The purpose of this lab was to synthesize and purify aspirin. The theoretical yield was calculated to 3. 766g. The actual yield of pure aspirin was 2. 863g with a yield of 76%. The percent yield indicates that our synthesis was a success but the yield is low and indicates that some of the aspirin was lost during synthesis. Some reasons for loss can result from human error such as loosing product from sticking on the spatula and the Buchner funnel and several weighings. Also, when transferring the crude aspirin into the vacuum filtration, some of the crystals stayed in the flask.
Some steps that would produce a better yield for future experiments is rinsing the flask with cold water to make sure that all product is transferred. Something else that can improve the experiment is allowing more than fifteen minutes for the Acetyl Salicylic acid to sit in the ice bath; this will allow for more product to crystallize out from solution and will allow for a higher percent yield. Another reason for product loss may be due to the temperature of the water that was used to wash the crude and purified aspirin.
Although aspirin is not soluble in water, everything has some solubility in water and in this case aspirin is soluble in higher temperatures and large amounts of water. The water that was used may not have been chilled enough and product was lost. For future experiments the temperature of the water for washings should be taken to verify that it is cold enough to use and a less amount of water should be used. Ferric chloride was used as a colorimetric test to analysis the purity of aspirin. When FeCl3 was placed into the crude aspirin the solution turned yellow with spots of purple.
This indicated that impurities (phenols) were present in the crude aspirin. When we tested the pure aspirin the solution turned yellow. This indicated that our aspirin was pure and did not contain any phenol groups. Since aspirin is an acid, 5% aqueous sodium bicarbonate was used to deprotonate the acetyl salicylic acid which produces a charged compound or an ion. Ions are soluble in water and since acetyl salicylic acid becomes an ion it is able to dissolve in water. The impurities in aspirin are not acidic and will not deprotonate when mixed with a base, they will remain solid.
The impurities are removed from the crude aspirin through gravity filtration; when aspirin is mixed with 5% aqueous sodium bicarbonate the deprotonated acetyl salicylic acid moves through the filter paper and the impurities remain solid and stick to the filter paper. The deprotonated acetyl salicylic acid is then mixed HCl, this protonates the acetyl salicylic acid turning it back into a solid, and through this pure aspirin is obtained.
References 1) D. A Straus Visual Organic Chem Lab Website 2) Merck Index (1984) Merck & Co, NJ 3) Handbook of Chem and Physics, 1972 (Weast, B. C. ed) CRC, Ohio.