Introduction
Recrystallization is a process that is used to purify a solid mixture. The preferred product is secluded from its contaminations by the solubility variances. Insoluble impurities and colored impurities are removed from the hot solvent using activated carbon and filtering. The soluble impurities will remain in the cold solvent after recrystallization (Pavia et al., 2005). The required product should be perfectly soluble in the hot solvent and very insoluble in low-temperature solvents. This purification process requires solubility of the impure solid in a heated solution and crystallization upon cooling.
There was an experiment with synthesizing acetanilide through the reaction of aniline and acetic acid. The other objectives were to purify the acetanilide by recrystallization from water and the purity check using the obtained melting point range. The apparatus and chemicals used were; hot plate, beakers, ice, stirring rod, spatula, Buchner funnel, Aniline, Acetic anhydride, filter paper, digital scales, Mel-temp apparatus, large test tube, 10- mL graduated cylinder and a medicine dropper.
Procedure
In a large test tube, using a medicine dropper, ten drops of aniline were placed and the weight determined to the nearest milligram. 5 ml of distilled water was added to the test tube, then 20 drops of acetic anhydride were added (Pavia et al., 2005). Then, the mixture is stirred using a stirring rod for about five minutes until solid formation. After the solution has crystallized, 5 ml of water was added then the test tube was heated in a hot water bath while stirring until the whole solid dissolved. The solution was then left to cool for about 5 minutes and then placed in an ice bath to chill.
After crystallization, the product was collected by vacuum filtration method using a Buchner funnel and a suction flask. The sample is given time to dry completely. The product is weighed, and the percentage yield calculated, and the melting point determined. The percentage yield of acetanilide is the mass of acetanilide recovered, divided by the theoretical mass of acetanilide multiplied by 100.
Data Observations
The solution was stirred, and a precipitate was formed. Water was added to the crystalized product, and after heating the mixture in a hot water bath with occasional stirring, the solid dissolved (Pavia et al., 2005). The solution was then left to chill in an ice bath, and crystals were again observed. After filtration, a flake-like solid was obtained and a clear solution. The solid is then purified acetanilide.
Data Analysis and Calculations
The number of moles is the given mass, divided by the molar mass. Therefore, aniline’s moles are calculated by taking the given mass of aniline, dividing it by the molar mass of aniline. The mass of the beaker was 150.85 g, and the test tube was 10.0g. The total weight of the apparatus and solution was 161.05 g. Hence the mass of aniline was 0.20 grams. The number of moles (0.20/93.023 = 2.15x 10-3).
The theoretical moles of acetanilide is 2.15 x 10-3, and the molar mass is 135.17g/mol. Therefore the theoretical mass of acetanilide is the theoretical mole multiplied by the molar mass (2.15x 10-3 x 135.17 = 0.291g). The analysis of the data for recrystallized acetanilide was as follows. The mass of the recrystallized acetanilide plus weighing paper was 0.54015g. The absolute mass for the recrystallized acetanilide was 0.27025g. Thus, the percentage yield from the experiment equals the mass recovered divided by the theoretical mass. (0.27025 /0.291) x 100 = 92.87%.
The melting point of the pure acetanilide from the experiment was found to be 112 degrees Celsius. (112 0 C). Which is within the literature range of acetanilide between 112-1150C (Pavia et al., 2005). Thus, the acetanilide produced is of extremely pure quality, and the amount of impurities is minimal. Melting points are always used to determine the purity of a product.
Acetanilide is an odourless chemical solid of flake-like form. This chemical can be produced by reacting acetic anhydride with aniline. When aniline reacts with acetic anhydride, another product, acetic acid, is also formed. In this chemical reaction, the acetic anhydride is broken between the center oxygen, and carbon is connected.
A proton breaks off the amine group of aniline. The electronics from this separation form a bond with a secondary carbon in the acetic anhydride. When the bond is broken between the secondary carbon and the center oxygen of the acetic anhydride, the electrons are used to attach the broken off proton to that oxygen. This forms acetanilide and acetic acid. The structural formula is then obtained from the following equation.
C6H7N + (CH3CO)2O → C8H9NO + C2H4O2
Aniline + Acetic anhydride → Acetanilide + Acetic acid.
Conclusion
The amount of acetanilide produced in this experiment from the synthesis of aniline and acetic anhydride was good. Likewise, the quality was higher than expected. The sound quality was obtained due to the care in ensuring all instruments used were thoroughly clean, and each step was done with careful precision. The lost aniline was likely to result from the mixture not being stirred vigorously enough to produce all the crystals that could have been made. In conclusion, purification by recrystallization allows for the purification of an impure product. The high gain in purity makes up the slight loss in the yield. The purity of a product obtained can be correlated to the sharpness of the melting point data observed during the experiment.
Reference
Pavia, D. L., Lampman, G. M., Kriz, G. S., & Engel, R. G. (2005). Introduction to organic laboratory techniques: A small scale approach. Cengage Learning.