Synthesis 5: Synthesis of the Dye Para Red from Aniline

[Mohrig, J.R.; Morrill, T.C.; Hammond, C.N.; Neckers, D.C. Experimental Organic Chemistry; Freeman: New York, NY, 1997; pp 456-67]

Introduction A lot of early organic chemistry was devoted to synthesizing dyes. This synthesis produces the brilliant red dye called Para Red. Most of the mechanisms in this synthesis are known to you (you may want to read about diazotization reactions on pp 971-921 in McMurry).


Acetanilide: Measure 5.0 g of aniline into a 125-mL Erlenmeyer flask. Add 40 mL of water to the flask. Stir the contents while adding 6.5 mL of acetic anhydride. After the addition, stir for an additional 10 min and then vacuum filter the product. Spread the solid out on a large piece of filter paper in a petrie dish to dry quickly. Save a little for characterization and use the rest in the next step.

p-Nitroacetanilide: Add 3.0 g of acetanilide to 5 mL glacial acetic acid in a 25-mL Erlenmeyer flask. Carefully pour 5 mL concentrated sulfuric acid into the mixture while stirring constantly. After the acetanilide has dissolved, place the flask in an ice-salt bath and cool it to 5 ºC. Prepare a solution of 2 mL concentrated nitric acid and 1.3 mL concentrated sulfuric acid. While stirring the acetanilide solution, add dropwise the nitric/sulfuric mixture at a rate such that the temperature does not rise over 20 ºC. After the addition, let the mixture stand at room temperature for 20 min. Pour the reaction mixture into 25 mL of cold water with 15 g of ice. Collect the solid that forms by vacuum filtration and wash the solid with three 30-mL portions of cold water. Recrystallize the solid from 95% ethanol and characterize it.

p-Nitroaniline: Add 2.2 g of p-nitroacetanilide and 5 mL of water to a 25 mL round bottom flask. Slowly add 5 mL concentrated sulfuric acid while swirling. Reflux the mixture for 30 min. Cool the reaction mixture and pour the contents into a mixture of 15 g ice in 20 mL water. Slowly with stirring add 9 M sodium hydroxide until the pH of the mixture reaches 4-5. Cool the mixture in an ice bath and collect the precipitate by vacuum filtration. Wash the solid four times with 20 mL portions of water. Characterize the p-nitroaniline.

Diazonium salt: Place a solution of 1 mL of concentrated sulfuric acid in 10 mL of water in a 50 mL Erlenmeyer flask. Stir the solution while adding 1.0 g of p-nitroaniline. Cool the mixture to 5 ºC in an ice bath. While stirring the mixture, slowly pour in a solution of 0.5 g sodium nitrite in 2 mL of water. Keep the temperature under 10 ºC.

Para Red: Prepare a solution of 1.0 g of 2-naphthol in 10 mL of 2.5 M sodium hydroxide. Cool this solution to 10 ºC and pour it into the flask that has your diazonium salt in it with stirring. Continue stirring the mixture vigorously for a few minutes and then acidify the mixture with 1 M sulfuric acid. A red precipitate forms which can be vacuum filtered. Wash the product with water and characterize it.