Synthesis of Aspirin and Determination of Melting Point

Synthesis of Aspirin and Determination of Melting Point

Belardo, Pia Jobelle J.

Department of Chemical Engineering, Faculty of Engineering

University of Santo Tomas

España, Manila

Abstract

Aspirin was prepared from the reaction of salicylic acid and acetic anhydride. Phosphoric acid was used as a catalyst. Upon addition of cold water, acetic acid was formed and thus eliminated. Other impurities like salicylic acid were removed upon the process of re-crystallization. Pure aspirin was obtained after filtering out the impurities and excess reagent through filter paper. A method to check a solid compound's purity after re-crystallization is to check its melting point. Impurities will always lower the melting point of a sample. Aspirin has a theoretical melting range of 134-136°C. The crude product had a melting range of 118°C-121°C, and the final product one of 133°C-136°C.

Introduction

Organic synthesis is the process where a desired organic compound is constructed or prepared from commercially available materials. The objective of organic synthesis is to design the simplest synthetic routes to a molecule.

Since ancient times, the bark and leaves of willow trees have been used as a pain killer. The active component, salicylic acid (SA), can, however, cause stomach upset because its acidity (pKa= 2.97) can be higher than the pH of the human stomach (pH ~ 4 after digestion is complete).

Salicylic acid is a diprotic organic acid with two acidic functional groups: a carboxylic acid and a phenol. (Acidic hydrogen atoms are blue.) In comparison, the monoprotic acetylsalicylic acid (ASA, aspirin) is less acidic (pKa= 4.57). The reason: an ester has replaced the acidic phenol in ASA.

The product’s purity can be analyzed using the melting point. Melting point is a physical property inherent to a substance’s identity. Point is actually a misnomer, substances actually melt over a range. The purer the substance, the smaller the melting range. The...