EXPERIMENT 16- SYNTHESIS OF 1-PHENYLAZO-2-NAPHTHOL

HONEYLYN A. HERBIETO Department of Mining, Metallurgical and Materials Engineering, University of the Philippines, Diliman, Quezon City FEBRUARY 14 AND 19, 2014 MARCH 5, 2014

Abstract

Azo dyes, one of the most common dyes can be synthesized by diazonization followed by coupling reaction. In this experiment, 1-Phenylazo-2-Naphthol, also known as sudan-I an azo dye was synthesized from phenyldiazonium chloride and ß-naphthol. The amount of sudan-I obtained was 0.29g which is 52.73% of the theoretical yield of 0.55g. Error could be accounted by the decomposition of phenyldiazonium chloride which at temperature above 4°C could result in the formation of side products. Ingrain dying was also done in the experiment to observe how sudan-I dyes a cotton fiber.

Keywords: Azo dye, 1-phenylazo-2-naphthol, ingrain dying

I. Introduction

Dyes have functional groups called

chromophores that absorb light in the visible region, giving the dye a colour. Chromophores have conjugated double bonds system that consist of an alternating double and single bond structure, this allows some delocalization of electron to occur, absorbing the energy in light and consequently reflecting colour.[1]

Auxochromes are groups attached to the chromophores that influence dye solubility [2]; they deepen the colour of the dye and provide sites where the dye can chemically bond with the fabric [1]. Few examples of auxochromes are of the type –NHR, -NHR2, -OH and –OR.

Azo dyes, one of the most common dyes contain –N=N- which links two aromatic rings together, this conjugation allows visible light to be absorbed [3]. Azo dyes can be synthesized by converting a primary amine to a diazonium ion and reacting the product with an aryl compound through electrophilic aromatic substitution reaction.

The main objective of this experiment is to synthesize 1-phenylazo-2-naphthol or sudan-1, an azo dye, orange in colour from ß-naphthol and phenyldiazonium chloride. Ingrain dying will also be observed in the experiment.

II. Methodology

Phenyldiazonium chloride solution was first

prepared at the start of the experiment. 0.2 mL aniline was added with 0.35 mL water and 0.5 mL concentrated HCl in 50-mL Erlenmeyer flask. The solution was cooled

to 4°C and afterwards, 1-mL of ice cold distilled water was added. 0.3 g of NaNO2 crystals were added gradually to the solution keeping the temperature of the reaction below 5°C.

ß-naphthol was then prepared next in a 50-mL beaker by dissolving 0.35 ß-naphthol in 4.5 mL 5% aq. NaOH. This was then cooled to 4°C.

To observe ingrain dying, a cotton fabric of 2X3 cm was dyed in cold ß-naphthol solution that was still immersed in an ice bath, soaking the fabric for 2-3 minutes. After soaking the fabric, remove from the bath and pat dry between filter papers. The fabric was then immersed in the phenyldiazonium chloride solution and after several minutes was removed from the solutions and was rinsed well with running water.

In order to synthesize sudan-1, the remaining phenyldiazonium chloride solution was added to the cold ß-naphthol solution. Allow solution to stand for 1-5 minutes at 4°C. Product was filtered and washed thoroughly with small portions of cold water. Crude product was recrystallized using minimal amount of hot ethanol in a steam bath, melting point was determined using the recrystallized product.

III. Results and Discussion

In synthesis of sudan-1, it involves two reaction, diazotization and coupling reaction. Phenyldiazonium chloride, a diazonium salt is formed from the diazotization of aniline. This was then added with phenol to for 1-phenylazo-2-naphthol. The obtained data are as follows:

Weight filter paper 0.73 g

Weight Sudan-1 obtained 0.29 g

% yield 52.73%

Melting point 131-133°C

In the diazotization reaction, the aniline was

reacted with acid and NaNO2. The NaNO2 added with the acid would produce NO+ ions that would react with the nitrogen in aniline forming the intermediate, it then tautumerizes and with HCl forms the diazonium ion. After forming the intermediate, coupling reaction is then done by the addition of phenol. Diazonium ions are weak electrophile, and can react with aromatic compounds such as phenols.

For this experiment, ß-naphthol was added to the diazonium ion formed. ß-naphthol was prepared in basic solution because it reacts more readily in a couple reaction, although excessive base in the solution could lead to the formation of another ion (diazotate ion) instead.

Diazonium ion was supposed to attack the para

of the arylamine, however, since it is already occupied, it attacks the ortho position instead which is readily available.

Fig 1. Resonance structure of intermediate diazonium ion

In the experiment, the phenyldiazonium chloride was maintained to a temperature below 5°C, this is because at higher temperature, the salt is

unstable and tends to decompose. The water can react with the diazonium salt and could replace the N2, a good leaving group, with –OH from the water.

Dyes are known to be used to colour their substrate, so they must be designed to have a good affinity with the substrate and that afterwards, it would be permanent, not easily fading under water or sunlight [2]. In the experiment, ingrain dying was done for the cotton with sudan-1. The cloth was first immersed in ß-naphthol, pat dried and then added to the phenyldiazonium chloride (diazonium salt) and again pat dried. After the process, it was then observe that the cotton cloth now had the permanent colour of sudan-1, an orange-brown compound. Direct dying cannot be done on the cotton cloth because cotton is made up of cellulose which is not reactive to cations or anions, lacking the polarizing group. In order for effective dying to occur, sudan-1 is synthesized in the spaces between the fabric making the colour permanent.

IV. IV. Conclusion

The main objective of the experiment was to synthesize sudan-1. Only 0.29g of sudan-1 was obtained from the theoretical of 0.55g, which was only 52.73%. synthesizing sudan-1 involved multi-steps that needed the temperature below 5°C, error could have been made and some side products formed decreasing the total yield of sudan-1. Human errors could also be accounted for throught the experiment, such as not keeping temp low, contamination of solutions, contamination in recrystallization and others.

V. Reference

[1] "Experiment #1, Synthesis of an Azo Dye for Incorporation

Into Crystals". pages 14-16

[2] "General Introductions to the Chemistry of Dyes,

Principles of colour chemistry". pages 57-58.Accessed at

http://monographs.iarc.fr/ENG/Monographs/vol99/mono99-

7.pdf on March 1, 2014.

[3] "Experiment 11, Synthesis of Sudan-I".

Department of Chemistry, Chulalongkorn

University.2012.

[4] Solomons, T. W. (1996). Organic Chemistry (6th

Edition). New York: John Wiley & Sons, Inc.

[5] “Organic Chemistry Laboratory Manual:

Experiment 16: Synthesis of 1-Phenylazo-2-

Naphthol”, 2008 ed. Institute of Chemistry,

University of the Philippines Diliman, Quezon City 1101

VI. Appendices