SYNTHESIS OF THE PHARMACEUTICAL ASPIRIN (ACETYLSALICYLIC ACID)
Aim: The aim of this experiment was to react salicylic acid with ethanoic anhydride to produce Aspirin.
CH3CO)2O + HOC6H4COOH → CH3CO2C6H4CO2H + CH3COOH i Mass: 5g Volume: 7.5 cm3 Mass: 6.52g
RMM: 138g/mol RMM: 180g/mol
Moles: 5/138 = 0.0362 RMM: 102g/mol Moles: 0.0362
OBSERVATIONS –
First, Ethanoic acid was added to the salicylic acid which resulted in a cloudy solution. When adding a few drops of concentrated sulfuric acid, which acted as a catalyst, effervescence transpired and some of the salicylic acid dissolved. Thereafter, the solution was swirled causing the solution to turn cloudy-white. It was then heated on a steam bath for a period of 15 minutes after which the solution had become clear and the solid had fully dissolved. The solution was left to cool until it reached room temperature during which crystallization occurred then 75 cm3 of distilled water was added. This caused a solid product to precipitate out which was then collected by vacuum filtration using a Buchner funnel. The solid was the crude product which released a noticeable smell of vinegar.
CALCULATION OF THE CRUDE YIELD –
Mass of the Crude (un-crystallized): 8.40g
Ratio of Salicylic acid: Ethanoic anhydride: Aspirin – 1:1:1 Hence, number of moles of Salicylic acid = number of moles of Aspirin
Number of moles = mass / relative molecular mass
= 5g / 138 = 0.0362 mole; based on the ratio, this is the same number of moles for aspirin
Mass of the theoretical yield (Aspirin): 0.036*180 = 6.52g
Percentage yield = (Actual yield / Theoretical yield)*100%
Percentage Yield: 8.40/6.52*100% = 128.8% → 129%
CALCULATION OF THE PURIFIED YIELD – give the mass and percentage yield of the purified (re-crystallised product), showing all workings out of equations.
Mass of watch glass: 60.61g
Mass of watch glass including product: 66.15g
Hence, mass of product: 66.15 – 60.61 = 5.54g
Mass of purified (re-crystallized product): 5.54g
Percentage Yield: 5.54/6.52*100 = 84.9% → 85%
MELTING POINT RANGE OF THE STARTING SALICYLIC ACID –
Due to lack of time, the melting point of salicylic acid could not be obtained.
LITERATURE VALUE –
158-161 °Cii
MELTING POINT RANGE OF THE RECRYSTALISED PRODUCT –
The melting point obtained of the recrystallized product was 132 °C.
LITERATURE VALUE –
The researched melting point of Aspirin was found to be 135°C.iii
COMMENT ON THE SUCCESS OF THIS REACTION –
The reaction was successful and pure white crystals of Aspirin were formed. However, the yield obtained of recrystallized product was 85% which means that 15% yield was lost; this could be due to many factors. The reactants may not have fully reacted forming less crystals and thereby, decreasing the mass. Minor losses may have occurred when transferring materials and during filtration which had an additive effect. Also, when washing out the solution into a vacuum filter, it is possible my partner failed to include all the solid by washing the spatula and flask out incorrectly. Errors also may have occurred due to inaccurate measurements when weighing the salicylic acid. On the other hand, the melting point of our aspirin product deviated from the literature value by a decrease of 3 degrees. The value for our melting point was found to be 132oC. Reasons for a lower value of melting point can be due to the presence of impurities in the product. Salicylic acid has a melting point range of 158-161oC which is significantly higher than pure aspirin which melts at 135oC. The remnants of Salicylic acid in my aspirin resulted in a decreased melting pointiv.
References:
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