Synthesis of 1-Phenylazo-2-Naphthol 1
Diane Layne Manansala Gomez
2
Justin Michael Apostol Naniong
1
Institute of Biology, College College of Science, University University of the Philippines, Diliman, Diliman, Quezon City 1101 Philippines Philippines
2
National Institute Institute of Molecular Biology and Biotechnology, Biotechnology, College of Science, Science, University of the Philippines, Philippines, Diliman, Quezon City City 1101 Philippines
ABSTRACT In this experiment, synthesis of Sudan- 1was obtained from the reaction between aniline and β naphthol, following the two processes: diazotization through formation of diazonium salts, and coupling reactions, or the electrophilic reaction of the charged diazonium ion to the aromatic substrates. Recrystallization was done on the product to obtain a suitable yield. The reaction of aniline and βnaphthol successfully synthesized the Sudan-1 red dye by the end of the experiment proper, and 0.20g of Sudan-1, which was 37.04% of the expected theoretical yield of 0.54 grams, was synthesized.
Introduction Azo compounds are compounds that bear the functional group R-N=N-R’. They are derivatives of diazene wherein both hydrogens are substituted by hydrocarbyl groups. They usually result from coupling reactions of phenols and arenediazonium salts and other amines. Azo compounds are brightly colored compounds due to their reactivity in optical matters with light and their chromophore functional group, which is due to pi delocalization. Aryl azo compounds are usually stable crystalline compounds that are vividly colored and used as dyes, known as azo dyes. Aromatic azo compounds can be synthesized by using an azo coupling reaction, which entails an electrophilic substitution reaction where an aryl diazonium cation attacks another aryl ring, especially those substituted with electrondonating groups. In this case, synthesis of 1-
phenylazo-2-naphthol is possible by reacting aniline (which forms the diazonium salt) and beta-naphthol . This product is also known as Sudan-1. Sudan-1, or 1-phenylazo-2-naphthol, which is synthesized in this experiment, is known also as a dyeing agent for textile. Ingrain dyeing is a method used for textiles which cannot undergo simple dyeing by absorption wherein the azo dye is made to go right in the middle of microspaces between the fabrics. The entirety of this experiment was based on the notion that aniline, through the use of NaNO2 in an acidic medium, would form a diazonium salt, which would react with the betanaphthol under very cold and basic conditions, to form 1-phenylazo-2-naphthol, a bright red dye. This dye would later be tested for ingrain efficacy by ingrain dyeing by using 1x1 cotton cloth and dipping it into the beta-naphthol
solution, patting it dry, and dipping it into the phenyldiazonium solution, and recrystallized and its melting point determined. The purpose and aim of this experiment was to understand the principles behind synthesizing an organic compound of one functional group from a compound of another functional group, in this case, two compound of another functional group, and using qualitative observation to determine the extent of the product formation. In this case, synthesizing an aryl azo compound from the diazotization and coupling reaction mechanism of an aryl diazonium cation and an aryl ring containing and electron-releasing group, under cold conditions. Also, in this case, to understand the mechanism of reaction of formation of diazonium salts and the concept of coupling reactions. Experimental Detail To begin the experiment, the materials and all other apparatus were all ready and prepared. First, phenyldiazonium chloride was prepared by mixing 0.2-mL aniline, 0.35-mL water, and 0.5-mL concentrated HCl in a 125mL Erlenmeyer flask. The solution was placed in an ice bath and was cooled to 4°C. Approximately 1 mL ice-cold distilled water was added. Then, 0.3 g NaNO2 crystals were gradually added to the solution.
β-naphthol was prepared by dissolving 0.35 g β-naphthol in 4.5 mL 5% aqueous NaOH in a 50-mL beaker and was also cooled to 4°C. A 2 x 3 cm cloth was cut and soaked into the β naphthol solution. After 2-3 minutes, the cloth was removed and patted dry using filter papers and was soaked in the phenyldiazonium chloride solution. After several minutes, the fabric was removed and rinsed in running water until a redorange dye was retained, which confirmed the formation of Sudan 1.
