Date Performed: September 14, 2011Synthesis of 1-Phenylazo-2-naphtholR.J. Bitare2, M.C. Endico1, & D. D. Orbe2Institute of Chemistry, University of the Philippines, Diliman, Quezon City 1101 Philippines1Department of Chemical Engineering, College of Engineering, & 2Department of Psychology, College of Social Sciences and Philosophy, University of the Philippines, Diliman, Quezon City, 1101 PhilippinesABSTRACTSynthesis of azo compounds can be achieved via the process of diazotization and subsequent azo coupling. In order to verify this reaction, 1-phenylazo-2-naphthol, commonly known as Sudan I, was synthesized via the reaction of a phenyldiazonium chloride solution with a basic -naphthol solution. The crude product obtained was then subjected to recrystallization and its melting point was obtained. Its property as an ingrain dye was also examined.Through the prescribed procedures, a 55.2 % experimental yield, presumed to be pure 1-phenylazo-2-naphthol, was obtained. The melting range obtained for the product was 8084 oC, far-off from the theoretical value of 131 oC. Despite not being as close as theoretically predicted, the occurrence of the said reaction is still confirmed by the obtained experimental results.

IntroductionAzo compounds are organic compounds which bear the functional group R-N=N-R in which the hydrocarbons can be either an aryl or alkyl; they are formally defined as derivatives of diazene (diimide), HN=NH, wherein both hydrogens are substituted by hydrocarbon groups. [8]Azo compounds are often vividly colored, mainly due to their highly conjugated structure. Because of this property, azo compounds are usually used as dyes. One such example is 1-phenylazo-2-naphthol, more commonly known as Sudan I, an orange-red powdered substance. It was previously used as dye for textiles and as food coloring but was subsequently banned for such uses due to its carcinogenicity.

Azo compounds are usually synthesized through diazodium coupling reaction, a type of electrophilic aromatic substitution.In order to synthesize an azo compound, arenediazonium salt is reacted with activated aromatic rings like phenols and other amines. The amine component of the compound must be less activated the coupling component. Meanwhile, the nitrogen in the salt serves as a bridge between the two rings in the reaction.The synthesis of 1-phenylazo-2-naphthol utilizing aniline and -naphthol is a two-step process involving diazotization and azo coupling, as shown by the following reaction diagram:[4]

Figure 1. General reaction to synthesize 1-phenylazo-2-naphthol via azotization and coupling reactionThis experiment aims to synthesize 1-phenylazo-2-naphthol via the aforementioned reaction.

Experimental DetailSynthesis of 1-phenylazo-2-naphthol is divided into two major procedural reactions: the diazotization reaction and the coupling reaction. As such, this experimental methodology follows the same trend with the addition of ingrain dyeing as an application procedure.There are two main preparations that were done during the course of this experiment: 1) preparation of the diazonium salt (phenyldiazonium chloride) solution and 2) preparation of the -naphthol solution.To prepare the diazonium salt solution, 0.2 mL aniline, 0.35 mL water and 0.5 mL concentrated hydrochloric acid were combined in a 50-mL Erlenmeyer flask strictly following the sequence of introduction as stated here. The solution was then subjected to a decrease in temperature via an ice bath to about 4 C. A cold mixture was produced after 1-mL of cold distilled water was combined into the mixture. To this resulting mixture, 0.3 g of NaNO2 crystals was gradually added while maintaining the temperature below 5 C.A -naphthol solution was prepared by simply dissolving 0.35 g -naphthol in 4.5 mL 5% aqueous NaOH in a 50-mL beaker. As with the diazonium solution, this preparation was cooled down and maintained at a temperature of about 4 C.To pre-test the synthesis reaction, ingrain dyeing was done. Because of the nature of the Sudan-1 (1-phenylazo-2-naphthol) dye, it doesnt react or bond well with cotton fabric because of the lack of functional groups in the fabric. As a counter method to this limitation, ingrain dyeing was developed. Instead of producing the dye independently then applying it to the fabric, Sudan-1 is synthesized inside the fabric so that it will be trapped in the individual fabric components where it will be formed. To do this, a 2 by 3 cm fabric was first soaked in the cold -naphthol solution for about 2-3 minutes. It was then removed, patted dry with filter papers and immersed in the phenyldiazonium chloride solution. As a result, a prominent red-orange dye formed within the fabric. After some more minutes, it was removed from the solution and rinsed well with running water. The red-orange dye remained in the fabric.After the ingrain dyeing, synthesis of Sudan-1 using the combination of what remained of the 2 preparations was set. Phenyldiazonium chloride solution was added into the -naphthol solution slowly while stirring. The mixture was still maintained at about 4 C for 1-5 minutes before filtering the product and washing it with small portions of cold water. This crude product was recrystallized using minimal portions of hot ethanol in a steam bath. The resulting red crystals were collected, dried and weighed along a pre-weighed filter paper to determine the over-all yield in the synthesis reaction.Finally, the melting point of the product was determined using the common hot oil immersion technique.Results and DiscussionsTable 1. Experimental ResultsWeight of Sudan I0.30 g

Theoretical Yield0.54 g

Percent Yield55.2%

Melting Point80-84 oC

The red-orange azo dye 1-phenylazo-2-naphthol was obtained through the reaction of a diazonium salt intermediate formed from aniline with -naphthol. 0.2 mL of aniline and 0.35 g of -naphthol produced 0.3 g of 1-phenylazo-2-naphthol upon experimentation. The expected yield was .54 g or 2.19 mmol. Therefore, the experimental yield is just 55% of what could have been produced in a total and perfect synthesis.Of a very conspicuous note would be the discrepancy of the experimentally-determined melting point and that of what was theoretically proposed. The experimenters posit 2 possible sources of error for this inconsistency: 1) a large amount of impurity in the final product and 2) a self-imposed miscalculation during the actual melting point determination. The latter may have been caused by an inefficient set-up while the former is being attributed to potential lapses during the filtration phases.For the particular method of synthesizing 1-phenylazo-2-naphthol perfomed in the experiment, the reaction mechanism is as follows:

