points of both crude and pure acetaminophen were taken, and found to be 165.9 - 170.9°C and 168.2 - 171.5°C, respectively. The literature melting point of acetaminophen is 169.5 – 171.0°C, indicating that our final product was pure.

Introduction

The synthesis of acetaminophen involves the attraction of the electrophilic carbonyl group of acetic anhydride to the nucleophilic NH2 of the p-aminophenol. This occurs because the NH2 group is a better nucleophile than the OH group attached on the opposite side of the p-aminophenol. A new nitrogen-carbon bond is formed, producing acetaminophen with acetic acid as a byproduct. During the synthesis of acetaminophen, it is necessary to dissolve all solid material, and heat the reaction at a high enough temperature and for a long enough time period to ensure completion. Cooling the mixture in an ice bath ensures that all crystals have formed, and drying removes any remaining solvent.

The synthesis of acetaminophen does not result in a pure product, so recrystallization is necessary to purify the substance. During recrystallization, it is important to dissolve all of the solid in order to remove all the impurities. Acetaminophen crystallizes slowly, so cooling the mixture adequately is necessary. Leaving the mixture in an ice bath for ten minutes ensures that the process is complete. Drying the crystals removes any remaining solvent. Once recrystallization has occured, comparison by melting point confirms the purification. The recrystallized product should have a melting point close to the literature value, with a narrow range, whereas the impure crystals...