The two solutions were combined as they are stirred continuously. It was then allowed to stand for 5 minutes at a temperature of 4°C. The product was then filtered and washed with cold water. It was dissolved in hot ethanol and it underwent hot filtration. Then, the solution was allowed to cool to recrystallize the product. The crystals were then allowed to dry on a pre-weighed filter paper. After drying, the filter paper and the crystals were weighed together and the mass of the filter paper was subtracted. This was how the experimental yield of the Sudan-1 product was determined. The percentage yield was then computed from the theoretical Sudan-1 yield and the yield gathered through experimentation. After computing the yield, its melting point was determined using the melting point determination set-up. Results and Discussion A. Data and Results Table 1. Quantitative Data of 1-Phenylazo-2Naphthol Obtained Through Experimenting
Weight (Sudan 1) 0.20 grams
Actual Yield 0.00081 moles
Theoretical Yield
% Yield (Sudan 1)
0.0022 moles
37.04 %
As presented in Table 1, which illustrates the quantitative data of the produced 1phenylazo-2-naphthol, or Sudan-1 (based on the theoretical moles of aniline and beta-naphthol used), including the weight of the produced Sudan-1, the yield in moles, and the theoretical yield in moles, and most importantly, how much of the Sudan-1 was actually produced (in the form of the percent yield), the percentage yield of product obtained was 37.04% of the theoretical amount of product synthesized. Through solution stoichiometry, the moles of 1-phenylazo-2naphthol from the grams of the compound
produced were determined. This is important to provide a comparison of the reactants and the product produced. Table 2. Comparison of Theoretical and Experimental Melting Points of Produced 1Phenylazo-2-Naphthol
Melting Point (theo)
Trial 1
Trial 2
131.0 ºC
131.5 ºC
132.0 ºC
Melting Point (expt’l)
As presented in Table 2, which illustrates the melting point comparison between the theoretical melting point, or the literature melting point of 1-phenylazo-2-naphthol, and the experimental melting point, or the obtained melting point of 1-phenylazo-2-naphthol, a suitable melting point of the product was obtained via melting point determination set-up. By comparing the two trials of melting point obtained from the produced Sudan-1 and the theoretical melting point, it can be concluded that the melting points are sufficiently close enough to draw the conclusion that the produced product is pure 1-phenylazo-2-naphthol. This is an imperative assumption when computing for the percentage yield of the 1-phenylazo-2-naphthol; we should consider that if impurities are present, their amounts are negligible and wouldn’t affect the yield of the produced 1-phenylazo-2naphthol. B. Discussion The experiment focused on the diazotization reaction and the coupling reaction that was undergone when aniline and betanaphthol were reacted when subjected to nitrous acid at low temperatures. Prior to the experiment proper, the experimenters prepared ice baths for the beta-
naphthol and the aniline reactions. Azo compound reactions must be performed under very cold conditions (not to exceed 5ºC). This is because when aniline reacts with the nitrous acid in the diazotization reaction, a phenyldiazonium salt intermediate is formed, which reacts with the beta-naphthol after ingrain dyeing. At normal or room temperatures, the diazonium salt intermediate decomposes. When this occurs, nitrogen gas is released, thus, the additional nitrogen cannot form triple bonds and the betanaphthol cannot properly react with the phenyldiazonium salt. This would cause a subsequent drop in the yield of Sudan-1, or Sudan-1 would not even be produced at the end of the reaction. To aid in cooling the ice-bath and lowering the temperature of the ice, rock salt was sprinkled on the ice bath. Rock salt is effective in lowering the temperature of ice because of the colligative property it exhibits, which is freezing point depression. Rock salt lowers the temperature that water will freeze. Water must be aligned properly to freeze. When salt is dissolved in it, it makes it harder for it to get into the proper arrangement so it freezes at a lower temperature. After preparing the ice-cold bath and cold distilled water for the diazotization and coupling reactions later, 0.2 mL of aniline, 0.35 mL of distilled water, and 0.5 mL of concentrated HCl were combined in a 50-mL Erlenmeyer flask and made to react in the prepared ice-cold bath to prevent decomposition of the diazonium ion to be formed later. After this, approximately 1mL of distilled water was added to the mixture to provide excess hydrogen ions. After, 0.3 grams of sodium nitrite was added to the solution. In this reaction, aniline is made to react with nitrous acid to produce the diazonium ion. The nitrous acid was produced in reaction by letting NaNO2 react with an acid. The nitrous acid is then protonated, releases
