Nitration of acetanilide (Lab report)

Introduction

Nitration is a type of chemical reaction which a nitro group is added to/substituted in a molecule. Basically it can be carried out by a mixture of concentrated nitric acid and sulphuric acid. Mixture is useful to obtain the active nitronium ion. Electrophilic aromatic substitution is a method used when a functional group is needed to be substituted on to an aromatic compound. In the nitration, nitronium ion acts as the electrophile that involves the attack of the electron-rich Benzene ring. In this experiment nitration is carried out using acetanilide.

Theory

Acetanilide displays moderately reactivity in electrophilic aromatic substitution. Also another advantage is, it’s not oxidized by nitric acid. Principally, acetanilide gives Ortho and Para mono nitroacetanilides. This position of nitronium ion is directed by the –NHCOCH₃ group attached to the benzene ring. This is due to the resonance delocalizing the benzene ring by nitrogen lone pair. Therefore Ortho and Para positions are more resonance stabilized than the Meta. Acetanilide undergoes ready nitration giving mainly the colourless P-nitroacetanilide, mixed with much smaller proportion of the yellow colour O-nitroacetanilide.

Procedure

·         About 2g of powdered acetanilide and 2mL of Glacial acetic acid were mixed well in a 100mL beaker.  4mL of con.H₂SO₄ was added to the mixture.

·         The beaker was placed in crushed ice until the temperature of the mixture was dropped down to 0-5⁰C.

·         4mL of con.H₂SO₄ was added drop wisely while stirring the viscous mixture continuously keeping the temperature below 10⁰C.

·         Afterwards the beaker was removed from the freezing conditions and allowed to stand 30 minutes at room temperature.

·         The mixture was poured onto about 20g of crushed ice and stirred to obtain crystals. The beaker was rinsed with 10mL of water containing few fragments of ice and the solution was added to the main bulk of the product.

·         It was allowed to stand for about 20minutes.

·         Later it was filtered at the pump and washed thoroughly with cold water.

·         Afterwards the crude product was recrystalized with water.

·         Finally the melting point of the product was determined.

Observations

·         Acetanilide powder was white in colour.

·         Glacial acitic acid, H₂SO₄ and acetanilide mixture was initially in pale yellow colour.

·         In cold condition, white colour crystals were formed.

·         Final purified crystals were also white.

·         The determined melting point range was 150-152⁰C.

Conclusion

The initial compounds used were known. Therefore according to those the final product should be nitroacetanilide. As the colour of the product was white, it should be Para nitroacetanilide.  

Discussion

Through the use of electrophilic aromatic substitution, acetanilide is nitrated to nitroacetanilide. There are several key steps involved in the nitration. The first step of the reaction involved in the donation of an electron pair by the acetanilide to the eletrophile, the nitronium ion. This nitronium ion was formed by the reaction of sulfuric and nitric acids. Basically the whole mechanism undergoes as below; (Benzene is shown here instead of acetanilide )

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To prevent dinitration of the acetanilide, the nitrating mixture of concentratred nitric acid and sulfuric acids were added in small portions to the acetanilide solution, so that the concentration of the nitrating agent is kept at minimum.

Also the addition of nitric acid is exothermic. Therefore the mixture would get too hot exceeding the temperature range suitable for the nitration. To avoid this, the addition of HNO₃ acid should be done very slowly, dropwisely.

Cold temperatures were used to slowdown the reaction rate and help to avoid over nitration.

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.

At the end, traces of acid should be removed because hydrogen ions catalyze the hydrolysis of the amide to p-nitroaniline. Acid is removed by pouring the mixture onto ice and water and filtering.

The melting point is determined to characterize the product. Theoretical melting point of the Para nitroacetanilide is found to be 214-216⁰C. The observed value was 150-152⁰C and it is much lower than the theoretical values and can be accounted for impurities in the product. Some impurities might be Ortho and Meta directing substances. Also there can be some experimental errors occurred during the experiment such as not controlling the exact temperatures mentioned for the reactions.