Thermal and mass spectral characterization of novel azo dyes of p-acetoamidophenol in comparison with Hammett substituent effects and molecular orbital calculations

Four novel azo compounds were synthesized: o-phenylazo-(C₁₄H₁₃N₃O₂) (I), p-bromo-o-phenylazo-(C₁₄H₁₃BrN₃O₂) (II), p-methoxy-o-phenylazo-(C₁₅H₁₆N₃O₃) (III), and p-nitro-o-phenylazo-p-acetamidophenol (C₁₄H₁₃N₄O₄) (IV). These compounds were carefully investigated using elemental analyses, IR, and thermal analyses (TA) in comparison with electron ionization (EI) mass spectral (MS) fragmentation at 70 eV. Semi-empirical MO calculation, PM3 procedure, has been carried out on the four azo dyes (I–IV), both as neutral molecules and the corresponding positively charged molecular ions. These included molecular geometries (bond length, bond order, and charge distribution, heats of formation, and ionization energies). The mass spectral fragmentation pathways and thermal decomposition mechanisms were reported and interpreted on the basis of molecular orbital (MO) calculations. They are found to be highly correlated to each other. Also, the Hammett’s effects of p-methoxy, p-bromo, and p-nitro-substituents of phenyl azo groups on the thermal stability of these dyes (I–IV) are studied by experimental (TA and MS) in comparison with MO calculations, and the data obtained are discussed. This research aimed chiefly to throw more light on the structures of the four prepared azo derivatives of acetoamidophenol (p-cetamol). The data refering to the thermal stability of these dyes can be used in industry for effective dyeing purposes under different thermal conditions.