Spectroscopic investigation of azo-hydrazo tautomerization in naphthalene-based azo dyes using 1D and 2D NMR

Abstract

An eco-friendly method for synthesizing naphthalene diazonium salts and azo dyes has been developed, offering a sustainable alternative to the traditional techniques requiring strong acids and extensive cooling. Aryldiazonium ions derived from nitroanilines are effective for vivid color labeling and precursors for durable azo dyes. In this method, naphthylamines (1-naphthylamine, 4-nitro-1-naphthylamine, and 2,4-dinitro-1-naphthylamine) undergo diazotization using tert-butyl nitrite and p-toluenesulfonic acid in an organic solvent at room temperature. The products obtained are bench-stable for several months and have been utilized as intermediates for azo dye formation by coupling with 2-naphthol at room temperature. A complete spectroscopic technique using 1D and 2D NMR with the mass spectrometric characterization of the dyes reveals that they exist as azo-hydrazone tautomers. The percentage hydrazone tautomer was calculated using the ¹⁵N chemical shift, ¹J_NH coupling constant, as well as the predicted ¹³C chemical shift. The result obtained shows that the synthesized dyes are predominantly present in hydrazone form (71%, 90%, and 94% for compounds 1c, 2c, and 3c, respectively). The electrospray ionization mass spectrometry analysis confirmed the mass of the dyes at m/z 298, 343, and 388, respectively. The environment-friendly and thermostable diazonium salts have been synthesized and also successfully employed as intermediates in synthesizing azo dyes with precise structural elucidation.

Graphical abstract

An eco-friendly synthesis of naphthalene diazonium salts and azo dyes was developed using tert-butyl nitrite and p-toluenesulfonic acid. 1D and 2D NMR spectroscopy revealed dominant azo-hydrazone tautomerism, with hydrazone content reaching up to 94%. This method offers sustainable alternatives for stable dye synthesis with precise structural elucidation.