Azo dyes form a class of compounds that are widely used as food additives as well as coloring agents in textile industry (Gordon and Gregory, 1983; Fierz-David and Blangey, 1949). They constitute sixty to seventy percent of the dyes used for industrial purposes. The term “Azo” comes from the French term “Azote” which denotes nitrogen. The general formula of azo dye is R-N=N-R’, where N stands for Nitrogen and R stands for an alkyl (groups containing methyl, ethyl, propyl or other similar moieties) or aryl (aromatic moieties especially benzene) groups. The group -N=N- is called the Azo group. Azo dyes may have more than one azo group. Azo dyes come in a number of hues; however, the yellow and red ones are in preponderance. The different colors are generated by using different compounds for the reaction.
Azo dye synthesis using a two step reaction:
First step: The first step is called diazotization. Several compounds are used for diazotization such as aniline, para and meta nitroaniline, para and meta anisidine, para and meta toluidine and sulfanilic acid. In this step, the amine group can be replaced by any other group such as fluoride, chloride, bromide, iodide, hydroxyl, hydrogen, and cyanide. The reaction takes place between a primary aromatic amine and sodium nitrite under low temperature (preferably zero to five degrees Celsius) and acidic (aqueous) condition. The reason for carrying the reaction in low water conditions is to minimize the formation of phenol which occurs at higher temperatures. Sodium nitrite accepts hydrogen ions in solution and produces water and Nitrosonium ion. The unprotonated aromatic amine has a lone pair of electron that is nucleophilic and attacks the nitrosonium ion which is electrophilic. The reaction between primary aromatic amine and Nitrosonium ion generates N-nitrosoammonium ion. This then accepts a proton from water to generate N-nitrosamine and hydronium ion. The N-nitrosamine undergoes a tautomerization (rearrangement reaction) reaction to generate a diazenol. This then accepts another proton (hydrogen ion) and after internal arrangement generates a diazonium ion and water. The aromatic amine may have additional groups such as alkyl, halogen, nitro, hydroxyl or sulfonyl. When the same reaction is done to an aliphatic amine, it decomposes by releasing nitrogen.
Experiment for Diazotization
In the following example (Figure 1) diazotization reaction occurs in 2, 6 dibromo-4-methyl aniline (also known as 2, 6 dibromo-para- toluidine). The compound to be diazotized, in this case 2, 6 dibromo-4-methyl aniline is dissolved in 3 molar hydrochloric acid by slightly warming the solution. The acid reacts with 2, 6 dibromo-4-methyl aniline to form 2, 6 dibromo-4-methyl anilinium chloride intermediate. The whole setup is cooled to five degree Celsius or less and 1 molar sodium nitrite is added while continuously stirring the solution. The intermediate then reacts with sodium nitrite to form 2, 6 dibromo-4-methyl diazonium chloride which is unstable. The end point of the reaction is predicted with a starch-iodide paper test which should turn blue-violet. The salt should never be dried because it is extremely explosive. As soon as the diazonium salt forms, it should be used to generate the azo dye using step 2.
Figure 1: Step 1 for azo dye synthesis (diazotization)
Recent studies have shown that it is possible to perform diazotization in non-aqueous conditions using glycol nitrite in ethylene glycol instead of sodium nitrite (Kraska and Boruszczak, 1990). It is known that glycol solutions of diazonium salts have a good amount of stability. The advantage of using glycol nitrite in ethylene glycol is that it is environment friendly and can tolerate high temperatures. Studies have shown that diazotization reaction can be carried out using an aromatic amine with two amino group. Such a compound is 2, 6-diaminobenzothiazole which give rise to a 2-chlorobenzothiazole-6-diazonium intermediate upon diazotization (Desilets and Hamer, 1993). This reaction is very useful because dyes prepared this way are very stable.
Second step: The second step is called azo-coupling. The azo-coupling step involves a reaction between the diazonium salt and a coupling agent (phenol or amine). The diazonium ion is highly electrophilic (electron loving) whereas coupling agents are nucleophilics. The coupling reaction usually takes place in the “para” position. In a situation in which the para position is filled, it takes place in the “ortho” position. Different coupling agents are used such as Aniline, N,N-Dimethylaniline, meta-Phenylediamine, N-Methylaniline, Resorcinol, Phenol, 1-Naphthol and 2-Naphthol. The azo-coupling step undergoes minor modifications based on the coupling agent used. If the coupling agent is acidic, it is dissolved in 1 molar sodium hydroxide. The suspension is the cooled and to this the diazonium salt is added while stirring regularly. The solution is then allowed to stand for fifteen minutes for the azo dye to precipitate out and crystallize. If crystallization does not occur, the pH of the solution is adjusted with sodium hydroxide or hydrochloric acid to initiate the formation of azo dye crystals. If the coupling reagent is basic, 1 molar hydrochloric acid is added and the solution is cooled. To this solution diazonium salt is added while stirring continuously. The solution is then neutralized with 3 M sodium carbonate which is basis and the solution is allowed to stand for fifteen minutes for the azo dye crystals to separate out. The reason for using acid or basic solutions to dissolve the coupling agent is because the azo-coupling reaction is very sensitive to pH fluctuations and does not occur unless the pH is neutral or less than neutral (low acid pH).
Experiment for Azo-coupling
In the following example, 2, 6 dibromo-4-methyl diazonium chloride generated in the previous experiment is used to react with para-cresol (also known as 4- methyl phenol) to produce azo dye and hydrochloric acid. In this case since phenol is acidic, it is dissolved in 1 molar sodium hydroxide, cooled and diazonium salt added to it to produce the azo dye.
Figure 2: Step 2 for Azo dye synthesis (Azo-coupling)
COSSH data and precautions:
- Eye protection and gloving should be done.
- Phenol and Hydrochloric acid are extremely corrosive and should not be allowed to come in contact with skin. In case of exposure skin should be washed with large amounts of water. Accidental spills can be contained by using sodium bicarbonate in case of hydrochloric acid..
- Diazonium salts are highly unstable and pose an explosive hazard and should not be allowed to dry, are toxic when accidentally ingested.
- Sodium nitrite is very toxic and irritates the eyes, skin and lungs, maybe a carcinogen.