The objective of the experiment was to produce two azo dyes: p-nitrobenzene azoresorcinol and methyl orange. The initial step involved mixing p-nitroaniline and resorcinol to create p-nitrobenzene azoresorcinol. In order to form the diazonium salt, we combined a solution containing p-nitroaniline dissolved in hydrochloric acid and water with a sodium nitrite solution. Subsequently, the resulting diazonium was mixed with a combination of resorcinol in water and sodium hydroxide.
Upon the addition of excess hydrochloric acid to the mixture, a red color was observed. The resultant product, obtained via suction filtration and drying, took the form of a red-brown powder. To produce methyl orange, sulfanilic acid and N,N-dimethylaniline were mixed together. By diazotizing a combination of sulfanilic acid and sodium hydroxide with a solution of sodium nitrite, sodium diazobenzenesulfonate was generated. This compound was then cooled.
Combining sodium diazobenzenesulfonate and N,N-dimethylaniline in a hydrochloric acid solution resulted in a coupling reaction. This reaction initially produced helianthin, a bright red variant of methyl orange. Later, when introducing a diluted solution of sodium hydroxide into an alkaline medium, helianthin transformed into the sodium salt of methyl orange, exhibiting an orange color.
INTRODUCTION
The dyes Para-nitrobenzeneazoresorcinol and methyl orange are categorized as “azo colors” and contain the azo group bonded to two aromatic nuclei. The color and water solubility of these dyes, as well as their ability to bind to specific fabrics, depend on the nature of the aromatic substituents flanking the azo group (-N=N-). To enhance color intensity and facilitate attachment to fabrics, these dyes must also possess salt-forming auxochromes such as hydroxyl, amino, sulfonic acid, or carboxyl groups. These auxochromes often interact with a mordant, a polyvalent metal, to form a chemical lake.
The process of creating an azo dye involves two steps. First, an aromatic substance containing a primary amino group is diazotized. Next, a solution of either an aromatic amino compound in dilute acid or a phenolic substance in dilute alkali is prepared. The solutions are then mixed, resulting in the formation of the dye through a reaction called coupling. However, for coupling to occur, the solution must be alkaline or slightly acidic.
The objective of this experiment is to create p-nitrobenzeneazoresorcinol and helianthin dyes. Their color is related to their structure, and the balanced chemical equations for the synthesis of the dyes are also provided. The necessary conditions for a successful diazotization are described. Additionally, the term “chemical lake” is defined in relation to the synthesis of p-nitrobenzeneazoresorcinol.
EXPERIMENTAL DETAILS
p-nitrobenzene Azoresorcinol Dye (S. & O. Reagent)
A mixture of 2 mL of concentrated hydrochloric acid in 25 mL of water was added to 1 g of finely powdered p-nitroaniline.
The solid was dissolved by heating it. The resulting solution was then cooled to a temperature of 2 °C and kept at that temperature while the reaction occurred. As the solution cooled, a fine precipitate formed by stirring. A separate solution containing 0.6 g of sodium nitrite in 15 mL of water was added slowly to the cooled mixture while stirring. Once the reaction of diazotization was complete, the resulting solution was slowly added with continuous stirring to a cold solution composed of 1.0 g of resorcinol, 25 mL of water, and 4 mL of 8N NaOH solution.
The mixture obtained was cooled and then combined with an excess of concentrated HCl, being sure to stir thoroughly throughout the addition. Light suction was used to collect the solid that formed on a Büchner funnel. This collected solid was washed multiple times with water until it was free from acid and then dried. After complete drying, it was finely ground into a powder. No additional purification steps were taken, though a test was performed to check for the formation of an Mg2+ lake using the prepared azo dye. A tiny amount of the product was dissolved in 6N sodium hydroxide.
The reaction was observed after adding 2-3 drops of Mg2+ solution. Methyl Orange was prepared by dissolving 4 grams of sulfanilic acid in 10 mL of 2N NaOH and adding a solution of 2 grams of NaNO2 in 20 mL of water to the mixture. The resulting solution was cooled and poured into 10 mL of cold 2N HCl. The sodium diazobenzenesulfonate solution was then mixed with a prepared solution of 2.5 grams of N,N-dimethylaniline in 20 mL of 1 M hydrochloric acid. The mixture was made alkaline by adding NaOH.
The product, which was the sodium salt of the dye, separated into orange-brown crystalline leaflets and was then left for a few hours. Afterward, it was filtered to be as dry as possible.
RESULTS AND DISCUSSION
The p-nitroaniline was initially yellow-brown and dissolved in a concentrated HCl solution in water with the application of heat. After cooling, a yellow precipitate formed and remained at a temperature of 2°C. The reaction required specific temperature control between 0-2°C when introducing the sodium nitrite solution to produce the diazonium salt.
To maintain the stability of the aqueous diazonium salt solution and prevent hydrolysis, it is crucial to keep the temperature constant. Even at ice bath temperature, the diazonium salt gradually breaks down. Therefore, we used the solution immediately after preparing it. Furthermore, in solid form, diazonium salts can be highly unstable and potentially explosive. Prior to azo coupling, a violet liquid was created by dissolving resorcinol in sodium hydroxide. The resulting solution consisted of water, sodium hydroxide, and resorcinol which turned violet when combined with the cold diazonium salt solution.
