SYNTHESIS OF 1-PHENYLAZO-2-NAPHTHOL Dizon, Niera Kamille M. 1, Gelvezon, Marie Angelique T. 2 1 National Institute of Molecular Biology and Biotechnology, 2 Department of Food Science and Nutrition, University of the Philippines, Philippines, Diliman, Quezon City May 12, 2009
ABSTRACT This experiment aims to show the process of synthesizing Sudan 1, or 1-phenylazo-2naphthol, naphthol, from adding benzenediazonium benzenediazonium ion to 2-naphthol. First, phenyldiazonium phenyldiazonium chloride solution was prepared from mixing aniline, water and concentrated hydrochloric acid. Then, βnaphtho naphtholl was prepared prepared and added added to the phenyldiaz phenyldiazoniu onium m chloride chloride solution solution and from that, that, Sudan 1 was formed. The product was then subjected to recrystallization process so that a purer purer sampl sample e would would be obtai obtaine ned. d. After After obtai obtaini ning ng the final final produ product, ct, its meltin melting g point point was was determined to be around 125-131°C. Its theoretical melting point is between the range of 131133°C which is not far from the obtained result; thus, making our recorded data somehow accurate.
INTRODUCTION Azo-compounds, Azo-compounds, compounds compounds with general general formula formula patterns patterns are coup coupli ling ng prod produc ucts ts from from the the reac reacti tion on of aren arened edia iazo zoni nium um salt salts s with with phen phenol ols s and and other other amine amines. s. Azo Azo dyes dyes are easily prepared using the coupling mecha mechanis nism m resul resultin ting g to its chea cheap p large large-scale manufacturing cost. Azo compounds are are brig bright htly ly colo colore red d comp compou ound nds s due due to optical reactivity with light of it chromopore functional group, . Dyein Dyeing g the the texti textile le using using azo-dy azo-dyes es requires different techniques in accordance to the the natu nature re of the the text textil ile e fabr fabric ic to be processed. A textile which cannot be dyed using simple adsorption undergoes ingrain dyeing wherein the azo dye is made right in the middl middle e of micromicro-sp space aces s betw betwee een n the fabrics. fabrics. Many synthet synthetic ic dyes, dyes, often often called called diazo dyes, are based on diazonium chemistry that has been used for over 100 years, and this experiment is an example of the synthesis of one such dye, 1-phenylazo2-naphth 2-naphthol ol or more simply, simply, Sudan Sudan 1. The reaction involves two steps, the first being the preparat ration of a solution of the
diazonium salt, benzenediazonium chloride from aniline. In the second step, which is ofte often n call called ed the the coup coupli ling ng reac reacti tion on,, the the positively-charged diazonium ion serves as the electrophile that attracts highly activated aromat aromatic ic subst substrat rates es to give give the the coupl coupling ing product. Although Although the use of Sudan 1 in food is now banned in many countries, it is still an imperativ imperative e to complete complete an experim experiment ent which which deals deals with its synthesi synthesis, s, applica application tion as a dye and purification because it is still import importan antt to study study the chemis chemistry try behi behind nd dyein dyeing, g, which which is a major major glob global al indus industry try.. Also, it is important important to know the chemistry chemistry of a widely used substance – as a carcin rcino ogenic foo food additiv itive e and dye. Diazotization Diazotization products are powerful reaction intermediate that can be used to synthesize many mono-substituted aromatic comp compou ound nds. s. Synt Synthe hesi sis s of butt butter er yell yellow ow (artificial colors of butters), for example, can also be produced using the usual azotiza azotization tion and couplin coupling g reactio reaction. n. Butter Butter yellow is primarily used nowadays to study carcinogenesis and other medicine related chemical reactions.
The experiment had three parts and three basic objectives. The objectives of the experiment were for the students to be able to synthesize Sudan 1, dye a cotton fabric using the synthesized Sudan-1 and lastly, to be able to purify the product using the process of recrystallization.
METHODOLOGY
cloth, which confirmed the formation of Sudan 1, the two solutions were combined while stirring. The mixture was allowed to stand for 5 minutes at 4°C. Then, the product was filtered and washed with cold water. The product was dissolved in hot ethanol and it underwent hot filtration. Then, the solution was allowed to cool to recrystallize the product. It was then dried and its melting point was determined using the melting point determination set-up. RESULTS AND DISCUSSION Aniline is reacted with NaNO2 crystals under extremely acidic solutions using HCl or H2SO4 in a very cold temperature. The phenyldiazonium intermediate easily decomposes back into its aniline counterpart at slightly higher temperature, thus the solution is maintained in an ice bath a constant temperature of 5OC. The reaction for diazotization process is:
Figure 1. The general reaction pattern for the azotization-coupling reaction includes two-step process. First, phenyldiazonium chloride was prepared by mixing 0.2 mL aniline, 0.35 mL water, and 0.5 mL concentrated HCl in a 125 mL 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 NaNO 2 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, it was removed and patted dry using filter papers and then it was soaked in the phenyldiazonium chloride solution. After several minutes, the fabric was removed and rinsed in running water. After obtaining a red-orange dye on the
Figure 2. The formation of azonium salt is achieved by the azotization reaction at 0 OC. The diazonium salt by-product is very reactive. It can be used to yield different kinds of substituted benzene products like aryl halides, aryl amide and phenols. Alkyl amine can also be synthesized by reacting it with NaNO 2 or HNO2 to yield alkanediazonium product; however, the reaction by product cannot be isolated due to its high reactivity. The azotization process starts with the bimolecular exchange of proton and N, in which the nitrogen lone pairs create triple bonds with the N from NaNO 2.
