Synthesis of 1-Phenylazo-2-Naphthol or Sudan-1

Results and Discussion Ever since William Henry Perkin, an 18 year old research assistant, accidentally synthesized “mauve” from aniline, while he was trying to obtain quinine from allytoluidine, the world of dyes was never the same. After his discovery, fellow chemists began experimenting with aniline and its reaction with the many component compounds of coal tar. This research led into some fundamental knowledge about these reactions1. Synthetic dyes, otherwise known as azo dyes, are created by the reaction between a diazonium salt with a phenol in the presence of sodium hydroxide2. In this experiment, 1phenylazo-2-naphthol, or more commonly known as Sudan-1, was synthesized from aniline and β-naphthol in cotton.

aniline. This will create an N-nitroso derivative (Figure 1)2. H

H

H

N

-H+

+

-

N

In this experiment, phenyldiazonium chloride, a diazonium salt, was prepared from aniline. Since nitrous acid is unstable, it was generated by the reaction of sodium nitrite (NaNO2) with hydrochloric acid (HCl)3. It is important to note that this reaction should take place at around 0°C, since the diazonium salt will decompose immediately at higher temperatures to form other compounds2. Due to this, the whole synthesis took place in an ice bath. Diazotization occurs in three simpler steps, which will produce the desired phenyldiazonium chloride from aniline. In the first step, the acidificied nitrite solution (NaNO2 + HCl) produces the nitronium ion which will replace a hydrogen in the amine group of

O

O

Figure 1. A nitrosonium ion electrophilically replaces a hydrogen from the amine group The second step in the diazotization process includes the N-nitroso derivative tautomerizing into its hydroxydiazo form (Figure 2)2.

N O

H

N

N Diazonium salts are prepared in a process called diazotization. In diazotization, primary amines are oxidized by nitrous acid (HNO2). It is important to note that aliphatic diazonium salts react immediately via SN1, E1 and/or SN2, since molecular nitrogen is one of the best leaving groups. However, aromatic diazonium salts can be isolated and be used in other reactions, due to the fact that benzene rings do not readily undergo SN1 and SN2 reactions3.

N N

N O H

Figure 2. The N-nitroso derivative tautomerizes into its hydroxydiazo form In the final step of the process, the hydroxydiazo form of the N-nitroso derivative yields the phenyldiazonium ion, under acidic conditions (Figure 3)2. N

+

N OH

N

N

+

+H

Figure 3. The final step of diazotization, forming the phenyldiazonium ion After the phenyldiazonium chloride solution was prepared, the phenol part of the dye was then prepared. To create Sudan-1, βnaphthol was dissolved in NaOH, to facilitate its reaction with phenyldiazonium chloride1.

The process, in which the components, phenyldiazonium chloride and β-naphthol, react, is called coupling. The coupling reaction is an electrophilic substitution, where the diazonium ion reacts at the position with the greatest electron availability2. Since the diazonium ion is only weakly electrophilic, the coupling component should have strong, activating electron donating groups, such as OH2 or NR21. To understand the process of coupling between the phenyldiazonium ion and βnaphthol, the properties of dyes and fabrics will be discussed. Not all dyes are suitable for any kind of fabric. In general, a good dye should have two important properties: fastness and levelness. Fastness refers to the dye’s ability to bond with the fabric material. A good dye should not fade after multiple washings or when exposed to light. Levelness refers to the uniformity of the dye on the material. A level dye should be able to evenly color a fabric after application4. The mechanism behind the bonding between the dye and the fabric used can be explained by the structures of both the dye and the fiber4. In this experiment, cotton was used as the material in which Sudan-1 is applied to. Cotton (Figure 4), made up of only hydroxylic groups in its structure, does not bond well with simple azo dyes, like Sudan-1. Although simple azo dyes cannot directly bond with cotton, it can be still be to dye cotton via ingraining. This means that the two components of the dye will diffuse into the pores and spaces of the fiber. The fully formed dye will not be able to do this, as it is too large, and will only bond to the surface of the fiber. When the components of the dye react inside the fiber, the dye will be trapped inside, or “in the grain” of the fibers4. Thus, to apply Sudan-1, to the cotton fabric, it should be developed via ingrain dyeing. The cloth was first dipped in the phenyldiazonium chloride solution, then the βnaphthol solution. After the ingrain dyeing, the cloth developed an orange-red color (Figure 5).

Figure 4. The general structure of cotton

Figure 5. The cloth after ingrain dyeing After ingrain dyeing, the remainder of the two solutions were mixed and the pure product was allowed to recrystallize. 1phenylazo-2-naphthol was created by a coupling reaction between the phenyldiazonium chloride and the β-naphthol solution (Figure 6). An intermediate with resonance structures was formed (Figure 7), making the reaction relatively stable. It can be observed that the phenyldiazonium ion attacks on carbon 1 of βnaphthol. As said earlier, electron donating groups are needed to attract the weakly electrophilic diazonium ion. Since electrondonating groups are ortho/para directors, the diazonium attacked carbon 1, which was in the ortho position to the –OH group. +

N

HO

HO N N

N

+ Figure 6. The general reaction between the phenyldiazonium ion and β-naphthol

HO N

+

+

HO N

N

N

N

N

N

O

-

OH-

N H C+

H

Figure 7. Two of the resonance structures of the coupling reaction intermediate.

Figure 10. Formation of diazotate ion, due to high pH

It should be noted that certain conditions should be followed to successfully synthesize 1phenylazo-2-naphthol. If a condition is not followed, side reactions and unwanted products may occur. One condition is the presence of excess hydrochloric acid during diazotisation. If less hydrochloric acid is used than intended, coupling will occur between the diazonium salt and the amino group in the aniline to give diazoamino compounds. Thus, in the case of the experiment, phenyldiazonium chloride and aniline will yield diazoaminobenzene (Figure 8)2.

References

+

N

HN

H2N

N

NH

NH

+ Figure 8. Formation of diazoamino compounds due to lack of HCl Another condition is the pH in which the reaction takes place. A low pH will keep the amine from reacting, caused by the protonation of the amino group (Figure 9), while a high pH will cause the diazonium salt to transform to a diazotate ion, which is incapable of coupling (Figure 10)1. H2N

+

+

H3N

H

Figure 9. Protonation of the amino group, due to low pH

[1]

[2]

[3]

[4]

Lehman, J.W. Operational Organic Chemistry: A Laboratory Course, 2nd ed.; Allyn and Bacon, Inc.: Needham Heights, MA, 1988; pp. 400-404. Furniss, B.S., Hannaford, A.J., Smith, P.W.G., Tatchell, A.R. Vogel’s Textbook of Practical Organic Chemistry, 5th ed.; John Wiley & Sons, Inc.: New York, NY, 1989; pp. 920-921; 946-949. Loudon, G.M. Organic Chemistry, 3rd ed.; The Benjamin/Cummings Publishing Company, Inc.: Redwood City, CA, 1995; pp. 1139-1144. Pavia, D.L. Introduction to Organic Laboratory Techniques: A Contemporary Approach; Saunders College: Philadelphia, PA, 1982; pp. 254-260.