water, and produces the nitrosonium ion. This is then attacked by the nucleophilic amine of aniline, ultimately producing the diazonium ion. Although aliphatic diazonium ions readily decompose, aromatic diazonium ions can be isolated due to delocalization of the positive charge on the nitrogen. Throughout the course of the experiment, even after reacting the aniline with the nitrous acid, the temperature of the phenyldiazonium solution was kept below 4ºC. This is due to the fact that at higher temperatures, the salt becomes unstable, and a possible side reaction occurs, where the diazonium salt releases nitrogen gas and the aromatic ring becomes protonated. Due to the presence of water and chlorine ions from the dissociation of concentrated HCl in aqueous environments, the protonated aromatic ring becomes susceptible to nucleophile attack from the OH from the water and the Cl from the HCl. When this occurs, phenol and chlorobenzene are formed as side products. Next, 0.35 grams of beta-naphthol is dissolved in 4.5 mL of 5% sodium hydroxide in a 50-mL beaker. Sodium hydroxide is present because since beta-naphthol has a phenol part, and as coupling agents, they react more readily to weak bases. This is why a strong base is required. However, only 5% NaOH is used because if the solution becomes too basic, the salt is converted into a diazotate ion when the beta-naphthol-NaOH solution is added to the phenyldiazonium salt. The formation of this side product compromises the ability for coupling reactions to occur, and it is possible that Sudan-1 will not be formed in its pure form, or even that lower yield of product would occur. After the reaction of the beta-naphthol and the 5% NaOH in ice-cold conditions, ingrain dyeing was performed. Cotton fabric, about 1x1 cm was dipped in the cold beta-naphthol
solution for 2 minutes and patted dry with folded filter paper. After, it was dipped in the phenyldiazonium chloride solution, taking extra care not to tip the beaker in the ice bath. After this was done, change of color in the white cotton fabric was observed. The formation of a dark-red color was observed in the fabric. This is the 1-phenylazo-2-naphthol product to be formed later. A good dye is the one that bonds perfectly well with the fibers involved. An ideal dye should then contain groups that bonds with most types of fabrics. The presence of functional group responsible for the intermolecular attraction of the dye molecules and the fiber ensures adhesion of the dye to the textile. “Sudan-1” contains molecules incapable of direct bonding with the cotton fiber. This is because the molecules of the Sudan-1 cannot diffuse readily into the cotton fibers because of their size. The structure of the cotton fiber molecules ensures that it is incapable of strong interactions with 1-phenylazo-2-naphthol. It is only capable of hydrogen-bonding with other cellulose molecules but not with the azo dye molecules. The adhesion between the Sudan molecules and the cotton fiber is very weak. This is why direct dyeing is not advised when done on cotton with 1-phenylazo-2-naphthol. After ingrain dyeing is performed, the remaining phenyldiazonium chloride solution and betanaphthol are combined and made to react to form the 1-phenylazo-2-naphthol. Beta-naphthol has an OH - group attached to the beta carbon, which is located 2 carbons away from the naphthol carbon shared between the two aromatic rings. Reaction between the diazonium ion and the betanaphthol ring is primarily carried out in the carbon 1 position. This is due to the f act that this reaction yields the most stable carbocation intermediate, due to the presence of resonance structures. In the attack on carbon 1 by the diazonium ion, the other ring is aromatic, and there is one double bond conjugated to the
aromatic ring. Electrons from the 1,2 carbon bond shift out to the nitrogen as electrons from the oxygen in the hydroxyl group shift in to form + C=OH .