Figure 3. The mechanism of formation of azonium ion by the bimolecular attack of nitrite to the aniline. The phenyldiazonium salt by-product then undergoes either nucleophilic substitution or diazonium-coupling reaction. In diazonium-coupling reaction, the diazonium salt attacks the carbon in the para- and ortho- position with respect to the activating substituent of the benzene. For this experiment, β-naphthol couples with the diazonium salt forming a resonancestabilized intermediate.
Figure 4. The coupling reaction of the diazonium salt with β-naphthol occurs with the attack of the diazonium salt with on the para- carbon of the β-naphthol. The attack of the diazonium salt the carbon next to the –OH in the β-naphthol is favored due to the presence of more resonance representation for the products.
Figure 5. Some of the resonance structure of the coupling reaction intermediate.
The final coupling product is “Sudan1” or 1-phenylazo-2-naphthol azo dye. Rock salt is added in the ice bath because in induces the colligative property of solution called freezing point lowering . Thus, ice melts at a much lower temperature below O°C. Among the possible side reactions in the experiment is the direct substitution of any nucleophile present in the interacting solution. In this case the diazonium salt in the reaction can be made into a phenol by the direct substitution of –OH in the solution. Other nucleophiles such as –NR 2 or –NH2 can also easily substitute the azo-group in the intermediate salt.
Also, the phenyldiazonium chloride solution must be acidic. If not, OH- readily attacks the N+ ion making a pink side product. As for the β-naphthol solution, the temperature must be kept below 4°C to prevent back reaction.
Figure 8. Possible back reaction. Lastly, the β-naphthol must be basic or else it may be protonated.
Figure 6. Side reaction due to the nucleophilic substitution of a nucleophile such as –OH or –NR 2 with the azo-group in the diazonium salt. The decomposition of diazonium salt also occurs if the solution is not kept at the advised temperature. The diazonium salt easily reverses back to aniline a slightly higher temperature than the boiling point. The overall result is the low percent yield for the azo compounds. Individually for the two reactants, there are certain conditions that must be maintained to prevent the formation of side products. For the phenyldiazonium chloride solution, the temperature must be kept below 5°C to prevent decomposition of the phenyldiazonium ion.
Decomposition Figure 7. phenyldiazonium ion.
of
the
Figure 9. Protonation of β-naphthol. For a dyeing process to be successful, the dye and fabric to be used must be appropriate for each other. A good dye is the one that bonds perfectly well with clothes’ fiber. This property is called fastness. Another property of a good dye is levelness or the uniformity of color. Going back to fastness, 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. An ideal dye should then contain groups that bonds with most types of fabrics. “Sudan-1” contains molecules incapable of direct bonding with the cotton fiber. The adhesion between the Sudan molecules and the cotton fiber is very weak. In cases like this, ingrain dyeing is the most appropriate for this application. In the
process of ingrain dyeing, the Sudan dye is synthesized right in the spaces between the fibers such that they are permanently trapped inside the fiber spaces of the cotton. CONCLUSION HERE. For the experiment to be more accurate, successful and relevant, some recommendations must be taken into consideration. To broaden the scope of further experiments, it could be more educational if different types of dyeing were employed and compared like direct and mordant dyeing. Also, different types of fabric other than cotton could be used for contrasting purposes. Lastly, the dye can also be analyzed by visible light spectrophotometry or IR for the intensity of the color as a function can be measured, or more specifically, the wavelength.
REFERENCES Alsoph, C. et al. (1996). Elements of Organic Chemistry as Revealed by the Scientific Method . Massachusetts: Addison-Wesley Publishing Company. Brown, W. H. et al. (2005). Introduction to Organic Chemistry 3 rd edition. New Jersey: John Wiley and Sons. Fryhle, C. & Solomons, T. (2003). Organic Chemistry. United States of America: Wiley Publishing. McMurry, J. (1984). Organic Chemistry . United States of America: Wadsworth. Organic Chemistry Laboratory Manual. University of the Philippines Diliman, Quezon City. 2006 ed.
APPENDIX
** See attached paper for Data Sheet