Alginic acid as a green agent for diazotization and azo dye synthesis

Abstract: Diazotization is a key process in organic synthesis, enabling the conversion of primary aromatic amines into diazonium salts, which are important intermediates in, for example, the synthesis of azo dyes. Traditional diazotization methods utilize strong mineral acids (e.g., hydrochloric acid/HCl), which pose safety, corrosion, and environmental risks due to their toxicity and handling hazards. Furthermore, these mineral acids are typically used in stoichiometric or excess amounts, are not readily recoverable or reusable after reaction, and require neutralization, resulting in substantial acidic and salt waste. To promote green chemistry principles, this study examines the use of a monomeric form of alginic acid (AA), a renewable, biodegradable, and low-toxicity biopolymer derived from brown algae, as an eco-friendly alternative acid medium for diazotization reactions that can be potentially recycled for subsequent use. Diazotization was carried out using aniline, 4-aminobenzenesulfonic acid, and other aromatic amines at both 0–5 °C and room temperature, followed by coupling with β-naphthol to produce various azo dyes. All seven dye systems successfully underwent diazotization under alginic acid conditions, with yields of 62–89% at 0–5 °C and 23–60% at room temperature. The recyclability and recovery of alginic acid were evaluated across all dye systems, with success observed for only C.I. Acid Orange 7 (6 reaction cycles). The stability of the diazonium salts derived from 4-methoxy aniline and 4-aminobenzene sulfonic acid was further evaluated at 30 min, 24 h, and 72 h, and the diazonium salts of 4-aminobenzene sulfonic acid showed high initial coupling efficiency and short-term stability up to 24 h, followed by rapid degradation between 24 h and 72 h. In contrast, (E)-1-((4-methoxyphenyl)diazenyl)naphthalen-2-ol, derived from the 4-methoxy aniline diazonium salt, showed lower initial yields but greater stability over time (up to 72 h), consistent with slower diazonium salt decomposition. These results indicate that alginic acid is a promising, safer, and more sustainable substitute for conventional mineral acids in diazotization, while highlighting the need for further work to optimize the process, evaluate scalability and environmental impact, and expand its applicability across diverse azo dye systems.