Chemistry Laboratory Report: Acetanilide Recrystallization

Abstract

The primary objective of this experiment was to purify crude acetanilide through recrystallization. Crude acetanilide was synthesized through the acetylation of aniline by acetic anhydride. To achieve a pure compound, the impurities in crude acetanilide, including acetic acid and unreacted acetic anhydride, were removed using activated charcoal, filtration, and recrystallization. The choice of recrystallizing solvent was determined by conducting solubility tests with methanol, hexane, and water. Water was selected as the recrystallizing solvent due to its suitable solubility characteristics. The experiment yielded 1.

2 grams of pure acetanilide, resulting in a 40.54% yield.

Introduction

Recrystallization is a fundamental technique used to purify solid organic compounds. Compounds obtained from natural sources or chemical reactions often contain impurities, which can include insoluble, soluble, and colored impurities. To obtain a pure compound, these impurities must be separated and removed in a stepwise manner during the recrystallization process.

In this experiment, we aimed to synthesize pure acetanilide by first performing the acetylation of aniline using acetic anhydride.

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The resulting crude acetanilide contained impurities in the form of acetic acid and unreacted acetic anhydride. To purify the crude acetanilide, we employed a series of techniques, including activated charcoal treatment, filtration, and recrystallization.

Experimental

A. Compounds tested

Aniline

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.

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.

Preparation of Aniline:

C6H5NO2 + 3 H2 → C6H5NH2 + 2 H2O

Acetic anhydride

Acetic anhydride is clear, colorless liquid with a strong, pungent, sour vinegar-like odor, lachrymator. Acetic anhydride is an important solvent and acetylation agent. 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. 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:

Step 1:

CH3I + CO → CH3COI

Step 2:

CH3COI + CH3COOCH3 → (CH3CO)2O + CH3I

B. Procedure

1. Choosing the Recrystallizing Solvent: A small amount of pure acetanilide was placed in three test tubes, and methanol, hexane, and water were added to separate tubes. After shaking and warming in a bath, the solubility of acetanilide in each solvent was assessed.
2. Acetylation of Aniline by Acetic Anhydride: Aniline solution was prepared by mixing 2mL of aniline with 20mL of distilled water. 3mL of acetic anhydride was added to the solution to initiate acetylation. The solution was then cooled to induce crystallization of crude acetanilide.
3. Purification of Crude Acetanilide by Recrystallization: The crude acetanilide crystals were filtered, dried, and weighed. In a separate flask, the crude acetanilide was dissolved in 20mL of the chosen recrystallizing solvent (water) and heated until complete dissolution. Activated charcoal was added to remove impurities, and the solution was heated until it became colorless. The hot solution was filtered, and the filtrate was cooled to obtain pure acetanilide crystals, which were subsequently washed and dried.

Results and Discussion

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

The solubility tests conducted with various solvents—methanol, hexane, and water—revealed that pure acetanilide was insoluble in hexane and water at room temperature. However, it became soluble when heated and returned to an insoluble state upon cooling. Methanol, on the other hand, exhibited solubility throughout the experiment. As a result, water was chosen as the recrystallizing solvent due to its favorable solubility characteristics.

2. Acetylation of Aniline by Acetic Anhydride

The acetylation of aniline using acetic anhydride resulted in the formation of crude acetanilide. The experiment yielded 7.2 grams of crude acetanilide.

3. Purification of Crude Acetanilide by Recrystallization

Pure acetanilide was obtained through the recrystallization of the crude acetanilide. From the initial 7.2 grams of crude acetanilide, 1.2 grams of pure acetanilide was isolated.

Calculations

Theoretical Yield:

Aniline:
2mL x 1.0217 g/mL = 2.0434 g

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

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

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

Therefore, aniline is the limiting reagent.

Percentage Yield:

Percentage Yield = (Actual Yield / Theoretical Yield) x 100

= (1.20 grams / 2.96 grams) x 100

= 40.54%

The experiment resulted in a 40.54% yield of pure acetanilide.

Conclusion

The experiment successfully demonstrated various chemical processes, including the selection of a suitable recrystallizing solvent (water), the acetylation of aniline using acetic anhydride to produce crude acetanilide, and the purification of crude acetanilide through heating, activated charcoal treatment, and recrystallization. The final yield of pure acetanilide was 1.2 grams, representing a 40.54% yield.

Recommendations

For future experiments, it is advisable to explore alternative recrystallizing solvents to compare their effectiveness in purifying acetanilide. Additionally, further optimization of the purification process could potentially enhance the yield of pure acetanilide.

References

• Acetylation of Aniline using Acetic Anhydride. http://prolabscientific.com/Acetylation-of-Aniline-using-Acetic-Anhydride-p-23917.html.
• Agency for Toxic Substances & Disease Registry: Toxic Substances Portal – Aniline. http://www.atsdr.cdc.gov/toxfaqs/tf.asp?id=449&tid=79.
• 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.
• Chemical Book: Acetic Anhydride. http://www.chemicalbook.com/ChemicalProductProperty_EN_CB2852742.htm.
• Encyclopædia Britannica: Aniline. http://global.britannica.com/EBchecked/topic/25473/aniline.
• Gauthier-Lafaye et al. Carbonylation of methyl acetate. http://www.google.com.ph/patents?hl=en&lr=&vid=USPAT4500474&id=ymY2AAAAEBAJ&oi=fnd&dq=carbonylation+of+methyl+acetate&printsec=abstract#v=onepage&q=carbonylation%20of%20methyl%20acetate&f=false.
• Recrystallization. http://www.chem.umass.edu/~samal/269/cryst1.pdf.