Preparation of Azo Dyes

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NAME: Maniwang, Ma. Aiza C.DATE: March 21, 2011

COURSE: BS Chemistry IIAPPROVED: ___________



In this experiment, the azo dyes p-nitrobenzene azoresorcinol and methyl orange were prepared by the azo coupling reaction. The p-nitrobenzene azoresorcinol dye was prepared from p-nitroaniline and resorcinol. The diazonium salt formed was from the reaction of the cold solution of dissolved p-nitroaniline in hydrochloric acid and water with the solution of sodium nitrite. Azo coupling was then followed by letting the diazonium react with a solution of resorcinol in water and sodium hydroxide. The resulting mixture was treated with excess hydrochloric acid turning the mixture to a color red. After suction filtration and drying, a red-brown powder was obtained as a final product.

Furthermore, synthesis of methyl orange was prepared from sulfanilic acid and N,N-dimethylaniline. The sodium diazobenzenesulfonate was made by the diazotization of the sulfanilic acid and sodium hydroxide mixture with a solution of sodium nitrite which was then cooled. Coupling reaction occurred when the solution of sodium diazobenzenesulfonate was mixed with a solution of N,N-dimethylaniline in hydrochloric acid. The first product obtained from the coupling was a bright red acid form of methyl orange called helianthin. In basic medium, through the addition of a dilute solution of sodium hydroxide, helianthin is converted to the orange sodium salt known as the methyl orange.


Para-nitrobenzeneazoresorcinol dye and methyl orange belong to a class of dyes known as “azo colors” which contain the azo group linked to two aromatic nuclei. The nature of the aromatic substituent’s on both sides of the azo group (-N=N-) controls the colors of the azo compound as well as the water- solubility of dyes and how well they bind to a particular fabric. In addition to the azo group, the dyes must contain salt-forming groups such as hydroxyl, amino, sulfonic acid or carboxyl groups (or generally termed as auxochromes) which usually intensify the color and at the same time enable the molecule to attach itself to the fabric, or combine with a mordant, a polyvalent metal, to form a chemical lake.

The preparation of an azo dye is done by first, diazotization of an aromatic substance containing a primary amino group, then the preparation of a solution of some aromatic amino compound in dilute acid, or a solution of a phenolic substance in dilute alkali followed by the mixing of the above solutions when the reaction called coupling takes place with the formation of the dye. Before coupling can occur, the solution must be alkaline or only slightly acidic.

This experiment aims to synthesize p-nitrobenzeneazoresorcinol and helianthin dyes, their color correlated to their structure, and the balanced chemical equations in the synthesis of the dyes. Necessary conditions are made to be described for a successful diazotization to occur. And along with the synthesis of p-nitrobenzeneazoresorcinol, the term “chemical lake” is to be defined.


p-nitrobenzene Azoresorcinol Dye (S. & O. Reagent)

A solution of 2 mL of concentrated hydrochloric acid in 25 mL of water was poured into 1 g of finely powdered p-nitroaniline. The solid was dissolved by warming. The resulting solution was cooled to 2 °C and was maintained at that temperature as reaction proceeded. Fine precipitate was produced by stirring the solution while it was cooling.

A solution made up of 0.6 g of sodium nitrite in 15 mL of water was added slowly, with stirring, to the cooled mixture. When the diazotization had reached completion, the resulting solution was added slowly, with constant stirring, into a cold solution made from 1.0 g of resorcinol, 25 mL of water and 4 mL of 8N NaOH solution. The resulting mixture was chilled, and then added...
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