Efficient and Less Hazardous Synthetic Procedure to Obtain Arenodiazonium Salts

MANUEL I. VELASCO; LAURA I. ROSSI; RITA HOYOS DE ROSSI.

Venecia, Italia

Congreso; 10th Summer School on Green Chemistry; 2008

Consorzio Interuniversitario " La Chimica per l´Ambiente " (INCA)

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 la Torre, esq Medina Allende, Córdoba, Argentina. E-mail: mvelasco@fcq.unc.edu.ar     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 5ºC, amines are insoluble, and many by-products are obtained. In classical chemistry, the manufacture of this industrially important family of compounds is traditionally associated with large volumes of hazardous and colored aqueous wastes. The release of this sewage in natural environments is very problematic to aquatic life and mutagenic to human, and “end of pipe” treatments techniques do not provide a complete solution to the problem. So, new synthetic procedures to obtain azo dyes that are environmentally benign are desired. 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