Figure 1. Resonance structures formed during coupling reaction of diazonium ion and betanaphthol The second resonance structure of the diazonium ion above has a positive charge on the terminal nitrogen. This positive charge makes the ion slightly electrophilic. However, their electrophilicity is only weak because of the delocalization of the positive charge. Hence, they would only react with highly activated compounds, like naphthol, since the hydroxyl group is a strong activator. Being an electron donor, the hydroxyl group is also an ortho/paradirector. However, the para position cannot be attacked since there are no hydrogen atoms to be substituted. Hence, another reason why carbon 1 is attacked by the diazonium ion. As the phenyldiazonium chloride solution is added to the beta-naphthol solution, constant stirring is to be employed. As always, the conditions should be low temperatures, not to exceed 5ºC, so that the diazonium salt will not be degraded. Conditions are to be normalized
only after all of the diazonium has reacted with the beta-naphthol. The product is filtered and washed with cold water to remove to let excess diazonium ions to react with the beta-naphthol. The product is allowed to recrystallize with hot ethanol and fluted filter paper set-up. The set-up and the containing flask are also heated. This is done to prevent premature recrystallization of the 1-phenylazo-2-naphthol. After the product was recrystallized, it was allowed to dry prior to melting point determination, which was done to ensure that the recrystallized product was pure 1-phenylazo-2-naphthol (we are allowed the assumption that the impurities have a different melting point from the obtained 1-phenylazo-2naphthol). After the experiment was conducted, the experimenters gained new knowledge on the concept of diazotization and coupling reactions to form highly conjugated products in the form of intensely colored dyes. A few examples of dyes include the following: [1]
[2]
[3]
Their corresponding amines and coupling components that would serve as their starting materials are the following: Table 3. Amines and Coupling Components With Their Corresponding Dye Productss
Dye
Amine
Coupling Component
[1]
[2]
[3]
Conclusion This experiment was conducted to gain an understanding in principles behind synthesizing 1-phenylazo-2-naphthol from reacting aniline with sodium nitrite to produce a diazonium ion to react with beta-naphthol in low temperature. This knowledge was gained by the experimenters. Results have showed that 1-phenylazo2-naphthol was successfully produced from aniline and beta-naphthol after undergoing diazotization and coupling reaction using sodium nitrite in acidic conditions as the source of the additional nitrogen to form the diazonium salt. Recrystallization and melting point determination was also utilize to extract the pure yield of the 1-phenylazo-2-naphthol and those concepts were mastered by the experimenters. It
is also possible that human errors and minor discrepancies arose from the recrystallization set-up, and even in the formation of the Sudan-1 product, as these stages could possibly affect the yield of the desired product, and it is possible that, since the set-up was left to dry for two weeks, stray impurities could have found their way to the recrystallized Sudan-1. Recommendations for the methodology of the experiment would include purification by distillation. This should be done to ensure that the synthesized product is pure in nature (however, the molecular weight and the condensation points of the products and the intermediate formed are to be considered first). Purification by distillation should, however, be done prior to melting point determination, to ensure that the melting point obtained is as close as possible to the theoretical melting point of the desired product. On the whole, the experiment was a success in the sense that the students have successfully achieved the objectives of the experiment, and a sensible yield was obtained. The students have learned the concepts of synthesis experiments, the mechanism of diazotization and coupling reactions of amines, and the effect that different factors can have on the yield and the purity of the product. References [1] H. T. Clarke and W. R. Kirner (1941), "Methyl Red", Org. Synth.; Coll. Vol [2] McMurry, John, et al. Organic Chemistry (7th ed.). Thomson Learning, Inc., 2008,344-345 [3] http://clem.mscd.edu/~wiederm/oc2chp/ , Retrieved on September 2011
APPENDIX Sample Calculations: Limiting Reagent: Aniline 0.1 mL x 1.022 grams x 1 mole x 1000 mmol = 2.19 mmol 1 mL 93.13 g 1 mole Excess Reagent: Beta-Naphthol 0.35 grams x 1 mole x 1000 mmol = 2.43 mmol 144.17g 1 mol yield of 1-phenylazo-2-naphthol: 2.19 mmol x 1 mole x 248.30 grams = 0.54 grams yield of Sudan-1 1000 mmol 1 mole Percentage yield: Theoretical yield x 100% = 0.20 grams of Sudan-1 x 100% = 37.04% Actual yield 0.54 grams of Sudan-1