Diazonium coupling of aromatic diazonium ions normally happen only with active substrates such as amines and phenols, both of which are strongly activating ortho-para directors. Considering the size of the attacking species (in this aromatic electrophilic substitution), substitution occurs mostly in the para position relative to the activating group. The ability of amines to give ortho and para products shows that even in acidic medium, they are reacting in un-ionized forms. However, at very highly acidic concentrations, no reactions occur because the concentration of free amines becomes too small. This is the reason why concentrations of acidity are controlled in this experiment. However, this may also be a source of error as the pH levels werent monitored in the entire experiment. [7]

Figure 2. Resonance structure of the attack of the diazonium ion on carbon 1 of -naphthol.As shown in the figure above, there are 2 possible ways of coupling with the -naphthol. However, of these 2, the attack on carbon 1 is preferred because it favors retaining the resonance-stabilized structure of the ring and sets the positive charge at a position favorable to the entire synthesis.In addition to our product of interest, there are also 2 possible side reactions that may result into 2 unwanted product formations. The first one is the direct substitution of a nucleophile present in the interacting solution. The nucleophile may substitute the nitrogen in the diazonium salt and may form other aromatic compounds like phenol. The second one is the possibility for a decomposition reaction due to heightened temperatures. In this case, the product reverts back to aniline (the reverse reaction of what is in Figure 1).

Figure 3. Side reaction wherein a nucleophilic substitution involving nucleophiles such as OH occurs.Practical applications for the main product of this synthesis reaction was said to be in the field of dyeing. A dye is considered ideal if it is absorbed by the target fibers efficiently. For this to happen, the dye must be perfectly bonded with the material of the cloth. The functional groups present in the dye determine the strength of adhesion between the dye and the textile. The best dye is the one which is capable of being absorbed by most fibers. This dye may have a structure ideal for bonding with the material of mostly used textiles. As discussed earlier, Sudan 1 is not an efficient dye because its particle sizes and its reactivities are not efficient for use with certain textiles (i.e. cotton). This is particularly due to unmatched structures that inhibit direct bonding. Consequently, it was classified as a category 3 carcinogen[10]. Meaning, the substance is potentially harmful and may cause cancer in humans. Aside from 1-phenylazo-2-naphthol, which can be prepared by using aniline as the amine and -naphthol as the coupling component, other dyes that can be prepared via a similar format include:1. ,

which can be prepared by using as the amine and as coupling component;2. ,

which can be prepared by using as the amine and as coupling component; and3.

which can be prepared by using as the amine and as coupling component.ConclusionThe final results show that the experiment was able to give a 55.2% yield of a probably still impure sample of Sudan-1.Regardless of the percent yield, the fact that 1-phenylazo-2-naphthol was produced and had given a positive qualitative result (red-orange dye) validates the theoretical reaction of diazotization and its corresponding coupling reaction. This, in correspondence, tells us that the primary objective of this experiment to prove that an azo dye synthesis exists via the said reactions has been satisfactorily reached.Despite all this, a complementary test (melting point determination), seem to be a problematic issue in this experiment. Not that this only severs an empirically tested and theorized physical property of Sudan 1 (melting point), but it also opens up concerns about sound experimental techniques used by the experimenters during the course of the experiment. With this at hand, the experimenters recommend further investigation on more efficient ways of a total organic synthesis for 1-phenylazo-2-naphthol.

References[1] Bristol, T. S.-U. (2002). Azo Dyes. Retrieved September 19, 2011, from Chemistry of Dyes: http://www.chm.bris.ac.uk/webprojects2002/price/azo.htm [2] Brown, W. H., Foote, C. S., Iverson, B. L., & Anslyn, E. V. (2005). Organic Chemistry (5th ed.). United States of America: Brooks/Cole Cengage Learning.[3] Carey, F. A. (1996). Organic Chemistry (3rd ed.). USA: McGraw-Hill.[4] Chemistry, P. S. (n.d.). Synthesis of the Dye Sudan I. Retrieved September 20, 2011, from http://courses.chem.psu.edu/chem36/Web%20Syn06/Exp31Syn06.pdf[5] Georgia, U. o. (n.d.). Diazotization reactions. Retrieved September 19, 2011, from http://asmehta.myweb.uga.edu/diazotization.html[6] Institute of Chemistry, University of the Philippines, Diliman. (2008). Organic Chemistry Laboratory Manual. Quezon City: Institute of Chemistry, UP Diliman.[7] March, J. Advanced Organic Chemistry, McGraw-Hill, 1968, 398-400. [8] McMurry, J. E., Organic Chemistry (7th ed.). Singapore: Thomson Learning, 2008, 941-945.[9] Mohrig, J.R., Hammond, C. N., Morrill, T. C., Neckers, D.C., Experimental Organic Chemistry. W.H. Freeman, 1998, 719-720, 289-291.[10] Refat N.A., Ibrahim Z.S., Moustafa G.G., Sakamoto K.Q., Ishizuka M, Fujita S. (2008). "The induction of cytochrome P450 1A1 by sudan dyes". J. Biochem. Mol. Toxicol. 22 (2): 7784.

AppendixCalculations:

.Theoretical yield:

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