Adding an excess amount of concentrated HCl caused a change in the color of the solution from violet to red. The azo coupling reaction had already occurred in this specific part. This reaction is most efficient when the solution is either near neutrality or slightly acidic. Azo coupling refers to an organic reaction that takes place between a diazonium compound and aniline, phenol, or other activated aromatic compounds, resulting in the formation of an azo compound. In this reaction, the diazonium salt acts as an electrophile while the activated arene acts as a nucleophile in an electrophilic aromatic substitution.
Helianthin (Methyl Orange) is an azo dye that is commonly used as an acid-base indicator. It forms orange crystals and exhibits a yellow color in its anion form and a red color in its acid form. The synthesis of this dye involves a diazonium coupling reaction between sulfanilic acid and dimethylaniline. [pic] Figure 1 illustrates the correlation between the dye’s structure and its colors. The initial step of the synthesis involves an acid-base reaction, where sulfanilic acid is dissolved in an aqueous solution using either NaOH or sodium carbonate, resulting in a colorless solution.
When hydrochloric acid (HCl) was added, sodium nitrite reacted with it to form the nitroso ion. This nitroso ion then reacted with the amine to produce a nitrosoammonium adduct. Under acidic conditions and after proton transfer, this adduct lost water and formed the diazonium salt. Aromatic diazonium salts are stable at low temperatures because of their electron-deficient terminal nitrogen. Strong nucleophiles can easily attack this electron-deficient nitrogen.
Dehydrating the nitrous acid resulted in the formation of a nitroso ion, which then added to the primary amine in sulfanilic acid to create a diazonium ion.
To create a dimethylaniline salt, N,N-dimethylaniline was dissolved in hydrochloric acid (HCl).
The N,N-dimethylamine substituent’s addition resulted in the compound being neutralized and becoming a nucleophile. This activation aided nucleophilic attacks, with the attack happening at the para position because of hindrance caused by the bulky dimethylamine substituent.
[pic] Figure 2. Chemical equation for synthesizing methyl orange
Azo compounds have an electron system that is widely delocalized, encompassing both benzene rings and the two nitrogen atoms connecting them. This delocalization can also extend to any groups attached to the benzene ring.
When white light illuminates a molecule, certain wavelengths are taken in by the delocalized electrons. The color visible to the eye is determined by the wavelengths that are not absorbed. These groups responsible for delocalization and light absorption are referred to as chromophores. By altering the groups within the molecule, both the absorbed light and the perceived color can be influenced.
The delocalization in the molecule changes as the hydrogen ion is lost or gained, leading to a shift in the wavelength of absorbed light. This shift in delocalization is what causes the formation of methyl orange.
CONCLUSION
The process of producing an azo dye involves treating an activated aromatic amine or phenol (or other activated arenes) with nitrous acid to create a diazonium ion intermediate. This step is known as diazotization. The equation for this reaction is: Ar-NH2 + HNO2 + HCl ? Ar-N+ N:Cl- + 2H2O. The diazonium ion is both an electron deficient (electrophilic) intermediate and an aromatic compound. It can be added to by an aromatic compound that is suitably rich in electrons (nucleophilic). Aromatic amines and phenols are the most commonly employed substances for this purpose.
Both types of compounds, phenols and amines, are typically more nucleophilic at carbon atoms in a ring structure compared to nitrogen or oxygen atoms due to resonance. In solution, diazonium ions often decompose quickly, so it is important to complete the coupling reaction promptly. The process of adding an amine or phenol to the diazonium ion is known as the diazonium coupling reaction. This reaction occurs most rapidly in slightly acidic or neutral solutions, especially with highly activated benzene derivatives like phenols and amines. In the case of amines that contain a hydrogen atom on nitrogen, coupling initially takes place on the nitrogen atom.
The coupling reaction predominantly occurs in the para position, except when this position is already occupied, in which case it occurs in the ortho position. If both ortho and para positions are occupied by the amino or hydroxyl group, no coupling reaction takes place. The azo grouping, known as a chromophore, is responsible for the color of these compounds. This chromophore contains the N=N bond and provides a vibrant color to these compounds.
The color of the substance changes due to substituents in the benzene rings. It can act as a dye and attach to fabric when there are salt-forming groups in the ring (auxochrome groups).
SUPPORTING INFORMATION
Azo dyes are a significant and extensive category of synthetic dyes. The formation of the azo linkage allows for various combinations of ArNH2 and Ar’NH2 (or Ar’OH), resulting in dyes that exhibit a wide range of colors, such as yellows, oranges, reds, browns, and blues. The synthesis of azo dyes, triphenylmethane dyes, and mauve utilizes anilines (Aniline, o-, m-, and p-toluidine) and aromatic substances (benzene, naphthalene, and anthracene).
Coal tar, a by-product obtained by distilling coal, contains all of these substances. It was originally considered a foul smelling nuisance, but Perkin’s discovery resulted in the development of a lucrative industry centered around coal tar. Nowadays, these substances can also be extracted from crude oil or petroleum as by-products during gasoline refining. Although coal tar is no longer used, many of the dyes created from it are still extensively utilized.
