Recrystallization is a technique used to purify organic solids. This method involves dissolving of a solute in a solvent and inciting the solute to produce a precipitate from a solution. In this experiment, acetic anhydride was added to the mixture of 2mL aniline and 20mL of distilled water. The mixture was cooled in an ice bath and filtered through filter paper resulting to the crude acetanilide. The pure acetanilide was then produced by the filtered solution of crude acetanilide and recrystallizing solvent. Choosing of the recrystallizing solvent was done by placing a small amount of pure acetanilide in three test tubes, each containing: water, 95% ethanol and hexane. The final weight produced by the pure acetanilide is 1.1g contrary to the weight of the crude of the acetanilide which is 0.5g. Quantitative analysis showed that the pure acetanilide produced 37.04% percentage yield.
Recrystallization is a process of purifying organic compounds obtained from nature that contain soluble, insoluble and colored impurities. This is based on the premise that solubility increases on the increase of temperature while solubility decreases on the decrease of temperature. This is the process behind the formation of crystals. Recrystallization can be broken down into several steps. A suitable solvent is chosen followed by the compound being dissolved in a minimum solvent. Insoluble impurities are removed and crystallization follows upon cooling. The crystals are collected and dried.
Choosing the best recrystallizing solvent would lead to the solubility of the impure substances in the compound. The boiling point of the recrystallizing solvent should be lower than the compound to be recrystallized. There are several criteria in determining the best recrystallizing solvent. The compound being purified must be insoluble in the solvent at room temperature. It should be insoluble in the solvent at high temperature. Upon low temperature, it should be soluble.
Acetanilide can be obtained by acetylation of aniline undergone by nitration at low temperature. For acetylation of anilines, acetic anhydride is commonly used. Also called acetic acid anilide, acetanilide is an odorless, white crystalline powder which is soluble in hot water, alcohol, ether, chloroform, acetone, glycerol and benzene. It has a melting point of 114 °C and boiling point of 314°C. It is a combustible solid. When digested, it is moderately toxic. Exposure to body tissues would lead to irritation. For the production of rubber accelerators, dyes and camphor, acetanilide is used as an intermediate. It also used in penicillin synthesis and other pharmaceuticals like painkillers and intermediates. It is incompatible with strong alkalis and oxidizers but remains stable under normal conditions. Also, it is an amide.
This experiment aims to purify crude acetanilide water, the best recrystallizing solvent, which is determined by the solubility of the solid compound, to produce pure acetanilide and to calculate the percentage yield of the weight of the pure acetanilide. Also, it aims to prove the process of recrystallization in purifying compounds.
Materials and Methods
The materials used were test tubes, Erlenmeyer flask, Bunsen burner, tripod, wire gauze, beaker, filter paper, aniline compound, hexane compound, acetic anhydride and methanol.
The experiment began on the choosing of the recrystallizing solvent. Three test tubes that individually contained 1mL water, 95% ethanol and hexane were added a corn grain amount of pure acetanilide. Water was the chosen recrystallizing solvent. In an Erlenmeyer flask, 2mL of aniline and 20mL of distilled water were mixed to be followed by the slow addition of 3mL acetic anhydride. This produced the crude acetanilide. The crude acetanilide was weighed. 20mL of recrystallizing solvent was added to the crude acetanilide. The solution was heated in the water bath until the solid dissolves. Activated charcoal is added to produce a colorless solution. While still hot, the solution was filtered through the filter paper. The crystals produced are collected, washed with distilled water and dried between filter papers. The resulting pure acetanilide is weighed
Results and Discussion
Choosing the recrystallizing solvent
3 test tubes containing 1mL water, 95% ethanol and hexane were subjected to different temperatures for 1-5 minutes in order to test the solubility of the pure acetanilide. In room temperature, water and hexane were insoluble; whereas, 95% ethanol was soluble. In high temperature or during heating, water and 95% ethanol are soluble while hexane was insoluble. In low temperature of upon cooling, water and 95% ethanol are both insoluble while hexane remained insoluble.
Table 1 shows the result of the solubility of pure acetanilide in different temperatures provided.
Table 1: Summary of the Solubility of Pure Acetanilide in Different Solvents of Varying Temperatures
Solvent| At room temperature| During heating| Upon cooling| Water| Insoluble| Soluble | Insoluble|
95% ethanol| Soluble| Soluble| Soluble|
Hexane| Inoluble| Insoluble| Insoluble|
In order to attain the best recrystallizing solvent, this table explains that the compound should be very soluble in high temperatures and insoluble in room temperature. The difference in hot and cold temperatures is essential for the process of recrystallization. It would not dissolve if at high temperatures the compound in the chosen solvent is insoluble. It would not crystallize in pure form if the compound in the solvent is soluble. The desired compound may be lost during recrystallization that is why the solvent should not react with the compound being purified. Unwanted impurities should be either very soluble in room temperature of insoluble in hot temperature. After the solution cools, the desired compound crystallizes and the remaining impurities will remain dissolved. After the compound has crystallized, the solvent should be volatile enough to be removed from the solvent after the compound. Easy and rapid drying of the solid compound after it has been isolated follows.
Table 2 shows the weight of the crude acetanilide and the pure acetanilide.
Table 2: Summary of the Weight of Crude and Pure Acetanilide
Acetanilide| Weight (g)|
Crude acetanilide was formed from the synthesis of acetanilide. Pure acetanilide was collected after recrystallizing the crude acetanilide.
Quantitative Analysis of Acetanilide thru Determination of Percentage Yield The percentage recovery of pure acetanilide was computed using the formula:
%Yield=Actual YieldTheoretical Yield x 100
Substituting the values:
Therefore, the percentage yield of pure acetanilide is 37.04%.
Percentage yield is used in cases where chemical transformation occurs. In calculating the percentage yield, the following is needed:
1. The molar ratio of product to starting material
2. The molecular weights of product and starting material 3. Limiting Reagent
Determination of Theoretical Yield
2mL Aniline (C6H5NH2) x 1.0271 mole x 1 mole93.13g x 102.09 g1 mole = 2.24g
3mL Acetic Anhydride (C4H6O3) x 1.0821 mole x 1 mole102.09g x 93.13g1 mole = 2.97g
The limiting reagent is Aniline since 2.97g was needed to make 3 milliliters of Acetic Anhydride.
From The Internet:
(1) http://www.chem.umass.edu/~samal/269/cryst1.pdf. Recrystallization. Electronic References (2) http://www.chemistry.sc.chula.ac.th/bsac/Org%20Chem%20Lab_2012/Exp.2.pdf. Purification by Recrystallization (3) http://www.chemicalbook.com/ChemicalProductProperty_EN_CB9444812.htm. Chemical Book (4) http://www.epcc.edu/OfficeofRiskManagement/Documents/EPCC%20MSDS/Acetanilide.pdf. Flinn Scientific Inc. (2001) (5) http://sites.psu.edu/lburns/files/2013/04/ch.6-formal-report.pdf. Bortiatynski, Jackie, M’Mechan, J.C.