Organic Chemistry Experiment 4 - Recrystallization

Recrystallization is the primary method for purifying solid organic compounds. Compounds obtained from natural sources or from reaction mixtures almost always contain impurities. The impurities may include some combination of insoluble, soluble, and colored impurities. To obtain a pure compound, these impurities must be removed. Each is removed in a separate step in the recrystallization procedure. [8]

Acetylation of aniline by acetic anhydride was performed to synthesize the crude acetanilide. The obtained crude acetanilide contained acetic acid as well as unreacted acetic anhydride.

The said impurities of the crude acetanilide were removed using activated charcoal, filtration and recrystallization.

A. Compounds tested
Aniline is a clear to slightly yellow liquid with a characteristic odor. It does not readily evaporate at room temperature. Aniline is slightly soluble
in water and mixes readily with most organic solvents. Aniline is used to make a wide variety of products such as polyurethane foam, agricultural chemicals, synthetic dyes, antioxidants, stabilizers for the rubber industry, herbicides, varnishes and explosives.

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[2] Aniline is prepared commercially by the catalytic hydrogenation of nitrobenzene or by the action of ammonia on chlorobenzene. The reduction of nitrobenzene can also be carried out with iron borings in aqueous acid. [6] Preparation of Aniline:

C6H5NO2 + 3 H2 → C6H5NH2 + 2 H2O

Figure 1. (a) Hydrogenation of nitrobenzene (b) Bechamp reduction

Acetic anhydride
Acetic anhydride is clear, colorless liquid with a strong, pungent, sour vinegar-like odor, lachrymator. [5] Acetic anhydride is an important solvent and acetylation agent. [4] It is used in the manufacture of acetyl compound, cellulose acetates, acetylizer and solvent in examining wool fat, glycerol, fatty and volatile oils, resins, detection of rosin, in organic synthesis, such as dehydrating agent in nitrations, sulfonations and other reactions where removal of water is necessary.

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[5] Acetic anhydride is prepared by the carbonylation of methyl acetate. There is a two-stage process for the preparation of acetic anhydride, in which, in a first step, methyl bromide or, preferably, iodide is carbonylated to provide the corresponding acetyl halide, such acetyl halide in turn being reacted with methyl acetate, in a second step, to provide acetic anhydride, which corresponds to the following reaction scheme, in the event that methyl iodide is the starting material: [7] Step 1:

Step 2:

Figure 2. Carbonylation of methyl acetate

B. Procedure
1. Choosing the Recrystallizing Solvent
A corn-grain amount of pure acetanilide was placed into each of three test tubes. Methanol, hexane, and water were added to each test tube respectively. Each was shaken and was placed in a warm bath (37oC-40oC) for 1-5 minutes and then was cooled.

Figure 3. Pure acetanilide in test tubes containing different solvents – methanol, hexane, water respectively.

2. Acetylation of Aniline by Acetic Anhydride

Aniline solution was made by mixing 2mL of aniline with 20mL of distilled water. 3mL of acetic anhydride was then added to the solution to form acetylation. The over-all solution was then placed in an ice bath to form crystals of crude acetanilide.

Figure 4. 2mL of aniline mixed with 20mL of distilled water

Figure 5. 3mL of acetic anhydride added to the aniline solution

Figure 6. Crystallization of crude acetanilide in an ice bath 3. Purification of Crude Acetanilide by Recrystallization

The crude acetanilide crystals were filtered through a wet filter paper. The residue was then dried and weighed. In a separate Erlenmeyer flask, the crude acetanilide residue was placed and 20mL of the recrystallizing solvent was added. The solution was then heated on a hot plate until the entire solid dissolved completely. When the solution became colored, it was removed from the heat and enough amount of activated charcoal was added. The heating process continued until the solution became colorless. While the solution was still hot, it was quickly filtered using a fluted filter paper. Its filtrate was then cooled by placing the receiver in a beaker with cold water. The crystals that formed were collected and were washed. When the
crystals completely dried up, it was then weighed.

Figure 7. Filtration of the crude acetanilide crystals

Figure 8. Crude acetanilide residue mixed with the recrystallizing solvent

Figure 9. Heating process of the solution

Figure 10. Addition of the activated charcoal to the solution

Figure 11. Continued heating process of the solution until colorless

Figure 12. Filtration and recrystallization of the acetanilide

Figure 13. Weighing of the dried acetanilide crystals

1. Choosing the Recrystallizing Solvent
Among the three recrystallizing solvents – methanol, hexane, and water, water was chosen to be used in the recrystallization of the acetanilide. Table 1. Solubility of pure acetanilide in various solvents

| At room temp.| During heating| Upon cooling|
Water| Insoluble| Soluble| Insoluble|
Methanol| Soluble| Soluble| Soluble|
Hexane| Insoluble| Insoluble| Insoluble|
2. Acetylation of Aniline by Acetic Anhydride
The crude acetanilide was obtained by the acetylation of the aniline. The crude acetanilide obtained was 7.2 grams in its weight. 3. Purification of Crude Acetanilide by Recrystallization

Pure acetanilide was obtained by the recrystallization of the crude acetanilide. From the 7.2 grams of crude acetanilide, 1.2 grams of pure acetanilide was gathered.

Theoretical Yield:

2mL x 1.0217 g/mL = 2.0434 g

2.04 g x 1 mol x 135.17 g =
93.13 g 1 mol

2.96 g Acetanilide

Acetic Anhydride
3mL x 1.082 g/mL = 3.246 g

3.246 g x 1 mol x 135.17 g =
102.09 g 1 mol

4.30 g Acetanilide

∴ Aniline is the limiting reagent.

Percentage Yield:

= Actual Yield x 100
Theoretical Yield

= 1.20 grams x 100
2.96 grams

= 40.54 %

The experiment conducted was able to perform different chemical processes. In choosing the recrystallizing solvent, solubility test was conducted among various solvents – methanol, hexane, and water. The solubility test resulted to the conclusion of choosing water as the recrystallizing solvent. The experiment also included the acetylation of aniline by acetic anhydride to
form the crude acetanilide. In the experiment, 7.2 grams of crude acetanilide was obtained. Since the crude acetanilide has impurities, it was purified by several processes – heating of solution, activated charcoal addition, and recrystallization. When recrystallization has already occurred in the solution, the pure acetanilide was then obtained. 40.54% of the pure acetanilide was gathered with its weight of 1.2 grams.

[1] Acetylation of Aniline using Acetic Anhydride. [2] Agency for Toxic Substances & Disease Registry: Toxic Substances Portal – Aniline. [3] Bayquen, A. V., Cruz, C. T., de Guia, R. M., Lampa, F. F., Peńa, G. T., Sarile, A. S., & Torres, P. C. (2009). Laboratory Manual in Organic Chemistry. 839 EDSA, South Triangle, Quezon City: C & E Publishing, Inc. [4] Chemical Book: Acetic Anhydride. [5] Chemical Book: Acetic anhydride (108-24-7). [6] Encyclopædia Britannica: Aniline. [7] Gauthier-Lafaye et al. Carbonylation of methyl acetate. [8] Recrystallization.

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Organic Chemistry Experiment 4 - Recrystallization. (2016, Apr 01). Retrieved from

Organic Chemistry Experiment 4 - Recrystallization

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