Department of Medical Technology Faculty of Pharmacy University of Santo Tomas, España, Manila

Organic Chemistry Laboratory
Recrystallization of Acetanilide
Experiment 4
Author: Janina Erika G. Sese
Group 8 – 2C Medical Technology (A.Y. 2015-2016)
Members: Kathleen Danielle Marie A. Robles, Amiel C. Sabangan, Hanz Jefry A. Saliendra, Andrea Betina M. Vega, Anna Denise Z. Yang
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Date Submitted: October 28, 2015
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ABSTRACT
Recrystallization is the simplest and most widely used operation for purifying organic solids that differ in their solubility at different temperatures. In this experiment, three test tubes with water, hexane and, ethanol, respectively, were used to dissolve pure acetanilide. These test tubes were placed in a water bath for one to five minutes, then, were cooled through running water for the selection of the best solvent to use in recrystallization. Crude acetanilide was obtained by mixing two milliliters aniline and 20 milliliters distilled water with three milliliters acetic anhydride, which was cooled with running water. It was then filtered and dried for the procurement of crude acetanilide. This was mixed with 20 milliliters of the chosen solvent and heated through water bath until the solid was dissolved. Next, it was immediately filtered while hot. The filtrate was collected and was cooled with running water leading to the recrystallization of pure acetanilide. This was again filtered with a tared filter paper to obtain pure acetanilide. This pure acetanilide was dried then placed in a capillary tube for the determination of its melting point. The melting point was 115 °C.
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Keywords: Recrystallization, pure and crude acetanilide, filtrate

INTRODUCTION
Recrystallization is the process usually used to purify compounds that are solid at room temperature. It is the process of dissolving the solid to be crystallized in a hot solvent and then cooling the solution slowly. The

dissolved solid has a decreased solubility at lower temperatures and separates from the solution as it is cooled. A small seed crystal is formed initially and then it grows layer by layer. This process depends on the difference in solubility at different temperatures.
Crystallization is a purification method usually used to remove impurities from solids. Recrystallization is named as such because it involves dissolving a solid that, in most cases, had originally been crystallized from a reaction mixture or another solution, then causing it to crystallize again from the solution. During the first crystallization, a crude product will be formed. This product still has impurities that need to be removed. This is the reason for doing crystallization for the 2nd time. The product of recrystallization will have fewer impurities, close to none, than the crude product. The level of purity of the product can be checked through its melting point range. Looking closely at the crystals, they formed a lattice structure. Lattice is a fixed and rigid arrangement of atoms, molecules or ions. Formation of crystals is due to the difference in solubility of the solid with the solvent. Solutes or compounds tend to be more soluble in hot solvents than they are in cold solvents.

Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a solution of the solute in the solvent.

Figure 1: Structural formula of Acetic Anhydride

Acetic Anhydride is a clear, colorless liquid with a very pungent, penetrating, vinegar-like odor that combines with water to form acetic acid. It is soluble in ether, chloroform and benzene. It reacts with alcohols. Acetic anhydride is prepared commercially in either of two ways. Acetaldehyde is converted into acetic anhydride by atmospheric oxidizing the liquid acetaldehyde in the presence of metal acetate as the catalyst (Oxidation process). The ketene (or acetylene) created form hot vapor of acetic acid is converted into acetic anhydride by reacting with acetic acid (Ketene process). The acyl groups (RCO) in organic anhydrides favor wide range of organic synthesis. They react with water to give carboxylic acids, with alcohols or phenols to give esters, and with ammonia and amines to give amides. Acetic anhydride is used in the manufacture of cellulose acetate having the application as a base for magnetic tape and in the manufacture of textile fibers. Also, it is heated with salicylic acid to produce acetylsalicylic acid (aspirin). It is also used in the manufacture of pigments, dyes, cellulose and pesticides etc.

Figure 2: Structural formula of Aniline
Aniline, phenylamine or aminobenzene is a toxic organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Its main use is in the manufacture of precursors to polyurethane and other industrial chemicals. Like most volatile amines, it possesses the odor of rotten fish. It ignites readily, burning with a smoky flame characteristic of aromatic compounds.

Figure 3: Structural formula of Acetanilide

Acetanilide is an odorless, white flake solid or crystalline powder (pure form); soluble in hot water alcohol, ether, chloroform, acetone, glycerol, and benzene; melting point 114 C and boiling point 304 C; can undergo self-ignite at 545 C, but is otherwise stable under most conditions. Acetanilide, which can be obtained by acetylation of aniline, undergoes nitration at low temperature and yields highly the para-nitro products. Acetyl group can then be removed by acid-catalyzed hydrolysis to yield para-nitroaniline. Although the activating affection of the amino group can be reduced, the acetyl derivative remains an ortho/para-orientation and activating substituent. Acetanilide is used as an inhibitor of peroxides and stabilizer for cellulose ester varnishes. It is used as an intermediate for the synthesis of rubber accelerators, dyes and dye intermediate and camphor. It is used as a precursor in penicillin synthesis and other pharmaceuticals including painkillers and intermediates. Phenylacetamide structure shows analgesic and antipyretic effects. But acetanilide is not used directly for this application due to causing methemoglobinemia (the presence of excessive methemoglobin which does not function reversibly as an oxygen carrier in the blood). Acetaminophen (4'-hydroxyacetanilide) is the analogue widely used as a nonprescription drug with analgesic and antipyretic effects similar to aspirin.

