Preparation and Recrystallisation of Aspirin

Topics: Acetic anhydride, Acetic acid, Aspirin Pages: 7 (1808 words) Published: December 13, 2012
Preparation and Recrystallisation of Aspirin

Contents Page
1.1 Objective1
1.2 Background1
3.1 Materials2
3.2 Preparation 2
3.3 Recrystallisation 3
3.4 Determination of Melting Point3
4Results and Calculation4
4.1 Mass4
4.2 Percent yield4
4.3 Melting Point4
4.4 Appearance4
6Conclusion 5


This report is based on preparation and recrystallization of aspirin. The objective of the experiment is to conduct the synthesis of aspirin and reinforce the skills of recrystallization and technique of melting point determination. Both experimental successes and errors were analyzed. The mass of recrystallized aspirin recorded was 0.85g, which led to a low percent yield of 27.1%. The melting point was 135.5 – 138.3 ˚C, but had a quite a wide range which could be due to impurities present. The aspirin had an appearance of a white, needle-like crystalline solid. If time permitted, the experiment could be repeated to improve the results, by reducing the errors which was analyzed. Overall, the experiment can be deemed as quite successful.

1.1 Objective
The objective of this experiment is to learn to conduct the synthesis of aspirin, reinforce the skills of recrystallization and determination of melting point.
1.2 Background
The active ingredient in aspirin, acetylsalicylic acid, is a synthetic derivative of a compound, salicin found commonly in willow trees. Ancient Greeks had noted the medicinal effect of willow tree extracts, yet its active ingredient salicylic acid was only isolated until the 1800s. However, it was strongly acidic making it hard to swallow and caused irritation to the lining of the mouth and stomach. Only later in 1897, the process to synthesize an ester of salicylic acid, acetylsalicylic acid was discovered. Thought it was not as strong as salicylic acid, they had the same medicinal properties. Aspirin is among the most effective, fairly non-toxic, pain killers, commonly used to treat mild to moderate pain, to reduce fever or inflammation, and sometimes used to treat or prevent heart attacks, strokes and angina. It reduces substances in the body that causes pain, fever and inflammation. It also reduces blot clots and small doses can be used to prevent heart attack and stroke. However, some people might get irritation to the stomach membranes or lose blood in the stomach lining from the consumption of aspirin.

Aspirin can be made from the reaction between salicylic acid and several different acetylating agents. In this experiment, acetic anhydride was used as it is cheap, and forms a non-corrosive by-product, acetic acid which can be reused to make more acetic anhydride.

Figure 1: Reaction between salicylic acid and acetic anhydride to produce aspirin (Cassandra Tan, 2012)

Recrystallization was used to obtain a pure compound of acetylsalicylic acid since the ester is in a solid state and impurities may remain in the mixture. It is important to recrystallize as the impurities will stay in the solvent, producing the purest final product which will be safe for human consumption. A mixture of ethanol and water is a suitable solvent for this experiment. Concentrated sulfuric acid was added as a catalyst to speed up the reaction. The theoretical number of moles of aspirin is calculated based on the molecular equation. Salicylic acid is used as a limiting reactant hence giving a mole ratio of 1:1of salicylic acid to acetylsalicylic acid produced. Therefore, the theoretical mass of acetylsalicylic acid is equivalent to the mass of salicylic acid used. Percent yield can be calculated by dividing the actual mass of aspirin over the theoretical mass and multiplying it by 100%.

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