Development of a more efficient and less hazardous synthetic procedure to produce azo dyes
Velasco, M.I., Rossi, L.I. y Hoyos de Rossi, R.
INFIQC, Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba. Haya de
Arenodiazonium salts are important intermediates in the synthesis of aromatic substituted compounds [1] with application in many industrial areas as textile, digital impressions, photography, food colorant, pharmaceutical, cosmetic, molecular recognition and nanocircuit design [2] . The most important method for the preparation of diazonium salts is treatment of aromatic amines with sodium nitrite in the presence of a mineral acid. However, this synthetic procedure involves aqueous media and these represent several disadvantages. In water these salts are unstable at temperatures higher than
A synthetic alternative for the generation of nitrogen dioxide or nitrire or nitrous acid, depending on the media, is using metallic nitrates from different metals of the first transitios series with the correspondeing halide.[3] In these conditions, selective sulfoxidation of organic sulfur can be achieved.[4]
Using different source of nitrites as Fe(NO3)(s)/HCl(aq), HNO3(aq)/HCl(aq), HNO3(aq)/HCl(g); and acetonitrile as solvent, diazotation of a wide variety of aromatic amines was studied. The diazotated amines were used in coupling reaction with a-naphthol and b-naphthol to obtain the corresponding azo dye.
These reactions were compared to those prepared under the classical methodology[5]. From this comparison we can conclude that this new procedure present longer reaction times, reaction rates are comparatuive or lower than those in the classical procedure, but yields and purity are significantly improved. Moreover, in these reaction medias by-products are not observed so substrates and solvent can be re-used. As the molar ratios are improved, amounts of reactants are reduced: in classical synthesis the molar ratio aniline:naphthol is 3:1 while in our system it is 1,5:1 or 1:1. All this reduce the environmental impact because no phenolic wastes are generated, no colored water is discharged with the effluents and the organic solvent can be recovered. Amines that are insoluble in water require large volumes of acid to dissolve them, but they are freely soluble in ACN so solvent amount is reduced, decreasing the total volume of wastes. All the above mentioned makes the presented procedure ?Greener? than the classical.
[1] A. Roglans, A. Pla-Quintana, M. Moreno-Mañas. Chem. Rev. 2006, 106, 4622. and cites therein.
[2] H.S. Freeman, A.T. Peters, Colorants for Non-Textile Applications, 2000, Elsevier
[3] a) L.I. Rossi, S.E. Martin. Appl. Catal. A: Gen. 2003, 250, 271. b) A.R. Suárez, A.M. Baruzzi, L.I. Rossi. J. Org. Chem. 1998, 63, 5689.
[4] C.O. Kinen, L.I. Rossi, R.H. de Rossi. Appl. Catal. A: Gen. 2006, 312, 120.
[5] A.Vogel. A textbook of practical organic chemistry. Longman, 4th edition. Londres 1978