P-NITRO ACETANILIDE FROM ACETANILIDE Aim: To prepare p-nitro acetanilide from acetanilide. Principle: The aromatic nitration of acetanilide is an exothermic reaction; the temperature must be carefully controlled by chilling, stirring, and the slow addition of reagents. Acetanilide is first dissolved in the solvent, glacial acetic acid. Glacial acetic acid is used because it is a polar solvent capable of dissolving acetanilide and the acetate ion is a poor nucleophile so no substitution is possible. After the solution is cooled, sulfuric acid is added. Both the acetanilide solution and the nitrating solution (a mixture of HNO 3, and H2SO4) must be chilled c hilled to about 10°C before the reaction re action is begun.
In this step the nitronium ion form in situ by dehydration of nitric acid. Sulfuric acid is the dehydrating agent. The nitronium ion is a very powerful electrophile and will react with the π−electrons of the aromatic ring of aniline. These reactions are shown below.
To prevent dinitration of the acetanilide, the nitrating mixture is added in small portions to the acetanilide solution (and not vice versa) so that the concentration of HNO 3 is kept at a minimum. After all the HNO 3, H2SO4 solution has been added, the reaction mixture is allowed to warm slowly to room temperature. If the reaction mixture has been kept excessively
cold during the addition, there will be a relatively large amount of unreacted HNO 3, present, which may cause the temperature to rise above room temperature. If this should happen, the mixture must be rechilled. The work-up procedure consists of removal of the acids and crystallization of the product. Every trace of acid must be removed because hydrogen ions catalyze the hydrolysis of the amide to p-nitroaniline or its protonated cation. Most of the acid is removed by pouring the reaction mixture onto ice and water, then filtering the flocculent yellow precipitate of p-nitroacetanilide. If all of the acid was removed, the product will be light yellow. A deep yellow to yellow-orange product is indicative of the presence of P-nitroaniline from hydrolysis. Unfortunately, p-nitroaniline is difficult to remove from p-nitroacetanilide by crystallization.
Apparatus: Beakers, Ice-salt bath, measuring cylinder, funnel, tripod stand, filter papers.
Reagents: Acetanilide, Concentrated H 2SO4, Concentrated HNO 3, Glacial acetic acid
Procedure: In a 250 ml beaker take 5 gm of finely powdered acetanilide and add 5 ml glacial acetic acid. Stir well until its get dissolve. Now add 10 ml concentrated sulphuric acid and stir it. Solution becomes warm but clear. Now place it in ice cold water bath. Prepare nitrating mixture in another beaker or test tube by taking 2.5 ml of conc. HNO &1.5 ml Conc. H 2SO4. Cool the mixture.
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Add the cold nitrating mixture into acetanilide solution drop wise with constant stirring; maintain the temperature of mixture below 10°c. After the complete addition of mixture, remove the beaker from freezing mixture. Keep it at room temperature for half an hour. Pour the reaction mixture on ice cold water bath with stirring. Cure p-nitro acetanilide gets precipitated. Filter it with gravity filtration. Wash the beaker with two 30-mL portions of cold water. Finally, wash the filter cake with an additional 50 mL of cold water. Press the filter cake with a spatula or clean cork to remove as much water as possible. Determine the yield.
Observation table: Mass of Product
Melting Point
Theoretical Yield (grams)
Result: Report the result as mass of acetanilide synthesized.
Precautions: A mixture of concentrated nitric and sulfuric acids is used as the nitrating mixture. Use extreme caution when preparing and using this mixture. Nitro compounds are toxic and can be absorbed through the skin. You may wish to wear disposable plastic gloves during portions of this experiment.