Acetanilide can be produced by reacting acetic anhydride with aniline:

C6H5NH2 + (CH3CO)2O → C6H5NHCOCH3 + CH3COOH

In this experiment, the percentage yield of acetanilide can be acquired with the theoretical yield. We can get this when we compute for the limiting reagent in the reaction. The limiting reagent is the reactant that is completely used up in a reaction, and thus determines when the reaction stops. From the reaction stoichiometry, the exact amount of reactant needed to react with another element can be calculated. If the reactants are not mixed in the correct stoichiometric proportions (as indicated by the balanced chemical equation), then one of the reactants will be entirely consumed while another will be left over. The limiting reagent is the one that is totally consumed; it limits the reaction from continuing because there is none left to react with the in-excess reactant. The limiting reagent is divided with the actual yield to get the percentage yield. On the other hand, to get the percentage recovery, the crude acetanilide is divided by the pure acetanilide and multiplied by 100.

The objectives of the experiment are to synthesize acetanilide by the acetylation of aniline and to purify crude acetanilide product by recrystallization.

METHODOLOGY

The samples used in the experiment were pure acetanilide, aniline, acetic anhydride which were used to produce crude acetanilide. Water, hexane and ethanol were also used for the determination of the best solvent for acetanilide. Cooking oil was also used for melting point detemination. Apparatus used were Erlenmeyer flask, test tubes, Bunsen burner, tripod, wire gauze, beaker, filter paper, iron stand, thermometer, iron ring, and capillary tubes.
A corn-grain amount of pure acetanilide was placed on three test tubes containing water, hexane and ethanol, respectively. These test tubes were then shook and placed on a warm water bath at 37°C - 40°C for 1-5 minutes. Next, they were cooled and obsevations were recorded.
Two milliliters of aniline and 20 milliliters of distilled water were mixed in an Erlenmeyer flask with three milliliters of acetic anhydride slowly added to the mixture. This was then cooled in running water to hasten crystallization. The solution was filtered through a filtration setup. The filter paper with the residue was left to dry then was weighed. Crude acetanilide was obtained.
20 milliliters of the chosen recrystallizing solvent, water, was added to the obtained crude acetanilide. This mixture was heated in a water bath until the entire solid was dissolved. This was then immediately filtered while it was hot. The filtrate was cooled with running water to hasten recrystallization. Crystals were again filtered with a tared filter paper and were left to dry, obtaining the pure acetanilide. The pure acetanilide was placed in a capillary tube to determine its melting point.
RESULTS AND DISCUSSIONS
One of the vital steps in recrystallization is choosing the right recrystallizing solvent. A compound usually exhibits one of three general solubility behaviors: (1), the compound has a high solubility in both hot and cold solvent, (2), the compound has a low solubility in both hot and cold solvent, and (3), the compound has a high solubility in hot solvent and a low solubility in cold solvent. Solvents which exhibit the first two behaviors are not useful for recrystallizing a compound. A solvent showing the third behavior, that is, high solubility at high temperatures and low solubility at low temperatures, is one that is suitable for use as a recrystallization solvent.

Table 1: Solubility of Pure Acetanilide in Three Different Solvents Solvent | Room Temperature | During Heating | Upon cooling | Water | Insoluble | Soluble | Insoluble | Ethanol | Soluble | Soluble | Soluble | Hexane | Insoluble | Insoluble | Insoluble | In this case, three solvents were tested namely, water, hexane and ethanol. Water was chosen to be the solvent because it shows high solubility in hot temperature and low solubility in cold temperature which is said to be the the ideal recrystallizing solvent because it shows the desired solubility behavior.
The pure acetanilide was not dissolved when first placed in water but when placed in a warm water bath, it slowly dissolved. On the other hand, when placed in running water the acetanilide recrystallized. Weight of crude acetanilide | 2.27 g | Weight of pure acetanilide | 0.20 g | Percentage yield | 6.57% | Percentage recovery | 8.81% |
Table 2: The actual yield of pure and crude acetanilide and percentage yield and percentage recovery of pure acetanilide
For the next part of the experiment, the weight of crude acetanilide was acquired through the acetylation of aniline and acetic anhydride. The 2.27 grams of crude acetanilide acquired may contain some impurities which could possibly affect the weight of the pure acetanilide which is why it was mixed with water, the chosen recrystallizing solvent, and was put in a hot water bath until all the solid dissolved. It was then filtered while hot. The filtrate acquired was cooled with running water for the recrystallization. This was dried then weighed, acquiring 0.20 grams of pure acetanilide. From this process, we were able to compute the percentage yield and the percentage recovery.
Calculations for percentage recovery:
% recovery = Mass of pureMass of crude × 100%
% recovery = 0.20 g acetanilide2.27 g acetanilide×100% =
8.81%
8.81%

Calculations for the limiting reagent of acetanilide:
2.0 ml of aniline
2.0 ml aniline x1.02g aniline1 ml anilinex1 mol aniline93g anilinex
1 mol acetanilde1 mol anilinex135 g acetanilidemol acetanilide=2.96 g acetanilide
3.0 ml of acetic anhydride
3.0ml acetic anhydride x 1.08 g acetic anhydride1 ml acetic anhydride x 1 mol acetic anhydride102 g acetic anhydride x 1 mol acetanilide1 mole acetic anhydride x135 g acetanilidemol acetanilide=4.17 g acetaniide
Calculations for percentage yield:
% yield = Actual yieldTheoretical yield x 100%
% yield = 0.20g2.96g x 100%= 6.57%
Lastly, to confirm if the acetanilide was pure, the melting point was determined at 115°C.
CONCLUSION
To conclude, recrystallization is an effective way to purify solid substances like acetanilide in this experiment. This was proved through the determination of melting point which was 115°C, very close to the theoretical melting point of acetanilide which is 114.3°C. The percentage recovery of pure acetanilide is 8.81% and its percentage yield is 6.57%.
LIST OF REFERENCES
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