Preparation of Azo Dyes

In this experiment, the azo dyes p-nitrobenzene azoresorcinol and methyl orange were prepared by the azo coupling reaction - Preparation of Azo Dyes introduction. 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.

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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. INTRODUCTION 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. EXPERIMENTAL DETAILS 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 with concentrated HCl in slight excess, stirring thoroughly during the addition. Using very light suction, the solid that was formed was collected on a Büchner funnel. The collected solid was washed repeatedly with water until it was free from acid and then dried. When it has completely dried, it was grind into a fine powder. No further purification was done, though a test was made for the formation of an Mg2+ lake using the prepared azo dye. A very small pinch of the product was dissolved in 6N sodium hydroxide.

It was then added with 2-3 drops of Mg2+ solution and the result of the reaction was properly noted. Methyl Orange Four grams of sulfanilic acid was dissolved in 10 mL of 2N NaOH and a solution of 2 g of NaNO2 in 20 mL of water was added to the sulfanilic acid-sodium hydroxide mixture. The resulting solution was cooled in an ice bath and poured into 10 mL of cold 2N HCl. The produced solution of sodium diazobenzenesulfonate was mixed with a previously prepared solution of 2. 5 g of N,N-dimethylaniline in 20 mL of 1 M hydrochloric acid. The mixture was made alkaline with the addition NaOH.

The sodium salt of the dye separated into orange-brown crystalline leaflets. The product was then left for a few hours, and filtered as dry as possible. RESULTS AND DISCUSSION The yellow-brownish p-nitroaniline was dissolved in a solution of concentrated HCl in water upon heating. The yellow solution obtained was cooled to form yellow precipitate and maintained at 2 °C. Temperature was a very important factor to be considered in this reaction. The temperature had to be kept between 0-2 °C when the solution of sodium nitrite was added to form the diazonium salt.

The temperature of aqueous diazonium salt solution should not rise, or else it will be hydrolyzed to a phenol, thus reducing the yield of the desired product. Since diazonium salt slowly decomposes even at ice bath temperature, the solution was used immediately after preparation. They are also very unstable and tend to be explosive as solids. Before proceeding to azo coupling, the resorcinol was first dissolved in sodium hydroxide giving a violet liquid. When the solution of diazonium salt was poured into the cold solution made from resorcinol, water and sodium hydroxide, a violet solution was obtained.

Adding concentrated HCl in excess converted the violet solution to red solution. The azo coupling reaction had already taken place in this part. This reaction occurs most rapidly near neutrality or in slightly acidic solution. An azo coupling is an organic reaction between a diazonium compound and aniline, phenol or other activated aromatic compounds, which produces an azo compound. In this reaction, the diazonium salt is an electrophiles and the activated arene is a nucleophile in an electrophilic aromatic substitution.

Helianthin (Methyl Orange) Methyl orange is an azo dye that forms orange crystals and is used as an acid-base indicator. The anion form of this azo dye is yellow and the acid form is red. It is synthesized from sulfanilic acid and dimethylaniline using a diazonium coupling reaction. [pic] Figure 1. Correlation between the colors of the dye with its structure The first step is simply an acid-base reaction. In order to dissolve the sulfanilic acid in the aqueous solution, NaOH or sodium carbonate was added yielding a colorless solution.

When HCl was added, the nitroso ion was formed from sodium nitrite and this reacted with the amine to form a nitrosoammonium adduct which loses water under the acidic conditions after proton transfer. This gave the diazonium salt. Aromatic diazonium salts are stable at low temperature. The terminal nitrogen of the diazonium salt is very electron deficient and can be attacked by good nucleophiles. Dehydration of the nitrous acid created a nitroso ion, which added to the primary amine in sulfanilic acid to form a diazonium ion. N,N-dimethylaniline is dissolved in HCl to form a dimethylaniline salt.

This was the neutralized and hence, became a good nucleophile due to the activating effect of the N,N-dimethylamine substituent. Attack was in the para position due to the hindrance at the ortho position by the bulky dimethylamine substituent. [pic] Figure 2. Chemical equation for the synthesis of methyl orange Azo compounds contain a highly delocalized system of electrons which takes in both benzene rings and the two nitrogen atoms bridging the rings. The delocalization can also extend to things attached to benzene ring as well.

If white light falls on one of these molecules, some wavelengths are absorbed by these delocalized electrons. The color one can see the result of the non-absorbed wavelengths. The groups which contribute to the delocalization (and so as to the absorption of light) are known as chromophore. Modifying the groups present in the molecule can have an effect on the light absorbed, as well as the color one can see.

As the hydrogen ion is lost or gained, there is a shift in the exact nature of the delocalization in the molecule, and that causes a shift in the wavelength of ight absorbed (shown in Figure 1). [pic] Figure 3. The mechanism of the formation of methyl orange CONCLUSION Producing an azo dye requires an activated aromatic amine or phenol (or other activated arenes) treated with nitrous acid to give a diazonium ion intermediate. This process is called diazotization. Ar-NH2 + HNO2 + HCl ? Ar-N+ N:Cl- + 2H2O The diazonium ion is an electron deficient (electrophilic) intermediate and an aromatic compound, suitably rich in electrons (nucleophilic), will add to it. The most commonly used substances are aromatic amines and phenols.

Both of these types of compounds are usually more nucleophilic at a ring carbon than at either nitrogen or oxygen due to the resonance that has taken place. Many diazonium ions decompose rapidly in solution, so the coupling reaction should be completed quickly. The addition of the amine or the phenol to the diazonium ion is called the diazonium coupling reaction. With highly activated benzene derivatives such as phenols and amines, this reaction occurs most rapidly near neutrality or in slightly acidic solution. With amines having a hydrogen atom on nitrogen, coupling occurs initially on nitrogen.

The coupling reaction goes almost exclusively para position unless this position is occupied, in which case it will occur in the ortho position. No coupling reaction would occur when both ortho and para positions to the amino or hydroxyl group are occupied. The azo grouping is responsible for the color of these compounds and such group is known as a chromophore. This azo group contains the N=N bond and this group is a strong chromophore that imparts a brilliant color to these compounds.

The shade of the color is affected by substituents in the benzene rings, and the substance is able to function as a dye and become ttached to cloth only when salt-forming groups are in the ring (auxochrome groups). SUPPORTING INFORMATION Azo dyes are both the largest and the most important group of synthetic dyes. In making the azo linkage, many combinations of ArNH2 and Ar’NH2 (or Ar’OH) are possible. These combinations give rise to dyes with a broad range of colors, encompassing yellows, oranges, reds, browns and blues. The azo dyes, the triphenylmethane dyes, and mauve are all synthesized from the anilines (Aniline, o-, m-, and p-toluidine) and aromatic substances (benzene, naphthalene and anthracene).

All of these substances can be found in coal tar, a crude material that is obtained by distilling coal. Perkin’s discovery led to the formation of a multimillion dollar industry based on coal tar, a material that was once widely regarded as a foul smelling nuisance. Today, these same materials can be recovered from crude oil or from petroleum as by-products in the refining of gasoline. Although we no longer utilize coal tar, many of the dyes are still widely used.

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